Discussing mold optimization from perspective of injection molding process

Discussing mold optimization from perspective of injection molding process - Design and optimization

Time：2024-10-15 09:05:03 / Popularity： / Source：

For previous article, please refer to "Discussing mold optimization from perspective of injection molding process - Design and optimization 1".

I. Understanding role of runner system

1. Runner system is channel before molten material enters mold cavity;
2. Ensure that molten material does not cool prematurely in runner;
3. Eliminate cold material generated by molten material in runner;
4. Adjust and control viscosity of molten material when it enters mold;
5. Adjust and control cooling time of injection molding;
6. Adjust and control flow resistance of molten material;
7. Adjust and control balance of glue feeding of multi-cavity molds;
8. Surface of runner (gate material) has a heat preservation effect;
9. Transfer molten material pressure to various parts of mold cavity;

II. Ten principles to be followed in design of runner system

1. Layout of mold cavity should be symmetrically distributed (compact size, balanced expansion force);
2. Molten material should not flow in runner. It is better to cool prematurely;
3. Pressure drop of molten material in main and branch runners is small;
4. Ensure that molten material is evenly fed into glue, reaches deep and corners of cavity at the same time;
5. Branch runners should be distributed in a balanced manner as much as possible;
6. Cold material well and exhaust channel should be opened at the end of main and branch runners;
7. Distribution of runners is conducive to smooth discharge of air in mold;
8. Prevent occurrence of poor injection molding (such as: lack of material, shrinkage, flow lines, injection lines, etc.);
9. Transition between main and branch runners should be chamfered R corners;
10. Under conditions of injection molding, runner material consumption is small and cooling time is as short as possible;

III. Design basis and optimization of runner system

Design of runner system needs to scientifically set size of runner according to type of plastic, viscosity of molten material, wall thickness of plastic part, requirements of product quality, etc.

No	Runner design basis	Optimization of runner	Purpose and function
1	Highly viscous plastics	Main and branch runners need to be thicker	Prevent melt from cooling early and reduce flow resistance
2	Crystalline plastic	Main and branch runners should be larger in size	Prevent runner from freezing and sealing too early
3	Transparent plastic parts	Main and branch runners should be larger in size	Prevent cold material and flow lines
4	High requirements for appearance quality	Main and branch runners should be larger in size	Improve appearance quality of plastic parts
5	Low viscosity plastic	Main and branch runners can be smaller in size	Do not affect its fluidity and shorten the cycle
6	Thick wall plastic parts	Main and branch runners need to be larger in size	Require long holding time (large glue filling channel)
7	Thin wall plastic parts	Main and branch runners can be smaller in size	Shorten cycle and save raw materials
8	Low appearance quality requirements	Main and branch runners can be smaller in size	Shorten cycle and save raw materials
9	Glass fiber reinforced plastic	Main and branch runners need to be thicker	Prevent melt from cooling early and reduce flow resistance
10	High dimensional accuracy requirements	Main and branch runners should be larger in size	Improve stability of product size

IV. Injection defects caused by poor runner size

A. Problems caused by too thin or too long runner
1. Large flow resistance loss of molten material;
2. Difficult to maintain pressure and fill glue (molten material is easy to cool);
3. Surface of plastic part is easy to shrink (difficult to adjust machine);
4. Plastic part is not full of glue (easy to lack of material);
5. Weld line of plastic part is obvious (temperature of molten material drops quickly);
6. Plastic part is prone to flow lines (cold material);
7. Surface gloss of plastic part is poor (poor reproducibility);
8. Dimensional stability of plastic part is poor;
9. Product has large internal stress and is easy to deform;
10. Floating fiber on the surface of plastic part is obvious;
11. Strength of injection molded part becomes lower;

Runner balance and cold well

I. Importance and optimization of runner balance

Unbalanced runners will lead to unbalanced filling, pressure holding and cooling of multiple cavities or single cavities, resulting in following problems:
1. Molten material cannot fill all corners of cavity at the same time (inconsistent filling);
2. Amount of glue in each cavity or each part is different, weight, strength, density, etc. of plastic parts will be different;
3. Shrinkage rate of plastic parts will be different, and size of plastic parts will be inconsistent;
4. Some plastic parts or parts will produce flash (flash);
5. Some plastic parts or parts are prone to shrinkage (dents);
6. Some mold cavity plastic parts are prone to sticking to mold;
7. Injection molding production process is difficult to adjust machine and stability is poor.

2. Function and optimization of cold well

Cold well is located at the end of main channel and branch channel. Its functions are:
1. Collect "cold material" at the front of molten material to prevent "cold material" from entering mold and causing "flow lines" on plastic parts;
2. Diameter of cold well is slightly larger than diameter of main channel at large end, and length is about diameter of main channel at large end; cold well must be available for plastics with high viscosity, glass fiber reinforced plastics, crystalline plastics and transparent parts, and it should be as large as possible. Cold well should have exhaust.
12. Viscosity of molten material increases and fluidity is poor;
B. Problems caused by too thick runner
1. Large loss of gate material (most of it can be reused);
2. Runner cooling time is long and injection efficiency is low (low output);
For later reading, please refer to "Discussing mold optimization from perspective of injection molding process - Design and optimization 3, 4, 5, 6, 7, 8".

Gud Mould Industry Co., Ltd