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Plastic mold manufacturers talk about why mold products crack
Edit:Taizhou Huangyan Yige Machinery Co., Ltd.   UpDate:2019-01-22

Cracking often occurs in the molding process of plastic molds, which is a relatively common disadvantage of plastic products. The main reason for this is the stress and deformation. There are mainly residual stress, external stress, and stress deformation occurring in the external environment. Let the plastic mold manufacturers explain in detail the causes of cracks and the corresponding solutions:

First, cracks caused by residual stress:

Residual stress is mainly caused by the following three conditions, namely, overfilling, ejection from the mold, and metal inserts. As the crack that occurs under the condition of excess filling, the solution can be started in the following aspects:

1. Because the pressure loss of the direct gate is small, so if cracks occur mainly near the direct gate, you can consider using the multi-point distributed point gate, side gate, and shank gate method;

2. On the premise of ensuring that the resin is not differentiated and not deteriorated, appropriately increasing the temperature of the resin can reduce the melt viscosity and improve the fluidity, and can also reduce the injection pressure to reduce stress;

3. Under normal conditions, stress is easy to occur when the mold temperature is low, 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;

4. Stress will also occur when the injection and holding time is too long. It is better to shorten it appropriately or perform th holding pressure switching.

5. Non-crystalline resins, such as AS resins, ABS resins, PMMA resins, and other crystalline resins, such as polyethylene and polyoxymethylene, simply cause residual stress, which should be paid attention to;

a. When the mold is ejected, the ejection force is too large and the stress is generated because of the small mold release gradient and rough mold and convex mold. Sometimes, whitening or cracking occurs around the ejection rod. Just by carefully observing where the cracks occur, you can determine the cause;

b. When the metal parts are embedded together with injection molding, zui simply undergoes stress, and cracks occur only after passing for a period of time, which is extremely harmful:

This is mainly because the thermal expansion coefficients of metals and resins are very different, and stress occurs, and over time, the stress exceeds the strength of the gradually degraded resin material and cracks occur. In order to prevent the cracks that occur, as a rule of thumb, the wall thickness is 7 ”and the outer diameter of the embedded metal.

c. General-purpose polystyrene is basically not suitable for inlays, and the impact of inlays on nylon is small. Because glass fiber reinforced resin material has a small thermal expansion coefficient, it is more suitable for inserts;

d. Pre-heating the metal inserts before molding has a better effect;

Cracks caused by external stress:

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

Third, the crack caused by the external environment:

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


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