6 problems and solutions for injection molding products

2022/11/04

Plastic products are not perfect in the process of injection molding, and there will always be some small problems of one kind or another. Only through continuous debugging can we achieve high-quality production status. So what problems will the product encounter in the process of injection molding, and how should the staff solve such problems?


1. Cracks


Cracking is a common defect in plastic products, and the main cause is due to stress deformation. There are mainly residual stress, external stress and stress deformation caused by the external environment.


(1) Cracks caused by residual stress


Residual stress is mainly due to the following three conditions, namely overfill, ejection and metal inserts. As the crack generated in the case of excessive filling, the solution can be mainly started in the following aspects:


①Since the pressure loss of the sprue is the smallest, if the cracks are mainly generated near the sprue, you can consider using the multi-point distribution gate, side gate and shank gate instead.


②On the premise of ensuring that the resin does not decompose or deteriorate, appropriately increasing the resin temperature can reduce the melt viscosity and improve the fluidity, and at the same time, the injection pressure can be reduced to reduce the stress.


③ Under normal circumstances, when the mold temperature is low, it is easy to generate stress, and the temperature should be appropriately increased. But when the injection speed is high, even if the mold temperature is lower, the stress can be reduced.


④Excessive injection and pressure holding time will also generate stress, and it is better to shorten it appropriately or perform multiple pressure holding switching.


⑤ Non-crystalline resins, such as AS resin, ABS resin, PMMA resin, etc., are more prone to residual stress than crystalline resins such as polyethylene, polyoxymethylene, etc., which should be paid attention to.


When demoulding is pushed out, due to the small demoulding slope, the roughness of the mold cavity and the punch, the pushing force is too large, resulting in stress, and sometimes even whitening or cracking around the pushing rod. The cause can be determined by looking closely at the location of the crack.


When inserting metal parts during injection molding, stress is most likely to occur, and it is easy to crack after a period of time, which is extremely harmful. This is mainly because the thermal expansion coefficients of metal and resin are very different to generate stress, and over time, the stress exceeds the strength of the gradually degraded resin material and cracks occur.


In order to prevent the resulting cracks, the general-purpose polystyrene of the wall thickness and the outer diameter of the embedded metal parts is basically not suitable for inserts, and the inserts have the least impact on nylon.

Because the thermal expansion coefficient of glass fiber reinforced resin material is small, it is more suitable for inserts. In addition, preheating the metal inserts before forming also has a good effect.


(2) Cracks caused by external stress


The external stress here is mainly due to the stress concentration caused by the unreasonable design, especially at the sharp corners.


(3) Cracks caused by the external environment


Chemicals, water degradation caused by moisture absorption, and excessive use of recycled materials will deteriorate physical properties and cause cracks.


2. Insufficient filling


The main reasons for insufficient filling are as follows:


a. Insufficient resin capacity.


b. Insufficient pressure in the cavity.


c. Insufficient resin fluidity.


d. The exhaust effect is not good.


As improvement measures, we can mainly start from the following aspects:


①Longer the injection time to prevent the resin from flowing backward before the gate is solidified due to the short molding cycle, which makes it difficult to fill the cavity;


②Increase the injection speed;


③Increase the mold temperature;


④Increase the resin temperature;


⑤Increase the injection pressure;


⑥ Enlarge the gate size. Generally, the height of the gate should be equal to 1/2 to 1/3 of the wall thickness of the product;


⑦The gate is set at the largest wall thickness of the product;


⑧ Set up exhaust grooves (average depth 0.03mm, width 3-5mm) or exhaust rods, which is more important for smaller workpieces;


⑨Leave a certain buffer distance between the screw and the injection nozzle;


⑩Choose materials of low viscosity grade;


⑪Add lubricant.


3. Folds and pockmarks


The cause of this defect is essentially the same as underfilling, but to a different degree. Therefore, the solution is also basically the same as the above method. Especially for resins with poor fluidity (such as polyoxymethylene, PMMA resin, polycarbonate and PP resin, etc.), it is necessary to pay more attention to properly increasing the gate and proper injection time.


Fourth, shrink the pit


The reason for the shrinkage pit is also the same as the insufficient filling. In principle, it can be solved by overfilling, but there is a danger of stress. In the design, attention should be paid to the uniform wall thickness, and the reinforcement ribs, convex columns and other places should be reduced as much as possible. wall thickness.


Five, overflow


The focus of the overflow processing should be mainly on the improvement of the mold. In terms of molding conditions, it is possible to reduce fluidity.

Specifically, the following methods can be used:


①Reduce the injection pressure;


②Reduce the resin temperature;


③Choose high-viscosity grade materials;


④ Reduce the mold temperature;


⑤ Grinding the mold surface where overflow occurs;


⑥Using harder mold steel;


⑦ Improve the clamping force;


⑧ Adjust the joint surface and other parts of the accurate mold;


⑨ Increase the mold support column to increase rigidity;


⑩ Determine the size of different exhaust grooves according to different materials.


6. Welding lines


Weld lines are caused by the cooling of the front end portion of the molten resin from different directions and the incomplete fusion at the joint. Under normal circumstances, it mainly affects the appearance, and has an impact on painting and electroplating.

In severe cases, it will affect the strength of the product (especially in the case of fiber-reinforced resin). The following items can be improved:


①Adjust molding conditions to improve fluidity. For example, increase the resin temperature, increase the mold temperature, increase the injection pressure and speed, etc.;


②Adding an exhaust groove and setting a push rod at the place where the weld line is generated is also conducive to exhausting;


③ Minimize the use of release agents;


④Set the process flash as the place where the weld line is generated, and then cut and remove it after forming;


⑤ If only the appearance is affected, the gate position can be changed to change the position of the weld line.

Or the part where the weld line is generated is treated as a dark glossy surface, etc., to be modified.





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