The combination of an injection molding press and a twin-screw compounding extruder is a new development for processors. The primary reason to combine compounding & injection molding is to control fiber length by minimizing fiber breakage, as longer fibers lead to higher part properties. This process, initiated ten years ago, helped reduce costs of molding long-glass thermoplastic composites for structural automotive parts. It has come a long way in the past decade, finding application in non-automotive applications, other types of compounding and alloying tasks. Creating their own materials as they mold them - these multi-million-dollar machines have established a growing niche by providing new opportunities for materials cost savings and flexibility to customize formulations for individual parts. Almost four dozen machines have been sold, and the number of suppliers has grown to three, with a fourth supplying a different but related approach.

In-line compounding and injection molding was initially driven by the recent gains in DLFT or DLFRT technologies. Direct Long-Fiber (Reinforced) Thermoplastic processes for in-line compounding-and-molding; either compression or injection�were aimed at helping thermoplastic composites compete more cost-effectively with thermoset SMC (also a long-fiber process). DLFT was developed to be lower cost than glass-mat thermoplastics (GMT) and long-fiber pellets produced by thermoplastic pultrusion methods. Both GMT and long-fiber pellets require sophisticated, proprietary processes to produce the intermediate raw material in sheet or pellet form. Additionally, GMT has the disadvantage of generating significant amounts of non-reusable scrap. This scrap cannot be reused in that process.
The earliest DLFT processes separated the steps of compounding and molding processes and required a manual or robotic transfer of the hot compound to the press. A standard injection unit and screw impart damaging shear to the glass, causing extensive fiber breakage, resulting in shorter fiber lengths and lower mechanical properties. Fiber lengths in parts made with long-fiber pellets on standard injection presses typically average 2-3mm, though the starting length can be 50mm or more. As per the latest development compounding and molding are integrated on an injection machine to provide greater automation and shorter cycles than can be achieved with compression molding. The role of the injection unit changes with the presence of the compounder. The twin-screw is charged with melting and mixing, and the injection unit acts only as a piston to pump material into the mold. IMC (Injection molding-compounding) systems using a twin-screw extruder are typically configured as two-stage plunger injection machines�totally eliminating the screw and check ring, thus minimizing fiber damage significantly. The longer lengths bring higher mechanical properties to a molded part. The increased performance allows injection molded parts to compete against metal, SMC, GMT, and pre-compounded pellets for auto interior, exterior, and underbody parts. Having the molder compound raw glass fibers and related additives, binders, and polymers into a thermoplastic part can deliver both significant cost savings and product-performance benefits. These benefits are finding a ready reception in the automotive market.

Material cost savings up to 35% are common; IMC machines offer opportunities for parts consolidation with lower weight and faster cycles. IMC offers other benefits:

• A single heat history for the material and the glass can reduce thermal degradation.
• Changing glass content in a compound is as simple as changing an input parameter on the machine controller.
• IMC can offer more design freedom for complex parts with non-uniform thickness, ribs, and holes than the more expensive GMT process. The same can be true of inserts. There is also no need for post-mold trimming, punching, or milling.
• The compounding extruder can deliver higher throughput than a reciprocating screw.
• Twin-screw plasticating with self-wiping screws may enable faster color or material changes.

All IMC equipment suppliers emphasize the need for gravimetric feeding to ensure consistent formulations. Even the large glass creels can be mounted on a platform scale, and motion sensors on each roving strand can detect breaks. KraussMaffei says its IMC system records real-time process documentation that can be traced from the barcode for each part.
IMC occupies a niche in a long-fiber injection market dominated by use of precompounded pellets. More than 80% of U.S. LFT automotive applications use pellets on standard presses. The remainder is split between in-line and direct methods or compounding with concentrates.

Most commercial IMC automotive applications are for polypropylene and glass, though new uses are being explored with other materials. IMC systems can easily cost 50% to 75% more than a standard injection machine. IMC typically requires applications for large parts with shot weights of around 2 kg or more to be cost-effective. There is general consensus that an application requiring around 1 KT of material provides the needed economic return from lower part cost compared to long-fiber pellets in a standard machine. IMC is at work in both non-appearance and first-surface appearance parts, from underbody applications to interior and exterior parts. Chief among them are front-end modules, door modules, bumper beams, roof racks, instrument panels, running boards, tailgates, lift gates, seat pans, and seat backs. There is also a growing interest in non-automotive sectors, such as transportation pallets or fuel-cell plates.

KraussMaffei is by far the leading supplier of IMC presses, having sold 42 systems from 220 to 3520 tons. Four machines are in the U.S. , seven in Canada , and one in Mexico . All but three of the rest are in Europe . Automotive customers include Intier, Faurecia, and Aksys.
The core elements of its system are a corotating twin-screw extruder (its own design) mounted over an MX series two-platen press with a plunger. The compounding extruder operates continuously. During the injection and holding phases of the cycle, melt from the extruder is diverted to a heated accumulator. A shutoff nozzle and valve in the injection system redirects the melt from charging the injection plunger to charging the accumulator. The accumulator gets emptied fully on each shot, mixing its contents with additional melt direct from the extruder during charging of the plunger.
KraussMaffei also offers IMC with the extruder mounted at right angles on its smaller CX hydraulic machines for non-fiber or non-automotive applications. Automotive applications include an Audi front-end module carrier that replaces GMT, a door module for the Chrysler Sebring convertible, and an instrument-panel structural duct assembly for the Jeep Liberty/Dodge Nitro, and a sound-insulation application for BMW. KM's Caprio says potential auto exterior applications include tailgate parts now made of PC/ABS, PC/PBT, or nylon/PPO with 10% glass. IMC could reduce material costs and even allow for a color change with every shot. Back-molding against in-mold decorative films is another possible use. Nonautomotive applications include shipping containers and pallets, a clothes-washer drum, and a clothes-washer counterweight. The latter, which replaced concrete with a mix of 10% PP and 90% iron oxide, is an example of the wider range of materials and compounding uses developing for IMC. Sound-damping components have been molded on the IMC with high levels of barium sulfate filler. Besides glass, KM has done some development work with natural fibers and carbon fiber. Pallets have been molded of 100% recycled PET or HDPE. The IMC has also compounded TPOs from PP and EPDM, as well as PC blends and reactively compatibilized blends of nylon and PP. At the recent K 2007 show in Dusseldorf , KM demonstrated compounding TPU with a liquid crosslinker to achieve properties similar to vulcanized rubber in a thermoplastic part.

Engel is the latest machinery supplier to add IMC to its product line. Engel has since partnered with Leistritz for use of its corotating twin-screw compounder. Engel created a new modular implementation of IMC, whereby the individual machinery components can be separated and run independently. Like KraussMaffei, Engel operates the extruder continuously and has an accumulator (adapted from its PET preform press) and related shut-off valves to redirect melt during the injection and holding steps. Baeck says the melt is emptied from the shot pot on each cycle.

Husky's In-Line Compounder (ILC) system is offered with presses from 550 to 5940 tons and the Coperion ZSK twin-screw corotating compounder. It incorporates �shooting pot� injection technology that has been used for more than 30 years in Husky's PET and packaging systems. A unique feature of the Husky ILC is that the extruder operates in discontinuous (start/stop) mode, like a conventional injection machine.

PlastiComp supplies equipment and licensing for a technology called Pushtrusion that differs from other IMC approaches. Its system is available as a retrofit for standard injection machines. It starts with an extra reciprocating-screw unit mounted above the main injection unit that plasticates unfilled resin and feeds it to a proprietary cylindrical chamber called an entrainment die. Here, the melt wets out continuous fibers as they enter the chamber from a spool or creel. As the melt is injected into the chamber, it enters an L-shaped channel. It makes a right-angle turn where it encounters the fiber strand. The high-pressure melt stream encapsulates the fiber and pushes it toward a cutter at the end of the entrainment die. The cutter chops the wetted fibers to a programmed length. The hot material then drops down into the throat of the main injection unit, which has a modified screw that provides no plasticating action but just a gentle metering.

An add-on extruder might be required for initial plastication in place of the reciprocating screw for long part. In such a case, the extruder would run continuously, thereby requiring an accumulator and switching valves.
(Source Courtesy: PlasticsTechnology)