New rules for gas-assist

Gas-assisted injection molding applications are on the rise in North America, thanks to the waning of court actions associated with the process, removal of licensing fees in some cases and falling equipment prices. Processors considering using the process can now concentrate on the more practical, essential questions: In what instances is it best to use gas-assisted molding and how can the process be optimized in both the design stage and in production?

“There’s no general rule as to what part can or can’t be converted to gas-assist,” says Paul Dier, technical specialist with Bauer Plastics Technology, a supplier of gas-assist molding equipment based in Norfolk, VA. “Each part has to be individually evaluated. We usually start by asking a customer a series of questions about what they are trying to achieve. If a customer is running a ten-second cycle time on a part, there may not be any advantage in going to gas-assist.”

In addition to reducing cycle time, gas-assisted molding can result in a reduction of part weight, resin usage, overall clamp tonnage needed to mold a specific part, as well as reduce the number of gates and/or the need for a hot runner system. Gas-assist molding is also frequently used to prevent or correct surface defects, most typically warpage and sink marks arising on thin-walled sections of parts containing relatively thick bosses and ribs.

Long panels or parts with panel-like sections are ideal for gas-assist applications, according to Peter Zuber, manager of aesthetic process technology at GE Plastics. Molding such parts by conventional injection methods can be difficult because the pressure drop of the melted resin after it leaves the gate often results in freeze-off before material reaches the extremities of the tool cavity. Higher packing pressures can sometimes suffice to fill the cavity, although gas-assist is an alternative that can often produce more consistent results. Unlike melted resin, the pressure of injected gas stays constant inside the cavity, which in turn pushes the melt front in to all surfaces of the tool before freeze-off.

Plastic handles or tubular-like parts with relatively thick walls are also ideal types of parts to mold with gas assist methods, says Zuber. In this case, making a good part can depend on the proper orientation of the tool, especially if the resin is a crystalline, lower viscosity material. For example, Zuber notes, in order to mold the roll bar used to load paper in a typewriter using gas-assist, one molder had to design the mold cavity in the vertical rather than of the horizontal orientation. In the horizontal position, material entering the mold cavity tended to settle in the bottom creating a lopsided wall or, in the worst case, gas blow out. A more even distribution of material was achieved by orientating the cavity to the vertical, shooting resin from the top and injecting gas from the bottom.

Gas delivery retrofits changing design rules

One relatively new gas-assist application being adopted by more molders involves modifying existing tooling in order to apply gas to a specific area of the part that requires it, reports Bauer’s Dier. The process, sometimes referred to as full-shot gas-assist molding, is most typically used to eliminate some surface defect such as sink marks or warpage in thin-walled sections of a part.

“We’ll ask a customer to send us a part and evaluate it to determine if there are any interferences we need to work around for placement of gas injector pins,” says Dier. “Once that’s determined, we can basically cross drill through the mold and insert a gas injector pin and apply pressure to a localized area of the part needing extra pack out.”

In one specific example, Dier reports, a company was experiencing problems with sink marks on a computer tower bezel molded from a PC/ABS blended material. The part is 24 to 26 in. long by about seven in. wide with a nominal wall thickness of approximately 0.080 in. The company attempted to eliminate the sink marks by making numerous conventional tool changes, costing about US$80,000. Dier says Bauer investigated the problem and suggested making modifications to the tool to allow gas to be injected in a localized area where the sink marks were occurring. The changes were made for about $2000 and the problem was eliminated.

Dier reports this same company is now planning to change every tool in its shop to accommodate gas injector pins.

“The problem with thinner walled parts is that if you run a gas channel across the product surface, you risk having a differential in gloss level, called read through. By applying gas directly to a localized area you can design a thicker boss or rib on a class A surface and get away with it. This method changes the rules of tool design.”

Simulation aids gas-assist design

According to Stewart Barton, Moldflow technical services engineer, making a good part entails evaluating the effects of four key criteria — material, part design, mold design and process conditions. The selection criteria apply whether the process used to make the part is conventional injection molding or gas-assisted injection molding. Moldflow’s C-MOLD Gas Assisted Injection Molding Simulation software allows the designer to make changes to one or more of these factors and obtain a model of the result.

Says Barton: “Typically we start out looking at conventional mold filling on the gas-assist part and mold design. We look at the polymer shot just before gas injection and observe the filling patterns. Then we run through the gas simulation and make sure there are no catastrophic problems like gas blow out or short shots.”

Barton says the biggest difference between simulations for gas-assist and conventional injection molding results from the addition of gas channels, which change pressure distributions and fill patterns in the mold cavity. The advantage of using simulation software is that you can quickly run through a number of design options — such as changing the location or size the gas channels — and see how each change will likely affect the part before cutting steel.

“Once we do the trouble shooting we will look at the final gas penetration within the mold,” says Barton. “The gas may not take the path we want it to or there could be problems such as ‘fingering’, which is when gas leaves the desired thicker areas and seeps into the thin-wall areas.” Barton says fingering can be caused by incorrect placement of gates or gas channels, but is basically due to gas seeking the path of least resistance.

Zuber concurs: “One incorrect assumption molders and designers often have about gas- assist is that the gas is going to go to the thickest area of the part. Gas, however, simply wants to go from areas of high pressure to areas of low pressure.” Zuber says that proper positioning of the gates, gas channels and gas injection inlets relative to one another is the key to obtaining a defect-free part with gas-assist.

Richard Goralski, technical specialist with Bauer, recommends computer simulation as an aid for designing gas-assist tooling, but cautions that the results obtained from modeling can sometimes differ from what actually happens at the machine.

“Some customers will agree to run a part using gas-assist and then find out there’s going to be a small amount of gas permeation into certain wall areas,” Goralski reports. He says that gas permeation can be minimized and in most cases it does not affect the performance of the part.

Barton allows that computer simulation can sometimes under-predict gas penetration, especially along the corners of adjoining wall sections. This is mostly due to the inability of current computer algorithms to accurately model what is happening in these areas.

“Where two walls come together in reality it makes a thicker area that the computer doesn’t see,” Barton says. Still, he believes computer simulation provides invaluable information the molder can’t obtain by any other means. “The critical things you learn in doing simulation mold filling for a gas-assist process are the placement of gas channels
, the size of gas channels, the location of the gates and the location of the gas injection pins.”

Applications demonstrate benefits

Milacron Inc.’s patented Airpress III gas-assist process was used to mold the side mirror for the Porsche Boxter sports car. The process produced a seamless one-piece molding with a deep hollow lip requiring tight geometry transition without thin-walling. According to Hermann Plank, managing director Ferromatik Milacron, which holds patents for the process in Europe, the distinguishing trait of Airpress III in comparison to other gas-assist methods is that the mold cavity is completely filled with plasticized resin before the introduction of gas.

“Completely filling the cavity with material ensures that entire part surface is covered,” notes Plank. “Then, with controlled gas injection, we blow out the liquid core and push the material back into the barrel.” The main advantages to the process, says Plank, are materials savings, uniform wall thickness and the elimination of flow marks and other surface defects, as gas is not being used to force the melt against the mold wall.

Barrie-Ont. based Injectech Industries entered the gas-assist market about four years ago. According to president Gert Walter, the company has gas-assist capability for up to five presses with both Battenfeld’s modular Airmould system and Cinpress gas assist equipment (see Canadian Plastics, News section, June 2000). In one current instance, Injectech is using gas assist to mold a 42 in. by 14 in. by 3 in. automotive seat pan with wall thickness ranging from one mm to 7.5 mm. The part is being molded warp-free on an 800-ton press. Without gas-assist, Walter notes, it would take at least 1200 tons of clamping force to mold the same part.

Bauer’s Dier says that in addition to clamp tonnage reduction, which can save a molder hundreds of thousands of dollars in the investment of a larger tonnage press, one of the biggest benefits provided by gas assist is the elimination of gates. He cites the example of a five and one-half ft. long headlight mounting module molded by gas assist through a single feed gate. Molded conventionally, Dier says, the part would have required a $125,000 hot runner system for each of the three molds.

“Not only did gas-assist save on the cost for three hot runner systems but it eliminated knit lines by going to a single gate,” notes Dier.

With advanced design tools and better understanding of the process, more and more molders are tapping gas assist in order to provide customers with just such benefits.