injection moulds

7/29/2019 Injection Moulds

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Design of Injection Moulds1) Given Part Design

2) Order Quantity

3) Target Production Costs4) Selection of Moulding Machine

5) Estimate Cycle Time

6) Calculation of Optimum no. of Cavities

7) Selection of Type of Mould8) Preliminary Cavity Layout

9) Runner & Gate Layouts

10) CAE Filling Analysis, Shrinkage / Cooling Analysis

11) Recheck Runner & Gate Design

12) Layout Cooling Channels

13) Mould Layout & Details Mould Base

14) Layout Ejection & Venting

15) Final Drawings & Bill of Materials


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Plastics Materials : General Characteristics

1. Verstality2. Relatively Easy to Mould into Complex Shapes

3. Low Specific Gravity

4. Transparency & Colouring5. Relatively low energy requirements for processing

6. Chemical resistance

7. Mechanical performance

8. Good Electrical & Thermal insulation

9. Flammability

10. Poor weather resistance

11. Relatively high coefficient of Thermal Expansion


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Feed-System

Flow-way to connect machine nozzle to each impression.


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Gates for Injection Moulds

Gate is a channel or orifice connecting the runner with the impression.

Allows the molten-plastic to flow from the runner or from the sprueinto the cavity.

Much smaller than Runner.

Large enough for suitable filling & small enough to seal-off and preventbackflow or packing.

Should be located for economical removal (de-gating) & finishing of thepart.


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Gate : Small Cross-Sectional Area

Small cross-section area is necessary for :-

1. Gate freezes soon after impression is filled

2. Allows for simple de-gating

3. Small witness mark on the moulding

4. Better control of the filling of multi-cavities

5. Over-packing of cavities is minimized


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Gate-Size

Depth, Shape, Size, Thickness

Start with 0.6 mm in depth

Gate thickness = 40 to 60% of part thichness

Optimum size of the gate will depends on :-

1. Flow characteristics of the plastic material

2. Wall-thickness of the moulding

3. Volume of material to be injected into the cavity

4. Melt-temperature

5. Mould-temperature


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Balanced Gate System

For multi-impression moulds

Impressions near to sprue will fill earlier

Over-packing of nearest cavities

Farthest cavities may not fill fully

Two methods :-

1. By varying the land length2. By varying the cross-sectional area


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Gate PositionThe position of the gate should be such that :-

1. Even flow of the melt in the impression

2. Uniform filling of the impression

3. All extremities fills at the same time

Central Gating :- results in even flow & the core is held centrally

Jetting should be avoided

Weld-lines :- by keeping the cooling away from weld-line areas,

melt temperature may be increased slightly

for proper knitting of melt-fronts.


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Types of Gates

Twelve Types :-

1. Sprue-Gate 2. Rectangular-Edge-Gate

3. Overlap-Gate 4. Fan-Gate

5. Tab-Gate 6. Diaphragm-Gate

7. Ring-Gate 8. Film-Gate

9. Pin-Point-Gate 10. Round-Edge-Gate

11. Subsurface-Gate 12. Winkle-Gate


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Injection MouldingRecommended Runner Sizes

S.No. Plastic Size - mmMin Max

1 ABS, SAN 4.7 9.52 Acetal 3.1 9.53 PMMA 7.5 9.54 Cellulosics 4.7 9.55 Nylon 1.5 9.56 PC 4.7 9.57 PE 1.5 9.58 PP 4.7 9.59 PPO 6.3 9.510 PolySulfone 6.3 9.511 PS 3.1 9.512 PVC 3.1 9.5


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Runners for Injection Moulds

A channel machined into the mould plate to connect the sprue with

the gate (entrance) to the impression.A Channel that connects the Sprue with the Cavity Gate.

Can be machined on either half of the mould.

Preferably on the Moving-half.

Highly Polished to reduce friction ( molten-plastics & metal )

Polished in the line-of-draw

Runner Design Considerations :-1). Shape of the Cross-Section 2). Size of Runner

3). Runner Layout


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Runners : Shape of Cross-Section

Efficient Runner Design : CriteriaRunner should provide :-

a maximum Cross-Sectional Area for pressure-transfer

&

a minimum Contact on the periphery for heat-transfer.

Runner Efficiency = ( Area / Periphery )


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Types of Runners : Shape of Cross-Section

Six Types :-1. Full-Round

2. Half-Round

3. Quarter-Round

4. Square

5. Trapezoidal

6. Modified Trapezoidal


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Runner-Size

As small as possible

Empirical (based on practical-experience) formula :-For moulding weighing 200 gm & wall-thickness 3mm

D = ( W x4

L ) / 3.7

D = Runner Dia, mm W = Wt. of moulding, gm

L = Height of runner, mm

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