Dissatisfaction in injection molding jargon is also called undershot or short shot, insufficient filling, and part dissatisfaction. It refers to the partial incompleteness at the end of the injection plastic flow or the partial filling of a mold with multiple cavities, especially the thin-walled area or the end of the flow path. area. The manifestation is that the melt is condensed before the cavity is filled, and the melt is not completely filled after entering the cavity, resulting in a lack of material in the product.
What causes dissatisfaction with injection molding?
The main reason for the short shot is that the flow resistance is too large, which makes the melt unable to continue to flow. Factors affecting the melt flow length include: part wall thickness, mold temperature, injection pressure, melt temperature and material composition. If these factors are not handled well, they will cause short bets.
Hysteresis effect: also called stagnation. If there is a relatively thin structure at a position close to the gate or a position perpendicular to the flow direction, usually stiffeners, etc., then the melt passes through this position during the injection molding process It will encounter relatively large forward resistance when the flow direction of the main body is smooth, and the flow pressure cannot be formed. Only when the melt is filled in the main body direction or enters the holding pressure will sufficient pressure be formed. The stagnant part is filled, and at this time, because the part is very thin, and the melt does not flow, there is no heat supplement, and it has solidified, thus causing short injection.
How to solve the dissatisfaction of injection molding?
1. Elimination method of short injection defect:
Increase the fluidity of the melt
Reduce the addition of recycled materials
Reduce gas decomposition in raw materials
The location of the gate is designed to ensure that it fills the thick wall first to avoid stagnation and cause the polymer melt to harden prematurely.
Increase the number of gates and reduce the process ratio.
Increase the size of the flow channel and reduce the flow resistance
The position of the exhaust port is set appropriately to avoid the phenomenon of poor exhaust (see whether the undershot part is burnt)
Increase the number and size of exhaust ports
Add cold material well design to discharge cold material
The distribution of the cooling water channels should be reasonable to avoid causing the local temperature of the mold to be low
——Injection molding machine
Check whether the check valve and the inner wall of the barrel are severely worn. The above-mentioned wear will cause serious loss of injection pressure and injection volume.
Check whether there is material at the feeding port or whether it is bridging
Check whether the ability of the injection molding machine can meet the molding requirements
Increase injection pressure
Increase injection speed and increase shear heat
Increase injection volume
Increase the barrel temperature and mold temperature
Increase the melt length of the injection molding machine
Reduce the amount of cushioning of the injection molding machine
Extend injection time
Reasonably adjust the position of each section of the injection and the speed and pressure of the injection
Part wall thickness design is too thin
The part has stiffeners that cause stagnation
There are large differences in the thickness of the parts, causing stagnation to appear locally, which cannot be avoided by mold design
2. Measures to solve the problem of stagnation and under-injection:
1) Increase the thickness of stagnant parts, and the thickness of the parts should not be too different. The disadvantage is that it is easy to cause sink marks;
2) Change the gate position to the end of the filling to make pressure build up at this position;
3) When injection molding, first reduce the speed and pressure, so that a thicker solidified layer is formed at the front of the material flow at the early stage of filling, and it is believed that increasing the melt pressure is our common measure;
4) Use materials with good fluidity.