Use Gas-assist Molding to Consolidate Parts

Gas-assist injection molding is especially appropriate for three main categories of plastic molded parts:

• Tube- or rod-shaped parts
• Large, cover-shaped structural parts
• Complex parts with both thin and thick sections, typically resulting from part integration.

A medical device OEM approached us with a part design that clearly fell into the last category. The customer had already designed the part, which is a keyboard surround for an ultrasound system. Their goals were to integrate several different parts into a single, load-bearing component, and cut costs to improve their competitive position. It was our job to determine how to mold the new design as effectively and efficiently as possible. Here’s what we were looking at.

Originally, this keyboard surround was made of nearly a dozen distinct parts. The new design integrated everything into one plastic part measuring 29″L x 21″W. The result was a thick-walled injection-molded part with molded-in handles. The main wall thickness was 0.6″. It also featured built-in thick-wall part details and a tubular detail. The thick tubular sections, close to 2.0″ in some areas, were primarily located on the perimeter of the part, making it a complex application from a processing perspective.

Two important rules
If you have a series of parts, whether they differ in thickness or size, and they have to be assembled using inserts, screws or other types of connectors, there is really no limitation to what you can combine using gas-assist injection molding as long as you follow the rules for filling the part and gassing it out.

Rule #1: When filling the part, you can only push the resin x number of millimeters in length per millimeter of wall thickness. In this instance, using PC/ABS resin, we could push the resin 100-120 millimeters in length for each millimeter of wall thickness.

t is very important to understand and appropriately design the filling pattern before injecting the nitrogen gas. The design should be adapted according to the planned or existing gas channel network. The gas channel layout depends on the size, shape, course and length of the gas channels. It also depends on the location of the injection point, part geometry and the needle position for gassing. You can obtain this information through iterative testing or by using mold-filling simulation programs. Here, the processing parameters, as well as rheological and thermal behavior of the polymer, are very important.

>Rule #2: Maintain uniform wall thickness, aided by the nitrogen gas, throughout the whole part. If you allow too much wall thickness, warpage can occur, as well as longer cycle times, thus excessive cost.

As mentioned above, you have to achieve a certain filling pattern. In this case, all the thick sections are arranged on the perimeter of the part. So we had to fill the part 100 percent with the polymer melt, and then allow the gas to hollow out the thick sections and help compensate for shrinkage. In the end, we used a single gate for introducing the resin, six gas pins, and two overspill cavities.

Now that I have emphasized the rules, I will also admit that they can be bent slightly if you go from an amorphous resin, like PC/ABS, to a semi-crystalline resin, like nylon. Semi-crystalline resins will usually set up at a higher temperature. As an example, if you make a thick part out of a nylon resin, you can achieve a thicker handle. There is a direct correlation between resin and wall thickness, so the resin family you choose will determine how thick you can go.

If you’d like to learn more about gas-assist injection molding, contact Michael Hansen, PhD, senior technical development engineer, Mack Molding, michael.hansen@mack.com.