Coinjection versatility spurs automotive potential.

Coinjection Versatility Spurs Automotive Potential

In recent years, coinjection or sandwich molding has become increasingly accepted in many areas of industry. This is especially true in automotive, which has an ongoing need to improve part performance and reduce weight and cost. Coinjection is increasingly able to satisfy this need by combining different materials to tailor-make product characteristics. This article will review recent technological developments that increase the range of versatility of the coinjection technique and broaden its applications potential.

From the time of my last article assessing the progress of the coinjection technique, up to the present, the main use of coinjection has been in housings for computers and business equipment (see PT, Aug. '84, p. 87). Today, however, the most promising prospects for new coinjection applications appear to be in the automotive field, both in Europe and the U.S., although automotive programs are still mostly at early stages of development. Volkswagen in West Germany has a coinjection machine; a German custom molder is working on coinjected bumpers with Opel; Daimler Benz is actively pursuing coinjected hard/soft combination body side moldings; and BMW is also investigating potential uses of the technique. In the U.S., General Motors Corp. has a 700-ton machine at its Advanced Development Center in Pontiac, Mich., and another machine is expected to be delivered to a major automotive development center by year-end.

The total number of Battenfeld coinjection machines in place or on order in North America is about a dozen. Three of those presses are at development facilities of major materials suppliers - Dow Chemical, GE Plastics, and Monsanto Co. - and at least five are at custom molders. All told, there are perhaps 150-200 Battenfeld coinjection machines around the world.

THE COINJECTION PROCESS

In the process of coinjection, two materials are injected simultaneously, so that one forms a skin around the part and the other a core in the center. Different from "interval" molding, in which separate materials are injected alternately, the simultaneous coinjection technique was first developed some 15 years ago by ICI in England. The process was first used to eliminate splay and poor surface quality in structural-foam parts by removing the blowing agent from the skin component. It was then found that coinjection could also be used to produce parts with a solid skin and solid core, where the two could be of different materials, or where virgin skin material could be combined with regrind or off-spec core material. This possibility, along with equipment developments by Battenfeld, such as the two-channel nozzle and other refinements, opened the way for rapid commercialization in many application areas.

Because it can combine different skin and core materials, coinjection offers a range of alternatives for overall part properties, which can be utilized to advantage in both thick-and thin-walled parts. These include:

* High-quality surface finish with no sink marks due to a foamed core;

* High-quality surface, low-cost core;

* Reinforced skin, unreinforced core - or vice versa;

* Soft outer skin, hard core - or vice versa;

* Parts with barrier properties;

* Unfilled skin and conductively filled core for EMI shielding.

Several of these combinations lend themselves to automotive parts. One that has been commercial for a few years now is headlamp reflectors for Spanish SEAT and French Citroen cars, made of PBT with a glass-reinforced core for heat resistance and unreinforced skin for mirror-like smoothness. This sort of core/skin structure was investigated for PP fenders in a development program between ICI of England and Austin Rover, though the program did not proceed beyond the prototype stage (see PT, Nov. '85, p. 106). Trunk lids and hoods with smooth skins and reinforced-core ribs have also been tested by Ford Motor Co. in Cologne, W. Germany; and similar concepts for a hood and engine cover were experimentally molded by Plastic Omnium of France for the now-discontinued Pontiac Fiero. Besides good surface finish, such parts also show undiminished impact strength, unlike fully reinforced parts.

An alternative approach for non-appearance parts has been investigated with glass-reinforced ABS skins and unreinforced ABS for the core. Tests on these parts revealed the same flexural strength as similar parts produced conventionally from fully reinforced material. Consequently, materials cost savings should be possible.

HARD/SOFT COMBINATIONS

Numerous types of automotive parts require a hard inner core and a soft skin. These include dash panels, consoles, glove compartments and compartment doors, door handles, caps, door panels, and more. Besides interior components, bumpers, spoilers, and body side moldings can also utilize such a structure. Many methods have been used to produce such parts, from molding two separate parts and then assembling them, to using the two-color injection process to alternately inject the hard and soft materials.

Coinjection offers an alternative that is both technically and economically attractive. Whereas conventional two-color injection requires two molds and, therefore, a larger clamping unit, coinjection requires only a single mold and can produce the finished article in a single operation.

Adhesion between skin and core materials places limits on the possible material combinations for coinjection. Until recently, it was believed that compatibility in this regard was solely a matter of selecting the appropriate generic types of plastics, such as a flexible PVC soft skin over a rigid ABS core. Although some combinations of this type do adhere well, it has been discovered that the precise choice of flexible PVC and ABS grades greatly influence the results. Not just any grade of either will do. There is as yet no sure means, other than trial and error, to arrive at an optimum selection.

Although coinjected hard/soft parts such as instrument panels have been produced experimentally, one limitation of the technique is that in parts such as these with multiple openings, the core material does not weld together where flow fronts meet, although the skin material does weld successfully. In order not to compromise mechanical strength, coinjection might necessitate leaving the openings to be trimmed out of the part after molding.

HOT-RUNNER AND

'BREAKTHROUGH' TECHNIQUES

Conventional hot-runner systems cannot be used for coinjection, as the two materials will mix in the hot-runner nozzles. Battenfeld recently developed a special hot-runner system for coinjection (see accompanying diagram), which has proven successful in two years of production molding of headlamp reflectors.

For hot-runner coinjection, it's necessary to keep the materials separate until they enter the cavity. Small, individual shutoff nozzles are located in the mold at the hot sprue bushing, rather than in the nozzle as is normally the case for coinjection. This system does take up some added space in the mold, but it has been proven successful with two-cavity production of headlamps and other products, and eight-cavity hot-runner coinjection of small nonautomotive parts is being considered.

A more recent development makes it possible to allow the core material to deliberately "break through" the skin material at a specified location in the mold. Normally, core breakthrough is undesirable, but selective breakthrough could be of interest in parts with hard cores and soft skins, but which require external rigidity at certain locations. An example is the car heating/air conditioning baffle shown in an accompanying photo, where the hard core is allowed to break through to form a hinge.

This is achieved after initial filing of the mold, in which the core is completely encapsulated by the skin; then, a core pin is retracted, creating a void that is filled by additional core material. Coordinated sequencing of the mold action and the twin injectors is required to accomplish this.

NEW FOAMING TECHNIQUE

A new process, first introduced by Battenfeld at the Interplas '87 show in Birmingham, England, permits thick-walled parts to be produced with a smooth, solid outer skin and an unusually low-density foam core (see PT, Jan. '88, pp. 53-54). Called Multi-Foam, the process involves injecting a compressed gas through the coinjection nozzle along with the chemically foamed core component. The gas pressure permits filling the cavity with a smaller amount of plastic core material. The gas pressure in the mold provides holding pressure and ensures that the part skin is adequately pressed against the cavity wall to provide good surface finish. After the mold is filled, the pressurized gas is vented back through the nozzle, allowing the still-molten core material to expand toward the center of the part.

The Multi-Foam process solves one of the main problems associated with low-pressure structural foam, where surface quality deteriorates in direct proportion to density reduction. Multi-Foam parts offer uniformly excellent surface quality with no sink marks, and lower overall densities than are possible with previous coinjection techniques. The process also offers the potential for cycle-time reduction, since less material is used.

The Multi-Foam process can be retrofitted on all Battenfeld coinjection machines, with a conversion kit involving a special nozzle and process controller. The controller can be integrated into the Unilog 8000 and 9000 machine control systems. Battenfeld recommends use of nitrogen gas, rather than air, to prevent material degradation.

Although a handful of Multi-Foam systems have been sold (at around $30,000-50,000 for the conversion kit), applications for Multi-Foam are still in the development stage. Part wall thicknesses must be at least 0.4 in. As wall thickness increases, part density and cycle time can be reduced in greater proportion. Processors might consider Multi-Foam for parts such as arm rests with soft outer skins and hard but lightweight foamed cores, where rigidity can be achieved without inserts, and lightweight handles with soft outer skins and soft foamed cores.

PHOTO : Prototype fender for Rover in the U.K. was coinjected with a glass-filled PP core for heat resistance and unreinforced PP skin for Class A surface.

PHOTO : Coinjected headlamp reflectors on some European cars have a reinforced PBT core for heat resistance and unreinforced PBT skins for smooth surfaces.

PHOTO : Relatively new, two-channel hot-runner system keeps core/skin melt streams separate until they reach sprue bushings, which have a valving system.

PHOTO : Car heater/air conditioner baffle uses special mold design that allows rigid core to break through the soft skin material just where a stiff hinge is needed.

PHOTO : Two hard/soft combinations of a rigid ABS core with a flexible PVC skin: body side molding and instrument panel.

PHOTO : Proprietary Multi-Foam process yields unusually low density in thick-walled parts. First, compressed nitrogen gas is injected along with the chemically foamed core material. Gas presses skin againts the mold to form a smooth surface. Then, the gas is vented, allowing the core to foam toward the center.

PHOTO : Car heater/air conditioner baffle (left) uses special mold design that allows rigid core to break through the soft skin material just where a stiff hinge is needed.

PHOTO : Two hard/soft combinations of a rigid ABS core with a flexible PVC skin: body side molding (center photo) and instrument panel (bottom).

PHOTO : Proprietary Multi-Foam process yields unusually low density in thick-walled parts. First, compressed nitrogen gas is injected along with the chemically foamed core material (top). Gas presses skin against the mold to form a smooth surface. Then, the gas is vented, allowing the core to foam toward the center (bottom).