4.0 Number Of Cavities Calculation.ppt

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 DEPEND ON :--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

CALCULATION OF NUMBER OF CAVITIES IN MOULD EXAMPLE : -- Following are data available Components to be produced per day = 5000 Effective production time per shift = 7 Hours Total no. of shifts per day = 2 Component Data : -Material = Poly propylene Density = 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 machine Injection 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 machine And also meet the daily Production is calculated as shown below : -Machine’s safe Shot capacity = Vs X ρ X C X ή Vs – 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 / shot One 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 =120 Hence no. of cavities as per plast. capacity, = 5. 6 X1000 / (120x4.5) = say 10 cavities

Contd.

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 0.85 No 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 hours Cycle time being 30 sec, no. of shots/ hour = 120 Therefore production /day = 14 x 120 x 4 = 6720 components

SHRINKAGE AND ITS APPLICATION SHRINKAGE :-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 the part dimension causing SHRINKAGE of plastic. SHRINKAGE of plastic depends on material and its processing. Various factors that affect shrinkage are :--1. Basic plastic material 2. Filler used and percentage 3. Part wall thickness 4. Melt temperature 5. Mould temperature 6. Injection pressure 7. Injection time 8. Hold on pressure 9. Hold on time 10. Gate area

SHRINKAGE MOULDING SHRINKAGE IS AFFECTED BY THE FOLLOWING : --

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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

( 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)

SHRINKAGE percent 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

mm / mm 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 R3

Material : ABS Shrinkage: 0.005 mm/mm

Component Drawing with shrinkage added R3.02

EXAMPLE DIA. 50 ON COMP. BECOME (50+50X0.005)=50.25

CORE INSERT

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MATERIAL : MOLD STEEL 45 – 50 HRc

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DIM WITH INDICATE SHRINKAGE ADDED

CAVITY INSERT MATERIAL : MOLD STEEL 45 – 50 HRc

*INDICATES DIM. WITH SHRINKAGE ADDED

Assy. of core & cavity Cavity Insert Core Insert