1, Brittle defects
Embrittlement refers to that the parts are easy to crack or break in some parts. Brittleness is mainly due to the degradation of materials, which leads to the chain breaking of macromolecules, reduces the relative molecular weight of macromolecules, and thus reduces the overall physical properties of polymers.
2, Causes of brittle plastic products
A large part of the brittleness of products is caused by internal stress. There are many reasons for brittle products, including:
1. Equipment
(1) There are dead solution or obstacles in the barrel, which is easy to cause the degradation of molten materials.
(2) The plasticizing capacity of the machine is too small, and the plastic is not fully plasticized in the barrel; the plasticizing capacity of the machine is too large, and the plastic is heated and sheared in the barrel for too long, so the plastic is easy to age, making the product brittle.
(3) The ejector is inclined or unbalanced, and the cross-sectional area or distribution of ejector rod is improper.
2. Mold
(1) If the gate is too small, it is necessary to adjust the gate size or add auxiliary gate.
(2) If the shunt channel is too small or improperly configured, it should be arranged in a reasonable balance or increased in size.
(3) The abnormal injection cycle is caused by the bad mold structure.
3. Process
(1) The temperature of barrel and nozzle is too low, so it should be adjusted high. If the material is easy to degrade, the temperature of the barrel and nozzle should be increased.
(2) Reduce the back pressure and rotation speed of screw pre molding, make the material slightly loose, and reduce the degradation of plastic caused by shear overheating.
(3) The mold temperature is too high, it is difficult to demould; the mold temperature is too low, the plastic cools too early, the fusion of fusion joints is poor, and it is easy to crack, especially for high melting point plastics such as polycarbonate.
(4) The core of the cavity shall have a proper demoulding angle. When the core is difficult to demould, the cavity temperature shall be increased and the cooling time shall be shortened; when the cavity is difficult to demould, the cavity temperature shall be reduced and the cooling time shall be prolonged.
(5) Try to use less metal inserts, such as polystyrene brittle hot and cold plastic with large capacity, not to add inserts injection molding.
4. Raw materials
(1) Raw materials mixed with other impurities or mixed with inappropriate or excessive solvents or other additives.
(2) Some plastics, such as ABS, will react with water vapor in catalytic cracking when heated under the condition of damp, which will cause large strain of the parts.
(3) Too many times of plastic regeneration or too high content of recycled materials, or too long heating time in the barrel, will promote the brittle fracture of the parts.
(4) The poor quality of plastic itself, such as the large relative molecular weight distribution, the large proportion of components with uneven structure such as rigid molecular chain, or the contamination by other plastics doping, bad additives, dust and impurities, is also the cause of brittleness.
5. Product design
(1) The products have sharp corners, notches or parts with large thickness differences that are prone to stress cracking.
(2) The product design is too thin or too hollow.
3, See the following for improvement methods
Materials - set appropriate drying conditions before injection molding; if the temperature of plastic drying is too high for several days, the volatiles and other substances can be removed, but at the same time, it is easy to cause material degradation, especially heat sensitive materials.
Reduce the use of recycled materials, increase the proportion of raw materials; select high-strength plastic, mold design, increase the size of main flow channel, shunt channel and gate, too small main flow channel, shunt channel or gate is easy to lead to too much shear heat, resulting in polymer decomposition;
Injection molding machine: select well-designed screw to make the temperature distribution more uniform during plasticization. If the temperature of the material is uneven, it is easy to accumulate too much heat in the local area, leading to the degradation of the material.
Process conditions - reduce the temperature of barrel and nozzle;
Reduce the back pressure, screw speed and injection speed, reduce the generation of excessive shear heat, and avoid polymer decomposition; if the brittleness is caused by the insufficient strength of melting trace, the strength of melting trace can be improved by increasing the melt temperature and injection pressure
