Gas injection demonstrates automotive large-parts potential.

The first commercial gas-assist injection molded front and rear bumpers, which debuted on the 1993 Mazda Lantis passenger car, may be a landmark demonstration of the capabilities of this process in large automotive parts. The Mazda application followed four years of development work in Japan by Mazda Motor Corp., which licensed the Cinpres gas-injection process from Mitsubishi Gas Chemical, the main Cinpres licensee in Japan.

Gas injected bumpers are also under development in Europe by Ford and Renault-Peugeot, according to a source at Cinpres Ltd.'s U.S. office in Ann Arbor, Mich.

Although at least one fairly sizeable trim part was produced by gas injection some years ago, it does not appear that anything on the scale of a bumper has been attempted commercially before. Although sources at Detroit's Big Three auto makers are somewhat closed-mouthed these days about their gas-injection plans, most of the activity they will discuss seems aimed at smaller parts (see box, P. 23).

PROCESSING CHALLENGES

OVERCOME

Mazda says it developed the gas-assist bumper with three basic goals: 1) to integrally mold the structural parts of the bumper, reducing part and assembly costs and making disassembly easier; 2) to reduce press tonnage requirements, thereby diminishing warpage and distortion; and 3) to reduce the thickness of the bumper facing, achieving weight and material savings.

The gas-assist bumpers, made of painted PP with 10% mineral filler, were actually a retrofit design. Gas assist reportedly was chosen when the original bumpers, designed for conventional injection molding, exhibited surface-finish and stiffness problems.

Until now, the occurance of sink marks at the point where the gas is injected, along with plastic flow problems when filling long, narrow parts limited the use of gas injection to small and medium-sized automotive components, according to Mazda. The Cinpres II ("in-article") method used for the Mazda bumpers injects the gas directly into the cavity through multiple ports, which is said to overcome flow problems.

In the bumper mold, the gas channels act as flow leaders, and the gas packs out the part, eliminating sink marks, according to Cinpres. The gas-injection technique used was the full-shot method developed by Mitsubishi, in which gas is injected only after the polymer has completely filled the mold cavity. Advantages of the full-shot method are said to be sink reduction and lower tonnage requirements.

In developing the bumper, Mazda found that the proportion of the component occupied by the hollow sections can change dramatically depending on the material used and on molding conditions. Experiments were conducted using PP resin, and a relationship between pressure, specific volume, and temperature (pvT) was established. The ratio of the hollow sections to the whole component volume was found to depend on the specific volume and fluidity of the melt. It was possible to predict that ratio from the pvT characteristics of the material and the molding conditions.

LAYOUT OF GAS CHANNELS

During the development stage when a number of adjacent gas channels were incorporated into the part, it was found that some of the channels were easier to inject with gas than others, since channels with lower resin pressures filled first. Under normal circumstances, the higher resin pressure in the other channels subsides during cooling; however, in the original gas-channel lay-out, the reduction in gas pressure was impeded by the gas pressure from the adjacent channel. To eliminate the interference of pressure between the gas channels as well as the problem of gas-injection failure, Mazda altered the layout of the channels, which also enhanced the cooling of the resin.

The use of gas injection on the bumpers greatly reduced surface shrinkage - from 300 microns with conventional injection molding to just 1-2 microns with gas-assist molding. The pressure of the gas, which was maintained until the end of the cooling cycle, helped to account for the reduction in shrinkage. Yet because of its strict surface-finish requirements, even this small amount of shrinkage was unacceptable. The problem could not be fixed by changing molding conditions because of the risk of introducing bubbling or overflow. Instead, a new cross-section was developed that would eliminate shrinkage almost entirely by altering the shape of the gas bubble from round to triangular, thus avoiding sudden changes in thickness of the channel wall.

HIGH RIGIDITY, LOW PRESSURE

The gas-injected bumpers, which were molded on a 2600-ton press, reduced the required molding pressure to 74% of that required in conventional molding. The bumpers included lattice-like ribs on the rear of the bumper facing - each rib connected to a flow channel hollowed out by the gas - and were 60% more rigid than the conventionally molded bumper.

During the development of the bumper, Mitsubishi Gas Chemical developed a computer-aided analysis technique for gas-injection molding that Mazda says lays the foundation for applying gas-assist technology to other mass-produced automotive parts.

Detroit's Plans for Gas Assist

How far long is gas assist in domestic automotive applications? Each of the Big Three car makers is conducting research into gas-assist applications, and two - General Motors and Chrysler - are using gas assist commercially on selected moldings. A look at the number of gas-assist projects at one automotive molder, Mega-Tech Engineering, part of the Becker Group in Warren, Mich., suggests that interest in the technology from domestic automotive OEMs is on the rise. Mega-Tech molds parts for Ford, GM, and Chrysler. It has 25 injection presses equipped for gas assist and is currently molding 12 to 13 gas-assist automotive parts.

Automotive OEMs are reluctant to discuss specifics, making it difficult to pinpoint the focus of much of the development work. Ron Powers, a Chrysler Corp. engineer involved in purchasing exterior parts, says Chrysler sees use of gas-assist parts increasing in selected applications where lighter weight, improved appearance, or reduced cost can be demonstrated.

One typical justification for using gas assist is simplified tooling, which can account for up to 20-30% tool cost savings, says Powers. A current example is a new body side molding with a very narrow cross-section. In this case, gas assist allowed the part to be molded without use of mold lifters to create seal surfaces (used to attach the part to the car), thus lowering tool costs.

GM's Inland fishers Guide Div. in Troy, Mich., which supplies lighting and seating components, interior trim, instrument panels, and door modules, has been investigating gas-assist molding for over four years. commercial applications have so far been limited to one interior part - a grip handle on the Geo Prizm and Toyota Corolla (1993 to current model years), according to project leader Suresh Shah.

Inland Fisher Guide has several more gas-assist applications under development, says Shah, three of which are close to production. For future applications, this GM division is focusing on structural applications as well as interior parts. One complex structural part, planned for three 1997 car lines, replaces several metal components with one plastic molding.

One area of current development at Inland Fisher Guide is creation of design guidelines for gas-assist molding, which will cut lead times in part development, according to Shah. The division is also focusing its efforts on better gas-nozzle design to improve control of the process and tool-design flexibility.

Although Chrysler doesn't do any in-house molding, it does specify gas-assist injection molded components from its suppliers in certain instances. Commercial gas-assists applications at Chrysler include door handles, grilles, body side moldings and other selected parts - particularly where problems are encountered with conventional molding. Bill Nelson, development manager for GAIN Technologies, Sterling Heights, Mich., says that gas assist is also being applied to several interior parts in the Chrysler S-body Minivan.

Currently, Ford does not use gas assist in commercial parts, although the company has several projects in development, according to Don Ray in plastics R&D. "It's not all that its most vocal proponents crack it up to be, but it has a lot of good aspects. We are trying to find out where it fits for our particular kinds of parts." Declining to give specifics, Ray says that the Ford, which recently signed an eight-year development agreement with GAIN, is working with university consortia and resin suppliers to investigate the advantages of gas assist.