In-mold textile laminating gives boost to injection-compression.

In-mold lamination is expanding the potential for injection-compression molding (ICM) beyond the optical parts that have been its mainstay.

ICM, a family of processes that inject the melt into a partially open mold before compressing the cavity to complete the filling stage, it has served in a handful of niche applications for more than 20 years. In thermoplastics, the process has well-known ability to reduce molding pressures and clamp-tonnage requirements (see table) and increase flow lengths in flat, simple shapes. These advantages have made ICM a natural choice for optical parts, which rarely involve complex geometries and which require low molded-in stresses to combat warpage and bire-fringence. But ICM holds promise for less well-explored large-part applications as well, especially for in-mold decoration.

Here, ICM molds over a fabric, metal-foil, film, or foam sheet. "There is a great deal of interest right now among auto manufacturers and tier-one automotive suppliers," says Jim Moran, sales manager for Engel Machinery Inc. in York, Pa. That interest centers on inner door panels and other interior parts. Moran points out that in-mold decoration is a glueless, single-step process, while most current production methods for these parts involve separate molding and lamination steps.

Most work on in-mold laminating in this country is still pre-commercial, adds Moran. But the interest level is increasing. Engel has already sold a couple of its "TecoMelt" systems, which can use either vertical or horizontal clamps. Mannesmann Demag also has sold ICM systems for in-mold decorating in Europe and offers them through Van Dorn Demag in Strongsville, Ohio.

Among resin suppliers, GE Plastics of Pittsfield, Mass., has done extensive work with large-part ICM, including evaluations of potential jobs for its automotive customers.

TABLE 1 - STANDARD MOLDING vs. ICM ON A LARGE FLAT PLAQUE

                    Conventional   Injection-Compression
                    Molding        Molding

Cavity Pressure,
psi                   22,560          7980
Clamp Force
Required, tons          1000           400

Source: GE Plastics.

GE evaluates ICM on 550- and 1100-ton standard horizontal machines from Krauss-Maffei Corp., Florence, Ky., and on the huge, 5500-ton "Alpha One" vertical machine built by Krauss-Maffei and Dieffenbacher in Germany. The latter can perform injection molding, ICM, or straight compression molding.

Papers from the recent Society of Automotive Engineers (SAE) annual conference in Detroit outline the use of ICM for in-mold textile applications in Japan, where using ICM for this purpose has been more popular.

A FAMILY OF PROCESSES

Rather than being a single process, ICM actually comes in several varieties distinguished by differences in injection and compression sequences, the size of mold gap used in the injection phase, and whether the compression step is performed by the clamp or by an in-mold actuation system (special cores or even ejector pins). Peter Zuber, a GE Plastics senior technology development engineer who has worked extensively with ICM, finds it handy to divide ICM processes into three realms based on the gap size:

* A small gap (less than part thickness) promotes low-stress packing when warpage or optical quality are critical issues.

* A medium gap (one to two times part thickness) helps fill large parts whose flow length and wall thickness would otherwise require too much injection pressure in standard injection molding.

* A large gap (up to about 30 mm) is suitable for in-mold decoration.

In all three ICM variants, the beneficial effects on injection pressure, clamp tonnage, and flow-length come about primarily because the mold gap increases the effective wall thickness at which filling takes place, according to Zuber. He explains that in a typical injection molding job, high injection-and clamp-pressure requirements spring from a mutually reinforcing combination of high melt viscosity, long flow distances, and thin walls. By creating the effect of a thicker wall, ICM cuts the pressure needed to fill the tool, which also helps keep residual stresses to a minimum.

Filling by compression rather than injection pressure alone contributes to lower clamp pressures and thus allows use of smaller machines. "In general, the larger the gap, the bigger the reduction in cavity pressures," Zuber says, noting that injection pressure drops to a lesser degree. Rather than the normal uniaxial flow orientation, ICM creates a biaxial flow inside the tool, which further reduces residual stress and improves impact properties.

WHERE IN-MOLD FITS

On the other hand, larger mold gaps can adversely affect surface quality. "You can see a halo effect radiating out from the center of the part because the larger gaps make it difficult to keep the melt front at a constant velocity," Zuber notes. ICM also has some severe limitations in dealing with complex parts due in part to the biaxial flow it promotes. Zuber sums up ICM's applications potential: "Flat is perfect; curves are a problem. The further you go from flat in part geometries, the further you go from ICM. The wrong part will have no advantages at all."

ICM's limitations don't inhibit its use for in-mold decoration. Many of these auto-interior parts are fairly flat, and the halo effect from large mold gaps is typically hidden by the fabric. And many in-mold fabrics and vinyl foams can't tolerate the high pressures and temperatures of standard molding. "Conventional injection molding can make some textiles as hard as a rock," notes Engel's Moran.

WHAT'S COMMERCIAL?

Although ICM in-mold decorating is new to the U.S., it has already achieved commercial acceptance overseas, as evidenced by recent SAE conference presentations. Last year, a paper from GE Plastics Japan detailed the successful use of ICM to make automotive sunroof shades for Nissan. Normally, these shades would need a secondary step to apply the ceiling fabric. And at this year's conference, a paper from Idemetsu Petrochemical Co. Ltd. of Japan showed how two types of automotive door panels coated with leather and with cloth, respectively, could be molded with ICM.

Engel's Moran adds that he knows of five molding shops in Europe that specialize in ICM in-mold textile applications. And resin and machinery suppliers report there are at least two nascent efforts in this country (by Ford Motor Co. and United Technologies Automotive, Dearborn, Mich.).

RELATED ARTICLE: What You Need for ICM

Most injection molding machines can handle ICM as long as they have clamp-position profiling capability. What's typically needed, according to machinery suppliers, is beefed-up hydraulics, including an accumulator and pump dividers to allow simultaneous injection and clamp movements. Additional control screens and software to control single- and multi-stage clamp movements are also needed. Engel Machinery Inc., York, Pa.; Krauss-Maffei, Florence, Ky.; Van Dorn Demag, Strongville, Ohio; Battenfeld of America, Warwick, R.I.; and others supply ICM packages consisting of the necessary hydraulic upgrades and clamp-control software.

A new ICM upgrade kit comes from a resin supplier - Idemetsu Petrochemical Co. of Japan - which has developed a self-contained ICM unit for use with toggle or hydraulic clamps. Mounted between the moving platen and the mold, the kit's hydraulically actuated wedges engage to provide compression force after the platens have closed the mold to the desired gap.

OKAY FOR TOGGLES

Although they're sometimes considered inappropriate for ICM, toggle machines have successfully been able to run this process. The challenge is to keep the mold from blowing open before the toggle has engaged fully, according to Engel's Jim Moran. Engel's solution consists of an accumulator and additional proportional valves. Likewise, Van Dorn Demag's ICM option for its HT Series toggles of 85-650 tons includes an additional manifold and proportional valve to hold the clamp before lock-up. JSW Plastics Machinery Inc., Santa Fe Springs, Calif., also supplies toggle machines for ICM applications.

GE Plastics' Peter Zuber notes that hydraulic-clamp machines have the advantage of permitting multi-stage coining - i.e., the clamp force can be increased and decreased in steps during the compression stage. Zuber gives one reason for profiling the coining pressure from low to high during compression: "Sometimes the best results are achieved by letting the mold blow open a bit after compression has started."

In horizontal ICM applications, the tooling must be designed to retain the melt while the mold is partially open. A shear edge is one popular approach. Another can be termed a "moveable parting-line mold," in which the two spring-loaded mold faces come into contact while the cavity remains open.