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Gas vs Water Assisted Injection Molding

As someone who first became involved with Water Assisted Injection Molding in 1998 (and who initially actively tried to promote it) we soon realized that the principle benefit of the technology (fast cooling of the plastic in contact with the water bubble) is also its limiting factor for certain part geometries.

Parts with no attached wall sections (i.e. pipes and media ducts) can successfully be produced with water assisted injection molding at cycle times that are up to 40% shorter than gas assisted injection molding. However, even with these types of parts, if the pipe wall section is sufficiently large you will tend to get "voiding" in the wall section. This is due to the water producing an instantaneous frozen skin when it comes into contact with the molten polymer. Thus, even if the water is at 300 BarG pressure, that pressure cannot be transmitted through the polymer melt as it is already contained within a "frozen" tube. If this tube (water channel) is attached to a wall section (chair molding) it also means that the surrounding plastic cannot be pressurized by the water channel, this can lead to dimensional issues and also sinkage if there are any nearby bosses or ribs.

With Gas Assisted Injection Molding, the pressure of gas inside the gas channel can be transmitted to the surrounding plastic (wall section) as there is no instantaneous frozen skin formed as the gas bubble "tunnels through" the molten polymer. This is the reason why gas assisted injection molding is used for many plastics chairs / stadium seats. Unfortunately you can't fight the Laws of Physics!

You first need to consider why think you need the gas assist technology. While at first thought, you may think for weight reduction. However, this is rarely the end result if you attempt to retrofit a molded part designed as a solid part. The advantage of the gas assist injection molding allows for a part designed for gas assist to be stiffer than a similar part made as a solid part. Essentially, you are replacing a solid part with a rib structure for a gas assist part without the ribs.

However your part is more thick, but with hollow channels. The design requirements for each process are unique to the process. The advantage or disadvantage depends upon the desired part design. Furniture has been produced successfully using both processes. However, the success rate of converting a solid part to a gas assist part rather low. If you're looking for overall material cost reduction you may consider talc or calcium carbonate filler to the PP. But be careful, the density of the filled material will go up, adding to part weight and shipping costs.

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