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

Oct 15, 2018 8:00:00 PM

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 color powder diffusion during the injection molding process, incomplete cleaning of the feed barrel, mixture with other colors in the raw materials, the unstable proportion of recycling, and poor melt plasticization. The reasons for analysis and improvement are as follows:

Reasons and Solutions

Reason analysis	Improving methods
Improper melt plasticization	Improve plasticizing condition and quality
Color powder clumps or poor diffusion	Grind or replace the color powder
Low material temperature or low back pressure	Increase material temperature, back pressure, and screw speed
Uncleaned cylinder (containing other residues)	Thoroughly clean the cartridge
The inner wall of screw rod and feed tube of injection was damaged	Repair or replace damaged screw/barrel or machine
Too little dispersant	Properly increase the dosage of the dispersant or replace the dispersant
The compatibility of plastics and color powder is poor	Replace the plastic or color powder
The recycled nozzle material has a mixed color	Check/replace raw material or water outlet material
Residual melt is retained on the head of the nozzle	Clean the residual melt on the outside of the nozzle

Gloss differences

What is gloss differences ？

When the plastic part is formed, the finished surface loses its original luster of the material and forms an opalescent film, or is in a fuzzy state, which is called gloss differences, shown here:

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 matter and foreign matter mixed in (polltion), this also is a cause that produces differences. Specific analysis and improvement methods are as follows:

Reasons and Solutions

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

Is quality something you care about in your production?

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