Analysis of design process of injection mold

Time:2023-11-23 20:14:32 / Popularity: / Source:

1. Composition of running system

Ordinary runner system, also known as runner system, or pouring system, is necessary passage for molten plastic from nozzle of injection machine to mold cavity. Runner system includes primary runner, sub-runner and gate.
design process of injection mold 
1. Main runner
Also known as sprue or down runner, it refers to runner from part where nozzle of injection machine is in contact with main runner bushing of mold to runner. This part is the first part where molten plastic flows through after entering mold.
2. Runner
Also called sub-runner. With mold design, it can be further divided into first runner and secondary runner. Sub-runner is transition area between main runner and gate, which can make flow direction of molten plastic change smoothly; for multi-cavity mold, it also has function of uniformly distributing plastic to each cavity.
3. Gate
Also called feed port, it is narrow opening between runner and mold cavity, it is also the shortest and thinner part. Its function is to use shrinkage of flow surface to accelerate plastic. High shear rate can make plastic good fluidity (due to shear thinning characteristics of plastic); heating effect of viscous heating also has effect of increasing material temperature and reducing viscosity.
After molding is completed, gate is the first to be cured and sealed to prevent plastic from reflowing, and to prevent pressure of cavity from falling too fast, which causes molded product to shrink and dent. After forming, it is convenient to cut off to separate runner system and plastic parts.
4. Cold slug well
Also called cold slug. Purpose is to store and replenish cold plastic wavefront at initial stage of filling, to prevent cold material from directly entering mold cavity, affecting filling quality or blocking gate. Cold slug well is usually set at the end of main runner. When length of branch runner is long, cold slug well should also be set at the end.

2. Basic principles of gating system design

1. Consideration of Cavity Layout
1) Try to use Balances Layout;
2) Layout of mold cavity and gate opening should be symmetrical to prevent uneven load of mold from being unevenly loaded, and problem of mold flashing;
3) Layout of mold cavity is as compact as possible to reduce size of mold.
2. Flow guidance considerations
1) It can smoothly guide molten plastic to fill mold cavity without generating eddy currents, and can smoothly exhaust;
2) Try to avoid impact of plastic melt on core and metal inserts with a smaller diameter to prevent core shift or deformation.
3. Consideration of heat loss and pressure drop
1) The smaller heat loss and pressure drop, the better;
2) Process should be short;
3) Cross-sectional area of runner should be large enough;
4) Try to avoid bend of runner and sudden change of flow direction (change direction at an arc angle);
5) Surface roughness should be low during runner processing;
6) Multi-point pouring can reduce pressure drop and required injection pressure, but there will be suture problems.
4. Consideration of flow balance
1) When filling a mold with multi-cavity (Multi-Cavity), runner should be balanced, and try to make the plastic fill each cavity at the same time to ensure quality consistency of molded products of each cavity;
2) Try to use natural-balanced layout of runners (Naturally-Balanced Layout);
3) When natural balance is not possible, artificial balance method is used to balance flow channel.
5. Scrap considerations
Under premise of smooth filling without affecting flow and pressure loss, reduce flow channel volume (length or cross-sectional area) to reduce waste production and recycling costs of flow channel.
6. Cold material considerations
Appropriate cold slug wells and overflow troughs are designed on runner system to supplement colder plastic wave front at initial stage of filling and prevent cold material from directly entering mold cavity and affecting filling quality.
7. Exhaust considerations
Plastic should be smoothly guided to fill mold cavity and allow air in mold cavity to escape smoothly to avoid problem of scorching of encapsulation.
8. Consideration of quality of molded products
1) Avoid short shots, burrs, encapsulation, suture lines, flow marks, jets, residual stress, warpage deformation, mold core deviation and other problems;
2) When runner system has a long process or multiple gating, warpage and deformation of finished product caused by unbalanced flow, insufficient pressure or uneven shrinkage should be prevented;
3) Appearance of product is good, it is convenient to remove and trim gate, gate mark does not damage appearance and application of plastic part.
9. Consideration of production efficiency
Minimize required post-processing, shorten forming cycle and improve production efficiency.
10. Consideration of ejection point
It is necessary to consider proper ejection position to avoid deformation of molded product.
11. Considerations for using plastics
For plastics with higher viscosity or shorter L/t, avoid using too long or too small runners.

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