Carbonated beverages can tell! What is ‘gas counter-pressure molding’?

In the world of injection molding, there is a slightly special but very reliable molding technology called ‘Gas Counter Pressure Moulding’. Today, we will explain this technology in an easy-to-understand manner, using the familiar example of carbonated beverages.

1. what are ‘bubbles’, the cause of molding defects?

First of all, it is important to understand why this technology is necessary.

In injection molding, molten resin is injected into the mold at high pressure, and one of the troublesome phenomena that occur at this time is ‘bubbles (voids)’ and ‘flow marks (sink marks, weld lines, etc.)’. Not only do these make the product look bad, but they also have a negative impact on strength and dimensional accuracy.

So why do air bubbles form? This is because air and gases are trapped when the resin is poured into the mold, or the volume of the resin shrinks when it cools, creating a cavity inside.

2. try to remember the moment when you open a carbonated drink

Here, think of a carbonated drink taken out of the fridge. The moment you open a chilled bottle of soda, you hear a ‘psshhhh!’ sound and see fine bubbles rising all at once. sound and fine bubbles rise up all at once.

In fact, this is a phenomenon in which carbon dioxide (CO₂) has dissolved in the container at high pressure and comes out all at once as a gas at the moment the pressure drops when the lid is opened.

This principle of bubbles being generated at the moment the pressure drops is exactly the same thing that happens in injection molding.

As an aside, bubbles also appear when ice is added to a carbonated drink because the impact of adding ice causes the bubbles to escape, or because they become supersaturated when the temperature drops and have no choice but to escape out of the liquid.

Now, this is where ‘gas counter pressure molding’ comes in.

This technology is a method of suppressing the formation of bubbles and surface defects by pre-filling the inside of the mold with a ‘high-pressure gas’ such as nitrogen gas and applying a certain pressure before the resin is injected into the mold.

The image is as follows:

If a plastic bottle of a carbonated drink is covered with a lid, bubbles will not appear (high pressure suppresses the formation of bubbles).

Even in molding, if pressure is applied to the mold with gas, the gas contained in the resin will be prevented from expanding rapidly

In other words, **"gas pressure keeps the lid on and prevents air bubbles from getting out of control ’**.

4. what are the advantages?

Using gas counter pressure molding offers the following advantages:

✅ Improved surface gloss Smoother flow and reduced weld lines and flow marks.

✅ Internal bubbles and voids are reduced Voiding and sink marks, which are a major enemy, especially in thick-walled products, can be significantly reduced.

✅ Can also be applied to foam molding Supercritical foam molding (e.g. MuCell®), which forms microcells inside the resin, is also effective with gas counter pressure. The foam size becomes smaller and more uniform.

✅ Improved dimensional accuracy due to stabilised resin flow The pressure inside the mold is kept constant, resulting in less unbalanced resin flow.

• 

5. when to use it?

This technology is particularly useful in the following situations:

- Home appliance parts where high appearance is required (TV frames, printer covers, etc.)

Thick-walled molding (prone to sink marks)

In combination with supercritical foam molding.

Surface roughness measures when glass fibre reinforced resin (GF) is used.

6. are there any precautions?

Of course, it is not a panacea. The following points require attention:

Cost of installing gas supply and pressure control equipment

Extension of the molding cycle (gas filling and venting time required)

Innovations in mold design (e.g. securing gas escape routes)

Also, ‘how much pressure to apply?’ and ‘When to release the pressure?’ and when to release the pressure are very important. This is where the skill of the craftsman is shown!