Updated Process Builds Reinforced Plastic Parts

March 12, 1999
Injection-compression molding yields stronger parts that warp less

Matt Miklos
Verton Product Manager
LNP Engineering Plastics
Exton, Pa.

Injection-compression molding (ICM) has been around for years — in fact, it’s how old vinyl records were made. However, with the recent availability of high-speed microprocessors and advanced software to precisely control the molding cycle, the process now works with long-fiber-reinforced thermoplastics. This offers the prospect of stronger and lighter parts, lower costs, and better part-to-part consistency. While standard injectionmolding produces reinforced- thermoplastic parts with good success, ICM can optimize the performance of long-fiber-reinforced thermoplastic materials.

The difference between ICM and conventional injection molding is that the shot is injected at low pressure into a partially open tool, as opposed to a closed one. The mold closes to compress and distribute the melt into the far reaches of the cavity, thus completing the filling and packing phase. This eliminates molded-in stresses resulting from high-injection pressures.

There are two basic types of ICM: sequential and simultaneous. In the latter, compression can begin at any point during injection, and cycle times are similar to those for conventional injection molding. With sequential ICM, the injection stroke ends before compression begins, and cycle times are 1 to 2 sec longer to accommodate secondary clamping motions.

ICM uses significantly lower injection pressures than standard injection molding so longer fibers remain in the finished part. That translates into better mechanical properties. For instance, ICM maintains the 0.5-in. fiber length of LNP’s Verton composites, resulting in finished parts with higher impact strength and more isotropic mechanical properties. In typical applications, impact strength improves 15 to 20% in 0.125-in. wall thickness and over 50% in 0.060-in. wall thickness.

Thus, one major benefit of ICM is that it can produce thinner-walled, long-fiber-reinforced parts previously unattainable with injection molding. This is an important consideration in automotive applications where companies want to reduce weight and still maintain high stiffness and impact strength. For instance, ICM lets engineers reduce wall-stock thickness in Verton parts from 0.080 in. to 0.060 in. with no appreciable loss in physical properties or impact strength — a material savings of 25%.

ICM also gives engineers the ability to in-mold-laminate long-fiber thermoplastics onto coverstock materials such as vinyl, thermoplastic olefins, cloth, and carpet, without adhesives. Thus, it can save time and money in assembly.

However, ICM start-up costs may be higher. The tooling, for example, depending on part and size could cost 10 to 15% more. That’s because it typically includes guide interlocks to ensure positive core/cavity alignment and hardened shear face and telescoping cores. ICM also requires a press with a second-stage compression stroke, as well as precise clamping, accurate shot-size control, and speed control during secondary clamping.

Costs to convert standard injection- molding presses to ICM vary with the type of machine, age, sophistication of the controller, and whether it has precision linear-position encoders to determine clamp and screw locations. Conversion may also require hydraulic accumulators and pump diverters to allow simultaneous ICM, additional control screens and software, and digital process monitoring. Many new machines can have this feature built in at a nominal fee. In most cases the cost to upgrade a newer injectionmolding machine is justified by the reduction in part cost.

ICM gating techniques are similar to those in conventional molding. To retain the glass fiber length, the mold should have large center gates and a symmetrical gating/flow pattern. Parts can also incorporate ribs, bosses, gussets, and through-holes. ICM is particularly suited to relatively flat parts, such as automobile load floors, sunroof liners, seat backs, and door panels.

© 2010 Penton Media, Inc.

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