How to Avoid Color Streaks and Gloss Difference in your Plastic Production

Today marks the last of our posts about common issues in your plastic production. This post belongs to a series aiming to describe the 20 most common issues you might encounter in your plastic production. If you want to start from the beginning, you can start here with our first post about short shot.

Color Streaks

What is color streaks?

The surface of the plastic part or the part where the flow direction changes will produce a local color deviation phenomenon, called color streaks (pictured below) : 

Color Streaks on blue plastic 

There are many reasons for color streaks including uneven pigment diffusion during the injection molding process, incomplete cleaning of the barrel, mixture with other colors in the raw materials, the unstable proportion of recycled material, and poor melt plasticization. 

Some ways to improve this defect are listed below:

Problem

Solution

Improper melt plasticization

Improve plasticity with additives or plastic quality

Pigment clumps or poor diffusion

Grind or replace the pigment

Too little dispersion aid

Properly increase the dosage of the dispersant or replace it

The compatibility of plastics and pigment is poor

Replace the plastic or pigment in masterbatch

Low material temperature or low back pressure

Increase material temperature, back pressure, and screw rotation speed

Unclean cylinder (containing other residues)

Thoroughly clean the cylinder

The inner wall of screw rod and feed tube of injection was damaged

Repair or replace damaged screw/barrel or machine

Residual melt is stuck on the head of the nozzle

Clean the residual melt on the outside of the nozzle

 

Gloss Differences

What are gloss differences ?

When the plastic part is formed, the finished surface can lose its original luster and form an opalescent film, or is in a fuzzy state. This is defined as having gloss differences.

An example of such error is shown below:

Gloss differences on blue plastic 

The surface gloss differences between plastic parts is mostly due to the surface state of the mold. If the surface of the mold is not polished well or has mold scale, the surface of the finished product will not get a good gloss. Excessive use of release agents or oil release agents is also a cause of poor surface gloss. If the material is hygroscopic or contains volatile or foreign matter mixed in (pollution), this can also cause gloss differences.

Specific analysis and improvement methods are as follows:

Reason analysis Improving methods
Mold temperature is too low or material temperature is too low Raise mold temperature or material temperature
The density of the melt is not enough or low back pressure

Increase pressure keeping pressure/time or increase back pressure appropriately

Excessive mold release agent in the mold Control the amount of mold release agent and wipe it clean
The surface of the mold is permeated with water or oil Wipe clean water or oil and check for leakage or oil
The inner surface of the die is not smooth Mold polishing or removing glue stains
The raw material is not sufficiently dry Fully dry ingredients
Mold cavity with mold scale/glue stain Remove mold scale/glue stain from the mold cavity
The molten material decomposes overheat or stays in the cylinder for too long Reduce melt temperature or residue
Too small runner and gate (cold material) Increase the size of runner and gate

Injection speed is too slow or the mold temperature is uneven

Improve injection speed or cooling system
The cylinder is not cleaned Clean the cylinder thoroughly

The cylinder is not cleaned

Clean the cylinder thoroughly

Excessive mold release agent in the mold

Control the amount of mold release agent and wipe it clean

The raw material isn’t dry enough

Fully dry ingredients

Mold temperature or material temperature is too low

Raise mold temperature or material temperature

Free Resource: 20 Most Common Problems in Your Plastics Manufacturing

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Ebook: 20 Most Common Quality Issues in Your Plastic Production

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Posted by Bo Li

Bo breaks down complex projects into turnkey-able parts. Educated in Optical Engineering, he is actually a veteran in plastic manufacturing.

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