The brittleness of products is largely caused by internal stress. There are many reasons for product brittleness, mainly including:
1, equipment
(1) There are dead solutions or obstacles in the barrel, which are easy to cause melt degradation.
(2) The plasticizing capacity of the machine is too small, and the plasticizing of plastics in the machine barrel is insufficient; The plasticizing capacity of the machine is too large, the plastic is heated and sheared for too long in the barrel, and the plastic is easy to age, which makes the products brittle.
(3) The ejection device is tilted or unbalanced, and the cross-sectional area or distribution of the ejector pins is inappropriate.
2, mold.
(1) The gate is too small. Consider adjusting the gate size or adding an auxiliary gate.
(2) If the shunt is too small or improperly configured, try to arrange the balance reasonably or increase the size of the shunt.
(3) Abnormal injection cycle due to poor mold structure.
3. Technical aspects
(1) The temperature of the barrel and nozzle is too low and should be increased. If the material is easy to degrade, the temperature of the barrel and nozzle should be increased.
(2) Reduce the back pressure and rotational speed of screw preform, make the material slightly loose, and reduce the degradation of plastic due to shear overheating.
(3) The mold temperature is too high, so it is difficult to demould; If the mold temperature is too low, the plastic will cool prematurely, the fusion seam will be poor, and it is easy to crack, especially the high melting point plastic such as polycarbonate.
(4) The cavity core should have proper demoulding inclination. When it is difficult to demould the core, it is necessary to increase the cavity temperature and shorten the cooling time. Love you; When the cavity is difficult to demould, it is necessary to reduce the cavity temperature and extend the cooling time.
(5) Use metal inserts as little as possible, such as polystyrene, which is a brittle plastic with large specific heat and cold capacity, and it is not suitable for injection molding with inserts.
4. Modified plastic raw materials.
(1) Raw materials are mixed with other impurities or doped with inappropriate or excessive solvents or other additives.
(2) Some plastics, such as ABS, will undergo catalytic cracking reaction with water vapor when heated in the presence of moisture, which will cause greater strain on the parts.
(3) Too many times of plastic regeneration, too high content of recycled materials, or too long heating time in the barrel will promote the brittle fracture of parts.
(4) The quality of the plastic itself is not good, for example, the relative molecular mass distribution is large, and the proportion of components with uneven structure such as rigid molecular chains is too large; Or contaminated by other plastics, bad additives, dust impurities, etc. It is also the reason for brittleness.
5. Product design.
(1) Products with sharp corners, notches or parts with large thickness differences are prone to stress cracking.
(2) The product design is too thin or too empty.
improve one's method
Materials-Set proper drying conditions before injection molding; If the plastic is continuously dried for several days, the drying temperature is too high, which can remove volatiles and other substances, but it is also easy to cause material degradation, especially heat-sensitive materials.
Reduce the use of recycled materials and increase the proportion of raw materials; Select high-strength plastic, design the mold, and increase the size of main runner, shunt runner and gate. If the main runner, branch runner or gate is too small, it will easily lead to excessive shear heat, which will lead to polymer decomposition.
Injection molding machine-choose a 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 locally, leading to the degradation of the material.
Process conditions-reduce the temperature of the barrel and nozzle;
Reduce back pressure, screw speed and injection speed, reduce the generation of excessive shear heat and avoid polymer decomposition; If brittleness is caused by insufficient strength of melt mark, the strength of melt mark can be improved by increasing melt temperature and injection pressure.
I hope the above information is useful to you, thank you.