During the injection molding process, if the gate is improperly located, the injection molding pressure or holding pressure is too large and the product is not ejected smoothly, which will lead to product cracking. The small crack reduces the strength of the product, and it is easy to break the product under the action of external force during the molding process or the use of the product.
Cracking is a common defect in plastic products. The location of the crack is mainly near the direct gate ,around the ejection pin and the insert, and the sharp corners of the product.
1. Improper control of injection molding process causes excessive strain of products.
1) When the temperature of the material is low, the melting viscosity becomes larger and the fluidity is poor, resulting in greater stress.
2) When the mold temperature is low or uneven, the product is easy to incur strain.
2. Mold design is unreasonable
1) The draft angle is small and the mold cavity is relatively rough.
2) Products with sharp corners and notches, or with inserts are easy to produce stress concentration.
3. Poor ejection of products
If the draft angle of the mold is small, and the mold cavity is relatively rough, the use of too much ejection force and too fast ejection speed will cause the product to incur stress, and sometimes even incur whitening or cracking around the ejection pin.
4. Solvent effect
The action of release agent and other chemical solvents, or resin hydrolysis caused by moisture absorption, will reduce the performance of the material and cause cracking.
1. Appropriately increase the barrel temperature and mold temperature.
Reducing the injection pressure and shortening the holding time can reduce or eliminate cracking.
2. Reasonable design of mold.
1) Improve the finish of the mold cavity. Usually finer the mold surface, easier the ejection is. Smaller the VDl index indicates finer surface. Usually less than 12 is fine enough to compensate small draft angle like 0.5 degree.
2) Reasonably design the size and position of the gate. The sprue needs to be small and pressure holding time needs to be short. Furthermore, the gate must be set at the thicker wall of the product, which will make injection pressure and holding pressure lower, hence smaller strain.
3) Increase the size of the runner to lower the injection pressure and shorten the injection time.
4) The cooling system of the mold should ensure uniform cooling and reduce the strain of the product.
5) Appropriately increase the draft angle of the mold, so that the product can be successfully ejected.
3. Reasonable design of product structure
1) The ratio of surface area to volume of the product should be as small as possible, because the thick product with a small ratio has slow cooling and small strain.
2) The wall thickness of the product should be as uniform as possible. The products with large wall thickness difference are easy to incur strain due to uneven cooling.
