How to avoid Dark Spots, Visible Ejector Marks and Scratches in your Plastic Production

If you've been following us at all, you might have noticed that avoiding quality issues in your plastic molding production is a major topic. Today we come back to you with three issues: dark spots, visible ejector marks, and scratches.


Dark Spots

What are Dark Spots

On transparent plastic parts, white plastic parts, or light colored plastic parts, dark spots often appear during the injection molding production, as shown in the picture below. The dark spots on the surface of plastic parts can affect the appearance quality of products, resulting in a high rejection rate, waste, and high cost in the production process.

Dark Spot 

Reasons and solutions

The problem of dark spots is a difficult problem in injection molding which needs to be controlled from the raw material, crushed material, ingredients, feeding, environment, stop, and production process to reduce the dark spot. The main reason for the dark spots on the plastic parts is that the plastic melt mixed with a foreign material that will degrade at high temperature resulting in dark spots on the surface of the products. The specific reasons and improvement methods are shown in the following table:

Reason analysis Improving methods
The raw material of the superheated decomposition attaches to the inner wall of the cylinder

1. Thoroughly shoot remaining glue;

2. Clean the pipe thoroughly;

3.Lower melt temperature;

4. Remove extra residue;

The raw material is mixed with foreign matter (dark spots) or the drying bucket is not cleaned

1. Check the raw material for dark spots;

2. The drying bucket should be thoroughly cleaned;

The thermally sensitive plastic gate is too small, injection speed is too fast

1.Increase gate size;

2.make the injection speed lower;

The dead Angle in the barrel causes the material superheated decomposition Check the nozzle, ring and feed tube for wear/corrosion or replace the machine;
Dust falls into the air during mold opening Adjust the wind and wind direction of the fan (it is best to turn off the fan), cover the injection molding machine with film

Color powder diffusion is poor, resulting in condensation point

Add diffuser or change the quality of the toner

Dust in the air enters the dryer

Add dust cover to the air inlet of drying barrel

The nozzle is blocked or the nozzle hole is too small

Remove non-melted material or enlarge the hole diameter

Impure or contaminated outlet material

Good control of water outlet materials (best dust free workshop)

Crusher/mixer not cleaned up

Thoroughly clean up the crusher/mixer


Visible Ejector Marks

What is visible ejector marks?

When the plastic part is ejected from the mold and the jacking method is used to remove the part, the jacking bar will often leave deep or shallow traces on the plastic part. If these traces are too large, it is called visible ejector marks. In the most serious case of this will be called a jacking explosion as shown in the following picture:


Visible Ejector Mark

Reasons and Solutions

Reasons analysis Improving methods
The back mold temperature is too low or too high Adjust the appropriate mold temperature
ejection speed too fast make ejection speed lower
There is a parting Angle Repair the mold
Unbalance of finished products (spring of broken thimble plate) Repair the mold(make the ejection balanced)
The ejector pin is not in sufficient quantity or position Increase ejector pin number or change ejector pin position
There is a vacuum in the die when it is unformed Remove stains from pin holes to improve air intake
the bone and pillar of the product are coarse(under-cut) polishing the bone and pillar
Excessive injection pressure or pressure maintaining pressure Reduce the pressure appropriately
After the finished mold release Angle is too small Increase the draft Angle of the back mold
Improper time or position of sideslip Overhaul mold (make core pulling operation normal)
The ejector pin size is too small or the ejection speed is too fast Increase ejector pin size or slow down ejection speed
Too fast injection at the end (burr) Slow down the last infusion



What is considered a scratch?

When the mold part is ejected and the erosion pattern on the side of the mold cavity is too thick or the deviation of the mold part is not enough, then the mold part will be separated from the core. This is called pulling up or scratching. This is shown in the following picture:



Reasons and Solutions

The main reason for the pulling is that the injection pressure or pressure maintaining pressure is too high, and there is an inversion inside the mold cavity. The specific reasons and improvement methods are shown in the following table:

Reason analysis Improving methods
Burr(under-cut) on the side of the mold cavity polishing the surface of the mold cavity
Excessive injection pressure or pressure maintaining pressure Reduce injection pressure or pressure maintaining pressure
Die cavity draft is not enough Increase the draft Angle of the die cavity
The inner side of the cavity is too rough Change rough grain to fine grain or smooth step structure
Excessive clamping force (mold cavity deformation) Reduce clamping force and prevent cavity deformation
Front mold temperature is too high or cooling time is not enough to Reduce cavity temperature or extend cooling time
The mold opens too quickly Slow down the mold opening starting speed
Mould locking end too fast (mould cavity impact collapse) Slow down the terminal locking speed to prevent the cavity from collapsing



Is quality something you care about in your production? 

NexPCB cares about this too.

NexPCB places high importance on quality assurance and standards. We offer full inspection before shipping which includes structural reliability and performance testing. All of our facilities are ISO9001 certified. Learn more about our quality assurance technologies here.

You can be assured of the quality of your PCB/PCBA and SMT production. 

Feel free to tell us your project needs and our team of experienced engineers would be glad to support you!


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