Mold Considerations

An injection mold performs several functions during the molding cycle. In addition to giving the polymer the desired shape, it distributes the molten polymer, cools it, and ejects the product. 

Between the injection nozzle on the end of the extruder and the gate that leads into the mold cavity the polymer flows through a channel known as a runner . It is vital that the polymer does not solidify in the runner before the mold is completely filled. We can prevent premature solidification in the runner in one of two ways we can use a large diameter unheated (cold) runner in which the polymer sohdifies after the polymer in the mold cavity, or we can use a heated (hot) runner in which the polymer does not solidify. Moldings that are produced 

When we injection mold small plastic items, it is standard practice to mold several identical parts simultaneously in a mold containing multiple cavities. In order to insure product uniformity, we must design runners that distribute the molten polymer evenly to each cavity. 

The flow properties of the polymer must be matched to the mold (or vice versa) in order to achieve optimum product quality. In particular, we must manage the pressure drop between the gate and the furthest extent of the flow path. Articles with thin walls, such as margarine tubs, require Io v viscosity resins. In other cases, the situation is more complex and the cavity may need to be designed so that the product is thicker near the gate than it is at its extremities. The development of complex products is aided by computer modeling of melt flow within the mold cavity. 

The simplest molds consist of two parts one fixed and one moveable. We use such molds to fabricate items, such as disposable stadium cups, compact disks, and coat hangers, which have simple designs with no undercuts. As the complexity of the product increases, so must the complexity of the mold. The molds used to produce large items, such as dishwasher tubs and automobile bumpers, may consist of multiple moveable components and weigh several tons. Even apparendy simple items, such as screw tops for botdes, require a complex ejection system that unscrews the product from part of the mold. In order to facilitate cooling, virtually all molds are designed with liquid cooling channels. 

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Pine Needles (100) Bagasse (100) 2.5 10 10 Hexame thy le ne-tetraxnine (10) Poor resin flow in mold. Considerable swelling after 2 days soaking in water. 

After all of these design and molding considerations were achieved, the end result was a mold that weighed 29,510 kg (65,000 lb). It was cut from P20 Tool Steel, chrome plated, and textured according to customer specifications. A CAD rendering of the bottom half of the mold is shown in Figure 6. 

Preservatives. Without control of yeasts (qv), molds, and bacteria, the food industry would experience considerable economic losses each year owing to spoilage. Sugar, salt, and wood smoke have been used for centuries to preserve food. These methods, however, are not compatible with all food products thus preservatives, also known as antimicrobials, are used. 

The product quaUty considerations for nonphotosynthetic microorganisms are similar to those for algae. Tables 6 and 7 present composition and amino acid analyses, respectively, for selected bacteria, yeasts, molds, and higher fungi produced on a large pilot-plant or commercial scale. Table 8 summarizes results of proteia quaUty and digestibiUty studies. 

Methacrylate polymerizations are accompanied by the Hberation of a considerable amount of heat and a substantial decrease in volume. Both of these factors strongly influence most manufacturing processes. Excess heat must be dissipated to avoid uncontrolled exothermic polymerizations. Volume changes are particularly important in sheet-casting processes where the mold must compensate for the decreased volume. In general, the percent shrinkage decreases as the size of the alcohol substituent increases on a molar basis, the shrinkage is relatively constant (35). 

Cones usually are pressed. These are easier to ignite than cast cones. Pressure used for cone formation is considerably lower than that used for candle formation to prevent excessive densification. The preformed cone can be added to the mold before pressing or casting the body of the candle, being held ia place by the resulting bond. Alternatively, a cavity is pressed iato the formed candle, and cone material is added and pressed iato place. The bond formed by this method is weak but generally sufficient for most purposes. 

Principal Uses. Groundwood pulp contains a considerable proportion (70—80 wt %) of fiber bundles, broken fibers, and fines in addition to the individual fibers. Principal uses of paper-grade pulps are in newsprint, magazine papers including coated pubHcation grades, board for folding and molded cartons, wallpapers, tissue, and other similar products. 

Forming-Die Alloys. The tonnage of slab zinc used in this appHcation is small. The use of zinc alloy dies started in the aircraft industry during World War II (119). Zinc-based alloys cast in sand and plaster molds continue to be used for short-mn dies for steel and aluminum stampings in the automotive and aircraft industries (120). Considerable cost savings are realized with these low melting zinc-based alloys which are easy to poHsh, machine, weld, and remelt. 

Phenohc resins (qv), once a popular matrix material for composite materials, have in recent years been superseded by polyesters and epoxies. Nevertheless, phenohc resins stiU find considerable use in appHcations where high temperature stabiHty and fire resistance are of paramount importance. Typical examples of the use of phenoHc resins in the marine industry include internal bulkheads, decks, and certain finishings. The curing process involves significant production of water, often resulting in the formation of voids within the volume of the material. Further, the fact that phenoHcs are prone to absorb water in humid or aqueous conditions somewhat limits their widespread appHcation. PhenoHc resins are also used as the adhesive in plywood, and phenohc molding compounds have wide use in household appliances and in the automotive, aerospace, and electrical industries (12). 

During the manufacture of glass, considerable dust, with particles averaging about 300 /xm in size, will be emitted. Some dusts may also be emitted from the handling of the raw materials involved. Control of this dust to prevent a nuisance problem outside the plant is a necessity. When glass is blown or formed into the finished product, smoke and gases can be released from the contact of the molten glass with lubricated molds. These emissions are quite dense but of a relatively short duration. 

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