Wood injection molding

Bleier, H., Kirsch, E. and Battenfeld, May 2002. Wood Injection Molding, Conf. Proc. Wood Plastic Composites A Sustainable Future, Vienna, Austria. 

Composites. The history of phenoHc resin composites goes back to the early development of phenoHc materials, when wood flour, minerals, and colorants were combined with phenoHc resins to produce mol ding compounds. In later appHcations, resin varnishes were developed for kraft paper and textile fabrics to make decorative and industrial laminates. Although phenoHcs have been well characterized in glass-reinforced composites, new developments continue in this area, such as new systems for Hquid-injection molding (LIM) and sheet-molding compounds (SMC). More compHcated composite systems are based on aramid and graphite fibers. 

Because of low injection pressure, some cost savings are possible in mold and press constmction. Mol ding cycles are somewhat longer than for injection molding. The part must be cooled in the mold long enough to be able to resist swelling from internal gas pressure. In stmctural foam parts there is almost a total absence of sink marks, even in the case of unequal section thickness. Stmctural foam has replaced wood, concrete, sohd plastics, and metals in a variety of appHcations. 

Compression and injection molding are used with amino resins to produce articles such as radio cabinets, buttons, and cover plates. Because melamine resins have lower water absorption and better chemical and heat resistance than urea resins, they are used to produce dinnerware and laminates used to cover furniture. Almost ah molded objects use fillers such as cellulose, asbestos, glass, wood flour, glass fiber and paper. The 1997 U.S. production of amino resins was 2.6 billion pounds. 

Phenolic phenol formaldehydes (PFs) are the low-cost workhorse of the electrical industry (particularly in the past) low creep, excellent dimensional stability, good chemical resistance, good weatherability. Molded black or brown opaque handles for cookware are familiar applications. Also used as a caramel colored impregnating plastics for wood or cloth laminates, and (with reinforcement) for brake linings and many under-the-hood automotive electricals. There are different grades of phenolics that range from very low cost (with low performances) to high cost (with superior performances). The first of the thermosets to be injection-molded (1909). 

Anisotropic materials have different properties in different directions (1-7). 1-Aamples include fibers, wood, oriented amorphous polymers, injection-molded specimens, fiber-filled composites, single crystals, and crystalline polymers in which the crystalline phase is not randomly oriented. Thus anisotropic materials are really much more common than isotropic ones. But if the anisotropy is small, it is often neglected with possible serious consequences. Anisoiropic materials have far more than two independent clastic moduli— generally, a minimum of five or six. The exact number of independent moduli depends on the symmetry in the system (1-7). Anisotropic materials will also have different contractions in different directions and hence a set of Poisson s ratios rather than one. 

Ore refiner Coal or wood burner Injection molding Reaction injection molding Membrane reactor Chromatographic reactor... 

Studies by a manufacturer of injection molding machines and experience with Scandinavian Wood Fiber have shown that wood fiber-plastic composites reduce the dwell pressure and cooling time by at least 40% compared with unfilled plastic. This means a 20 to 50% reduction in cycle time. 

Lap shear strength of the pieces of wood with plastic injection molded to them was tested on a Instron Model 4200 universal testing instrument. Experimental conditions were as follows room temperature, 23°C room relative humidity 50% crosshead speed, 2.54 mm/min with the sample in hand-fastened grips and an aluminum specimen holder. The lap shear strengths of the wood-plastic samples are summarized in Table 9. The copolymer samples were fractionated by benzene extraction. The reaction product was labeled product A. The benzene extract of the product became ben. ex. Whereas the benzene-insoluble portion of the product was labeled product B. 

These products of oligoesteriflcation are able to give, by compression molding or injection molding, various types of plastic-like crosslinked wood samples as shown in Fig. 1. 

In foam injection-molded parts the anooth-skinned resilient foam produces a tough energy-absorbing structure that is being used as a wood substitute in athletic products such as lacrosse-stick heads and hockey-stick blades. Protective structures for helmets and automotive parts are under development. All are being produced in densities ranging for 0.35 to 0.7 g/cc (22 to 44 Ib/ft ). In those applications where added sti ess is desirable the use of glass or titanate fiber reinforcement with the foamed structure is very effective (6). 

While polymers with moderately high performance qualities, such as cellulose in wood and cotton have been known for years, these linear polymers could not be extruded or injection molded. However, they were able to withstand the effects of moderately hostile environments. The classic reinforced thermosets could be compression molded and could also be classified as high performance plastics. In contrast, the general purpose thermoplastics, introduced in the 1930 s, were readily extruded and Injection molded but were not useful at boiling water temperatures. 

Polymers with moderately high performance qualities, such as cellulose and cotton and wood have been known for centuries. These linear polymers could not be extruded or injection molded but they did withstand the effects of moderately high hostile environments. 

A.K. Bledzki, V.E. Sperber, K. Specht, M.A. Letman and A. Viksne. Effect of defined waxes and coupling agents on moisture behavior of injection molded wood fiber-rein-forced PP composites. In Eighth International Conference on Woodfiber-Plastic Composites (and Other Natural Fibers), Madison, WI, May 23-25, 2005. 

Stndying microbial degradation of 50% WPCs by brown-rot and white-rot fungi (G. trabeum and T. versicolor, respectively), it was found that the decay was more prononnced as the wood particle size increased [9]. The authors found that decay in the pine-based composites was more sensitive to particle size than that in the maple-based composites. It is noticeable from the above examples that for the same soil block test, ASTM 1413 or similar to it, the weight loss figures are quite different, and vary from fractions of a percent to qnite high numbers. Indeed, as it was shown, extended composite material samples are mnch more sensitive to soil block test compared to compression-molded and injection-molded sample. After 12 weeks,... 

Extruded Versus Injection-Molded Wood-Plastic Composite Materials... 

TABLE 16.11 Shift of lightness (L in Hunter Lab color space) for an extruded (as is and planed) and injection-molded wood flour (50% w/w, 40 mesh)-filled HDPE (MFI 0.72)... 

Vectra LCPs consist of parallel, closely packed, fibrous chains in injection molded parts which give the polymer a wood-like appearance at fracture surfaces and also give it seU-reinforcing properties that are at least as good as those of conventional fiher-reinforced thermoplastics. ICI supphes LCPs (self-reinforcing polymers) under the trade name Victrex SRP. Excellent physical properties, low comhnstihility and smoke generation, resistance to chemicals and solvents, and stahihty towards radiation are some of the important properties of LCPs. 

Microcellular foamed WPCs are fighter and feel more like real wood [271]. Both extrusion and injection molding have been used to produce foamed WPCs [272], producing materials with a density reduction of approximately 25%. 

Re-New Wood, Wagoner, Oklahoma, injection molds wood-filled PVC shingles and does its own compounding. The shingles which are made to imitate wood shakes, a traditional roofing material, are wind, rain, and hail resistant. 

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