Process and Technology

Structural Foam molding is a low-pressure injection molding process to make large parts. Thermoplastic materials such as HDPE, Polypropylene, ABS and Styrene are used along with a blowing agent which produces a molded article with a cellular core and solid skins.

How large parts are molded using low-pressure comes down to several important differences between structural foam and conventional molding.

Other than their very large size and gas injection apparatus, low-pressure structural foam molding presses are similar to high-pressure injection molding machines.

The thermoplastic resin material is fed via a hopper into a barrel and screw heating unit before going into the injection stage.

This is where the low-pressure structural foam molding process begins to differ.

An inert gas, usually nitrogen, is introduced to the plastic melt prior to injection into the mold.

The inert gas expands within the mold cavity. This aids the plastic flow.

The gas and the action it produces is so efficient that only a partial short-shot is required. The foaming action and low-pressure work together to push the resin flow to the furthest extremes of the mold.

The plastic cells collapse or freeze as they come in contact with the walls of the mold. This forms a rigid and smooth outer skin.

Once the cavity is completely packed-out and the part begins to cool, the material in the core of the walls develops a cellular or honeycomb structure. The cellular structure, together with the solid outer skin gives the part its integrity and superior strength to stiffness property.

Structural Foam Low-Pressure Molding Process

Check out the graphic below to see how it works.
Click on the images to enlarge it and see it in greater detail!

Diagram of structural foam molding machine

The first stages of the low-pressure process are similar to high-pressure molding. The resin material is fed from a hopper into a barrel and reciprocating screw heating unit. The plastic melt then goes into the injection unit.

Before injection into the tool, inert gas is introduced into the melt.

Only a partial shot is used because along with the low machine pressure, the gas creates a foaming action that helps the flow, fill and pack of the mold.

The plastic cells collapse as they come into contact with the wall of the mold creating the solid plastic skin. The foaming action continues to push the leading edges of the melt into the furthest extremes of the mold.

Once the mold is completely packed out and begins to cool the ‘frozen’ outer skin becomes solid and rigid. The inner core develops a cellular or honeycomb structure when cooled.

Process Overview

Structural Foam molding is a way to injection mold large parts. Thermoplastic materials such as HDPE, Polypropylene, ABS and Styrene are used along with a blowing agent which produces a molded article with a cellular core and solid skins. The following is how the process works:

Prior to resin injection the mold is clamped with 1/4 ton per square inch minimum clamping pressure. The pre-blended material is held in the machine in an accumulator prior to injection. The machine injects a precisely measured short shot of material.

Structural Foam Process 1
Structural Foam Process 2

Structural Foam’s thicker wall section along with the blowing agent boost allows rapid and low pressure filling.

After resin injection the cavity pressure is isolated and the blowing agent pressure continues to fill the mold cavity.

Structural Foam Process 3
Structural Foam Process 4

The blowing agent pressure continues to pack the mold even after the mold cavity is completely filled. This blowing agent pressure compensates for volumetric shrinkage as the plastic cools. The blowing agent becomes an internal “cushion”.

Information above provided by Steve Ham Plastics, 537 Hickory Street Highlands, NC 28741 USA

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