Thixotropic molding

The hand lay-up or spray-up process, used universally for the production of laminar composites incorporating glass fiber reinforcement, is most efficient for the manufacture of large parts, such as boats, bathtubs, tanks, architectural shapes, and recreational accessories. Resins intended for spray-up processes are usually modified with thixotropic additives, such as fumed siHca (1%), to reduce the risk of drainage when appHed over large vertical mold surfaces. Molds are also made from ERP for short-mn products usually surfaced with a tooling gel coat to provide consistent surface quaHty and appearance. 

This process also called just bag molding. It is the conventional hand lay-up or spray-up that is allowed to cure without the use of external pressure. For many applications this is sufficient, but maximum consolidation may not be reached. There can be some porosity fibers may not fit closely into internal corners with sharp radii but tend to spring back. Resin-rich and/or resin-starved areas may occur because of draining, even with thixotropic agents. With moderate pressure (hand rollers, etc.) these defects or limitations can be overcome with significant improvement in mechanical properties. 

In addition, this system has been modified by adding dibenzyl sorbitol to give high thixotropicity to the system to prevent penetration of the system into foam cells. For these reasons the foam system is very suitable for pour-in-place application into small foam voids, e.g., 1 to 10 cm, for repairing surface air voids of molded automotive-foam seats to reduce the rejection rate, thereby resulting in lowering production costs. 

More recently,thin walled articles have been fabricated by blow-molding composites of liquid crystal polymer (LCP) and expanded porous polydetrafluoroethylene sheetingl l material. Container application examples include food and pharmaceuticals, automotive gas tanks, bottles, and other vessels. Unlike most other thermoplastic polymers, thermotropic LCP forms high-viscosity melts that have thixotropic characteristics. Applying shear force to the melt substantially alters the melt viscosity of LCP and the orientation of its polymer domains. These attributes are useful to... 

Acetal homopolymer resin (molding resin) 15.0 6 Semiflexible high-bond thixotropic 78.7 6... 

Fillers such as silica may also be used to make the resin thixotropic, for contact molding on vertical or inclined surfaces. Thixotropic is a... 

Uses Filler for plastics thixotrope, antisedimentation agent in epoxy casting resins, polymethylmethacrylate and acrylic casting, coating and molding materials, water-reducible coatings thickener for aq. systems, other polar liqs., cosmetics... 

Uses Polychloroprene for wire/cable, hose, gaskets, seals, tire sidewalls, molded/extruded goods, cellular prods., adhesives, sealants, protective coatings, contact adhesives, mastics Features Rec. for contact adhesives and mastics requiring thixotropic chars. 

Chem. Descrip. Oxidized polyethylene wax CAS 68441-17-8 EINECS/ELINCS 200-815-3 Uses Mold release agent for plastics melting agent, water repellent, thixotrope, and antisettling agent for paints abrasion-resist, printing ink emulsions/pastes/dispersions... 

An air spray gun includes a roller cutter that chops glass fiber rovings to a controlled length before they are blown in a random pattern onto a surface of the mold simultaneously with a spray of catalyzed resin (Fig. 7-7). The chopped fibers are coated with resin as they exit the gun s nozzle. The resulting, rather fluffy, mass is consolidated with serrated rollers to squeeze out air and reduce or eliminate voids. As in hand lay-up, the first layer of a gel coat may be applied over the mold, followed by successive passes of the sprayed-on composite before a final gel coat is applied. If required, inserts, and so on, can be included during the spraying operation. Thixotropic agents (Chapter 6) may be employed in the resin. 

Hand layup. The oldest method is a purely manual operation. (1) The open female mold is first treated with mold release. (2) Optionally, a pigmented gel coat is applied 15 to 20 mils thick and partly cnred. (3) Glass fiber mat, or woven or knitted cloth, is hand-laid into the mold or onto the gel coat. (4) Thixotropic polyester/styrene liquid, containing MEK peroxide and activator, is impregnated into the cloth. (5) The assembly stands and cures at room temperature. This gives a product with one good surface and somewhat irregular thickness. 

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