Cracking and cracking is a common defect in plastic products. The main reason for this is due to stress and deformation. There are mainly residual stress, external stress, and stress deformation caused by the external environment. SUSAN GIFTS CO.,LIMITED , https://www.susangifts.com
(-) The residual stress caused by residual stress is mainly due to the following three conditions, namely, overfilling, ejection ejection, and metal inserts. As a crack that occurs when filling in surplus, its solution can be mainly started in the following aspects:
(1) Since the pressure loss at the sprue is the minimum, if the cracks are most likely to occur near the sprue, consider using multi-point distribution gates, side gates, and shank gates.
(2) Under the premise of ensuring that the resin does not decompose or deteriorate, the proper increase of the resin temperature can lower the melt viscosity and increase the fluidity, and at the same time, the injection pressure can also be reduced to reduce the stress.
(3) Under normal circumstances, when the mold temperature is low, stress is likely to occur, and the temperature should be appropriately increased. However, when the injection speed is high, even if the mold temperature is lower, the stress can be reduced.
(4) The injection and holding pressure for too long will also generate stress. It is better to shorten it properly or perform Th holding pressure switching.
(5) Non-crystalline resins, such as AS resin, ABS resin, PMMA resin, etc., which are more crystalline, such as polyethylene and polyoxymethylene, are prone to residual stress and should be noted.
When the mold is ejected, due to the small mold release angle, the mold glue and the convex mold are rough, the pushing force is too large, stress is generated, and sometimes even whitening or cracking occurs around the ejection rod. As long as you carefully observe the location of the crack, you can determine the cause.
When metal parts are embedded in the injection molding, stress is most likely to occur, and cracks are easily generated after a period of time, which is extremely harmful. This is mainly due to the fact that the difference in thermal expansion coefficient between the metal and the resin causes stress, and over time, the stress exceeds the strength of the gradually deteriorating resin material and cracks are generated. To prevent the resulting cracking, as a rule of thumb, the general-purpose polystyrene with a wall thickness of 7" and an outer diameter of the embedded metal part is basically not suitable for the addition of the insert, and the insert has the least influence on the nylon. Due to the glass fiber reinforcement The resin material has a small thermal expansion coefficient and is suitable for inserts.
In addition, preheating the metal insert before molding also has a good effect.
(2) Cracking caused by external stress The external stress here is mainly caused by irrational design and stress concentration, especially in sharp corners.
(c) Cracks caused by the external environment Chemicals, degradation of water due to moisture absorption, and excessive use of recycled materials will deteriorate physical properties and cause cracking.
Second, insufficient filling The main reasons for insufficient filling are the following:
1) Insufficient resin capacity.
2) Insufficient pressure in the cavity.
3) Insufficient resin fluidity.
4) The exhaust effect is not good.
As an improvement measure, we can mainly start from the following aspects:
1) Increase the injection time to prevent the molding cavity from being filled with the cavity because the molding cycle is too short to cause the resin to flow back before the gate cures.
2) Increase the injection speed.
3) Increase the mold temperature.
4) Increase the resin temperature.
5) Increase injection pressure.
6) Increase the gate size. The general gate height should be equal to 1/2 to l/3 of the wall thickness of the product.
7) The gate is set at the maximum wall thickness of the product.
8) Set the exhaust groove (the groove depth varies from 0.01 to 0.03mm, the width is approximately 5 to 10mm, depending on the plastic type) or the exhaust rod. It is more important for smaller parts.
9) Use a low viscosity grade material.
10) Add lubricant.
Third, wrinkles and pockmarks The reason for this defect is essentially the same as filling, but to a different extent. 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.), care must be taken to properly increase the gate and proper injection time.
4. The reason for retracting pits is also the same as filling defects. In principle, it can be solved through surplus filling, but there is a risk of stress. Attention should be paid to the uniform wall thickness in the design. Ribs and ribs should be reduced as much as possible. The thickness of the column, etc.
V. Overflow
The emphasis on the handling of overflow edges should be mainly on the improvement of the mold. In terms of molding conditions, you can start to reduce the liquidity. Specifically, the following methods can be used:
1) Reduce injection pressure.
2) Lower the resin temperature.
4) Use high viscosity grade materials.
5) Reduce the mold temperature.
6) Grind the mold surface where the overflow occurs.
7) Use a harder mold steel.
8) Increase the clamping force.
9) Adjust the joint surface and other parts of the accurate mold.
10) Increase the mold support column to increase rigidity.
11) Determine the size of different exhaust slots based on different materials.
Six, weld marks
The weld marks are caused by the fact that the front part of the molten resin from different directions is cooled and fails to be completely fused at the joint. Under normal circumstances, it mainly affects the appearance and affects the painting and plating. In severe cases, it has an effect on the strength of the product (especially when it is a fiber-reinforced resin). Refer to the following items to improve:
l) Adjust the molding conditions to improve the fluidity. For example, increase the temperature of the resin, increase the mold temperature, and increase the injection pressure and speed.
2) The addition of an exhaust groove and the provision of an ejector rod at the place where the weld line is generated also facilitates the exhaust.
3) Minimize the use of release agents.
4) Set the process flash and use it as a weld mark. After molding, cut it and remove it.
Injection molding defects and their solutions