（33） Poor gloss of plastic parts (glass fiber precipitation) Injection Molding Process Defect Handling Manual

What is bad gloss?
Poor gloss refers to the molded product not adhering to the mold, which means poor replication.
Reasons for poor gloss
(2-1) Caused by spray marks or cold material
When spray marks or cold materials appear on the surface of the formed product, they appear as blurred as fog due to their differences from the surrounding environment. It often occurs near the gate and in uneven places.
(2-2) Due to gas
This is because the gas is blocked between the mold and the resin and hinders the adhesion between the two, resulting in poor gloss. It is more likely to occur when the holding pressure is low or the exhaust is not smooth.
(2-3) Due to insufficient pressure
Due to the low pressure, the extrusion pressure on the mold surface of the resin is weak, resulting in poor gloss. In materials containing fillers such as glass fiber, poor gloss becomes more apparent due to the fact that the filler tends to float out of the surface (resin tends to sink into the inside).
For example, when the injection speed is low, pressure cannot be transmitted to the end due to resin solidification, which can easily lead to poor gloss at the end of the molded product. At the same time, when the mold temperature is low and the overall curing speed is relatively fast, poor gloss is also likely to appear on the entire molded product.
(3) Countermeasures for poor gloss
First, try to increase the pressure holding setting value and the pressure holding time setting value. You should see the situation improving at this point. If not, it can be considered that the actual pressure increase in this part is insufficient due to other reasons. At this time, the following countermeasures should be used simultaneously:
Increase mold temperature
Increase barrel temperature (especially nozzle)
Increase gate size
Increase injection speed
Use a material grade with good fluidity
1、 Injection mold
"If the mold cavity is poorly processed, such as defects such as scars, micropores, wear, and roughness, it is bound to react on the plastic part, causing poor gloss of the plastic part. Therefore, the mold should be carefully processed to have a smaller roughness on the surface of the mold cavity, and if necessary, it can be polished and chrome plated.".
If there are oil and water stains on the surface of the mold cavity, or too much release agent is used, the surface of the plastic part will become dark and shiny. Therefore, it is necessary to promptly remove the oil and water stains, and use a limited amount of release agent.
If the mold release angle of a plastic part is too small, it is difficult to release, or the force applied during mold release is too large, resulting in a good surface gloss of the plastic part, the mold release angle should be increased.
4. If the mold exhaust is poor and too much gas stays in the model, it can also lead to poor gloss. Therefore, it is necessary to check and correct the mold exhaust system.
"If the cross-sectional area of the gate or runner is too small or suddenly changes, the shear force acting on the melt when flowing in it is too large, resulting in turbulent dynamic flow, resulting in poor gloss. Therefore, the cross-sectional area of the gate and runner should be appropriately increased.".
It must be checked whether the gas has been completely discharged.
Check whether the thickness and size of the exhaust port required for effective gas discharge are ensured, and whether they are contaminated by mold scale. Whether it is configured to the appropriate location is also important. If possible, it is recommended to use a short short fill method to understand the actual resin flow situation.
2、 Injection molding process
If the injection speed is too small, the surface of the plastic part is not dense and exhibits poor gloss, the injection speed can be appropriately increased.
"For thick wall plastic parts, if cooling is not sufficient, their surface will become hairy and shiny, and for this reason, the cooling system should be improved.".
If the holding pressure is insufficient and the holding time is too short, resulting in insufficient density and poor gloss of the plastic part, the holding pressure and holding time should be increased.
5. If the melt temperature is too low, resulting in poor fluidity and poor gloss, it is necessary to appropriately increase the melt temperature.
"For plastic parts made of crystalline resins such as PE, PP, POM, etc., uneven cooling can lead to poor gloss. Therefore, the cooling system should be improved to ensure uniform cooling.".
"If the injection speed is too high and the gate cross-sectional area is too small, the casting area will be dark and shiny near the casting. For this reason, it is advisable to appropriately reduce the injection speed and increase the gate cross-sectional area.".
3、 Raw materials
1. The particle size difference of raw materials is large, making it difficult to uniformly plasticize, and the gloss is poor. Therefore, the raw materials should be screened
"Too much recycled material or nozzle material is added to the raw material, which affects the uniform plasticization of the melt and causes poor gloss. Therefore, the amount of recycled material or nozzle material added should be reduced.".
Some raw materials will decompose and discolor during temperature adjustment, resulting in poor gloss. Therefore, raw materials with good temperature resistance should be selected.
"The moisture or volatile matter content in the raw material is too high, and when heated, it evaporates into gas, which condenses in the mold cavity and melt, resulting in poor gloss of the plastic part. Therefore, the raw material should be pre dried.".
5. Some additives have poor dispersibility, resulting in poor gloss of plastic parts. Therefore, additives with better flow performance should be used instead.
"The raw materials are mixed with foreign matters, miscellaneous materials, or immiscible materials, which cannot be uniformly mixed with their raw materials, resulting in poor gloss. Therefore, these miscellaneous materials should be strictly excluded in advance.".
If the amount of lubricant is too small, the fluidity of the melt is poor, and the surface of the plastic part is not compact, resulting in poor gloss. Therefore, the amount of lubricant should be appropriately increased.