4.0 Number of cavities calculation Shot weight calculation - Plasticizing capacity calculation -Cycle time calculation - Clamping force calculation - Determination of number of cavities - Determination by shot capacity - by plasticizing capacity- by clamping capacity

NUMBER OF CAVITIES IN MOULD

1. PRODUCTION REQUIREMENT TO MEET TARGET.

2. MACHINE’S CAPACITY a) shot b) plasticizing c) clamping

3. MAX. SIZE OF COMPONENT AND THE MOULD THAT CAN FIT INTO THE MOULDING MACHINE BETWEEN TIE RODS.

4. MAX. INJECTION PRESSURE REQUIRED TO FLOW AND FILL THE PARTICULAR PLASTIC INTO MOULD

DEPEND ON :---

CALCULATION OF NUMBER OF CAVITIES IN MOULD

EXAMPLE : -- Following are data available Components to be produced per day = 5000Effective production time per shift = 7 HoursTotal no. of shifts per day = 2

Component Data : -- Material = Poly propyleneDensity = 0. 9 gm. / cc. ; wall thickness = 2. 0 mm.Weight of one component = 3. 6 gm.Weight of feed system = 0. 9 gm.Max. projected area of component = 12. 5 cm2

Projected feed system area = 2. 5 cm2

Machine Data :--- SP 30 Windsor machineInjection pressure ( Max. ) = 1950 Kg. / cm 2

Shot capacity , Swept volume = 44 cm3

Plasticising capacity = 15 Kg / Hour

Safe no. of cavities in mould to utilize optimum capacities of machineAnd also meet the daily Production is calculated as shown below : --

Machine’s safe Shot capacity = Vs X ρ X C X ήVs – Swept volume cm 3; ρ --Density of plastic gm./ cm 3

C -- Material constant ( 0.85 for crystalline & 0.93 for Amorphous) ή – Efficiency of machine = 85 %Hence safe shot capacity of m/c = 44 x 0.9 x 0.85 x 0.85

=28. 6 gm / shotOne component wt. + feed system wt. = 3.6 + 0.9 = 4.5 gm.Hence no. of cavities as per shot capacity, =28.6 / 4.5 = 6

m/c’s safe Plasticising capacity (PP.) = Pl. capacity P.S.X QPS / QPP X ή = 15Kg./hr.X 57/ 130 X 0.85

= 5. 6 Kg./ hr.Assuming cycle time of 30 sec., no. of shots/ hr = 3600 /30 =120Hence no. of cavities as per plast. capacity, = 5. 6 X1000 / (120x4.5)

= say 10 cavitiesContd.

CALCULATION OF NUMBER OF CAVITIES IN MOULD (contd.)

Clamping tonnage = no.of cavity x proj. area x cavity press.For P.P. ,Inj. press. recommended is 700 to 1400 Kg./ cm2

Wall thk. being 2mm, pi .can be taken as 800Kg / cm2

Cavity press. = 1/ 2 or 1/ 3 of Inj.press. Since PP. is easy flow material ,cavity press. = 1/2X 800kg/ cm2

Therefore Clamping tonnage = N x 15 x 400 /1000 tons.Equating to machine’s safe clamp tons. of 30 x 0.85No of cavities as per clamp. N = 30 x 0.85 x1000 / 15x400 say N = 4 cavities The least no. of cavities 4, out of the three capacities is selected.Check for production rate per day :--Total productive time / day = 2x7 hoursCycle time being 30 sec, no. of shots/ hour = 120Therefore production /day = 14 x 120 x 4 = 6720 components

SHRINKAGE AND ITS APPLICATIONSHRINKAGE :-- All materials when heated expand and on cooling contract. The method of injection moulding also is subjected to expansion of plastic during injection phase and its contraction during cooling for solidification, Hence the volume change from mould dimension to thepart dimension causing SHRINKAGE of plastic.

SHRINKAGE of plastic depends on material and its processing.Various factors that affect shrinkage are :---1. Basic plastic material2. Filler used and percentage3. Part wall thickness4. Melt temperature5. Mould temperature6. Injection pressure7. Injection time8. Hold on pressure9. Hold on time10. Gate area

SHRINKAGE

MOULDING SHRINKAGE IS AFFECTED BY THE FOLLOWING : --

• Shrinkage increases with ----• Increase in melt temperature• Increase in mould temperature• Increase in wall thickness• Low injection pressure• Shrinkage decreases with ----• Low melt and mould temperature• High injection pressure• Long injection time• Filler material and its content

SHRINKAGE OF COMMON PLASTICS

PLASTIC MATERIAL SHRINKAGE

percent mm / mm

( A. B. S, )Polystyrene ( P.S.)

Nylon ( P.A.) Acrylic ( PMMA )

Polycarbonate ( P,C.) Polyacetal ( P.O.M.)Polypropylene (P.P.) Polyethylene (L.D.P.E) Polyethylene(H.D.P.E)

Poly vinyl chloride (PVC)

0.4 to 0.6 0.5 to 0.6

1.0 to 2.5

0.4 to 0.8 0.6 to 0.8 1.5 to 2.5 1.0 to 2.0 1.5 to 3.0 1.5 to 3.0 0.1 to 0.2

0.004 to 0.006 0.005 to 0.006 0.01 to 0.025 0.004 to 0.008 0.006 to 0.008 0.015 to 0.025 0.010 to 0.020 0.015 to 0.030 0.015 to.0.030 0.001 to 0.002

Application of Shrinkage-- Example

CUP

Material : ABSShrinkage: 0.005 mm/mm

R 3

Component Drawing with shrinkage added

EXAMPLE DIA. 50 ON COMP.

BECOME (50+50X0.005)=50.25

R3.02

CORE INSERT

MATERIAL : MOLD STEEL 45 – 50 HRc

DIM WITH *INDICATE

SHRINKAGE ADDED

*

*

CAVITY INSERT

MATERIAL : MOLD STEEL 45 – 50 HRc

**

*

*INDICATES DIM. WITH

SHRINKAGE ADDED

Assy. of

core & cavity

Cavity InsertCavity InsertCore Insert