How to Avoid Bulges and Vacuum Vesicles in Your Plastic Production

After analyzing the Short Shot, the Sink Marks, the Flashing and Bubbles issues. We decided to come back to some other  problems you can encounter during your injection molding plastic production. Specifically, we'll talk about bulges and vacuum vesicles.

The bulge

What’s the bulge on plastic products?

There are times that, when a part has been completed and released from the mould, the surface has an area where the volume has expanded more than the surrounding area. This situation is called bulging and looks something like the picture below: 

plastic Bulge issue 

Why and How?

The main reason for this bulge in the plastic is that, when the pressure on the mould is released, any areas that haven't dried at the same rate as other areas around it will release some of the gasses inside of the plastic. This causes the "bubble" look that characterizes the bulge. Some ways to mitigate this are to:

1. Ensure effective cooling by making the temperature of mold cooler, increasing the die sinking time, and/or decreasing the plastic injection temperature.

2. Make the mold filling speed slower, reduce molding cycle and/or reduce the flow resistance.

3. Increase the pressure protection and time.

4. Optimize the structure of plastic and avoid having areas where the plastic part is too thick or too thin.

Effectively balancing these variables can help in avoiding bulging in plastic. 


The Vacuum vesicle

What is the vacuum vesicle?

Shrinkage in the interior of a plastic part is often called vacuum vesicle. It is generally found in an areas where the plastic is very thick or where multiple channels come together to form a rib or a plastic wall. These areas are especially susceptible to uneven cooling or shrinking. Below is a cross-sectional picture of what this looks like at an area where a plastic outer section has a small support ridge and a vesicle has formed inside :

vacuum vesicle layout

What causes the vacuum vesicle?

When the part is cooling down after the injection is completed, areas of thicker plastic will tend to dry at a different rate. If the hardness of the plastic isn't high enough or there isn't enough melt supplement mixed into the plastic, vesicles can form within the plastic as it shrinks unevenly. This issue is similar to plastic shrinkage, but instead of being on the surface of the plastic, it's inside and not visible. 

This particular problem can affect the strength and the mechanical properties of the plastic. If the plastic is transparent, it can also affect the appearance of the part as you'll be able to see the vesicles inside. The key point in remediation is the control the mold temperature and design. Some specific issues that can cause this problem and some ways to improve are below :



The mold temperature is too low

Increase the mold temperature

The wall thickness is too large
(like the boundary of the rib or the brace to the wall)

Optimize the product structure design, try to keep the wall as thin as possible

The gate is too small or the location of gate is improper.

Make the gate larger or change the gate location

The runner is too long or too thin.

Shorten and/or enlarge the runner

Injection pressure is too low or injection speed is too slow

Increase the injection pressure or speed

Pressure protection or the time isnt enough

Increase the pressure and cooling time

The melt temperature is too low or short shot

Increase the temperature or increase amount of material to melt

Cooling time in mold is too long

Decrease the cooling time in mold or use hot bath cooling

 Back pressure is too low Increase overall back pressure on the mould


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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