Why Is the Gloss of Injection Molded Parts Poor?
The quality of the injection molding plastic product is determined by many elements. In some case, the surface is with poor gloss. As many year in this industry, we think the main reason as following:
1. Injection mold
(1) Poor processing of mold cavity – mold quality
If the mold cavity has defects such as scars, micro-holes, abrasion, roughness, etc., it will inevitably reflect on the plastic parts and make the plastic parts poor in gloss. Therefore, we must process the mold carefully to make the surface of the cavity have a smaller roughness. It can be polished and chrome-plated if necessary.
(2) Dirt on the surface of the cavity – mold use
If there are oil stains, water stains on the surface of the cavity, or too much release agent, the surface of the plastic parts will be dark and dull. For this, the oil and water stains should be removed in time. Besides, the release agent should be used in a limited amount.
And the mold maintenance is another related issue.
(3) The demolding angle of the plastic part is too small
If the demolding angle is too small, it will be difficult to be ejected. Or it will make the force applied during demolding too large, causing the poor surface gloss of the plastic part. For this, it is necessary to increase the demolding angle.
(4) Poor mold air-exhaust
If the air-exhaust is poor, too much air will stay in the cavity, and it will also cause poor gloss. For this, we should check and modify the mold air exhaust gate.
(5) Improper gate or runner design
If the cross-sectional area of the gate or runner is too small or suddenly changes, then the melt plastic will be subjected to too much shear force when flowing in it. And the turbulent flow is dynamic, all of those will result in poor gloss. For this, the cross-sectional area of the gate and runner should be appropriately increased.
2. Injection molding process
(1) The injection speed is too slow
If the injection speed is too slow, the surface of the plastic part will not be dense and the gloss will be poor. For this, we can increase the injection speed appropriately.
(2) Insufficient cooling of plastic parts
If the cooling of thick-walled plastic parts is insufficient, the surface will be hairy and the gloss will be dark. For this, we should improve the cooling system.
(3) Improper pressure holding operation
If the holding pressure is too low and the holding time is too short, the density of the plastic part is not enough and the gloss is poor. For this, we should increase the holding pressure and holding time.
(4) The melt plastic’s temperature is too low
When the melt temperature is too low, the fluidity is poor, and it is easy to cause poor gloss. For this, we should increase the melt temperature appropriately.
(5) Uneven cooling of crystalline resin
For the plastic parts made of PE, PP, POM, etc., if the cooling is not uniform, it will cause poor gloss. For this, we should improve the cooling system to make it evenly cooled.
(6) The injection speed does not match the gate area
If the injection speed is too high and the gate cross-sectional area is too small, the vicinity of the gate will be dark and with poor gloss. For this, we can reduce the injection speed appropriately and increase the gate cross-sectional area.
3. Raw materials
(1) Large difference in raw material particle size
The large difference in particle size makes it difficult to plasticize uniformly and so causes poor gloss. For this, we should sieve the raw materials.
(2) There are too many recycle material in the raw materials
If adding too much recycled material or runner, the uniform plasticization of the plastic melt will be affected and the gloss will be poor. For this, we should reduce the amount of recycled material.
(3) Poor temperature resistance of raw materials
Some raw materials will decompose and change color when the temperature is adjusted, resulting in poor gloss. For this, we should select the raw materials with better temperature resistance.
(4) The raw materials contain volatile matter
If the content of moisture or volatile matter in the raw material is too high, it will volatilize into gas when heated. And that will condense in the cavity and melt, resulting in poor gloss of the plastic part. For this, we should pre-dry the raw materials.
(5) Poor dispersion of additives
The dispersibility of some additives is too poor, and the gloss of the plastic parts is poor. For this, we should use the additives with better flow properties.
(6) Impure raw materials
If the raw materials are mixed with foreign matter, miscellaneous materials or incompatible materials, they cannot be uniformly mixed and melted with the raw materials, resulting in poor gloss. For this, these miscellaneous materials should be strictly excluded in advance.
(7) Too little lubricant
If the amount of lubricant is too small, the fluidity of the melt will be poor. Then, the surface of the plastic part will not be dense, resulting in poor gloss. For this, we should increase the amount of lubricant appropriately.
