Mold corner

Lecithin will function as a release agent, providing cleaner and faster pet food or biscuit parting from a stamp or mold. A reduction in the amount of pressure required to create an impression with a stamp is seen when lecithin is used in the mix. The ease of formation of intricate, stamped, surface appearance details, especially with low-fat products, is also improved with lecithins. When dough is being cut, lecithin improves release from the die, especially with rotary cutters. With superior release, foods and biscuits are formed with a better impression, and lower numbers of cripples (i.e., improperly formed product). Some pet biscuits are formed from a batter injected into a mold prior to cooking. Lecithin can improve flowability of the batter so that it fills all the mold corners and cavities. After cooking, lecithin improves release from the mold. 

Foams have limited use for these purposes. Rigid cellular PVC is good as a thermal barrier but aot for stmctural parts. Doors and frames of stmctural molded foam, eg, foamed high impact polystyrene, can be made by iajection mol ding, with recesses for hinges, striker plates, and miter corners. Sohd polystyrene and stmctural foam-molded polyurethane have been molded for door frames. 

Basically, in the vicinity of a sharp comer all fringes converge toward the apex. Having a high density of lines at this point indicates the presence of high stress level. At a rounded corner there will be considerably less concentration. Besides the molding problems, sharp corners often cause premature failure because of the stress concentration. To avoid these problems, inside comer radii should be equal to one-half the nominal wall... 

Sharp corner As reviewed, and never to many times, when a drawing does not show a radius, the tendency is for the toolmaker while manufacturing a mold to leave the intersecting machined or ground surfaces as they are generated by the machine tool. The result is a sharp comer on the molded product. Such sharp comers on the insides of products are the most frequent property detractors. 

The recommended radius not only reduces the brittleness effect but also provides a streamlined flow path for the plastic melt in the mold cavity. The radiused corner of the metal in the mold reduces the possibility of its breakdown and thus eliminates a potential repair need. Too large a radius is also undesirable because it wastes material, may cause sink marks, and may even contribute to stresses from having excessive variations in thickness. 

In order to simplify the analytical exercise, a particular material was selected for each. The single curved sheet is made of TS polyester fiber glass molded to the shape. The corner supported pan is molded from ABS plastics. The structural foam unit is molded from PP with glass fiber filler. 

The Izod impact test may indicate the need to avoid inside sharp corners on parts made of such materials. For example, nylon and acetal-type plastics, which in molded products are among the toughest materials, are notch-sensitive and register relatively low values on the notched Izod impact test. 

As reviewed there is much to consider. Examples include cooling as the product sets up results in different shrinkage rates for thicker versus thinner sections in the different processes. This results in either external waviness or sink marks, or warpage and internal voids, as the product contracts. Flat surfaces are difficult to maintain but not impossible to attain using certain processes. High speed of flow to fill the cavity of the mold is impeded going around square corners, so provision for radii and fillets are important. 

Great care is taken that we design blow molds to avoid thin or weak regions that could result in premature failure. Molds are designed to avoid excessive draw into corners that would result in locally thin areas. For this reason, blow molded products invariably have rounded corners. Another potential source of weakness is the pinch-off line. To compensate for this fact, it is common to program the parison to produce a thickened base. 

Why do blow molded products have rounded corners ... 

Vacuum forming has limitations due to the non-uniform wall thicknesses of its products. As the sheet is drawn into the mold, its thickness decreases, especially in the corners. For this reason, just as in blow molding, we design vacuum formed products to have rounded corners. If the depth of the cavity is excessive, walls can become locally so thin that they are unacceptably weak. One strategy that we use to alleviate this problem is to pump air into the cavity after the sheet has been clamped. This inflates the sheet, pushing it upwards and expanding its area approximately uniformly. When we subsequently apply a vacuum the expanded sheet is drawn back down into the mold. The finished product has a more uniform wall thickness than if we had applied the vacuum directly. 

Simple leaf mold containers can be made with netting and posts, or bought. There is no need for a lid or solid sides, nor is size critical—just big enough to hold your supply of leaves. Smaller quantities can be stuffed into plastic bags. Make a few air holes with a garden fork when the bags are full, and tie the top loosely. An even simpler method is to just pile the leaves in a sheltered corner and wait. 

It is a modified system of drape forming for producing more uniform wall thickness and minimizes the dangers of tearing over the corners of large moldings because of the protective cushion of compressed air... 

Plug assist is used principally when the process is likely to lead to undue variation in the product wall thickness. Plug assist supplies essentially a selective or localized stretch that is related to the specific demands of an individual mold cavity. It is likely to be beneficial when the draw ratio of a product feature is high, and when the product includes edges, corners and other features where excessive stretch and thinning is likely to occur. Plug assist is preferred for plastics with high enthalpy and low thermal conductivity such as polypropylene sheet formed in the solid phase. 

The semipositive mold (Figure 14.3c) is by far the most popular. It combines the best features of the positive and the flash molds. Since its design includes a plastic material well of larger diameter, with a tight fitting force above the cavity, the material is trapped fairly positively and the plastic is forced to flow into all corners of the cavity. As the material picks up more heat and becomes fluid, it escapes between the force and cavity sidewalls as flash, allowing the force to seat on the land area. 

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. 

This property makes bismuth useful for producing type metal. An alloy of bismuth is melted and poured into molds that have the shape of letters and numbers. As the type cools, it solidifies and expands to fill all the corners of the mold. The type formed is clear, crisp, and easy to read. Computer typesetting, however, has largely replaced bismuth metal typesetting. 

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