Transparent plastic parts such as light guide plates are discolored and yellow, mainly caused by overheating or over-shearing and decomposition of materials. The causes of these changes are specifically analyzed from four aspects: mold, machine, molding process, and use of materials.
First, the mold
1) The mold is poorly vented, adiabatic compression occurs when the mold is filled under high temperature and high pressure, and the plastic oxidizes.
2) The dimensions of the runner and the rubber inlet of the pouring system are too small, and the mold is filled under high temperature and high pressure. The shearing action occurs to discolor and yellow the plastic.
3) Inappropriate oil lubricants, mold release agents, etc. on the mold adhere to the surface of the product or discolor and yellow.
Second, the machine
1) Due to failure or damage of thermocouple, temperature control system, and heating system, the temperature of the cylinder is misaligned, resulting in local overheating of the cylinder, and the material (including additives, colorants) stays in the high temperature cylinder for too long and discolors and yellows. .
2) The screw is damaged and defective, the inner wall of the barrel is damaged, the nozzle is in poor conformity with the inlet of the main channel, and the gap between the screw and the barrel is too large, both of which cause the melt to stick in and accumulate, and undergo decomposition and discoloration due to prolonged fixed heating. Yellow.
3) Improper fit of the nozzle and the cartridge, and the resident material in the gap is decomposed and discolored and yellowed after being heated for a long time.
4) The clearance between the screw and the barrel is too small, or the eccentricity of the screw causes the material to discolor and yellow due to excessive shearing.
5) When the nozzle hole is too small, the shear discoloration and yellowing occur when high-speed commercial pressure filling.
6) The quality of the screw processing is poor, the surface is rough or the design is not good, causing the material to become discolored and yellow due to excessive shearing during the process of sticking the screw or stock material.
7) The screw material is poor, long-term work under high temperature and high pressure, high torque, screw deformation, resulting in the material in a certain part of the barrel over shear discoloration, yellowing
Third, the molding process
1) Screw speed is too high when melted, back pressure is too large, discoloration and yellowing occurs
2) The forming temperature is set too high, the molding cycle is too long, and when the injection is finished, the amount of residual material is too large, and the material stays in the cylinder for too long, leading to discoloration and yellowing of the material.
Fourth, molding materials
1) The performance of the molding material is unstable, too much recycled material, or mixed with miscellaneous materials, easily discolored by thermal decomposition, yellowing
2) The material is not dry and the moisture in the material is too high. A colored hydrolysate will form in the high temperature barrel, causing the part to change color and yellow.