hytel design guide
DESCRIPTION
Injection mould elastomer design guideTRANSCRIPT
Technical Information
Hytrel®Thermoplastic Polyester Elastomer
Start with DuPontEngineering Polymers ® DuPont registered trademark
Module V
Design information
®
Table of contents
Introduction
1. Physical properties of HYTREL®
1.1 Mechanical properties1.1.1 Tensile properties1.1.2 Compressive properties1.1.3 Elastic modulus1.1.4 Flexural modulus1.1.5 Creep modulus1.1.6 Compressive creep1.1.7 Useful temperature range1.1.8 Dynamic properties1.1.9 Resistance to fatigue1.1.10 Ross Flex (ASTM D 1052)1.1.11 DeMattia Flex (ASTM D 813)1.1.12 Friction and wear1.1.13 Impact resistance1.1.14 Notched Izod impact (ASTM D 256)1.1.15 Instrumented impact (ASTM D 3763)1.1.16 Brittleness temperature (ASTM D 746)1.2 Thermal properties1.3 Electrical properties1.4 Physical properties1.5 Other physical properties
2. Effect of environment
3. Regulatory compliance
4 Design concepts for HYTREL®
4.1 General considerations4.1.1 Defining the end-use requirements4.1.2 Design checklist4.1.3 Drafting the preliminary design4.1.4 Prototyping the design4.1.5 Testing the design4.1.6 Taking a second look4.1.7 Writing meaningful specifications4.1.8 Setting up production 4.1.9 Controlling the quality4.2 Multifunctional component integration4.2.1 Hydraulic piston seal4.2.2 Vacuum or pressure activated mechanism4.2.3 Linkage suspension4.2.4 Solenoid mount4.3 Controlling shock, vibration and noise4.3.1 Solenoid mount4.3.2 Engineered footwear4.3.3 Suspension system4.3.4 Strut rebound bumper
5. Designing for specific applications5.1 Bearings and seals5.1.1 Unlubricated bearings5.1.2 Lubricated bearings5.1.3 Design considerations5.2 Boots and bellows5.3 Rollings diaphragms5.4 Belts5.5 Coiled tubing and cables5.6 Reinforced hose5.7 Seals, valves and pumps
Design principles for HYTREL® thermoplastic polyester elastomers
HYTREL® thermoplastic polyester elastomers are high per-formance polymers characterized by:• Excellent flexibility at room and low temperatures.• Excellent flex crack resistance.• High resilience.• Excellent resistance to stress relaxation and creep.• Resistance to swell in oils and aliphatic and aromatic hydro-
carbons at moderate temperatures.
Conventional thermoplastic processing methods such as injec-tion moulding, extrusion, melt casting and rotational mouldingcan be utilized for producing tubing, rods, slabs, sheeting, filmor specific shapes. Machining operations like bandsawing,turning, milling, drilling and tapping can be employed as fab-rication or finishing techniques when necessary.
This brochure is intended to assist design engineers in the successful and efficient design of parts of HYTREL® thermo-plastic polyesterelastomers. Many of the same design considerations apply to HYTREL® as to metals and other engi-neering materials of construction.
It is common practice to use standard engineering equations for designing with HYTREL®. However, since all engineeringmaterials are affected to some extent by temperature, moistureand other environmental service conditions, it is necessary todetermine the extreme operating conditions and to design a partso that it will perform satisfactorily under all these conditions.
The selection of the best material for any application requiresa knowledge of the properties for all candidate materials andhow they satisfy the requirements of the application. HYTREL®
may be chosen for a job because of one or a combination of itsproperties.
Much of the engineering data needed in designing withHYTREL® is given in the following pages and should be help-ful to the designer. However, it is always important to test prototypes of a proposed design and material under realisticconditions before making production commitments.
Introduction
1. Physical properties of HYTREL®
1.1 Mechanical propertiesThe hardness of HYTREL® thermoplastic polyester elastomersspans the range from hard rubber at 40 durometer D to engi-neering plastics with a hardness of 72 durometer D. Typicalproperties for various types of HYTREL® are shown in Table 1.01.
1.1.1 Tensile propertiesTensile properties over a range of temperatures are shown inFigures 1.01 through 1.05. Figures 1.06 through 1.10 show ten-sile properties at low strain levels, which is the range most usedfor design purposes. HYTREL® polyester elastomer will yield, as most thermoplastic do, when strained and will take som permanent set. Values of tensile set versus strain are shown inFigures 1.11 and 1.11a.
1
Table 1.01 Typical mechanical properties
ASTM HYTREL® thermoplastic polyester elastomerProperty Method Units 4056 G4074 5556 6356 7246Hardness D 2240 Shore D 40 40 55 63 72Specific gravity D 792 – 1,17 1,18 1,20 1,22 1,25Melting point D 3418
Peak of endotherm °C 150 170 203 211 217Melt complete °C 170 183 220 230 232
Tensile strength D 638 MPa 28,0 13,8 40,0 40,0 45,8Elongation at break (23°C) D 638 % 550 207 500 420 360Flexural modulus D 790 MPa 62 65,5 207 330 570Resilience, Bashore – % 62 53 53 43 N.A.Compression set, 22 hrs at 70°C1
Constant load (9,3 MPa) D 395A % 27 28 4 2 2Tear strength D 624
Die B kN/m 110 102 164 185 238Die C kN/m 122 70 158 149 200
Shear strength D 732 MPa 24 17 37 48 52Resistance to flex cut growth
Ross (pierced) D 1052 cycles to failure >1 × 106 >1 × 106 >5 × 105 5 × 105 3 × 104
(unpierced) D 1052 cycles to failure >1 × 106 >1 × 106 >1 × 106 1 × 106 N.A.DeMattia (pierced) D 813 cycles to failure >1 × 106 >1 × 106 >1 × 106 5 × 105 N.A.
Vicat softening point D 1525 °C 108 112 180 195 207Heat deflection temperature D 648
0,46 MPa °C 54 50 90 115 1301,82 MPa °C N.A. N.A. 49 51 52
Water absorption, 24 hours D 570 % 0,6 2,1 0,5 0,3 0,3Poisson’s ratio – – 0,45 0,45 0,45 0,45 0,451 Can be improved by annealing.N.A. – Not Applicable.
2
Strain (%)
Tens
ile s
tres
s (M
Pa)
100 200 300 400 500 600 7000
30
10
20
50
60
70
40
–40°C
–20°C
0°C
23°C
65°C
100°C
120°C
Strain (%)
Tens
ile s
tres
s (M
Pa)
100 200 300 400 500 600 700 800 900050
60
60
60
60
60
60–40°C
0°C
65°C
150°C
–20°C
23°C
100°C120°C
Strain (%)
Tens
ile s
tres
s (M
Pa)
500200100 300 4000
20
0
10
40
30–40°C
–20°C
0°C
23°C
65°C
100°C120°C Strain (%)
Tens
ile s
tres
s (M
Pa)
600 700400 500300100 20000
20
10
40
30
50
70
60
–20°C23°C
65°C100°C
0°C
150°C
Dehnung (%)
Zugf
estig
keit
(MPa
)
100 200 300 400 500 600 700 800 90000
10
20
30
40
50
60
–40°C
–20°C 0°C 23°C
65°C
150°C
100°C120°C
Strain (%)
Tens
ile s
tres
s (M
Pa)
5 10 15 20 25 30 3500
1
2
3
4
5
6
7
8
9
10
11
12–40°C
–20°C
0°C
23°C
65°C
100°C120°C
Figure 1.01 Tensile properties of HYTREL® 4056 ASTM D 638
Figure 1.02 Tensile properties of HYTREL® G4074 ASTM D 638
Figure 1.03 Tensile properties of HYTREL® 5556 ASTM D 638
Figure 1.04 Tensile properties of HYTREL® 6356 ASTM D 638
Figure 1.05 Tensile properties of HYTREL® 7246 ASTM D 638
Figure 1.06 Tensile stress at low strain of HYTREL® 4056 ASTM D 638Strips: 6,35 mm wide – Strain rate: 25,4 mm/min
3
Figure 1.07 Tensile stress at low strain of HYTREL® G4074 ASTM D 638Strips: 6,35 mm wide – Strain rate: 25,4 mm/min
Figure 1.08 Tensile stress at low strain of HYTREL® 5556 ASTM D 638Strips: 6,35 mm wide – Strain rate: 25,4 mm/min
Figure 1.09 Tensile stress at low strain of HYTREL® 6356 ASTM D 638Strips: 6,35 mm wide – Strain rate: 25,4 mm/min
Figure 1.10 Tensile stress at low strain of HYTREL® 7246 ASTM D 638Strips: 6,35 mm wide – Strain rate: 25,4 mm/min
Figure 1.11 Tensile set ASTM D 412
Figure 1.11a Tensile set at low strain ASTM D 412
Strain (%)
Tens
ile s
tres
s (M
Pa)
5 10 15 25 30 352000
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15–40°C
–20°C
0°C
23°C
65°C
100°C
120°C
Strain (%)
Tens
ile s
tres
s (M
Pa)
105 2015 25 30 3500
2
4
6
8
10
12
14
16
18
20
22
24
26
28
30–40°C
–20°C
0°C
23°C
65°C
100°C
150°C120°C
Strain (%)
Tens
ile s
tres
s (M
Pa)
105 2015 3025 3500
10
20
30
40
50
60–40°C
–20°C
0°C
23°C
65°C
120°C150°C
100°C
Strain (%)
Tens
ile s
et (%
)
3 51 6 842 97 11 121000
0,2
0,4
0,6
0,8
1,0
1,2
1,4
1,6
1,8
2,0
7246
6356
5556
4056
HYTREL®
Strain (%)
Tens
ile s
et (%
)
6020 100 140 2001601208040 18000
20
40
60
80
100
120
140
7246 6356
5556
4056
See Figure 1.11a
HYTREL®
Strain (%)
Tens
ile s
tres
s (M
Pa)
105 2015 25 353000
20
10
40
30
50
70
60
–40°C
–20°C
0°C
23°C
65°C
100°C120°C150°C
1.1.2 Compressive propertiesCompressive stress/strain properties are shown in Figures 1.12through 1.16. Typical values for compression set of HYTREL®
under constant load and deflection are shown in Table 1.01.Method A (constant load) compression set values at differenttest temperatures are given in Table 1.12.
Compression set can be significantly improved by annealing.For best results, parts of HYTREL® 4056 or G4074 should be annealed for 24 to 48 hours at 100°C. A temperature of120°C for the same time period should be used for all othertypes.
Table 1.02 Compression set resistanceASTM D 395, method A, 9,3 MPa load
Compression set, % after 22 hours at:
Type of HYTREL® 23°C 70°C 100°C
4056 11 27 33
G4074 10 28 51
5556 <1 4 8
6356 <1 2 4
7246 <1 2 5
4
Strain (%)
Com
pres
sive
str
ess
(MPa
)
10 15 20 25 30 35500
2
4
6
8
10
12
14
16
18
20
22
24
26–40°C
–20°C
0°C
23°C
65°C
100°C
120°C
150°C
Strain (%)
Com
pres
sive
str
ess
(MPa
)
105 15 30 3520 250
10
20
30
40
50
60
0
–40°C
–20°C
0°C
23°C
65°C
120°C150°C
100°C
Strain (%)
Com
pres
sive
str
ess
(MPa
)
105 2015 3025 3500
2
4
6
8
10
12
14
16
18
20
22
24
26
28
30–40°C
–20°C
0°C
23°C
65°C
100°C
120°C
150°C
Strain (%)
Com
pres
sive
str
ess
(MPa
)
1510 2520 353050
20
10
40
30
60
50
80
90
70
0
–40°C
–20°C
0°C
23°C
65°C
120°C
150°C
100°C
Figure 1.12 Compressive properties of HYTREL® 4056 ASTM D 575
Figure 1.13 Compressive properties of HYTREL® G4074 ASTM D 575
Figure 1.14 Compressive properties of HYTREL® 5556 ASTM D 575
Figure 1.15 Compressive properties of HYTREL® 6356 ASTM D 575
1.1.3 Elastic modulusFigures 1.17 and 1.18 show values for elastic modulus in tension and compression versus temperature. These numbersare calculated from the linear portions of the stress-straincurves, that is, below the elastic limit, which is approximatelyseven to ten per cent for HYTREL®. Modulus changes with time under load, however, and this factor must be included in part design. See section entitled “Creep Modulus” for additional information.
1.1.4 Flexural modulusVariation of flexural modulus with temperature is shown inFigure 1.19. Differences in modulus values for tension, com-pression, and flexure will occur due to differences in strainrates shapes of samples, etc. Also, flexure tests emphasize thesurface of the sample, which will have moulded-in stresses thatare different from those in the interior of the sample, whichcools more slowly in the moulding process.
1.1.5 Creep modulusAn important factor to consider when designing with thermo-plastics is that the modulus of a given material will change dueto many factors including stress level, temperature, time andenvironmental conditions. Figures 1.20 through 1.23 are plotsof creep or apparent modulus versus time at various stress lev-els, all at room temperature. Generally, linear creep modulusplots can be extrapolated one decade of time with reasonablesafety. This has been done on the creep modulus plots and issignified by dashed lines. For critical applications, however,testing for the full expected life of the part should be done toverify these results. The highest stress level shown on each plot is the maximum recommended stress level for each material under long term loading. Higher stress levels mayresult in catastrophic failure of the part. In all cases, testingshould be performed on the fabricated part to verify satisfac-tory performance of the material in each application.
Figure 1.24 presents limited creep modulus data at 100°C.These plots are not extrapolated due to the unpredictableeffects that heat ageing under stress can have on materials.
5
Strain (%)
Com
pres
sive
str
ess
(MPa
)
105 2015 25 35300
20
10
40
30
60
50
80
70
100
90
0
–40°C
–20°C
0°C
23°C
65°C
100°C
150°C
120°C
Temperature (°C)
Flex
ural
mod
ulus
(MPa
)
20–20 600 12080 100 180140 16040–4010
2030406080
100
200300400600800
1000
2000
4056
HYTREL®
72466356
5556
Figure 1.16 Compressive properties of HYTREL® 7246 ASTM D 575
Figure 1.19 Flexural modulus versus temperature ASTM D 790
Figure 1.17 Elastic modulus in tension versus temperature ASTM D 638Strips: 6,35 mm wide – Strain rate: 25,4 mm/min
Figure 1.18 Elastic modulus in compression versus temperature ASTM D 575
Temperature (°C)
Elas
tic m
odul
us (M
Pa)
20 60 100 140 18080 1200 40–20 160–40
200300400600800
1000
10
2030406080
100
2000
4056
HYTREL®7246
6356
5556
G 4075
4056
G 4074
Temperature (°C)
Elas
tic m
odul
us (M
Pa)
20–20 400 60 80 100 120 140 180160–400
2030406080
100
200300400600800
1000
4056
HYTREL®
7246
63565556
G 4074
6
Time (hours)
Cree
p m
odul
us (M
Pa)
0,2 0,5 1 2 5 10 50 10020 500 1000200 5000 1000020000,11
2
34
68
10
20
3040
6080
100
1,4 MPa3,4 MPa3,8 MPa4,3 MPa4,8 MPa
5,5 MPa5,9 MPa
Extrapolated
Applied Stress
Time (hours)
Cree
p m
odul
us (M
Pa)
1
2
34
68
10
20
3040
6080
100
200
0,2 0,5 1 2 5 10 50 10020 500 1000200 5000 1000020000,1
Extrapolated
5,5 MPa
9,7 MPa11,0 MPa
8,3 MPa
3,4 MPa
Applied Stress
12,4 MPa
Time (hours)
Cree
p m
odul
us (M
Pa)
10
20
3040
6080
100
200
300400
600800
1000
0,2 0,5 1 2 5 10 50 10020 500 1000200 5000 1000020000,1
Extrapolated
Applied Stress
5,5 MPa8,3 MPa10,3 MPa
13,1 MPa
13,8 MPa
Figure 1.21 Tensile creep modulus of HYTREL® 5556 ASTM D 2990, 23°C
Figure 1.22 Tensile creep modulus of HYTREL® 6356 ASTM D 2990, 23°C
Figure 1.20 Tensile creep modulus of HYTREL® 4056 ASTM D 2990, 23°C
1.1.6 Compressive creepCompressive creep results for a load of 6,9 MPa at 23°C and 50°C are presented in Table 1.03. Creep in compression is much less than in tension, as can be seen by comparing thevalues for compressive creep with those for tensile creepshown in the same table. Values for tensile creep wereobtained by converting creep modulus data to creep strain with the formula:
Creep strain =
7
Time (hours)
Cree
p m
odul
us (M
Pa)
500,2 0,5 1 2 5 10 50 10020 500 1000200 5000 1000020000,1
20
3040
6080
100
200
300400
600800
1000
Extrapolated
Applied Stress
5,5 MPa
12,4 MPa
18,9 MPa
Cree
p m
odul
us (M
Pa)
10
20
30
40
50
60
70
80
90100110120130140150
200
Time (hours)0,2 0,5 1 2 5 10 50
HYTREL
10020 500200 10000,1
8,3 MPa
5,5 MPa
8,3 MPa
5,5 MPa
7246
7246
6356
6356
5556
Applied Stress
5,5 MPa
Figure 1.23 Tensile creep modulus of HYTREL® 7246 ASTM D 2990, 23°C
Figure 1.24 Tensile creep modulus at 100°C ASTM D 2990
stresscreep modulus
Table 1.03 Creep strain100 hours at 6,9 MPa stress
Compression creep, % Tensile creep, %Type of HYTREL® 23°C 50°C 23°C
4056 5,4 8,9 –G4074 6,0 11,5 –5556 0,6 1,3 8,06356 0,5 0,7 5,87246 0,5 0,5 2,5
1.1.7 Useful temperature rangeHYTREL® thermoplastic polyester elastomer exhibits excellentphysical properties over a broad temperature range. HYTREL®
4056 remains quite flexible down to –40°C. The harder gradesof HYTREL® retain good physical properties at temperatures ashigh as 180°C. Figure 1.25 presents information on dynamicelastic modulus versus temperature and is valuable for esti-mating the relative response characteristics of parts at varioustemperatures. This is a short term test, however, and does not consider the effects of heat ageing, so the information in Figure 1.25 should not be used for estimating part life at agiven temperature.
1.1.8 Dynamic propertiesFigures 1.25 and 1.26 present data on dynamic modulus anddamping factor (tan δ) versus temperature. This data is usefulin the design of parts used in dynamic applications such asmotor mounts and couplings.
1.1.9 Resistance to fatigueThe fatigue resistance of HYTREL® is excellent. Table 1.04 givesdata on the temperature rise due to hysteresis after twenty min-utes for two of the softer grades of HYTREL® when tested in aGoodrich Flexometer. The temperature rises fairly quicklyand then remains roughly constant for the balance of the test.
Table 1.05 lists the fatigue limits of four types of HYTREL®.Fatigue limit is defined by ASTM as the limiting value of stresswhich will yield a very large number of cycles before failure.Sample size and shape, frequency of flexing, ambient temper-ature and heat transfer all have significant effects on fatigue.For design purposes, tests simulating actual end use conditionsshould be performed to determine the expected fatigue limit.
One of the outstanding properties of HYTREL® thermoplasticpolyester elastomer is its resistance to cut growth in flexure. As can be seen from Table 1.01, HYTREL® can endure more thana million cycles without failure in the Ross and DeMattiapierced flex tests.
8
Temperature (°C)
Dyn
amic
ela
stic
mod
ulus
(MPa
)
–40–80 –60 20–20 0 8040 60 140100 120 160 220180 200–10010
20
3040
6080
100
200
300400
2000
30004000
600800
1000
HYTREL®
7246
6356
5556
4056
Figure 1.25 Dynamic modulus versus temperature ASTM D 2236
Figure 1.26 Damping factor versus temperature ASTM D 2236
Tan
(d)
–40–80 –60 20–20 0 8040 60 140100 120 160 220180 200–100
Temperature (°C)
0,02
0,01
0,2
0,3
0,03
0,040,050,06
0,08
0,1
HYTREL®4056
55566356
7246
1.1.10 Ross Flex (ASTM D 1052)A pierced strip test specimen of 6,35 mm thick is bent freelyover a rod to a 90° angle and the cut length is measured at frequent intervals to determine the cut growth rate. The cut isinitiated by a special shape piercing tool.
The test results are reported in Table 1.06 as the number ofcycles it took the specimen to grow five times the originalpierced length. These results are also reported in the brochure“General guide to products and properties of HYTREL®”.
1.1.11 DeMattia Flex (ASTM D 813)A pierced strip test specimen of 6,35 mm thick with a circulargroove restrained so that it becomes the outer surface of thebend specimen, with 180° bend, and the cut length is measuredat frequent intervals to determine the cut growth rate.
The test results are reported in Table 1.07 as the number ofcycles it took for the specimen to reach failure.
1.1.12 Friction and wear
Values for coefficient of friction of HYTREL® measured by twodifferent methods are shown in Table 1.08. As can be seen fromthe data, test conditions have a great influence on the results,therefore, it is difficult to predict frictional forces unless test-ing is performed under conditions that simulate the end use.
HYTREL® polyester elastomer has excellent wear properties inmany applications. Table 1.09 lists results from Taber and NBSabrasion tests. For information on wear in bearing applications,see “Bearings and Seals”, Chapter 3.
9
Table 1.04 Goodrich flexometer ASTM D 6362,54 mm stroke, 1,0 MPa static load, 23°C
Sample temperature after 20 minutesType of HYTREL® °C
4056 485556 66
Table 1.05 Flex fatigue ASTM D 671
Fatigue limite1
Type of HYTREL® MPa
4056 5,25556 6,96356 6,97246 11,01 Samples tested to 2,5 million cycles without failure.
Table 1.06 Resistance to flex cut growth, ross (pierced)ASTM D 1052Cycles to five times cut growth
Type of HYTREL® 23°CG3548 L >1 × 106
G4074, G4078 W >1 × 106
G4774, G4778 >1 × 106
G5544 8 × 105
3078 >1 × 106
4056 >1 × 106
4068 >1 × 106
4556 >1 × 106
5526, 5556 5 × 105
6356 5 × 105
7246 3 × 104
8238 –HTR4275 BK 5 × 104
5555HS 1 × 105
HTR5612 BK 6 × 105
HTR6108 6 × 105
HTR8068 –HTR8139 LV >1 × 106
HTR8171 >1 × 106
HTR8206 –
Table 1.07 DeMattia flex life (pierced) ASTM D 813Cycles to failure
Type of HYTREL® 23°CG3548 L 3,6 × 104
G4074, G4078 W 3,6 × 104
G4774, G4778 1,6 × 105
G5544 7 × 103
4056 >1 × 106
4068 1,7 × 105
4556 3,6 × 103
5526, 5556 >1 × 106
HTR4275 BK 5,4 × 104
HTR5612 BK 1,1 × 105
HTR6108 5,4 × 103
Table 1.08 Coefficient of frictionHYTREL® on steel HYTREL® on steelMoving sled – Thrust washer –ASTM D 1894 ASTM D 3702
Type of HYTREL® Static Dynamic Dynamic4056 0,32 0,29 0,855556 0,30 0,18 0,946356 0,30 0,21 0,907246 0,23 0,16 0,90
Table 1.09 Abrasion resistance ASTM D 1044 (mg/1000 rev)Taber abrasion NBS abrasionCS-17 H-18 ASTM D 1630
Type of HYTREL® Wheel Wheel (%)4056 8 109 590G4074 10 223 3005556 6 97 22506356 15 109 23407246 15 75 2620
10
1.1.13 Impact resistanceThe impact properties of polymeric materials are directlyrelated to their overall toughness, which is defined as the abil-ity of the polymer to absorb applied energy. Impact resistanceis the ability of a material to resist breaking under shock load-ing or the ability to resist the fracture under stress applied athigh speed.
Most polymers, when subjected to impact loading, seem tofracture in a characteristic fashion. The crack is initiated on thepolymer surface by the impact loading. The energy to initiatesuch a crack is called the crack-initiation energy. If the loadexceeds the crack-initiation energy, the crack continues topropagate. A complete failure occurs when the load exceeds thecrack-propagation energy. Thus, both crack initiation andcrack propagation contribute to the measured impact strength.
The speed at which the specimen or part is struck with anobject has a significant effect on the behaviour of the polymerunder impact loading. At low rates of impact, relatively stiff material can still have good impact strength; while at highrates of impact, even highly elastomeric material like HYTREL®
may exhibit brittle failure at low temperatures. All materialshave a critical velocity in which they behave as glassy, brittlematerials.
Impact properties are highly dependent on temperature. Generally, plastics are tougher and exhibit ductile modes of failure at temperatures above their glass transition tem-perature (Tg), and are brittle well below their Tg.
A notch in a test specimen, which creates a localized stressconcentration, or a sharp corner in a moulded part drasticallylowers impact strength. Only the harder grades show evidenceof notch sensitivity as shown in Table 1.11.
1.1.14 Notched Izod Impact (ASTM D 256)The objective of the Izod impact test is to measure the behav-iour of a standard notched test specimen to a pendulum-typeimpact load. The specimen is clamped vertically and theswinging pendulum is released with the notch on the oppositeside. The results are expressed in terms of kinetic energy consumed by the pendulum in order to break the specimen. The energy required to break a standard specimen is actuallythe sum of energies needed to deform it, to initiate its fracture,and to propagate the fracture across it, and the energy neededto throw the broken ends of the specimen.
1.1.15 Instrumented Impact (ASTM D 3763)One of the drawbacks of the conventional impact test methodis that it provides only one value, the total impact energy; itdoes not provide data on the type of fracture (ductile, brittle),dynamic toughness, fracture, yield loads or fracture behaviourbased on the geometry of the specimen.
The falling weight instrumented impact tester provides a com-plete load and energy history of specimen fracture mechanism.Such a system monitors and precisely records the entire impactevent, starting from the rest position to initial impact, plasticbending to fracture initiation and propagations to complete failure.
Measurement is done by mounting the strain gauge into thestriking tup and an optical device triggers the microprocessorjust before striking the specimen. The output of the strain gaugerecords the applied load variations to the specimen throughoutthe entire fracturing process. A complete load-time history(fracturing) of the entire specimen is obtained. The apparenttotal energy absorbed by the specimen is calculated and plot-ted against time.
Figures 1.27 to 1.31 show drop-weight-impact results for rep-resentative grades of HYTREL®. The plots show energy dissi-pated in rupturing the sample and the maximum force experi-enced by the tup as it punches through the sample.
1.1.16 Brittleness temperature (ASTM D 746)This test method establishes the temperature at which 50% ofthe specimens tested fail when subjected to the test conditions.The test evaluates long-term effects such as crystallization.Thermoplastic elastomers are used in many applicationsrequiring low-temperature flexing with or without impact.Data obtained by this test may be used to predict the behaviourof elastomeric materials at low temperatures only in applica-tions in which the conditions of deformation are similar to thetest conditions. Table 1.10 lists the brittleness temperatures forrepresentative grades of HYTREL®.
Table 1.10 Brittleness temperature ASTM 746Brittleness temperature
Type of HYTREL® °CG3548 L –60G4074,G4078 W –66G4774 –56G4778 –65G5544 –603078 <–1054056 <–1004068 <–1054556 <–1055526 <–705556 –1006356, 7246 <–708238 –92HTR4275 BK –1005555HS <–70HTR5612 BK –100HTR6108 –98HTR8068 –48HTR8139 LV <–100HTR8171 –63HTR8206 –67
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Temperature (°C)
Ener
gy (J
)
10
20
30
40
50
60
70
80
90
0–40 20–20 0 8040 60 100
7246
6356
HYTREL®
ductile failurebrittle failure
Temperature (°C)
Load
(N)
1000
2000
3000
4000
5000
6000
7000
0–40 20–20 0 8040 60 100
5556
4056
HYTREL®
Temperature (°C)
Load
(N)
1000
2000
3000
4000
5000
6000
7000
8000
0–40 20–20 0 8040 60 100
ductile failurebrittle failure
7246
6356
HYTREL®
Temperature (°C)
Bri
ttle
failu
res
(%)
–30–40 –10–20 100–50–600
10
20
30
40
50
60
70
80
90
100
7246
6356
HYTREL®
Figure 1.28 Drop weight impact failure energy versus temperature16 mm spherical tup – 32 mm diameter support
Figure 1.30 Drop weight impact failure energy versus temperature16 mm spherical tup – 32 mm diameter support
Figure 1.29 Drop weight impact failure load versus temperature16 mm spherical tup – 32 mm diameter support
Figure 1.31 Pourcentage of brittle failures versus temperature16 mm spherical tup – 32 mm diameter support
Temperature (°C)
Ener
gy (J
)
10
20
30
40
50
60
70
0–40 20–20 0 8040 60 100
5556
4056
HYTREL®
Figure 1.27 Drop weight impact failure energy versus temperature16 mm spherical tup – 32 mm diameter support
Table 1.11 Izod impact ASTM D 256, method A (J/cm)
Unnotched NotchedType of HYTREL® 23°C –40°C 23°C –40°C4056 >10,6 >10,6 >10,6 >10,6
No break No break No break No break5556 >10,6 >10,6 >10,6 >10,6
No break No break No break No break6356 >10,6 >10,6 >10,6 0,3
No break No break No break No break7246 >10,6 >10,6 2,1 0,4
No break No break
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1.2 Thermal propertiesThermal conductivity data are shown in Table 1.12 and coeffi-cients of linear thermal expansion are presented in Table 1.13.Figure 1.32 is a plot of specific heat versus temperature for fourtypes of HYTREL®.
Table 1.12 Thermal conductivityType of HYTREL® W/m ·°C4056 0,190G4074 0,1655556 0,1566356 0,1527246 0,149
Table 1.13 Coefficient of linear thermal expansionTemperature range Coefficient
Type of HYTREL® °C mm/mm/°C × 10–5
4056 –50 to –20 51–20 to +120 19
5556 –50 to –10 21–10 to +60 10+60 to +120 13
6356 –50 to –30 9–30 to +40 10+40 to +120 13
7246 –50 to +10 4+10 to +60 9+60 to +120 10
1.3 Electrical propertiesElectrical measurements show that HYTREL® polyester elas-tomers are suitable for low-voltage applications. High mechanical strength, coupled with excellent resistance tooils, solvents and chemicals, also makes HYTREL® suitable for many jacketing applications. The properties shown inTable 1.14 were measured on injection moulded plaques with the dimensions 76 × 127 × 1,9 mm.
1.4 Physical propertiesFor flow analysis, simulating mould filling, the physical prop-erties as shown in Table 1.15 are applicable. Viscosity data ofseveral HYTREL® grades is available in figures 1.33 to 1.36.
Table 1.15 Physical properties of HYTREL®
HYTREL® grade4056 5526 G5544 7246
Density kg/m3 1160 1200 1220 1250
Melt density kg/m3 1000 1020 1020 1050
Specific heat J/kg ·°C 2144 2186 2186 2152
No-flowtemperature °C 107 162 182 176
Freezingtemperature °C 50 122 152 130
Thermalconductivity melt W/m ·°C 0,16 0,15 0,15 0,12
Temperature (°C)
Spec
ific
heat
(kJ/
kg· °
C)
20 40–20 0 60 80 100 120 130–400
1
24056555672466356
HYTREL®
Figure 1.32 Specific heat versus temperature
Table 1.14 Electrical properties at room temperature and 50% R.H.ASTM HYTREL® HYTREL® HYTREL® HYTREL® HYTREL®
Property Method 4056 5526 5556 6356 7246Volume resistivity, ohm·m D 257 8,2 × 1010 1,2 × 1011 1,1 × 1010 9,7 × 1011 1,8 × 1012
Dielectric strength, kV/mm D 149 16,1 17,3 15,7 16,1 18,1Dielectric constant D 150
100 Hz 5,1 4,5 4,6 4,4 4,01 kHz 5,1 4,5 4,5 4,2 3,91 MHz 4,7 4,2 4,2 3,7 3,8
Dissipation factor D 150100 Hz 0,005 0,006 0,006 0,018 0,016
1 kHz 0,008 0,009 0,009 0,02 0,0191 MHz 0,06 0,04 0,04 0,04 0,036
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1.5 Other physical propertiesIn special cases, properties other than those discussed abovemay be of interest in designing parts of HYTREL®. A sample ofthe kinds of subjects covered by bulletins available fromDuPont are shown here.
Figure 1.34 Viscosity of HYTREL® 5526 vs. shear rate for three temperatures
Figure 1.35 Viscosity of HYTREL® G5544 vs. shear rate for three temperatures
Figure 1.36 Viscosity of HYTREL® 7246 vs. shear rate for three temperatures
Visc
osity
(Pa.
s)
10
Shear rate (s-1)102 103 104
100
1000
215235255
Visc
osity
(Pa.
s)
1000
10
100
Shear rate (s-1)102 103 104
210230250
Visc
osity
(Pa.
s)
1000
10
100
Shear rate (s-1)102 103 104
220240260
Figure 1.33 Viscosity of HYTREL® 4056 vs. shear rate for three temperatures
Shear rate (s-1)
Visc
osity
(Pa.
s)
102 103 10410
100
1000
190210230
Shear rate (s–1) Shear rate (s–1)
Shear rate (s–1) Shear rate (s–1)
Bulletin SubjectHYT-402 Injection moulding guideHYT-403 HYTREL® extrusion manualGeneral Design Principles(Module I) Machining of HYTREL®, design rules
2.2 Radiation resistanceThe increasing use of nuclear energy, for example in powerplants, military areas, and medicine, places new requirementson many rubber compounds as well as other materials. Somefactors of importance to market development of nuclear energyinclude, for example: the maximum dosage to which the mate-rial can be subjected without damaging effects, the possible useof additives to provide additional stabilization to radiation, andthe effect of radiation on physical properties.
Three uncompounded grades of HYTREL® thermoplastic poly-ester elastomers show excellent retention of physical proper-ties after irradiation at 23°C in air. (The combined effect ofheat-ageing or steamageing concurrent with radiation exposurewas not studied.)
Injection-moulded slabs of HYTREL® 4056, HYTREL® 5556,and HYTREL® 7246, 2 mm thick, were exposed to a 1,5 MeVelectron beam at Radiation Dynamics Ltd., Swindon, Wiltshire,U.K. The slabs were then tested by ASTM test methods.
For the most part, the radiation of prime interest from the stand-point of insulation damage has energy of the order of 1 MeV,which is principally gamma photons and fast neutrons. Damageis caused by collisions of this radiation with electrons and nucleiin the elastomer where the energy input from such collisionsmay be greater than the bond energies in the elastomer.
Most elastomers are embrittled by radiation exposure, whichinduces cross-links between molecules. This eventually givesa three-dimensional network, such as is seen in hard rubber orphenolic resins. A few polymers, notably butyl rubber, degradeby reversion to low-molecular-weight tars and oils.
Although upgrading changes can occur under controlled lowdosage (radiation cross-linked polyolefins), long exposurenormally produces degradation. Thus, the amount of change isdependent on radiation flux rate, total radiation dose, energy ofradiation, chemical composition of the polymer, environment(ambient temperature, air versus inert gas, steam exposure,etc.), and the initial properties of the elastomeric compound.The amount of change is independent of the type of radiationat equal energy*, whether alpha, beta, or gamma rays, or neu-trons. This is known as the equal-energy, equal-damageconcept.
Table 2.02 summarizes the effect of radiation on three hardnessgrades of HYTREL®. It will be seen that the exposure to 150kJ/kg (15 Mrad) produces very little change in the properties ofHYTREL®.
* R. B. Blodgett and R. G. Fisher, IEEE Transactions on power apparatus and systems, Vol. 88, No. 5,p. 529, (May 1969).
2. Effort of environment
Table 2.01 Permeabilitya of HYTREL® to gasesGas HYTREL® 4056 HYTREL® 5556 HYTREL® 6356 HYTREL® 7246Air 2,4 × 10–8 1, 8 × 10–8 – –Nitrogen 1,7 × 10–8 1,4 × 10–8 – –Carbon dioxide 3,5 × 10–7 1,8 × 10–7 – –Helium 15,7 × 10–8 9,9 × 10–8 – 3,2 × 10–8
Propane <0,2 × 10–8 <0,2 × 10–8 <0,2 × 10–8 –Waterb 3,1 × 10–5 2,4 × 10–5 – –a Units of permeability: cm3 (at standard temperature and pressure, STP)·mm/Pa·s·m2 at 21,5°C and ∆P = 34,5 kPa or cm3.b Values obtained at 90% RH, 25°C, assuming that permeability laws hold for water.
2.1 Gas permeabilityHYTREL® thermoplastic polyester elastomers have an unusualcombination of polarity, crystallinity, and morphology. As a result, they have a high degree of permeability to polarmolecules, such as water, but are resistant to permeation by non-polar hydrocarbons and refrigerant gases (see Table 2.01).
In permeability to moisture, HYTREL® is comparable to thepolyether-based urethanes and, therefore, is useful as a fabriccoating for apparel. Its low permeability to refrigerant gasesand hydrocarbons, such as propane, makes HYTREL® of inter-est for use in refrigerant hose or in flexible hose or tubing totransmit gas for heating and cooking.
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2.3 Resistance to mildew and fungusThe resistance of a high-performance 40 durometer D hardnessgrade of HYTREL® to certain fungi was determined according toASTM D 1924-63, using the following cultures.
Culture Observed growthAspergillus niger NoneAspergillus flavus NoneAspergillus versicolor Very slight, sparsePenicillin funiculosum NonePullularia pullulans NoneTrichloderma sp. None
Samples of the same grade HYTREL® were also buried for oneyear in Panama. Instron test results were as follows.
OriginalDurometer D hardness 40Tensile strength, MPa 25,5Elongation at break, % 450100% modulus, MPa 6,9300% modulus, MPa 8,8
Retention after 1 year soil burial in Panama, %Durometer hardness 98Tensile strength 82Elongation at break 82100% modulus 99300% modulus 98
The harder grades of HYTREL® were not included in these testsbut should show at least equivalent resistance, because they arebased on the same raw materials.
Table 2.02 Stability of HYTREL® thermoplastic polyester elastomer to radiationElectron Beam, 1,5 MeV, 23°C, 70% RH, radiation dosage in J/kg (rad)
ASTM test method HYTREL® 4056 HYTREL® 5556 HYTREL® 7246
OriginalTensile strength, MPa D 638 24,1 27,2 35,7Elongation at break, % D 638 550 390 430100% modulus, MPa D 638 6,8 14,4 22,0Hardness, durometer D D 2240 40 55 72
Exposure 5 Mrad, kJ/kg 50 50 50Tensile strength, MPa 22,8 28,3 36,6Elongation at break, % 510 470 410100% modulus, MPa 7,3 14,5 23,6Hardness, durometer D 40 55 72
Exposure 10 Mrad, kJ/kg 100 100 100Tensile strength, MPa 22,8 28,9 37,4Elongation at break, % 500 470 370100% modulus, MPa 6,2 14,5 23,9Hardness, durometer D 40 55 72
Exposure 15 Mrad, kJ/kg 150 150 150Tensile strength, MPa 22,1 30,3 38,6Elongation at break, % 490 490 390100% modulus, MPa 6,1 14,2 24,6Hardness, durometer D 40 55 72
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For use in many applications, a material has either to beapproved or must meet the requirements of various govern-mental or private agencies. This is mainly to protect the user,the general public or the environment.
Besides meeting such regulations, all products and/or theirconstituents have to be listed in the different chemical inven-tories. Specific regulations exist for certain areas like electri-cal applications or applications in contact with food.
DuPont makes sure that all materials supplied to its customerare in compliance with applicable regulations for the materialitself.
As a subscriber to the RESPONSIBLE CARE initiative,DuPont also has accepted to share information and help theproduct users to handle, process, use, recycle and dispose of itsmaterials safely and in an environmentally sound manner.
For selected specific application areas, DuPont has developedinformation which will enable the product user to obtain appro-vals from authorities or to certify compliance with regu-lations.
These areas are:
Compliance statements with European and non-European foodcontact regulations
Europe:The EU (European Union) Directive 90/128 and its subsequentamendments plus country specific regulations where applicable.
USA:FDA (Food and Drug Administration).The following grades of HYTREL® meet Food and Drug Admi-nistration (FDA) guidelines for food contact use in the U.S.The stabilizer system used in these grades is in full compliancewith FDA regulations.
The following grades of HYTREL® may be used in compliancewith the Federal Food, Drug, and Cosmetic Act, specifically:
Essentially all High Performance grades meet FDA guidelinefor bulk dry food contact use in the U.S. If the customer usescompliant additives and follows the temperature and alcoholcontent requirements of the application, then the applicationshould be in compliance with the repeated use regulation.
None of the High Productivity grades have FDA approval inthe U.S. – not because of the presence of toxic extractablematerial, but rather because they have not been tested due tothe enormous expense of extraction and animal feeding testsrequired by the FDA.
Canada:HPB (Health Protection Branch of Health and Welfare).
Other countries:Compliance statements can be established on request.
Compliance statements with European and non-Europeandrinking water regulations
Germany:The KTW (Kunststoff-Trinkwasser-Empfehlungen) recom-mandation.
Italy:The circular on plastics materials for pipes and auxiliariesintended to be used in contact with potable water.
The Netherlands:The Guideline: Quality of materials and chemicals for drink-ing water supply and KIWA (Keuringsinstituut voor Water-leidingartikelen) certification.
UK:The WRC (Water Research Council).
Support information for approval of application underEuropean and nonEuropean pharmaceuticalregulations.
Statements on the content of certain regulated chemicalsas required e.g. by the “Deutsche Dioxinverbotsverordnung” or the “Clean Air Act” in the USA.
Regulations are constantly adapted to new information avail-able, new test methods and also issues of concern developingin the public.
DuPont will adapt its products to the changing market needsor develop new products to satisfy new requirements. Thesame is true for information needed to support customers forregulatory compliance of their applications.
It is impossible in the frame of this bulletin to provide up-to-date information on all grades of HYTREL® meeting the variousspecifications. Our recommendation is therefore to consultwith your DuPont representative on the best material selectionfor a given application in an early stage of a development.
Repeated use contactContact with dry bulk food with fatty or wet food
HYTREL® (regulation 21CFR177.1590) (regulation 21CFR177.2600)
3078 √ √4056 √4069 √ √4556 √ √5526 √ √5556 √ √6356 √ √7246 √ √8238 √ √HTR6108 √
3. Regulatory compliance
17
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4.1 General considerationsAlthough the material properties of strength (stiffness) andflexibility (elasticity) are seemingly “mutually exclusive”,they do “cohabit” in a class of plastics that combines thestrength of engineering resins with the useful elasticity of rub-ber. They are known as “Thermoplastic Polyester Elastomers”(TPEs), produced by DuPont under the trademark HYTREL®.TPEs display an unusual combination of strength and flexi-bility. The resins are load bearing under conditions that alsorequire flexing, sealing, shock absorption or elastic recoveryfrom deformation.
Figure 4.01 illustrates the point graphically. Steel performs wellas a spring, provided the deformation is limited to a fraction ofone per cent (beyond this, the steel will yield). Engineeringplastics such as DELRIN® acetal resin retain spring characteris-tics to a strain of a few per cent. At the other end of the scale,rubber has very high elasticity, but minimal load bearingcapacity.
Replacing traditional conceptsHYTREL® thermoplastic polyester elastomer performs bothfunctions. It behaves as a spring up to 25 per cent strain(depending on the grade) and is 2 to 15 times stronger than rub-ber. This unique position defines HYTREL® as a high strengthplastic with a high capacity for flexing. Used in a thin section,therefore, HYTREL® can behave like rubber. In thick or ribbedsections, it provides structural functions similar to engineeringplastics or metals.
TPE resins close the circle of component integration by com-bining parts across the spectrum of functionality into singleunits.
A new design strategyFor innovative designers and engineers, this broad portfolio ofproperties defines a new degree of design freedom in compo-nent integration. A single part designed with HYTREL® can per-form tasks conventionally performed by separate componentsmade from different materials, and save considerable costs byeliminating or reducing multicomponent assemblies. Sepa-rate structural and flexible elements in hinges, springs, seals,fasteners, power transmission and shock absorption can occursimultaneously in one part. New design strategies for multi-functional component integration are now possible with the useof HYTREL®.
The following general stepwise procedure is intended to helpminimize problems during the growth of a design, and to aidin the rapid development of a successful commercial product.For design rules, see “Module I – General Design Principles”.
4.1.1 Defining the end-use requirementsAs an initial step, the designer should list the anticipated con-ditions of use and the performance requirements of the articleto be designed. He may then determine the limiting design fac-tors and, by doing so realistically, avoid pitfalls which cancause loss of time and expense at later stages of development.Use of the check list (below) will be helpful in defining designfactors.
4. Design concepts for HYTREL®
Useful elasticity (% strain)
Stre
ngth
5 25 1003
Metal
Rubber
Engineeringplastic
HYTREL®
Figure 4.01 Strength and elasticity of materials
4.1.2 Design checklistGeneral informationWhat is the function of the part?How does the assembly operate?Can the assembly be simplified by using HYTREL®?Can it be made and assembled more economically?What tolerances are necessary?Can a number of functions be combined in a single moulding
to eliminate future assembly operations and simplify design?What space limitations exist?What service life is required?Is wear resistance required?Can weight be saved?Is light weight desirable?Are there acceptance codes and specifications
such as SAE, UL?Do analogous applications exist?
Structural considerationsHow is the part stressed in service?What is the magnitude of stress?What is the stress vs time relationship?How much deflection can be tolerated in service?
EnvironmentOperating temperature?Chemicals, solvents?Humidity?Service life in the environment?
AppearanceStyle?Shape?Colour?Surface finish?Decoration?
Economic factorsCost of present part?Cost estimate of part in HYTREL®?Are faster assemblies and elimination of finishing operations
possible?Will redesign of the part simplify the assembled product and
thus give rise to savings in installed cost?
Manufacturing optionsShould the proposed design be machined, blow moulded, melt
cast, injection moulded or extruded considering the numberof parts to be made, design geometry and tolerances?
If injection moulding is chosen, how can mould design con-tribute to part design?
In subsequent assembly operations, can the properties of the cho-sen material be used further; e.g., spin welding, snap fits, etc.?
After preliminary study, several steps remain to convert designideas into production.
4.1.3 Drafting the preliminary designAfter considering end-use requirements, the designer is readyto define the part geometry. This may be done in severalstages with preliminary drawings indicating the basic designand functions. More detailed sketches provide information onthickness, radii and other structures, as worked out from end-use considerations.
4.1.4 Prototyping the designPrototypes can be prepared by several techniques. A commonapproach is to machine the part from rod or slab stock. (See “Module I, General Design Principles, MachiningHYTREL®”.) If machining operations are expected to be elabo-rate or expensive, it is sometimes advisable to x-ray the part toavoid using material with voids. A medical type unit willshow voids as small as 1,58 mm diameter and even greater res-olution can be obtained with some industrial units.
The melt stability of HYTREL® permits production of prototypesby melt casting, which is a process using an extruder to fill aninexpensive aluminium mould. This method can also beadvantageous for short production runs because set-up costs arelow. In any large and important development, the preparationof prototypes using the fabrication method intended for pro-duction can provide added assurance against failure in use. For injection-moulded parts, the use of an inexpensive alu-minium, brass or copper beryllium mould is frequently con-sidered an important step between conception and production.In addition, moulded prototypes provide information on gatelocation and on mould shrinkage.Additional reasons why the moulded prototype is preferred to the machined prototype are:• Machine marks may result in variable behaviour.• Orientation effects in the moulded parts resulting from gate
location or knockout pins may influence toughness.
4.1.5 Testing the designEvery design should be subjected to some form of testing whilein the prototype stage to check the accuracy of calculations andbasic assumptions.• Actual end-use-testing of a part in service is the most mean-
ingful kind of prototype testing. Here, all of the performancerequirements are encountered, and a complete assessment ofthe design can be made.
• Simulated service tests are often conducted with prototypeparts. The value of this type of testing depends on howclosely the end-use conditions are duplicated. An automobileengine part might be given temperature, vibration andhydrocarbon resistance tests; a luggage fixture could besubjected to impact and abrasion tests, and a radio compo-nent might undergo tests for electrical and thermal insulation.
• Standard test procedures such as those developed by theISO/ASTM generally are useful as a design guide but, normally, cannot be drawn upon to predict accurately the performance of a part in service. Again, representative fieldtesting may be indispensable.
Long-term performance must sometimes be predicted on thebasis of “severe” short-term tests. This form of accelerated test-ing is widely used but should be used with discretion becausethe relationship between the long-term service and the accel-erated condition is not always known.
4.1.6 Taking a second lookA second look at the design helps to answer the basic question:“Will the product do the right job at the right price?” Even atthis point, most products can be improved by redesigning forproduction economies or for important functional and aestheticchanges.
20
Weak sections can be strengthened, new features added andcolours changed. Substantial and vital changes in design maynecessitate complete evaluation of the new design. If thedesign has held up under this close scrutiny, specifications anddetails of production can be established.
4.1.7 Writing meaningful specificationsThe purpose of a specification is to eliminate any variations inthe product that would prevent it from satisfying the functional,aesthetic or economic requirements. The specification is a com-plete set of written requirements which the part must meet. It should include such things as: generic name, brand and gradeof material, finish, parting line location, flash, gating, locationswhere voids are intolerable, warpage, colour, decorating andperformance specifications.
4.1.8 Setting up productionOnce the specifications have been carefully and realisticallywritten, moulds can be designed and built to fit the process-ing equipment. Tool design for injection moulding should be left to a specialist or able consultant in the field, becauseinefficient and unnecessarily expensive production can resultfrom improper design of tools or selection of manufacturingequipment.
4.1.9 Controlling the qualityIt is good inspection practice to schedule regular checking ofproduction parts against a given standard. An inspection checklist should include all the items which are pertinent to satis-factory performance of the part in actual service at its assem-bled cost. The end-user and moulder should jointly establishthe quality control procedures that will facilitate production ofparts within specifications.
4.2 Multifunctional component integration4.2.1 Hydraulic piston sealThe standard design for a piston seal used in hydraulic cylin-ders, valves, transmissions, etc., is a multipiece assembly.The piston is usually metal or structural plastic. A rubber sealwith supporting washer is used at each end of the piston. Somedesigns feature concentric outer grooves for O-ring seals or V-shaped seals with back-up rings. Figure 4.02 demonstratesredesign of this assembly as a single part in HYTREL®. Incor-porating ribbed and thick sections allows the resin to meet thestructural requirements of the piston. The sealing function isachieved by thin sections (see inset), which provide functionalelasticity equivalent to rubber. This design is commercial inhydraulic cylinders rated to 3,45 MPa. Designs for operationat 6,9 MPa are possible.
4.2.2 Vacuum or pressure actuated mechanismVacuum actuated diaphragm mechanisms are used in industrialplants and automotive design. A typical application is openingand closing doors in heating and air conditioning systems. The traditional mechanism is made from two welded steel or plastic halves. A metal bracket connects the door to thespring/diaphragm assembly. When vacuum is applied to thediaphragm, it flexes and opens the door. Release of the vacuumallows the spring to return to its unstressed position.
Former designs (Figure 2.03) used eight separate pieces ofmetal, rubber and plastic. Since all of these materials can bereplaced by HYTREL®, a single blow-moulded part can bedesigned to replace the multipiece assembly. The TPE provideslongterm dynamic performance, spring action over a wide tem-perature range and it resists creep, ensuring long life of the part.This design reduces part cost between 50 and 75 per cent.
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Figure 4.02 Hydraulic piston seal
Figure 4.03 Automotive vacuum motor
Multipiece design
One-piece design in HYTREL®
One-piece design in HYTREL®
Normal position Under vacuum
Multipiece design (8 pieces)
4.2.3 Linkage “suspension”Electronic and computer equipment often use linkage “sus-pension” designs to control movement of recording and play-back heads. The function of the linkage is to keep two objectsin parallel planes. The traditional approach in metal or plasticinvolves a multipiece assembly with lubricated pivot points.Figure 4.04 illustrates a new one-piece design in HYTREL®
that replaces the former twelve-piece design in metal. The keyto successful use of TPEs is to mould integral hinges at the sixflex points. In addition to the significant cost savings fromreduced assembly labour, the new design offers these additionalfeatures:• silent mechanism• electrical insulation• use without lubrication.
4.2.4 Solenoid mountSolenoid relays must be protected from vibration to avoid arc-ing of the contacts and failure by “burnout”. The traditionaldesign assembles the solenoid to a plastic wire connector,which then snaps into a mounting box supported by rubbervibration mounts. This has required a five-piece assembly asshown in Figure 4.05.
Now, a single moulding of HYTREL® replaces wire connectors,the mounting box and the rubber vibration mounts. This inte-grates a five-piece assembly into a single moulding. In addi-tion, the superior spring quality of this design protects the sole-noids well enough for a 300 per cent improvement in solenoidlife!
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Figure 4.04 Movement control linkage
Figure 4.05 Solenoid mount, vibration control
Multipiece metal or plasticwith pin hinges
One-piece moulding ofHYTREL® with integral hinges
Solenoids
Nylon connectors
Nylon mounting box
Rubber vibration mounts
Snap-in moulding HYTREL®
Built-up assembly of plastic rubber
4.3 Controlling shock, vibration and noise4.3.1 Solenoid mount (see also 4.2.4)
Other applications: computer motherboardsand disk drives
4.3.2 Engineered footwear for running, walking and team sports
Characteristics:• High load capacity• Conforms to body shape; can be designed for localized
load.
Applications, see figures 4.06 and 4.07.
Benefits:• Shock absorption• Consistent properties over life of product – avoids the
“break-in” required by foam• Breathing action – air circulation• Thermal insulation• Longer life than foam – resistant to creep• Lighter weight than rubber• Consistent properties over a broad temperature range.
4.3.3 Suspension system for offroad equipment (heavymining truck)
The real job of a shock absorbing system is managing energy.A typical system consists of damper and spring, where• the damper absorbs mechanical energy, converting it to heat• excess mechanical energy is stored in the spring and metered
to the damper.
Spring design is critical for effective storage and metering ofenergy. With a unique combination of elastic properties andinjection mouldability, HYTREL® allows custom design ofspring characteristics into a part.
Application, see figure 4.08.
Other applications:Railroad car bumpers; crane bumpers.
23
Figure 4.06 Heel spring insert for walking shoe
Figure 4.08 “Solid state” strut
Figure 4.07 Cushion innersole for dress and golf shoes
Pads of HYTREL® thermoplastic polyester elastomer, separated by steel plates.
Friction damping... no fluids to leakCommercial on heavy-duty 170-ton mining trucks.
4.3.4 MacPherson strut rebound bumperRebound bumper cushions are activated when uneven road sur-faces cause full suspension travel of the wheel. The part is tested in the laboratory at 2,5 m/s entry rates with shock loads as high as 1120 kg. Vehicle durability tests require160000 km without failure.
U.S. automakers have adopted HYTREL® thermoplastic poly-ester elastomer on 1987 vehicles. Benefits of new system:• “33% improvement in rebound damping and entry noise
rating” over previous materials• Improved life• Improved ride quality.
24
Figure 4.09 Mac Pherson strut rebund bumper
5.1 BearingsHYTREL® thermoplastic polyester elastomer has been used in anumber of bearing and seal applications where flexibility,chemical resistance, or useful temperature range not found inother elastomers or plastics are required. In addition, HYTREL®
is suitable for use in handling of dry food and in potable waterapplications requiring approval by the FDA, National Sanita-tion Foundation, and National Water Council (U.K.).
A convenient way for assessing the suitability of a material foruse in unlubricated bearing applications is by determiningwhether the PV of the proposed bearing is lower than the PVlimit for the material under the operating conditions foreseen.The PV limit for a material is the product of limiting bearingpressure MPa and peripheral velocity m/min, or bearing pressure and limiting velocity, in a given dynamic system. It describes a critical, easily recognizable change in the bear-ing performance of the material in the given system. When thePV limit is exceeded, one of the following manifestations may occur:1. melting;2. cold flow or creep;3. unstable friction;4. transition from mild to severe wear.
PV limit is generally related to rubbing surface temperaturelimit. As such, PV limit decreases with increasing ambienttemperature. The PV limits determined on any given tester geo-metry and ambient temperature can rank materials, but trans-lation of test PV limits to other geometries is difficult.
For a given bearing application, the product of pressure andvelocity (PV) is independent of the bearing material. Wear isdependent on PV for any material.
The use of experimentally determined PV limits in specificapplications should be considered approximate, since all per-tinent factors are not easily defined. This means that a gener-ous safety factor is an important consideration in bearingdesign. Some factors known to affect PV limits are:1. absolute pressure;2. velocity;3. lubrication;4. ambient temperature;5. clearances;6. type of mating materials and7. surface roughness.
5.1.1 Unlubricated bearingsPlastic materials frequently show a wide range for the coeffi-cient of friction depending upon test conditions such as velo-city, temperature and humidity. Static coefficients of frictionare usually, although not always higher than dynamic coeffi-cients. Friction values for HYTREL® under two test conditionsare shown in Table 1.08 on page 9.
The following equation can be useful in interpreting wear testresults in terms of specific wear rate or wear factor. With cau-tion, experimentally determined wear factors can be used toestimate wear in a particular application.
W = KFVtwhere:
W = Volume of wear particles, mm3
F = Load supported, NV = Sliding velocity, m/mint = time, hK = Wear factor, mm3 · min/m · N · h
For two flat rubbing surfaces, the equations can be written:w = KPVt
with w = W/Aand P = L/A
w = Wear, mmP = Pressure, MPaA = Contact area, mm2
Unlubricated bearings operating below the PV limit willexhibit mild wear.
PV limits and wear factors for HYTREL® are shown in Table 5.01.
5.1.2 Lubricated bearingsLubrication can increase the PV limit by reducing the coeffi-cient of friction, and providing cooling and washing away ofwear debris.
Lubrication of bearings can be considered to be of two types:1. thick-film (hydrodynamic or hydrostatic);2. thin-film (boundary).
Thick-film lubrication describes lubrication by a fluid-film ofthickness sufficient to prevent contact between the solid slid-ing surfaces. Thus, if the lubricant is clean, neither surface willwear and friction coefficient will depend on lubricant proper-ties, bearing geometry and bearing PV.
In thin-film lubrication, contact can occur between the slidingsurfaces because the lubricant film is not sufficiently thick to prevent contact. Consequently, both lubricant properties and properties of the solid bearing surfaces are important. An effective boundary lubricant must wet the solid surfaces.
5.1.3 Design considerationsAs indicated previously, the PV limit will be decreased by anychange which results in increase of the coefficient of frictionor reduced heat dissipation from the bearing zone. This obser-vation and industrial experience leads to the following sug-gestions for bearing design.
25
Table 5.01 PV limit and wear factor ASTM D 3702PV limit Wear factor, K
MPa × × 10–3Type of HYTREL®
4056 2,1 22G4074 1,1 395556 2,1 2,06356 6,3 2,17246 8,4 0,48
mmin
mm3·minm ·N ·h
5 Designing for specific applications
1. Design bearing sections as thin as consistent with applica-tion requirements. This maximizes heat conduction throughthe plastic material adjacent to the bearing surface andreduces thermal expansion.
2. Metal/plastic bearing interfaces run cooler than plastic/plas-tic interfaces, because heat is conducted from the interfacemore rapidly by metal than plastic. Metal/plastic bearingshave higher PV limits than plastic/plastic bearings.
3. Provision for air circulation about the bearing can bringabout cooler operation.
4. Lubrication can greatly increase the PV limit depending ontype and quantity of lubrication. Where lubricants are used,these must be stable at the bearing temperature.
5. For unlubricated bearings of HYTREL® on metal, the metalshould be as hard and smooth as consistent with bearing liferequirements and bearing cost.
6. Bearing clearance is essential to allow for thermal expansionor contraction and other effects.
7. Surface grooves should be provided in the bearing so that wear debris may be cleared from the bearing area. For lubricated bearings, the grooves can increase the supplyof lubricant. Bearing pressure will increase with grooving.
8. For the bearing applications in dirty environments, use ofseals or felt rings can increase bearing life if they are effec-tive in preventing penetration of dirt into the bearing.
5.2 Boots and bellowsFigure 5.01 shows a blow-moulded automotive CVJ-boot.HYTREL® is replacing vulcanized rubber in boot and bellowsapplications because:
1. higher modulus permits a thinner wall with equivalentstrength and less weight and;
2. processing costs are much less for a blow moulded boot thanan injection moulded rubber boot.
The best design for a plastic boot differs from many existingrubber boots. The convolutes should be designed with nearlyflat sides and small radii at the peaks, as shown in Figure 5.02.It is very important that the boot does not buckle as it is flexed,as this will usually result in early failure. Excessive extensionof a boot of this type will cause it to buckle inward. If theapplication requires extension of the boot from some neutral position, the boot should be designed so that its length, asmoulded, is equal to the maximum extension length in service.The boot will then experience only compression and bucklingwill not occur.
Flexing of the boot results in bending stresses at the peaks ofthe convolutes. In any bending situation in which total deflec-tion is determined by the geometry of the application, stress inthe outermost layers of the material will increase as thicknessincreases. Flex life, therefore, will be maximized if materialthickness is kept to a minimum. HYTREL® CVJ boots have,compared to rubber boots, the advantage of higher impactresistance. HYTREL® also has higher fatigue resistance and bet-ter shape stability at high speed. At low temperatures, the prop-erties of HYTREL® remain outstanding.
5.3 Rolling DiaphragmsBecause of its flexibility and fatigue resistance, HYTREL® issuitable for use in many diaphragm applications. Its highmodulus compared to vulcanized rubber allows a thinnercross-section and possible elimination of fabric reinforce-ment, which, combined with thermoplastic processing, oftenresult in a lower cost part.
Pictured in Figure 5.03 is a rolling type diaphragm, which provides a longer stroke than a flat diaphragm. A plasticdiaphragm of this type must be designed so that there is no cir-cumferential compression of the diaphragm as it rolls from thecylinder wall to the piston, which would cause wrinkling orbuckling and results in early failure. There are two ways toaccomplish this:1. Use a piston with a tapered skirt to keep the compression to
a minimum, as shown in Figure 5.03.2. Design the system so that the piston moves only in the direc-
tion that will roll the diaphragm from the piston to the cylinder wall, as related to the moulded shape of thediaphragm.
26
Figure 5.01 Automotive CVJ-boot
Figure 5.02 Boot design
5.4 BeltsHYTREL® has proven to be an excellent material for powertransmission and conveyor belting. It can be made in “V”,round, flat and other configurations. Its high tensile moduluscompared to rubber eliminates the need for reinforcing cord inmany applications which means that belting can be extruded inlong lengths and stocked in rolls. When a belt is needed, alength is cut off and heat spliced to make a finished belt.
Belts of HYTREL® should be made to the same dimensions asthe belts being replaced. In applications involving large diam-eter pulleys and moderate speeds, belts of HYTREL® have out-lasted vulcanized rubber belts by a wide margin. Small diam-eter pulleys and high speeds should be avoided, as these resultin excessive heat buildup and failure of the belt.
Heat splicing of the belt is a simple process, but must be doneproperly for best results. A 45° bias cut will generally give thebest splice. After cutting, the ends to be spliced are heatedabove the melting point of the material with a heating paddleand then joined together. Two important points are:1. The belt ends must not be pushed so tightly together that the
melt is squeezed from between the ends;2. The ends must be held motionless until the melt has solidi-
fied. A fixture which will hold the belt ends properly willhelp insure a good splice. Flash is trimmed from the splicewith a knife or clippers.
Excessive moisture content will cause degradation of the meltin the splice as it does during any other processing operation.For best results, the ends of the belt should be dried beforesplicing or the belting should be stored in a dry atmosphere,such as a heated cabinet.
5.5 Coiled tubing and cablesTable 5.02 Heat setting temperatureType of HYTREL® Temperature °C
4056 1075556 1256356 1257246 150
Products such as coiled pneumatic tubing and coiled electricalcables are made from HYTREL® by winding an extruded profilearound a mandrel and heat setting. The coil will spring back tosome extent when released so the mandrel must be smaller thanthe desired final diameter of the coil. Exact mandrel size mustbe determined for each application by trial and error.
Recommended temperatures for heat setting are shown inTable 5.02. Parts must be held at the setting temperature onlylong enough to heat the entire cross section of the part to thesetting temperature. Parts may be cooled in room temperatureair and should remain on the mandrel until cool.
5.6 Reinforced hoseIn the design of reinforced hose, three important factors to con-sider in the choice of the tube and cover materials are: resistanceto the environment in which the hose must operate, strength, andflexibility of the material. Based on these factors and others,HYTREL® thermoplastic polyester elastomer has been chosen forseveral hose applications such as hydraulic and paint spray hose.As a cover, HYTREL® offers excellent resistance to abrasion andweathering. HYTREL® 20UV should be added to the cover mate-rial if it will be exposed to sunlight. Similarly, thin-walled hoselinings of HYTREL® must be protected from ultraviolet (UV) lightthat passes through the cover. Carbon black is an effectivescreen.
In fire hose and other lay-flat hoses, it is possible to use a tubeof HYTREL® which is thinner than a vulcanized rubber tube,making the hoses lighter and easier to handle. It can be used inSAE100R7 and R8 thermoplastic hydraulic hoses, which offerthe advantages of lighter weight and a wider colour selectionthan steel reinforced rubber hose.
In the design of thin-walled tubing of HYTREL® care must betaken that the expansion of the lining against the cover does notexceed the elastic limit of HYTREL®.
If the finished hose is to be coupled, creep, thermal expansion,and cut and/or notch sensitivity must be considered in the fit-ting design. If the possibility exists that the hose fittings aresubjected to corrosion in service, the use of steel as the fittingmaterial should be avoided and brass should be used instead.Creep data for HYTREL® may be calculated from the creep mod-ulus plots appearing elsewhere in this handbook. Sharp edgesand burrs should be avoided when designing fittings for hosesbased on HYTREL®. In all cases, the final fitting design shouldbe tested under actual or closely simulated service conditionsto insure satisfactory performance.
If the possibility exists that the hose fittings are subjected tocorrosion in service, the use of steel as the fittings materialshould be avoided and brass should be used instead.
27
Figure 5.03 Rolling type diaphragm
Figure 5.04 Tapered piston skirt
Dp + Convolution Width
Dp
Reprinted with permission from Bellofram Corp.
Reprinted with permission from Bellofram Corp.
28
5.7 Seals, valves and pumpsSpecific designs for seals and valve actuators are alreadyshown in Figures 4.02 and 4.03 (page 21).
It can be seen that it is not always possible to achieve full com-ponent integration as shown in Figure 4.03. There might be cir-cumstances when, for example, the housing must be stiffer,therefore glass-fibre reinforced plastic (or metal) is needed.
Figure 5.05 shows a typical design for a diaphragm pistonpump, including the previous “metal-rubber” solution. Suchpumps are used in the following sectors:• Automotive• Industrial• Medical/health• Nuclear energy• Sports/recreation (volleyballs, scuba tanks)• Other applications requiring clean, oil free operation
Requirements for such design solutions are:• Heat resistance comparable to epichlorohydrin rubber• Dimensional stability equivalent to fabric reinforced rubber• Strength and stiffness to replace metal or structural plastic
connecting rod• Thinner areas to replace rubber (A key design feature is a thin
convolution to allow flexing without overstraining theresin.) Cost savings with this design can be substantial.
Other design examples of seals are shown in Figures 5.06 – 5.08
Figure 5.05 Diaphragm piston pump design
Figure 5.06 Snap-together vacuum fitting
Figure 5.07 Crush seal with over-torque protection
Figure 5.08 Rotary shaft seals
Assembly
“Metal-rubber” solution
HYTREL® solution
Interference fit membrane
Built-in wear surface prevents overstrain of seals
Wipes lubricant two directions for low wear
09.98 Printed in SwitzerlandH-38344 ® DuPont registered trademark
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The information provided in this documentation corresponds to our knowledge on the subject at the dateof its publication. This information may be subject to revision as new knowledge and experience becomesavailable. The data provided fall within the normal range of product properties and relate only to the spe-cific material designated; these data may not be valid for such material used in combination with any othermaterials or additives or in any process, unless expressly indicated otherwise. The data provided shouldnot be used to establish specification limits nor used alone as the basis of design; they are not intendedto substitute for any testing you may need to conduct to determine for yourself the suitability of a spe-cific material for your particular purposes. Since DuPont cannot anticipate all variations in actual end-use conditions DuPont makes no warranties and assumes no liability in connection with any use of thisinformation. Nothing in this publication is to be considered as a license to operate under or a recom-mendation to infringe any patent rights.Caution: Do not use this product in medical applications involving permanent implantation in the humanbody. For other medical applications see “DuPont Medical Caution Statement”, H-50102.