It is characterized by a radial crack starting from the gate, and the depth of the crack is deep, which is generally transparent.
In addition to this, it is not a chatter mark. The cause of chatter marks is the anisotropy of rigid materials. When spraying glue in the middle gate, the strength of the material flowing in the longitudinal direction (tensile strength) is large, while the strength in the transverse direction (tensile strength) is small, and the stress caused by shrinkage will pull the product to break, and the break must start from the weakest point. It is the lateral area of the material near the gate with the highest internal stress.
Shock mark defects are very serious apparent defects, which are absolutely impossible to pass, so they must be resolved. The idea is as follows:
1. About the material:
The rigidity of the material is the main reason for the shock pattern, so when there is a long process of large products, try not to choose materials with too high rigidity and too low elongation at break, such as GPPS, AS, etc. In common materials, the order of rigidity from weak to strong, and the possibility of occurrence of shock marks from small to large is approximately: PE=>TPU=>PP=>PC=>ABS=>PA=>PVC=>PET=>POM=> PMMA=>AS=>PS. Generally speaking, materials with flexible groups added to the material can improve the vibration pattern. For example, rubber materials, SEBS, EVA, K materials, etc. are all beneficial.
2. About the mold:
The gate design of the mold is the key. Generally speaking, under the structure with large internal stress and long process flow, the gate ripple is easy to produce. Therefore, for large products, it is easy to take multiple gates and wide gates to reduce the resistance to glue and easy to flow. In general, point gates are prone to chatter marks. The side gate, fan gate, and overlap gate are a little bit. But other gates such as diving gates and diaphragm gates will not be used in this type of structure. Because most of the products with shock marks are transparent products, there is no need for diving mouths and diaphragm nozzles.
3. About the parameters:
The parameter measurement to solve the chatter marks is: 3.1 slow shooting speed, low shooting pressure 3.2 short holding time 3.3 mold temperature must be high, such as PS material, mold temperature can be set to 60 degrees.
4. Summary
Shock marks are very common defects in the production of transparent products of GPPS materials. If you do not pay attention to the treatment method, there may be more than 50% of the defects or all the defects. Only by mastering the above methods can you eliminate the defects and ensure stable and high-quality production.