How to Avoid Flashing and Bubbles in your Plastic Production

After analyzing the short shot the sink marks issues, we've decided to come back to some other issues you can encounter during your injection molding plastic production: Flashing and Bubbles.

If you are interested in Plastic Molding, you might want to have a look to our post about Adding Plastic Molding to your Project!

Flashing

What is Flashing?

During the injection process, sometimes a little bit of plastic can seep out of the die joint and make a small rib. This is called flashing.

Some examples are pictured below : 

PCB Flashing example 

How can Flashing be Avoided?

Flashing is not a good phenomenon in injection molding production, and it can cause die joint damage if the flashing can’t be immediately cleaned from the die joint and the clamp.  This type of damage will cause more evident flashing in future products. For this reason, it’s very important to look out for flashing marks in injection molding production and remove them before they turn into a persistent problem.

There are many reasons for flashing to occur in injection molding. Some examples include very high injection pressures or terminal injection speeds, and inadequate clamping force.

Below are the some culprits and ways to solve them.

Problem

Solution

The injection pressure is too high or the injection speed is too fast

Adjust the speed of the injection at the nozzle so as to inject the plastic at the correct rate for the chosen material

Clamping force is too high

Adjust the pressure of the clamps to loosen the grip

The joint face isn’t fit or the accuracy is poor

Repair and check the mold or make the joint face more precise

Mold clamping force isn’t enough (especially true if the part has flashing around edges)

Increase the clamping force

The clamping area of mold is too large

Change the machine to one with larger mold clamping force or the mold design to accommodate a smaller clamping area

Partial flashing due to gate blush

Rebalance the gate either naturally (preferred) or artifically.

Mold deformation

Use a harder material for the mold

Pressure drop before the material can set

Conserve the clamping pressure according to cooling temperature

The material of mold is bad or easy to wear and tear.

Choose better mold materials with proper heat treatment

The viscosity of the plastic is too low (like PA or PP).

Change the plastic material with a high viscosity one or adjust with a suitable filler

The material temperature or mold temperature is too high

Make adjustments to the respective temperatures of the mold and the material

 

Bubbles

How Do Bubbles Form in plastic?

When the mold cavity fills up, it is possible that the flow of the plastic is too quick and air can’t escape quickly enough from the mold. When this happens, bubbles can form in the materials.

The presence of air sets this issue apart from vacuum voids, which are caused by a similar yet not identical set of causes.

Some examples are pictured below:

 

 PCB bubbles examplePCB bubbles example

The bubbles in the first picture are ones from the vacuum vesicle and the other is small bubbles in the plastic. The reasons and improving method of the plastic with bubbles in the below table:

Problem

Solution

Back pressure is low or the melt temperature is too high

Increase back pressure or lower the melt temperature

The screw rotation speed or the injection speed is too fast

Decrease the speed of the screw or the injection pressure

The gas can’t be expelled quickly enough

Make the mold vent larger to allow air to escape

Incorrect gate size

Decrease the size of gate or use a different shape

Raw materials are not fully dried

Increase the intensity of the drying process

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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