Injection molding machines represent an established, reliable, effective means of making plastic parts, but use of the process becomes more challenging as the size of the part increases. The larger the part, the more tonnage required to clamp the part. In addition, common internal plastic part features such as reinforcing ribs, fastener bosses, component mounting placements, all become more problematic with larger parts because these features can create visible sink marks on the exterior side where aesthetics matter most. But now, an emerging technique is attracting attention because it can create a large plastic part that is ribbed for strength, yet is thin-walled and free of sink marks, ejection pin marks, warping and other part distortions.

The technique, called external gas molding, uses pressurized nitrogen gas that is injected into the mold cavity that has been nearly filled with plastic resins. The gas forms a micro-thin layer between the plastic and the adjacent mold surface and uniformly presses the plastic against the opposite mold surface, smoothing out sink marks that could form as the plastic cools and begins to solidify and shrink.

The technology has been licensed to three custom, injection-molding companies in the U.S. by Cinpres, a UK-based company that has exclusive rights to market the technology in North America and most of the world. In Japan, the Asahi Kasei Corp. has marketing rights. To date, the technology has been licensed to Bemis Manufacturing of Sheboygan Falls, Wis., Consolidated Metco of Portland, Ore., and Mack Molding of Arlington, Vt.

The image illustrates how the internal gas-assist molding process cored out the thick rim and bosses on this large truck panel, resulting in a structurally strong, yet light part. Photo by Mack

Steve Ham, a retired plastics consultant with more than 30 years of experience using plastic materials, has worked with Cinpres since 2002 to help develop the technology. He says the benefits of EGM range beyond just removing sink marks. The technique helps reduce molded-in stress, improve dimensional stability, shrink the amount of material used, and reduce the tonnage required to clamp the material and produce the part.

EGM is similar to the more familiar internal gas-assist technology in that it uses nitrogen gas and is injected into a mold cavity to press the plastic material and remove sink marks and other flaws. Like EGM, internal gas-assist is a low pressure molding technology. Unlike EGM, however, with internal gas assist, the gas is injected inside the melted resin creating a gas pocket inside the part, which makes it especially useful for the manufacture of hollow parts. The technique requires that an internal gas channel be designed into the mold to deliver the gas and that parts have a large enough gap to contain the gas bubble. Internal gas assist also often requires over-flow channels to be incorporated into the mold to help distribute weld lines and make room for plastic movement when the gas enters the mold cavity. “EGM has none of these problems,” says Ham.

The gas injected in the EGM process is used for packing pressure and works especially well for large, flat parts and flat panels. The pressure eliminates the sink marks, which are small depressions on the part’s surface. They typically appear opposite thick areas of plastic such as the supporting ribs on the underside of the part.

Shows the flip side or cosmetic side of the same truck panel, illustrating the high-gloss, Class A surface that was achieved by using external gas-assist molding technology.

“Initially,” says Ham, “I was intrigued by the sink mark elimination feature, but as I dug into it I found that there was so much more to it and designers had more freedom in creating their parts.”

For instance, when injection molding, Ham says that designs can be constrained by the ratio of the wall thickness to the thickness of the projections that are allowed off of that wall thickness. Conventional injection molding needs to have the profile of the ribs reduced by 0.7:1 or 0.8:1 (70 t o 80 percent of the wall thickness) to keep the mass down and eliminate the sink mark. Internal gas-assist molding can have thicker ribs, but the projections need to be much thicker in order to fully encapsulate the gas bubble.

With external gas molding, the ratio of wall thickness to rib base thickness can be 1:1. In essence, the rib-to-wall ratio can be increased for more strength and the ribs can be placed where the designer wants to place them.

Another benefit includes reducing molded-in stress. One of the main issues of injection molding is dealing with the plastic shrinkage that happens inside the steel mold throughout the cycle time and the stress that builds up. In general, the plastic linearly shrinks 5 to 20 thousandths of an inch per inch of the part. On a volumetric basis, that amounts to approximately 5 to 10 percent of the mold volume that is reduced by plastic shrinkage and for which it must be compensated. “With normal injection molding, additional material is used,” says Ham. “The machine will try and ram more material in there after the initial shot trying to compensate for that shrinkage,” says Ham.

The image from Mack Molding illustrates how large this truck panel is by photographing it with a ruler alongside. Photo by Mack

Reducing shrinkage in the mold can also help the finished product. Studies have been conducted on in-mold pressures that indicate that in an injection molding 50-second cycle, the pressure on the cavity side of the mold begins to drop after 10 seconds. Thus, the plastic starts to shrink inside the mold and it begins to pull away from the mold’s cavity side, says Ham, adding that EGM can solve this problem.

For example, if injection molding a 12-in. cubed box, made from a material like polypropylene that shrinks at 20 thousandths inch-per-inch, the part will try to shrink approximately 1/4 in. in all directions. By inflating the mold from the backside, the plastic is held in constant contact with the cavity throughout the cycle time. “It is still going to shrink when it comes out,” says Ham, “but now it is going to shrink uniformly throughout the part instead of trying to shrink more in the corners and less in other places causing the part to twist and warp. This way, it will shrink much more uniformly.”

The part will also eject from the mold easier, says Ken Kincaid, technical engineering manager, Mack Molding Co., Southern Division. In conventional injection molding, a molded part will often need ejector pins to push the part out and that can leave pin marks. With EGM, once the gas pressure is removed, the part loosens and is ready to eject from the mold.

A comparison of a large part. On the left are sink marks that are eliminated with external gas molding. Photo: Steve Ham Plastics.

Another major benefit of EGM as compared to conventional injection molding is that a part does not require as large of a press to make the part, says Kincaid. Typically, 3 to 5 tons of clamping pressure per square inch of part is required to hold the plastic inside the mold. A 100-square inch part would require a 300- to 500-ton machine to hold it closed.

With EGM, the required pressure is only that which is needed to fill the mold and inject the gas. After that, the pressurized gas will exert much of the needed pressure. “The tonnage can be reduced to about 1-ton per square inch,” says Kincaid. “That part that needed a 300 to 500 ton press can now be made with a 100 or 150 ton press, which can save significant press costs.”

For instance, one EGM project completed by Mack Molding featured a 195.5-sq.-in. part that was molded in a 300-ton press of PC/ABS resin. By comparison, conventional molding would have required a 600-ton to 850-ton press, says Kincaid.

A rectangular shaped test component measuring about 280 mm by 190 mm by 50 mm with apertures and various ribs of different heights and thicknesses. The ‘A’ surface of the mold has gloss, matte and grained finishes. Photo: Steve Ham Plastics.

The part was a front fascia for an automated teller machine manufactured by Triton, a Dover Co. based in Long Beach, Miss. This was the first commercial use of EGM, says Kincaid.

Previously, Triton had used structural foam to make the 11.5 in. by 17 in. by 4 in. fascia. The part required strength to withstand direct impact and day-to-day wear, and had to match the other front panels on the unit.

“Using the structural foam, they had to paint the outside of the part to get a good look,” says Kincaid. “They wanted to reduce the amount of paint that they needed without reducing the big, heavy features on the inside of the part. External gas molding was a way to mold a nice looking part on the outside with heavy mounting features, ribs and structural features on the inside. The result was a solid part with no gas holes, no voids and no read-through from gas lines on the show surface,” says Kincaid.

In another application, Mack Molding used EGM and internal gas assist to develop a Class A surface component. The part was the base of an external refrigeration unit for semi-trailer trucks that sits between the truck cab and the front of the trailer. It measured 76 in x 22.4 in. x 0.18 in., and weighed 15.3 lbs. The part previously had to be produced by twin-sheet thermoforming of ABS/PC with high gloss on one of the two sheets. In the field, the base suffered cracking and other failures and Mack Molding was charged with eliminating the cracking and producing a Class A surface.

External gas pressure is shown in green during the gas-packing phase. The still molten plastic shown in blue was completely injected into the mold in the prior phase. Gas can be introduced through individual gas pins or through a gas manifold system; either approach puts gas on the back side to allow uniform gas packing pressure. Integral seals in the piece part geometry provide the sealing. Another sealing technique is to “O” ring the parting line. The gas pressure “inflates” and holds the plastic against the cavity half of the mold during cooling. With other processes the plastic shrinks down on the core.

Because of  the size of the part, and the internal ribs and bosses, conventional injection molding was ruled out. Kincaid and his crew developed a process that combined internal and external gas-assist in the same cycle to mold the large structural part and produce a high-gloss, Class A surface.

Twenty-five seconds after injecting a full shot of resin into the tool, internal gas was injected to core out the part’s 2.5-in-thick rim and bosses. This displaced the material and created a gas channel of 1.6 in. diameter around the rim. The rim sealed off the periphery of the part, preventing gas seepage during the external gas phase. External gas was applied on the core side, forming a pillow of nitrogen that packed out the part and resulted in a 90-gloss rating on the cavity or show side, right out of the mold.

Kincaid says that this is an example of the technique’s overall potential. “Features can be designed into the mold tool to isolate the gas in just the areas where you need it, making it possible to take advantage of the benefits of both internal and external gas in the same part.”

For more information, email:
Cinpres Gas Injection Ltd: enquiries@cinpres.com
Mack Molding Co.: kkincaid@mackmolding.com
Steve Ham Plastics: Steve.ham@dnet.net