mold design - kocwelearning.kocw.net/contents4/document/lec/2013/chonnam/... · 2013-07-19 · mold...
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Bong-Kee LeeSchool of Mechanical Engineering
Chonnam National University
Mold Design
10. Cooling Systems
School of Mechanical EngineeringMold Design
Design of Cooling System
Mold Cooling System– a mold is a highly stressed heat exchanger and the
cooling phase of the injection molding cycle is extremely important
• to cool the polymer as quickly as possible to a temperature at which the molded part can be safely ejected
– the cooling phase can be up to 80% of the overall cycle and often the most expensive cost component of the molding
• to preserve the physical properties of the material and the required quality of the molded part
– especially, crystalline materials will warp and distort and suffer dimensional changes over a period of time because of the formation of undesirable structures (,such as random orientations) in the frozen layers
School of Mechanical EngineeringMold Design
Design of Cooling System
Mold Cooling System– rate of cooling is most important rather than the
cooling itself
– the temperature of the mold should be:• high enough to allow the cavities to fill without premature
freezing of the material• as uniform as possible to ensure the molded part is cooled
equally in all areas• high enough with crystalline materials to avoid an
unsatisfactory structure
School of Mechanical EngineeringMold Design
Design of Cooling System
Mold Cooling System– external systems
• chillers– air cooled, water cooled– to circulate the coolant into the cooling channels in the mold
• temperature controllers
School of Mechanical EngineeringMold Design
Design of Cooling System
Cooling Channels– the simplest method of supplying cooling fluid to the
mold• to drill holes in the mold plate around the cavities• layout, diameter, distance from the mold surface, and so on, of
the cooling channels ~ uniformity, efficiency
basic cooling circuit more efficient cooling circuit
School of Mechanical EngineeringMold Design
Design of Cooling System
Cooling Channels– positioning
• if channels are too far apart– danger of an uneven thermal gradient developing between them
• if channels are too close together– the thickness of steel between them may become too low
• if channels are too close to the cavity surface– localized over-cooling, resulting in irregular temperature control
part thickness W channel diameter d
2mm 8-10mm
4mm 10-12mm
6mm 12-15mm
c=2d~3d, b=3d~4d
guide to cooling channel positioning
School of Mechanical EngineeringMold Design
Design of Cooling System
Core Cooling– in many cases, core pins will get very hot if no cooling
is incorporated in them, resulting in prolonged cooling cycles → provide cooling inside core pins
– useful methods (for cooling cores and cavities)• baffle system• fountain system• angled hole• stepped hole• spiral cooling• heat rods• heat pipes• beryllium copper cores and cavities
School of Mechanical EngineeringMold Design
Design of Cooling System
Core Cooling– baffle system
• cooling of smaller cores, (or larger cores with array-form)• a hole is bored into the core and a strip of copper is inserted
into it
School of Mechanical EngineeringMold Design
Design of Cooling System
Core Cooling– fountain system
• more efficient than baffle systems (because of the parallel cooling circuitry)
• more uniform temperature control over the entire area• a tube is fitted into the center of a hole in the punch or core
pin
single fountain design multiple fountain design
School of Mechanical EngineeringMold Design
Design of Cooling System
Core Cooling– angled hole design
• drilled holes at angles in larger punches for them to interact each other to create a path for the coolant to flow through
• difficulty in manufacturing the holes to interact each other on a full diameter → restricted maximum length of the holes ~ 150mm
• obstructions inside the holes, expensive EDM technique
School of Mechanical EngineeringMold Design
Design of Cooling System
Core Cooling– stepped hole
• easier mold-making works than the angled hole design• plugs are prone to leak due to the injection pressure and
continued cyclic expansion and contraction• witness mark of ring shape or blush mark owing to a
differential cooling effect through the plug to the mold surface
School of Mechanical EngineeringMold Design
Design of Cooling System
Core Cooling– spiral cooling
• for lager cylindrical cores above 50mm diameter• basic design ~ a channel that is machined down the outside of
a centrally inserted tapered diameter and follows the path of a helix
• seals have to be used to prevent leakage
School of Mechanical EngineeringMold Design
Design of Cooling System
Core Cooling– heat rods
• for smaller punches and core pins• high thermal conductivity materials that are inserted into the
core pin• copper ~ ductile and excellent conductor of heat
School of Mechanical EngineeringMold Design
Design of Cooling System
Core Cooling– heat pipes
• for small core pins• copper tube filled either with water or with a low-boiling-point
alcohol• efficiency of heat transfer depends on the contact between the
outside of the heat pipe and the inner wall of the core pin
School of Mechanical EngineeringMold Design
Design of Cooling System
Cavity Cooling– to introduce cooling into cavity inserts
• on many jobs, cooling the cores without cooling the cavities can lead to non-uniform cooling of the component
– sealing to prevent cooling fluid leaking• O-rings or gaskets• copper pipe with a BSP taper thread
School of Mechanical EngineeringMold Design
Design of Cooling System
Cavity Cooling
cooling system for circular cavities cooling arrangement for rectangular cavities
School of Mechanical EngineeringMold Design
Design of Cooling System
Cavity Cooling
annular groove cooling
School of Mechanical EngineeringMold Design
Design of Cooling System
Cooling Circuitry– series cooling
• all coolant flows through one connected circuit• there is a high temperature differential between inlet and outlet• there is a high-pressure drop between inlet and outlet• any blockage in the circuit is easilyidentified
baffle system in series
School of Mechanical EngineeringMold Design
Design of Cooling System
Cooling Circuitry– parallel cooling
• all circuits are fed by a common supply at a similar temperature• there is a lower temperature differential across the mold• there is a lower pressure drop between inlet and outlet• the mold temperature is more uniform• circuit blockage are less easily detected
fountain system in parallel
School of Mechanical EngineeringMold Design
Design of Cooling System
Analytical Thermal Calculation– design steps
• computation of cooling time ~ minimum cooling time down to demolding temperature
• balance of heat flow ~ required heat flow through coolant• flow rate of coolant ~ uniform temperature along cooling line• diameter of cooling line ~ turbulent flow• position of cooling line ~ heat flow uniformity• computation of pressure drop ~ selection of heat exchanger,
modification of diameter or flow rate
School of Mechanical EngineeringMold Design
Design of Cooling System
Calculation of Cooling Time– until the mold opens and the molded part is ejected– permissible demolding temperature, TE
– 1-dimensional analysis ~ heat conduction• in the direction of part thickness• heat exchange between plastic part and coolant
ydiffusivit thermal:
2
2
pC
k
x
T
t
T
School of Mechanical EngineeringMold Design
Design of Cooling System
Calculation of Cooling Time– assuming that
• immediately after injection, the melt temperature in the cavity has a uniform constant value of TM
• the temperature of the cavity wall jumps abruptly to the constant value TW and remains constant
s
xe
TT
TT
s
xne
nTT
TT
ts
WM
WE
n
ts
n
WM
WE
sin4
12sin
12
14
2
2
2
22
0
12
nesspart thick :s
School of Mechanical EngineeringMold Design
Design of Cooling System
Calculation of Cooling Time
18ln
1
numberFourier : & rate cooling :
8ln
1or
8ln
8
re, temperatudemoldingmean :
222
2
22222
2
2
0
0
2
2
s
tFo
Fos
t
TT
TT
TT
TT
s
t
TT
TTst
eTT
TT
dx
dxTTT
c
c
WM
WE
WE
WMc
WE
WMc
ts
WM
WE
s
s
E
EE1
tc
α
1
Fo
School of Mechanical EngineeringMold Design
Design of Cooling System
Calculation of Cooling Time
– from the experimental works, demolding usually takes place at the same dimensionless temperature
WM
WE
E
TT
TT
T
ˆ
ˆ
part ofcenter demolding,at re temperatu:ˆ
TW
x
TM
t=0 TW
x
TE
t=tc
TE
part theof re temperatuaverage on the based :16.0
or center, in the re temperatumaximum on the based :25.0ˆ
School of Mechanical EngineeringMold Design
Design of Cooling System
Cooling Time of Thermoplastics– estimation
– cooling time diagram
nesspart thick :
][s/mm 3 to2 ~constant :
timecooling :2
2
s
c
t
sct
c
c
cc
School of Mechanical EngineeringMold Design
Design of Cooling System
Cooling Time of Thermoplastics– nomogram
parts lcylindricafor 7.0ln8.5
parts planefor 8
ln
2
22
2
WE
WM
effc
WE
WM
effc
TT
TTRt
TT
TTst
School of Mechanical EngineeringMold Design
Design of Cooling System
Cooling Time of Thermoplastics– typical material data
melt temp.(°C)
wall temp.(°C)
demoldingtemp. (°C)
avg. density(g/cc)
ABSHDPELDPEPA 6
PA 6.6PBTPPC
PMMAPOMPPPS
PVC rigidPVC soft
SAN
200-270200-300170-245235-275260-300230-270270-320180-260190-230200-300160-280150-210120-190200-270
50-8040-6020-6060-9560-9030-9085-12010-8040-12020-10010-8020-7020-5540-80
60-10060-11050-9070-11080-14080-14090-14070-11090-15060-10060-10060-10060-10060-110
1.030.820.791.051.051.051.141.141.300.831.011.351.231.05
School of Mechanical EngineeringMold Design
Design of Cooling System
Cooling Time of Thermoplastics– for other geometries
WE
WM
effc
WE
WM
effc
TT
TTst
TT
TTst
ˆ4
lnor 8
ln2
2
22
2
WE
WM
effc
WE
WM
effc
TT
TTDt
TT
TTDt
ˆ602.1ln
14.23or 692.0ln
14.23
22
WE
WM
eff
cWE
WM
eff
cTT
TT
LD
tTT
TT
LD
tˆ
040.2ln14.23
1or 561.0ln
14.23
12
2
2
2
s Qy
yD L>>DQr
yDL~D
Qr
Qz
School of Mechanical EngineeringMold Design
Design of Cooling System
Cooling Time of Thermoplastics– for other geometries
WE
WM
effc
WE
WM
effc
TT
TTht
TT
TTht
ˆ064.2ln
3or 533.0ln
3 2
2
2
2
WE
WM
effc
TT
TTDt
ˆ2ln
4 2
2
D
h
h h
School of Mechanical EngineeringMold Design
Design of Cooling System
Heat Flux Balance– mold for processing thermoplastics has to extract fast
and uniformly, so much heat from the melt injected into the cavity
ksQ
adQ
cQ
cvQ
radQ
cdQ
platens machine into conductionby exchangeheat :
mold theof side at theradiation by exchangeheat :
mold theof side at the convectionby exchangeheat :
coolant with exchangeheat :
runner)hot from (e.g.flux heat additional :
part molded thefromflux heat :
0
balance)flux (heat
cd
rad
cv
c
ad
ks
cdradcvcadks
Q
Q
Q
Q
Q
Q
QQQQQQ
School of Mechanical EngineeringMold Design
Design of Cooling System
Heat Flux Balance– heat flux from the molded part
• enthalpy difference between injection and demoling– related to the mass (with the average density and the volume) can
be converted to the amount of heat (which has to be extracted from the molding and conveyed to the mold during the cooling stage)
pVUH (enthalpy) timecooling :
part theof volume:
mold theinto injected mass :
temp.demolding andinjection between
differencedensity average :
differenceenthalpy specific :
c
ks
ks
c
ks
c
ksks
t
V
m
h
t
Vh
t
mhQ
School of Mechanical EngineeringMold Design
Design of Cooling System
Heat Flux Balance– heat exchange by convection (at the side of the mold)
– heat exchange by radiation (at the side of the mold)
mperatureambient te :
re temperatumold external :
K)8W/m ~motion slight (in t coefficienfer heat trans :
faces side mold theof area :
0
2
0
amb
m
s
s
ambmsscv
T
T
h
A
TThAQ
0.85)~rustedheavily
0.6,~rustedslightly 0.25,~clean 0.1, ~ polished :steel(for
surface mold theof emissivity :
KW/m105.670 ~constant Boltzmann -Stefan : 428-
440
ambmsrad TTAQ
School of Mechanical EngineeringMold Design
Design of Cooling System
Heat Flux Balance– heat exchange by conduction into machine platens
80)~steelalloy -high 100,~steelalloy -low 100, ~ steelcarbon (for
K][W/mt coefficienfer heat trans :
faces clamping mold theof area :2
0
c
c
ambmcccd
h
A
TThAQ
School of Mechanical EngineeringMold Design
Design of Cooling System
Heat Flux Balance– determination of external mold temperature, Tm0
channel cooling of etemperatur
coolant of etemperatur
molds unheated smallonly with epermissibl
0
0
0
ccm
cm
ambm
TT
TT
TT
material mold theofty conductivi thermal:
surface mold external and channel coolingbetween distance :
0
m
mcs
cdradcvccm
k
l
kAA
lQQQTT
School of Mechanical EngineeringMold Design
Design of Cooling System
Flow Rate of Coolant– coolant throughput
• permissible temperature rise of coolant due to the cooling of the molded part ~ 3 to 5°C
→ adequate homogeneous cooling along the cooling circuit
coolant of rise re temperatuepermissibl :
coolant ofheat specific :
coolant ofdensity :
coolant of rate flow :
c
pc
c
cpccc
T
C
Q
TCQQ
School of Mechanical EngineeringMold Design
Design of Cooling System
Diameter of Cooling Line– turbulent flow for efficient heat transfer– pressure drops inside the cooling line
cc
cc
cccc
c
cc
ccc
cc
Qd
d
Q
d
Q
d
Qd
vd
vAQdv
2300
4300,2
4Re
drops) pressure gconsiderinby 10,000(or 2,300Re flow,ent for turbul
44Re
4Re
2
2
School of Mechanical EngineeringMold Design
Design of Cooling System
Diameter of Cooling Line– based on fluid mechanics
tcoefficien resistance :
turnsofnumber :
line cooling theoflength :
pipesin factor friction :
36002
164
2
2
c
c
c
c
ccc
cc
cc
K
n
L
f
Knd
Lf
p
Qd
bending round 90for 4.0
bending sharp 90for 9.1
law) (Blasius' Re
3164.04/1
c
c
K
f
School of Mechanical EngineeringMold Design
Design of Cooling System
Diameter of Cooling Line– determination of heat transfer coefficient– based on convective heat transfer
factor correction :
number Prandtl :Pr
number Reynolds :Re
number Nusselt :Nu
8.0Pr8.1230Re0235.0Nu 3.08.0
f
fc
ccv
K
vL
k
hL
Kk
dh
School of Mechanical EngineeringMold Design
Design of Cooling System
Positioning of Cooling Line– layout of cooling line ~ uniform heat flow– distance from the mold wall
Tw
Tc
dc
l
1q
Tcc
2q
21
122
1
TThTThql
TTkTkq
cccc
ccwm
22
itypenetrabilheat :
contact before re temperatu wall:
contact before re temperatumaterial :
mperaturecontact te :
,
max,min,max,
max,
max,
coolantcwww
w
p
c
pwp
wwppc
TTTTT
b
T
T
T
kCbbb
TbTbT
School of Mechanical EngineeringMold Design
Design of Cooling System
Numerical Analysis– three-dimensional heat transfer
• for both mold base and plastic part• proper boundary conditions, including convection heat transfer
of coolant
• coupled analysis of heat transfer and fluid flows of plastic melt and/or coolant
rz
T
y
T
x
Tk
t
TCp
for
2
2
2
2
2
2