Analysis of common problems in injection molds
Analysis of the causes of black spots in discoloration and coking
The main cause of the black spots of the discoloration and coking of the injection molded products is that the plastic or the added ultraviolet absorber, antistatic agent, etc. decompose in the barrel due to overheating, or decompose and coke after staying in the barrel for too long, and then injected with the melt Cavity formation. analyse as below:
1. Machine aspects:
(1) The heating control system is out of control, which causes the barrel to overheat and cause decomposition and blackening.
(2) Due to the defects of the screw or the barrel, the melt is stuck and hoarded, and it is decomposed due to long-term fixed heating. The rubber tip kit should be inspected for wear and tear or metal foreign objects.
(3) Some plastics such as ABS are cross-linked and coked under high heat in the barrel. Under the condition of almost maintaining the original particle shape, it is difficult to melt and be entrained into the product after being crushed by the screw.
2. In terms of mold:
(1) The mold exhaust is not smooth, it is easy to burn, or the size of the pouring system is too small, and the cutting is too severe, causing coking.
(2) There are inappropriate oil lubricants and release agents in the mold.
Too much volatile matter, too much humidity, too much impurities, too much recycled material, and contamination.
(1) Excessive pressure, high speed, excessive back pressure, and excessively high speed will decompose the material temperature.
(2) The barrel should be cleaned regularly to remove additives that are less resistant than plastic.
Analysis of the reasons for delamination
Causes and elimination methods of layered peeling caused by injection products:
1. The material temperature is too low and the mold temperature is too low, resulting in internal stress and welding seams.
2. The injection speed is too low and should be slowed down appropriately.
3．Back pressure is too low.
4．Foreign material impurities are mixed in the raw materials, and the foreign materials should be screened or replaced with new materials.
Causes of swelling and bubbling
Some plastic parts quickly swell or blister on the back of metal inserts or in particularly thick areas after molding and demolding. This is due to the incomplete expansion of the gas released by the internal pressure of the hardened plastic.
1. Effective cooling. Reduce the mold temperature, extend the mold opening time, reduce the drying and processing temperature of the material.
2. Reduce the mold filling speed, reduce the molding cycle, and reduce the flow resistance.
3．Increase dwell pressure and time.
4．Improve the condition that the wall surface of the part is too thick or the thickness changes greatly.
Analysis of the Causes of Transparency Defects
Frost spots, silver streaks, cracked polystyrene, transparent parts of plexiglass, sometimes sparkling filament-like silver streaks can be seen through the light. These silver streaks are also known as flecks or cracks. This is due to the stress generated in the vertical direction of the tensile stress, which causes the polymer molecules to be oriented in the flow direction, so that the refractive index of the oriented portion and the unoriented portion are different, and light rays are refracted when they pass through the interface to generate silver streaks.
(1) Eliminate the interference of gas and other impurities, and fully dry the plastic.
(2) Reduce the material temperature, adjust the barrel temperature in stages, and appropriately increase the mold temperature.
(3) Increase injection pressure and decrease injection speed.
(4) Increase or decrease the pre-plastic back pressure and reduce the screw speed.
(5) Improve the exhaust conditions of the runner and cavity.
(6) Clean up possible blockages of nozzles, runners and gates. (7) Shorten the molding cycle, and remove the silver streaks by annealing after demolding: keep polystyrene at 78 ° C for 15 minutes or 50 ° C for 1 hour, and for polycarbonate, heat to 160 ° C for several minutes .