Pushtrusion direct in-line long fiber thermoplastic (D-LFT) compounding technology versus LFT pellets and GMT sheet
The opportunity to compound fiber-reinforced thermoplastics in-house allows processors and converters to capture a greater portion of the value chain. The flexibility of the technology does not favor a particular thermoplastic or fiber formulation and therefore allows compound customization using any combination of commercially available raw materials.
Figure 1: D-LFT Process Schematic.
Direct In-Line Compounding of Long Fiber Thermoplastic Compounds
An in-line chopper cuts the glass fiber imbedded in the molten thermoplastic resin as it exits the viscous entrainment die. The chopper’s cutting chamber is heated to maintain the cut mixture in a molten state. The resin and cut fiber mixture is then directed through a transition tube positioned directly above the injection molding press screw. Fiber chop lengths of 1/4 inch (6 mm) through several inches (50 mm) are possible.
The D-LFT process is capable of controlling glass fiber percentages within a narrow range. A total variation of less than +/-2% by weight is typical.
The significant advantage of PlastiComp’s D-LFT process over other D-LFT technologies is the ability to control fiber length. The patented chopper design allows fibers to be cut to a specified length. Other D-LFT technologies use twin screws to chop the fiber, which leaves fiber length up to random distribution.
Injection Molding Property Comparison (D-LFT Versus LFT Pellets)
Figure 2: Tensile and Flexural Strength.
Figure 3: Tensile and Flexural Modulus.
Figure 4: Notched IZOD Impact Strength.
This would account for some of the increase in properties seen in the D-LFT materials, but one would expect a property increase across the board (tensile, flexural, and impact) if the slightly higher fiber content was the determining factor. Since the largest gain was seen in impact strength, it can be assumed that longer fibers were retained in the test bars molded with material from the D-LFT process. The D-LFT compounds are never cooled and are introduced to the injection screw in an already molten state. No shear is needed to melt the material in D-LFT processing. This results in less fiber breakage than typically occurs when molding LFT pellets.
LFT pellets are typically chopped at a length no longer than 1/2 inch (12 mm). This is to prevent bridging problems in the hopper of injection molding machinery. The D-LFT system delivers its chopped compound to the injection machine in a molten state right to the injection screw, so the fiber length could be up to 2 inches (50 mm) long and not cause any feeding issues.
Fiber length becomes a more critical issue when molding larger parts on higher tonnage injection presses as fiber length retention will be greater on larger machines that have larger screw flights and larger mold gating. This should result in an even greater property increase for D-LFT materials.
Compression Molding Property Comparison (GMT Sheet Versus D-GMT)
Figure 5: Properties of D-GMT versus GMT Sheet.
By placing the charge of material at the edge of the 7″ x 7″ (18 cm x 18 cm) mold, the properties of the part were increased by approximately 30% or more when compared to properties of a part that had the charge placed in the center of the mold. This increase is due mainly to increased fiber orientation. This orientation effect provides processors with a level of control that is not possible when purchasing GMT sheet.
Cost Comparison (D-LFT ROI)
Where PlastiComp’s D-LFT system separates itself from other D-LFT technologies is with its size — its compact construction allows for the unit to be mounted on top of an existing injection machine, minimizing the need for additional floor space — and its price. PlastiComp’s D-LFT system can also be by-passed or removed when not in use so that a molder can process neat or other pre-compounded thermoplastics if needed. This increased flexibility reduces the risk of having underutilized, dedicated molding machinery assets on the production floor.
The D-LFT unit can be retrofitted to an existing injection molding machine, or an entirely new system can be purchased. With the large material costs savings that a D-LFT system provides, the return on investment (ROI) for this equipment is quite rapid. Figure 6 shows a payback scenario based on a retrofit unit.
Figure 6: PlastiComp’s D-LFT Retrofit Unit ROI.
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