FOAMING

Foaming technology is used to produce solid thermoplastic parts with a foamed cell structure inside. The cell structure is caused by nitrogen (N2) or carbon dioxide (CO2) gas which is compressed and mixed into the melt during the screw recovery. Both during and after injection, the gas begins to expand and builds a very delicate cell structure inside the part, barely visible to the naked eyes.

That cell structure brings following benefits:

  • reduced part density - weight/material savings
  • less warpage
  • reduced clamping force required
  • equal shrinkage throughout whole part

How does it work?

What is special about this technique? The gas is not just mixed with the melted plastic, but it is actually solved into the melt like salt dissolves into warm water. The amount of gas which can be solved in melt isn’t very big and is related to different factors like the type of the thermoplastic material or the environment. In order to get an equal and fine cell structure and to maximize the benefit of saving part weight it is needed to extend that boundary of solvent. An efficient way to increase this amount of solvent is to increase the pressure. Just like a soda bottle the gas stays solved in the melt in a greater amount as long as the increased pressure remains. At and after the injection the pressure drops, and the gas begins to dissolve out of the melt. Because the gas was solved in the melt and not just mixed with it, many small gas cells begin to appear within the melt matrix in equal distribution. This process is called cell nucleation.

To return to the prior example, it’s like opening the soda bottle and releasing the pressure. Suddenly the CO2 dissolves everywhere out of the water and rises to the top.

The more delicate and more evenly distributed the cell structure is, the smaller is the negative impact on mesh properties even at reduced part weight.

Due to the properties of this injection molding process, it is possible to produce a foamed part with completely closed surfaces. When the injected melt-gas solvent hits the mold surface, a thin layer of material instantly freezes due to the high heat of the metal surface. Therefore, the gas has no chance to expand or escape. Because of poor heat flow abilities through that layer, the melt in the center of the molded part remains in the plastic state longest. Letting the gas expand to compromise the shrink of plastic and provide an inner holding pressure.

What is needed to run MuCell®?

Initially a source of pure gas is needed. That could be a 200-bar bottle or a generator that filters molecules out of a pressured air supply. The most common way is to use nitrogen (N2) out of bottles due to the easier processing compared to carbon dioxide (CO2) and the smaller amount of gas needed for every part. These two gases are mainly used because they are so called inert gases which means that they won’t react with the plastic melt under the given circumstances. That’s the reason why it’s so important to have just a very little amount of other gases within the nitrogen or carbon dioxide. The biggest impurity in pure nitrogen is oxygen. Oxygen is not an inert gas and would causes a reaction (burn) in the melt. This would lead to degradation of the plastic material and burning marks on the part.

In the next step the gas moves into the gas dosing and gas injection unit. These units compress the gas into the desired state and manage it to provide a precise and repeatable amount of gas for every injection cycle. Through a shut off injector implemented into the barrel, the gas gets into the melt during screw recovery.

In order to distribute the gas completely within the melt, an additional mechanical mixing is needed. This is done by a special wiping and mixing zone on top of the screw. Because the length of that zones reduces the actual plasticizing zones of the screw, the whole screw is extended to a L/D ratio of 25. This partly compensates the decrease in plasticizing performance. The gas is injected during screw rotation and travels with the melt through this wiping and mixing zone. In this way the 2-phase mix becomes a single-phase substance. To keep it like that, the pressure inside the barrel needs to be hold at any time. This means that even when the screw recovery is done, the back pressure keeps pressing on the screw. To close the whole system a shut off nozzle on top of the barrel and a second check valve at the end of the wiping zone are necessary. That makes it possible to insert a certain amount of gas in every part regardless of cycle time or other variables. And that’s the superior point to all chemical foaming methods.