Injection Moulds
Transcript of Injection Moulds
-
7/29/2019 Injection Moulds
1/14
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
-
7/29/2019 Injection Moulds
2/14
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
-
7/29/2019 Injection Moulds
3/14
Feed-System
Flow-way to connect machine nozzle to each impression.
-
7/29/2019 Injection Moulds
4/14
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.
-
7/29/2019 Injection Moulds
5/14
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
-
7/29/2019 Injection Moulds
6/14
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
-
7/29/2019 Injection Moulds
7/14
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
-
7/29/2019 Injection Moulds
8/14
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.
-
7/29/2019 Injection Moulds
9/14
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
-
7/29/2019 Injection Moulds
10/14
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
-
7/29/2019 Injection Moulds
11/14
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
-
7/29/2019 Injection Moulds
12/14
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 )
-
7/29/2019 Injection Moulds
13/14
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
-
7/29/2019 Injection Moulds
14/14
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