Mold surface

The mold is usually prepared with orifices to permit air trapped between the sheet and the mold to escape and ensure uniform, close contact of the plastic with the mold surface. By clamping the sheet beyond the perimeter of the piece, plastic may be drawn from the peripheral areas into the mold, ensuring uniformity. Both pressure and vacuum are employed to force the softened plastic sheet into the mold. 

Lubricants. Lubricants are used to improve the melt flow, screw feeding, and mold release of nylons. Long-chain acids, esters, and amides are used together with metal salts, eg, metal stearates. Improved melt flow is mainly a function of molecular weight reduction during mol ding. Mold release is improved by waxes of limited compatibiHty with nylon, which migrate to and lubricate the mold surface. 

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. 

Gel coats are pigmented polyester coatings appHed to the mold surface and are an integral part of the finished laminate. Gel coats are used widely on hand lay-up and spray-up parts to enhance surface aesthetics and coloration as weU as to provide an abrasion-resistant waterproof surface that protects the underlying glass-reinforced stmcture. 

Hand Lay-Up and Spray-Up. In hand lay-up, fiber reinforcements in mat or woven form are placed on the mold surface and then saturated with a Hquid polymer, typicaHy a polyester resin, that has been chemically activated to polymerize (cure) without the addition of heat. Multiple pHes of reinforcement and multiple cure steps aHow very heavy waH thicknesses to be achieved. 

In the spray-up process a reinforcement, usuaHy glass fiber, is substituted for the mat and a special spray gun simultaneously chops the glass fiber and appHes it with catalyzed resin to the mold surface. Hand rolling techniques then consoHdate the fiber and resin to conform to the mold surface contours. The shorter chopped fibers aHow for more intricate design detaHs than do mats. Both processes rely heavily on the operators skiHs for product quahty. These two processes require the least capital investment and have the largest product size capabHity of aH the processes. A single-surface mold produces a part with one controHed (usuaHy the visible) surface. 

A special attribute of these processes is the abiHty to pre-position reinforcement, inserts, and core materials for stiffening ribs. Gel coatings can be apphed to the mold surface to eliminate post-mold finishing. Because both surfaces of the part are formed in a mold to close tolerances, accurate assemblies are possible, which is a requirement for many automotive or tmck body appHcations. 

The success of the compaction operation depends pardy on the effective utilization and transmission of appHed forces and pardy on the physical properties and condition of the mixture being compressed. Friction at the die surface opposes the transmission of the appHed pressure in this region, results in unequal distribution of forces within the compact, and hence leads to density and strength maldistribution within the agglomerate (70). Lubricants, both external ones appHed to the mold surfaces and internal ones mixed with the powder, are often used to reduce undesirable friction effects (71). For strong compacts, external lubricants are preferable as they do not interfere with the optimum cohesion of clean particulate surfaces. Binder materials maybe used to improve strength and also to act as lubricants. 

Unsaturated polyester resin powders can provide a colored and finished exterior molded surface or a finish ready for painting. Normally, a primer/sealer must be appHed to molded articles prior to painting. In addition to the unsaturated polyester resin, multifimctional unsaturated monomers such as triaHyl cyanurate (TAC) [101-37-1] or diaHyl phthalate (DAP) [131-17-9] suitable peroxide initiators (qv) or mixtures thereof, and mold release agents (qv) are used to formulate the coating powder (46). 

Ease of cure, easy removal of parts from mold surfaces, and wide availabiHty have made polyesters the first choice for many fiber-reinforced composite molders. Sheet mol ding compound, filament winding, hand lay-up, spray up, and pultmsion are all weU adapted to the use of polyesters. Choosing the best polyester resin and processing technique is often a challenge. The polyester must be a type that is weU adapted to the processing method and must have the final mechanical properties requked by the part appHcation. Table 1 Hsts the deskable properties for a number of fiber-reinforced composite fabrication methods. 

PPS resins are chiefly used for injection mol ding. The melt flow of the glass-fiUed resins is very stiff, and high injection pressures are required. Mold surface wear is heavier than for most other engineering plastics. Mol ding melt temperatures are near 330°C for optimum surface gloss and impact strength, mold temperatures of 130°C should be used. The resins are brown to brown-black. 

Process whereby layers of resin-impregnated fabrics are built up one layer at a time onto the mold surface, forming the product. Little or no pressure is required for laminate curing. 

The pressure distribution across the mold is a function of both the mold surface and the platen flatness. The larger the mold size and the platen, the more difficult it is to maintain the high degree of flatness required. The mold and platen flatness is also critical in the area of heat transfer from the platen to the mold itself. 

The choice of manufacturing technology for the fabrication of fiber-reinforced plastics or composite materials is intimately related to the performance, economics, and application of the materials. It also depends upon a number of factors, such as component numbers required, item complexity, number of molded surfaces, and type of reinforcement. 

Spray molding is the modification of the hand-lay process where the resin and glass fiber are deposited simultaneously on the molding tool. The fibers are mixed with the resin at the spray head before being deposited on the mold surface. Subsequent consolidation of the laminate is achieved by rolling in a similar manner to the hand-lay process. This method is suitable for large components. Here the capital cost is higher and the process is very operator sensitive. 

For RRIM-based composites, the main benefit is the case with which the properties can be varied across a wide range. Decorative finishes can be applied to RRIM molding by painting or, where color matching between components is less important, by self-coloring. However, color coating of the mold surface prior to RIM provides a better finish. 

Control temperature of mold surfaces, to chill plastic to rigid state From side holes, slots, undercuts, threaded sections... 

Low-pressure SF products can have characteristic surface splay patterns. However, the utilization of increased mold temperatures, increased injection rates, or grained mold surfaces will serve to minimize or hide this surface streaking. Finishing systems like... 

This low pressure process, also known as elastic reservoir molding, consists of making basically a sandwich of plastic-impregnated open-celled flexible polyurethane foam between the face layers of fibrous reinforcements. When this plastic composite is placed in a mold and squeezed, the foam is compressed, forcing the plastic outward and into the reinforcement. The elastic foam exerts sufficient pressure to force the plastic-impregnated reinforcement into contact with the heated mold surface. Other plastics are used. 

This term is used both to describe designs that are etched or engraved in the mold surface and the process of inserting a printed film into the mold, to be produced as an integral component of the finished product. Etched surfaces can be drawn both parallel and perpendicular to a parting line of molds or postforming in an extrusion line.. However, be alert with molds to the fact that parallel to the parting line additional draft is required. A wide selection of patterns is available and new ones can be readily created. 

After cooling, the final part is removed and the mold is recharged with resin. Since the mold surfaces are often complex, processors commonly apply mold release agents to allow the part to demold. Mold release agents are lubricants that coat the metal surface with a waxy material preventing the polymer from adhering to the mold s surface. Examples of mold release agents include silicone sprays or olefin based waxes. 

Several injection-molded polycarbonate (PC) disks used in a consumer product (i.e., a clock face) were analyzed, because the disks contained defects. The defects were analyzed initially and imaged using a stereoscopic microscope fitted with a digital camera. The defects present on the samples were easily identified. It was noticed that the defects were located on different regions of the molded part. This important observation eliminated a systematic problem or an imperfection at a specific site on the mold surface as the cause of the defect. 

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