Gas-injection molding: 'black art' or science?

As more molders take a serious look at gas-assist injection, the key question on many minds is: How well understood are all aspects of the process? Here's the experts' assessment of equipment, processing techniques, mold design, and flow analysis.

Few new technologies have stirred the imaginations of injection molders like gas-assisted molding. This "short-shot" approach uses localized injection of pressurized gas to pack out the mold. Gas-assist molding has demonstrated a broad range of benefits as its commercial applications increase. Over the last decade, these have extended beyond preventing sinks and coring out thick parts into less obvious and more sophisticated applications, such as using gas to eliminate lifters in tools or to allow parts to be molded at lower clamp tonnages.

Although gas injection reportedly has been pursued by dozens of molders in Europe and a smaller number in Japan, this promising technology has been slow to catch on among molders in the U.S., at least partly because of perceived threats of litigation over patent infringement. Most of the conflicting patent claims appeared to have been laid to rest by a court decision in 1989. Still, until recently the perception of risk has been enough to keep many molders and machinery OEMs from taking a closer look at the technology, which has limited the pool of available expertise. Combined estimates by Gain Technologies and Cinpres Ltd. (the two major licensors of gas-assist technology) of the number of molders in North America that have been issued licenses totals more than 50.

There are signs that the situation may be changing. North American resin suppliers such as GE Plastics and Dow Plastics have ongoing research programs and report steadily increasing interest in the process. The recent appearance of new gas-injection flow-analysis software packages promises to eliminate much of the guesswork in the critical areas of mold design and process set-up. There is even a new licensor of gas-assist technology in the market.

Does all this mean gas assist is finally ready for serious consideration by molders at large? And, for those who believe it is, what is the learning curve? Where are the pitfalls or gaps in our collective knowledge of the process? This look at today's answers to those questions updates our first major report on the process (see PT, June '90, p. 70).

MANY VARIATIONS ON A THEME

Gas-assist licenses are offered through several companies that own patents on the process. Hardware typically consists of a gas-generation unit, various shutoff pins and nozzles, and a controller. Depending on the license, gas can be injected through the machine nozzle, through the runner system, or directly into the part in the cavity ("in-article"). Gas may be metered either volumetrically or by controlled pressure and time. And there are various ways of venting the gas from the part, such as via sprue break or by evacuating the gas back out through the nozzle. Each of these methods has its advantages; licensors of the technology often offer more than one approach.

Michael Caropreso, senior process-development engineer at GE Plastics, Pittsfield, Mass., says both volumetric and pressure-controlled gas injection techniques can work effectively. The important thing, he says, is designing a part for the process that one has licensed.

"The first step in deciding whether an application is suited to gas assist is to ask yourself why you are choosing it," says Caropreso. He suggests looking at the potential benefits, such as lower clamp force, less expensive tooling, and avoidance of sink marks, to determine whether a particular job is a good candidate for gas assist.

Natalia Kassa, a consultant who until recently was a mold-filling analyst and materials specialist with gas-injection pioneer APT Corp. in Sterling Heights, Mich., cautions against over-enthusiasm about gas injection. "Sometimes designers tend to use gas where it is not needed at all. If it is used where it is needed, it's a great help and perhaps even a necessity. But there are costs involved in using gas."

A somewhat different view of those costs is offered by Bruce Tallmadge, development associate of Dow Plastics, Midland, Mich., who advises molders on gas assist: "It's important to understand that there is substantial capital involved--in licensing fees and equipment fees. These costs can be recaptured through improved operating efficiency--reduced part weight, lower cycle time, reduced scrap, and improved part quality. You want to spread the capital cost over multiple applications."

TECHNOLOGY LICENSORS

There are a number of different patents covering a variety of approaches to gas assist, with some companies holding several patents.

Gain Technologies, Sterling Heights, Mich., uses what it terms "phased pressure," rather than volume, to meter nitrogen gas into the part, according to v.p. and general manager Jon Erikson. The gas-intensification system provides pressures to 10,000 psi, and computer-controlled pressure profiles can be varied as needed--for example, initial high pressure followed by a decrease and then a pressure rise again during the cooling phase.

Gain reportedly owns over 30 patents, including ones covering gas injection through the nozzle, in the runner, and directly into the part, Gain reportedly has over 100 licensees worldwide, including around 30 U.S. molders.

The Cinpres II patent belonging to the British firm of Cinpres Ltd. (U.S. office in Ann Arbor, Mich.) specifies that gas is injected "in-article," i.e., at specific areas in the mold cavity, which allows local areas of the part to have separate gas-flow circuits. This approach reportedly allows greater design flexibility by feeding the gas precisely where it is needed. The company's original single-cylinder gas conversion unit has been supplemented by a range of multi-cylinder versions that enable gas to be injected into the molten polymer through nozzles at different pressures and several positions in the tool. The in-article approach is also said to be advantageous for family molds, in which each cavity may have different gas requirements.

Gas may also be injected through the runner system. The Cinpres method meters in a fixed volume of gas, which reportedly provides a high degree of consistency from shot to shot. Gas pressure reportedly can be controlled at any time with a hand valve.

Cinpres has over 150 licensees worldwide (around 25 of them North American molders) and its technology has been incorporated into 600-700 tools, estimates international technical sales manager John Heasman in Ann Arbor.

Nitrojection Corp., Chagrin Falls, Ohio, licenses gas-assist technology originally developed at Sajar Plastics, a custom molder in Middlefield, Ohio. Several patents cover gas injection both through the nozzle and in-article. Metering of the gas is by pressure over time. A key feature of the Nitrojection patents is the way in which gas is vented after part cooling, according to CEO Tom Johnson. Nitrogen gas is injected and vented through the same nozzle. One reported advantage of this approach is that venting is much smoother and more controlled than via sprue break, in which the nozzle is pulled back from the mold allowing the pressurized gas to escape in an uncontrolled burst. Also, with the Nitrojection method, the gas can be recaptured for use in the next shot. Johnson claims that sprue break is very rough on equipment.

So far, Nitrojection has restricted licensing of its technology to a "limited number" of U.S. and foreign molders, according to Johnson, although he expects to market the technology more aggressively later this year.

Recently, a new gas-assist technology has become available in North America from JPI Technologies, Palgrave, Ont. (PT, Mar. '94, p. 13). Developed two years ago by Kontor Moulding Systems Ltd. in the U.K., this approach is aimed at sink reduction. The technology directs gas into the cavity, usually through one or more vents in the core, after the mold has been totally filled with plastic and a thin skin of semi-cooled polymer has formed on the surface of the part. The gas is directed to the opposite surface of the part, where maximum surface finish is required. It is not used to achieve a hollowing or coring effect, says JPI v.p. Paul Boettger.

UNUSUAL VARIANTS

A quite different variation on gas-assist molding is the "structural web" process marketed by Johnson Controls' Plastics Machinery Div., Manchester, Mich. Structural web produces hollow sections with a solid skin and solid "webs" or irregularly shaped ribs supporting the skins. The exact configuration of the solid and hollow sections depends on the product's design. Parts can be hollow overall with randomly dispersed solid ribs, or they can be designed with localized hollow channels as in conventional gas-assist techniques, says Ed Hunerberg, Johnson Controls' director of engineering. Polymer and gas are injected through the same nozzle, which is also used to vent the gas.

The company sells a technology license along with its Uniloy-Springfield structural-web machines, eliminating a separate licensing fee for the technology. Hunerberg says that the low tonnage, wide platens, and big shot sizes (up to 150 lb) of the structural-web machines, make them well suited to conventional gas-assist molding, too. Fewer than 20 structural-web units have been sold.

The Helga technique, developed by Hettinga Equipment Co., Des Moines, Iowa, takes a different approach. It coinjects polymer and a proprietary liquid through the nozzle, and the liquid is converted into a gas from the heat of the plastic in the mold. Unlike other gas-assist methods, the gas is not vented; it reportedly remains dormant in the part and does not revert back to a liquid. The liquid is dispensed volumetrically under precise control during injection to direct it to designated areas of the mold, according to owner Siebolt Hettinga.

Hettinga sells the Helga system without need for a license. Siebolt Hettinga estimates he has sold "well over" 200 units worldwide. Most are retrofits of conventional injection machines.

FEAR OF PATENTS?

Injection machine suppliers have differing strategies for offering gas assist. Engel Canada, Guelph, Ont., does not market its own Gasmelt system in the U.S. but offers a Gain modular system and license. Battenfeld of America, West Warwick, R.I., does offer its own Airmould technology (it has about 15-20 installations in North America), but requires that molders first buy a Gain license. HPM Corp., Mt. Gilead, Ohio, offers Gain's gas-assist technology as an option on its machines and incorporates the cost of the license into the price of the press.

Licensors of the technology each claim that their own version does not infringe on those of their competitors. In fact, for all of the talk of lawsuits, only one patent-infringement case on this subject has ever been brought to court, according to Nitrojection CEO Tom Johnson.

GE's Caropreso advises molders to "read the patents and understand what they say, and once you buy the license, follow those rules. It's that simple."

STILL A 'BLACK ART'?

Although gas injection is acquiring a growing base of processing and design literature, technical papers, and guidelines, these don't necessarily tell molders everything they need to make a painless entry into this new technology, nor do they substitute for experience, says GE's Caropreso. He predicts that this year will be a big one for growth of new gas-assist applications and that molders' confidence level will rise along with the baseline of experience.

Molders experienced in the process say that the learning curve is substantial. Noland Broaddus, gas-injection engineer for Cascade Engineering in Grand Rapids, Mich., estimates that it took this custom molding firm about two years to become confident in using the technology. Cascade, which was the first Cinpres licensee in the U.S., has now used gas assist for about seven years. When it got started, there was much less cumulative experience to draw on than today. Broaddus says new entrants are now in a much better position: "The molder that wants to get into gas injection today can buy the expertise on the market. Whoever he licenses from is going to have to provide good technical support in order to help him get past that learning curve without making serious mistakes."

Anne Bernhardt, president of Plastics & Computer Inc., Dallas, which offers gas-injection flow-analysis software and consults with molders on the process, sees the pool of expertise from which newcomers can draw as still very small. She advises molders to start out with relatively simple projects and build up their own expertise from there.

Bernhardt says that, among molders, gas assist is poorly understood. "Part of the problem is that people have tried to put in a lot of black magic and obscure terminology. Relationships between pressure and volume in a gas are well understood; they don't change just because you are injecting gas into a plastic. The basic concept of the technology is very simple." She adds that gas-injection flow-analysis software is an effective and relatively inexpensive educational tool for molders.

MOLD AND PART DESIGN

Knowledgeable sources agree that gas-injection mold and part design are potential pitfalls for the uninitiated. According to Roger Harwood of RG Mold & Engineering, Grand Rapids, Mich., a toolmaker experienced in building gas-assist molds, "Every job is different. The first thing you look for is how to get gas into the desired area."

Designing the gas channels is dependent on the part design as well as the particular gas-assist process being used, says GE's Caropreso. "The part has to be gated in such a way that the areas last to fill with resin are at the ends of the gas channels in the thick sections," he says. "You can do this with anybody's process, but you may have to change your part design a little." After the part is designed correctly, he adds, gas-entry timing and pressure critically influence control of gas penetration.

Paul Siebert of Sajar Plastics says gas-channel layout is particularly critical when gas is injected through the nozzle and has to travel some distance to the desired place in the mold. "It's the design and sizing of the channels that helps to feed the gas. If the channels are too big, yon have a racetrack effect where the melt stays in the channels and does crazy things to your filling process. If the gas channels are too small, the gas goes all over the place and doesn't stay in the channel."

Siebert adds that it is important to consider the resin's properties and flow characteristics to help size the gas channels properly. A lot of people incorrectly design all gas channels to the same configuration, no matter what the material, he says. If they are working with an easy-flow material like PP or PS, they use the same size channels as they would use for a stiffer melt like glass-filled polycarbonate.

Another important design criterion is balanced flow. According to Timothy Lake of Moldflow (Europe) in the U.K., "Gas is very fickle and doesn't really head off in two directions at once. The moment it sees an imbalance, it will take that as the priority route." Lake adds that it's sometimes necessary to invert conventional design rules when designing for gas injection. "Typically, when designing a component, to get more melt into an area you would put in a flow leader or thicken up a section. But to get more gas into that same area, you would reduce the geometry."

POTENTIAL PITFALLS

Liam Burns, sales administrator of Sandretto Plastics Machinery Co., Wheaton, N.J., says improperly designed parts can result in thick sections where gas does not penetrate, thereby increasing cooling times and driving cycle times through the roof. Burns, who is experienced in gas-assist applications in Europe, says that gas-assist components are generally more successful if the molds are designed for gas from the outset. Simple parts, however, may lend themselves better to retrofit mold conversion, he notes.

Harwood of RG Tool & Engineering cautions that more lead time is needed to get gas-assist molds up and running production parts. "Hitting it on the first try is almost a miracle." Conventional molds might require two weeks to tune in and get parts to a customer, he says. "With gas injection, you are probably doubling that or more."

Although there have been cases where gas assist proved to be the only way to mold certain "impossible" parts, Dow's Tallmadge cautions that gas assist is not a panacea for molding problems. "Keep in mind that this is typically a short-shot technology, and machines should provide shot-size consistency to 0.5%." Process repeatability, he says, is a key factor to success in gas assist.

FLOW ANALYSIS TO THE RESCUE

The one major development that may take much of the guesswork out of gas-assist molding is the availability of gas flow-analysis software. Gas-assist flow-simulation packages are now offered by three suppliers: Plastics & Computer, AC Technology in Ithaca, N.Y., and Moldflow Pty. Ltd. in Shelton, Conn. (PT, Jan. '93, p. 21; Feb. '93, p. 17; May '93, p. 74). These packages are gradually becoming more accepted in the market. Moldflow introduced its MF/Gas software less than a year ago, and has now introduced its second generation of the product. Product marketing manager Mark Toussaint estimated the company has sold about 30 packages. Anne Bernhardt estimates that Plastics & Computer's faGaim software package has been used to design 200-300 parts that are now commercial. AC Technology has offered a commercial C-Gasflow product since 1990 and has packages in about 60 sites worldwide. The software is now in its second phase of development.

CAE is giving people a higher confidence level in successfully using gas assist in creating a part, says Peter Medina, president of AC Technology North America. In addition to helping sort through many design options that are highly dependent on each other, he says, it's also an economical and effective educational tool for the gas-assist process.

"Since you've doubled the number of variables |using gas assist~, you've quadrupled the number of processing options," says Plastics & Computer's Anne Bernhardt. Given the number of variables, it's difficult to get into gas assist unless the results can be simulated first and then duplicated consistently on the injection press, she adds.

Various software packages can simulate both pressure-controlled and volumetric gas-assist processes. They can tell the user at what point in mold filling to inject the gas, how much gas to inject, under what pressure (or tamped pressure profile), and for how long. Some will prescribe what process to use to achieve a user-specified skin thickness; others will automatically choose process conditions that will result in uniform advancement of the melt front in the cavity without sudden acceleration when gas pressure is introduced, which could result in surface imperfections. To accomplish the latter, the computer must match the gas pressure to the melt pressure in the cavity at each moment during filling--something a human engineer couldn't calculate on his or her own.

All the programs will display graphically a prediction of where the gas will go in a part under a specific set of molding conditions. Bruce Tallmadge of Dow, which also happens to be a beta site for developing Moldflow's MF/Gas, says the accuracy of these predictions is improving as the programs grow more sophisticated, especially in predicting the secondary penetration of the gas during the packing phase of the gas-injection cycle.

ROOM FOR IMPROVEMENT

Gas software is still evolving and users and suppliers alike say there are areas needing refinement. Gas penetration occurs in two phases: the first when the gas flows through the cavity and displaces the polymer forward, filling the cavity; the second occurs during packing when the gas takes up the shrinkage of the melt. Peter Zuber, senior process development engineer for GE Plastics, says the current generation of flow-analysis software needs to be more accurate in predicting the second phase. "The pvT |pressure-volume-temperature~ properties of the polymer need to be modeled in that phase."

Says Moldflow's Tim Lake, "More work needs to be done to understand the physics of the process, such as the finite details of gas-polymer interaction."

Bernhardt sees a need to simplify the analysis at the front end, permitting relatively quick and easy feasibility studies that tell molders if it's worthwhile to go into a full-blown analysis on some applications. Another need may be predicting shrinkage and warpage of gas-injected parts, she adds.

Suresh Shah, project leader at Inland Fisher Guide Div. of General Motors Corp. in Troy, Mich., and a proponent of plastic flow analysis for even slightly complex part design, wants the flow programs made faster to use. He suggests that molders could save hours in plastic/gas flow-analysis mold-analysis iterations if they could restart the gas filling or packing phase from any intermediate file. He also sees a need for predicting gas "fingering" or "bleed-through" into solid wall sections more precisely. Additionally, he wants the software to be to able simulate all the capabilities of gas-assist equipment--e.g., independent gas delay times for multiple gas pins or sequential, time-based opening and dosing of plastic hot-runner valve gates. Up-front part design and plastic/gas pin locations are the key to success to this technology, he says. "Even plastic flow analysis by itself can reduce the development lead time and cost."

Lawrence R. Schmidt, a consultant in Schenectady, N.Y., with experience in gas assist, says it's not well understood how gas bubbles move through the melt. Although the bubble often acts to push the melt forward, at times it appears to knife through the melt and rush ahead of it. "This needs to be understood for the next level of design expense."

RESIN-SELECTION ISSUES

Molders and resin suppliers say that gas assist is applicable to almost any thermoplastic, although some materials appear to be better suited to it than others. Says Schmidt, "It depends on the rheology of the material; often you can modify the rheology by changing the temperature." The key material characteristics for the process, he says, are melt viscosity and elasticity. GE's Caropreso adds that polyolefins, which are widely used in gas assist, have a higher degree of shrinkage and therefore more secondary gas penetration.

Thermosets have had limited use in gas assist. Battenfeld demonstrated gas assist on a thermoset at the K'92 show in Dusseldorf, but so far, there are few, if any, commercial applications.