Plastic parts are crisp in processing. What should we do?
I. Defect manifestation of brittleness
Brittleness refers to the easy cracking or breaking of parts in some parts. The main reason for brittleness is that the degradation of materials results in chain breakage of macromolecules, which decreases the relative molecular weight of macromolecules, thus degrading the overall physical properties of polymers.
2. Reasons for Brittleness of Plastic Products
The brittleness of products is largely caused by internal stress. There are many reasons for brittleness of products, mainly as follows:
1. Equipment
(1) There are dead solutions or obstacles in the barrel, which easily lead to the degradation of the melt.
(2) Machine plasticizing capacity is too small, plastics are not fully plasticized in the barrel; machine plasticizing capacity is too large, plastics are heated and sheared in the barrel for too long, plastics are easy to aging, brittle products.
(3) The ejector is inclined or unbalanced, and the cross-section area or distribution of the ejector rod is inappropriate.
2. Mold aspect
(1) If the gate is too small, adjusting the gate size or adding auxiliary gate should be considered.
(2) If the shunt is too small or improperly allocated, it should be arranged as far as possible in a reasonable and balanced manner or increase the size of the shunt.
(3) The abnormal injection cycle is caused by the bad die structure.
3. Technological aspects
(1) The temperature of barrel and nozzle is too low and should be raised. If the material is easy to degrade, the temperature of barrel and nozzle should be increased.
(2) Reduce the back pressure and rotational speed of screw pre-plastics, make the material looser, and reduce the degradation of plastics due to shear overheating.
(3) Mould temperature is too high, demoulding is difficult; mould temperature is too low, plastic is cooled too early, fusion seam is not good, easy to crack, especially for high melting point plastics such as polycarbonate.
(4) The core of the cavity should have an appropriate demoulding slope. When the core is difficult to demould, it is necessary to increase the cavity temperature and shorten the cooling time to love you; when the cavity is difficult to demould, it is necessary to reduce the cavity temperature and prolong the cooling time.
(5) Minimize the use of metal inserts, like polystyrene, which is a brittle plastic with large specific capacity of heat and cold, let alone insert injection.
4. Raw materials
(1) The raw materials are mixed with other impurities or adulterated with inappropriate or excessive solvents or other additives.
(2) Some plastics, such as ABS, will react with water vapor when heated under damp conditions, resulting in 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 cause brittle cracking of the parts.
(4) The poor quality of plastics itself, such as the large distribution of relative molecular mass, the excessive proportion of components with non-uniform structure such as rigid molecular chains, or the pollution of other plastics, bad additives, dust and impurities, is also the cause of brittleness.
5. Product design
(1) The products have sharp corners, notches or parts with different thickness which are prone to stress cracking.
(2) The product design is too thin or too hollow.
3. See below for improvement methods
Material - Set appropriate drying conditions before injection molding; if plastic is dried continuously for several days at excessive drying temperature, it can remove volatiles and other substances, but also easily lead to material degradation, especially thermal sensitive materials.
Reduce the use of recycled materials, increase the proportion of raw materials; select high-strength plastic, die design, increase the size of main channel, shunt and gate, too small main channel, shunt or gate easily lead to excessive shear heat, leading to polymer decomposition;
Injection Molding Machine - Select well-designed screw to make temperature distribution more uniform during plasticization. If the material temperature is not uniform, it is easy to accumulate excessive heat in the local area, leading to degradation of the material.
Technological conditions - lowering the temperature of barrel and nozzle;
Reducing back pressure, screw speed and injection speed, reducing excessive shear heat and avoiding polymer decomposition; If the brittleness is caused by insufficient melt trace strength, melt temperature and injection pressure can be increased to increase melt trace strength.
