Before everyone assumes gas-assist is the way to go [``Gas-assist focus turns to processing advances,'' April 15, Page 14] and invests a lot of money in soon-to-be-obsolete equipment, I'd like to mention a better route that has many advantages and does not encumber the molding machine area. By using foamable thermoplastic molding compounds, a modified machine cycle, a nozzle valve and a specific type of injec-tion/compression mold, more advantages are obtainable than with gas injection. We call this type of I/C molding ``Pause I/C,'' or PIC molding. It is being patented to protect sales of the special molding materials licensees.
When injection molding foamable materials, it is best to maintain enough back pressure to prevent appreciable foam formation (or cause such to be compressed to a negligible constant volume) and then bottom out the screw-ram each shot in order to get consistent shot control. Also, ``pause'' during mold closure at a point where the compression type mold forms an enclosing compartment but is substantially removed from full compaction closure.
By use of open runners and shearable open gates, mold fill can be almost explosively turbulent when the nozzle valves open at the same time full injection pressure is applied. Thus mold time is reduced substantially and, if full mold closure pressure is quickly applied to give full densification, little added time is needed and the total mold fill time can be somewhat less than with fill of preclosed molds. Substantially atactic, stress-free and weld-line-free parts can thus be obtained.
When a quickly densified foam is allowed to re-expand (after forming a dense glossy or other surface) as by retracting a portion of a mold, a foam-centered, laminatelike, hard-crusted part can be produced having interesting insulating and lightweight structural properties.
When long, and very long, 1-inch to 21/2-inch fiber-reinforced foamable materials (soon to be produced in quantity) are so molded, the hard-skinned, fiber-reinforced foam-centered parts can be revolutionary for our industry.
The PIC process overcomes the problems with weld lines that prevented me from introducing a primitive form of very long-fiber-reinforced pelletized thermoplastics in the 1960s. With current developments, it becomes possible to push back the frontiers, using just standard, modern reciprocating screw equipment with only a little juggling of machine cycles and some innovative mold making.
So get your ``gas'' from your molding materials supplier's foamable pellets and don't mess up the molding machine with unneeded expensive accessories.
Materials Research Innovations Corp.