The main cause of black spots in discoloration and coking of injection molding products

1. Machine:
(1) As the heating control system is out of control, the material barrel is overheated, resulting in decomposition and blackening.
(2) Due to the defect of the screw or barrel, the molten material is stuck and accumulated, and it is decomposed due to long-term fixed heating. Check whether the rubber head kit is worn or whether there is metal foreign matter inside.
(3) Some plastics, such as ABS, are cross-linked and coked by high heat in the barrel, and are difficult to melt under the condition of almost maintaining the original particle shape. They are entrained into the parts after being crushed by the screw.
2. Mold:
(1) The mold exhaust is not smooth, easy to be burnt, or the size of the pouring system is too small, and the shearing is too severe, resulting in coking.
(2) There are inappropriate oil lubricants and release agents in the mold.
3. Plastic:
The plastic volatiles are too much, the humidity is too high, the impurities are too much, the recycled material is too much, and it is polluted.
4. Processing:
(1) Too high pressure, too high speed, too high back pressure and too fast rotating speed will cause material temperature decomposition.
(2) The barrel should be cleaned regularly to remove additives that are less resistant than plastics.
Cause analysis of delamination
Causes and troubleshooting of delamination of injection molding products:
1. The material temperature is too low and the mold temperature is too low, resulting in the occurrence of internal stress and fusion joints.
2. The injection speed is too low. It should be slowed down properly.
3. The back pressure is too low.
4. If the raw material is mixed with impurities, the impurities should be screened out or replaced with new materials.
Cause analysis of swelling and blistering
After molding and demoulding, some plastic parts will soon appear swelling or blistering on the back of the metal insert or on the particularly thick part. This is due to the expansion of gases released by the plastic that has not been completely cooled and hardened under the action of internal pressure.
Solution:
1. Effective cooling. Reduce mold temperature, extend mold opening time, and reduce material drying and processing temperature.
2. Reduce the filling speed, reduce the forming cycle and reduce the flow resistance.
3. Increase the holding pressure and time.
4. Improve the condition that the wall of the part is too thick or the thickness changes greatly.
Cause analysis of transparent defects
Transparent parts made of melting spots, craze, cracked polystyrene and plexiglass can sometimes see some glittering fine silver lines through the light. These silver stripes are also called speckles or cracks. This is due to the stress generated in the vertical direction of the tensile stress, which makes the polymer molecules oriented in the flow direction, making the refractive index of the oriented part different from that of the non-oriented part. When the light passes through the interface of the two parts, it refracts and produces craze.
resolvent:
(1) Eliminate the interference of gas and other impurities, and fully dry the plastic.
(2) Reduce the material temperature, adjust the barrel temperature in sections, and properly increase the mold temperature.
(3) Increase injection pressure and reduce injection speed.
(4) Increase or decrease the back pressure of pre-molding and reduce the screw speed.
(5) Improve the exhaust condition of flow channel and cavity.
(6) Clean possible blockage of nozzle, runner and gate. (7) Shorten the molding cycle, and remove the craze by annealing after demoulding: keep polystyrene for 15 minutes at 78 ℃, or for 1 hour at 50 ℃, and keep polycarbonate for several minutes after heating to 160 ℃ or above.