Types of Molds

Molding. Many glass articles ate made by shaping a glass gob by forcing it under pressure against a mold. The source of pressure and type of mold vary with the application. 

As reviewed in other chapters, different plastics have different melt and flow characteristics. What is used in a mold design for a specific material may thus require a completely different type of mold for another material. These two materials might, for instance, be of the same plastic but use different proportions of additives and reinforcements. This situation is no different than that of other materials like steel, ceramics, and aluminum. Each material will require its own cavity shapes and possibly have its own runner system. 

Molds can be of three basic types. Open molds are used to define one surface of a product. They can be of the female (indenting) or male (projecting) type. Two matched female half-molds can be used to make hollow items, and a matched female and male pair is used to form a cavity that is to be completely filled. Shaping of the polymer inside the mold is usually achieved by applying pressure to cause the fluid to conform to one or both mold surfaces. The variation in types of molding is primarily due to the type of mold used and the method of melt delivery to the mold. 

The third region of flow near the front is of special interest. The important feature of this region is the fountain effect, which must be considered in modelling all types of mold filling. It is important not only for estimating the hydrodynamic flow pattern, but also because the deformation of the macromolecules near the front influences their orientation and the properties of the end product. 

A major application of these types of molded products would be for interior uses in automobiles, such as head liners, door panels, and dashboards. Although this is a low-cost, low-performance application, it represents a very laige-volume market. Indeed, wood is already utilized in applications of this type, but as a finely ground flour that serves as a filler (up to 40%) in extrusion-molded polyolefin products. The use of recycled fiber in this process and the one described above offers the potential of even greater cost reductions, combined with alleviation of solid waste disposable problems. 

The type of molding machine available has an important influence upon the blow opening system used. Some machines, for instance, use needle blowing exclusively. Hollow needle insertion at the mold parting line is considered when a very small opening is required that may even need closing or if the hollow object must be blown up on the side, e.g., on carousels with a series of molds. 

Different foundry casting techniques are used. Included are plastic-based binders mixed with sand. Various types of molds and cores are produced that include no-bake or cold-box, hot-box, shell, and oven-cured. Usual binders are phenolic, furan, and thermoset polyester. There is the foundry shell casting, also called dry-mix casting. It is a type of process used in the foundry industry, in which a mixture of sand and plastic (phenolic, thermoset polyester, etc.) is placed on to a preheated metal pattern (producing half a mold) causing the plastic to flow and build a thin shell over the pattern. Liquid plastic pre-coated sand is also used. After a short cure time at high temperature, the mold is stripped from its pattern and combined with a similar half produced by the same technique. Finished mold is then ready to receive the molten metal. Blowing a liquid plastic/sand mix in a core-box also produces shell molds. 

Draft on the cavity wall such as 1° is recommended to facilitate product removal. Lower-shrinkage plastics like PC and PMMA requires l1 to 2°. Undercuts are possible, but they should be kept to a minimum. Making provisions for undercuts or no drafts usually requires higher mold costs because the mold will require action such as core pulls or splitting a mold to allow separation parallel to the undercut groove. This type of mold action usually requires additional cycle time thus increasing cost. 

Three general types of molds are used for CM. In the positive mold (Figure 14.3a) all the material is trapped in the mold cavity. The pressure applied compresses the material into the smallest possible volume. Any variation in the weight of the charge will result in a variation in part thickness. In multicavity molds, if one cavity has more material than the others, it will receive proportionately greater pressure. Multiple cavities, therefore, can result in density variations between parts if loading is not done with some degree of precision control.1 278 284... 

Indoor mold measures might include air sampling or source sampling. Interpretation of either should not rely on total counts, but rather on the types of molds recovered and the rank order of types of mold spores (or colonies) recovered. 

Molds reproduce by releasing spores (essentially packets that contain the genetic material necessary for the formation of a new mold). These spores can float through the air and, if landing in a hospitable environment, can germinate to form a new mold. One of the essential components of a hospitable environment is moisture. The many types of mold all require a moist surface for growth. 

The doses necessary to establish effect levels for most types of mold exposures have not been established. Risk assessments are difficult due to the complex effects of mixtures of toxins and other physiologically active molecules produced by molds. 

Both CMOS and bipolar devices encapsulated with the new transfer molding compositions performed well under thermal cycling between -65°C and + 150°C and also under accelerated life testing at 145°C, 85% RH and 18V bias. Further optimization of this type of molding composition is expected to improve its performance as a protective material for integrated circuitry. 

These generalities will be illustrated through a discussion of the release agent technology in two different and important types of molded urethane product. These are automotive seat foams and RIM urethanes. Before that discussion, however, it seems useful to discuss why Options 1, 2, and 3 for releasing urethanes have not been successful so far. 

Certain types of molds can produce mycotoxin. A. flavus and A. parasiticus, under favorable conditions of temperature and humidity, can produce aflatoxins. Produces that are commonly contaminated with aflatoxins are groundnut, maize, chili, spices, and cottonseed (Cotty and Jaime-Garcia, 2007 Kumar et al., 2008). Mycotoxins can also occur in milk and milk products as a result of animals consuming mycotoxin-contaminated feed (Moss, 2002 Cotty and Jaime-Garcia, 2007). P. expansum, causing blue mold rot, can produce toxin called patulin (Barkai-Golan and Paster, 2008 Kumar et al., 2008). 

There are many other typical types of mold that can be found on wood and composite materials, such as Epicoccum sp., Fusarium sp.. Geotrichum sp., Stachybotrys chartarum, Ulocladium sp., and others. 

Properties of granular PTFE compounds are presented in this section. A few points should be noted about filled PTFE compounds. Shape and size of the filler and resin particles, type of molding process, and processing conditions all influence the properties of parts made from compounds. Properties of compounds made with non-spherical particles are... 

Moldy off-flavors are attributed to the effects of many types of mold. Contamination may start on the tree and continue during storage and treatment of the cork sheets and cork manufacture, as well as in the bottles of wine as they age in the cellar. The microbial flora at each of these stages are not necessarily the same. 

The same types of mold (particularly Penicil-lium) may find a similar environment (humidity, temperature, presence of chlorinated derivatives, etc.) in cork as well as wooden containers and structures in the winery. This explains the possible confusion between authentic cork taint and moldy off-flavors , which have similar smells, but different origins. When cork is responsible, only a few bottles are affected, whereas all the bottles may be spoiled if the defect is of external origin. 

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