Multicavity molds

Over the years the trend has been toward an increasing use of the more expensive elastomers. Concern has been expressed about the cost of waste compound when using a multicavity mold and injection molding, primarily due to the amount of elastomeric compound in the runner system. There are three approaches to overcome the problem ... 

The use of multicavity injection molds together with a cold runner system. The objective of the cold runner system is to prevent the runner system in the mold from curing during the molding process. The result is that the elastomeric compound held in the run-... 

Multicavity Molds. While the mold can be machined with a single cavity to produce a... 

With multicavity molds it is usual to employ a crosshead on which the blowing mandrels are arranged in series. Mandrel chains can also be used for product ejection. The channel for the blowing medium should be as large as possible. Angled radial orifices at the tapered end of the mandrel achieve a fast distribution of the cooling media and turbulence. 

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... 

SBC can also be molded into overcaps. These are injection molded clear caps that are often used on bottles for food and cosmetic packaging. Overcaps are typically made by injection molding in multicavity molds and designed for a snap fit. They must have high gloss and clarity and can often have a transparent color. 

Many processes consist of multiple streams, such as multiple-head fillers for powder fills or multicavity molds for tablet compression. Each stream must be considered a separate process. There are two general situations ... 

Flash Excessive injection pressure Excessive melt temperature Mold parting face faulty Insufiticient clamp force Foreign matter on mold parting face Flow restriction in one or more cavities of multicavity mold... 

Runners are the ehannels through which the polymer melt is fed into the mold cavities from the eyhnder nozzle. In a multicavity mold, it is necessary to fill all the mold cavities simultaneously and uniformly. Control of the size of the runners provides a means of controlling the flow resistance and balaneing the flow into the mold cavities. In most multicavity molds, the runners form part of the mold flame. Consequently, the ejected part is accompanied by the runner system, which must be removed and, in the ease of thermoplastics, reground for reuse. The use of the hot runner mold whereby the runner ehannels are heated to keep the polymer in the molten state, eliminates this need for plastic runner separation and avoids possible generation of scrap material. With proper machine operation, a hot rurmer mold requires a smaller amoimt of melt per shot than an equivalent cold runner mold, leading to redueed injeetion time and faster cycles. 

A three-plate mold design (Figure 2.10) features a third, movable, plate which contains the cavities, thereby permitting center or offset gating into each cavity for multicavity operation. When the mold is opened, it provides two openings, one for ejection of the molded part and the other for removal of the runner and sprue. 

Polyethylene has been irradiated to produce cross-links thus giving it improved strength. Drip irrigation for raw crops uses such irradiated LDPE tubing. For vine and tree crops and drop emitter is made of PE molded in large multicavity molds. 

Correct the mold after it is completed and tested the mismatching areas are blended by hand, by grinding the offending ridges. This is frequently done but not recommended because, in multicavity molds or where a number of identical molds are used for the same product, each product, from each cavity, may be different (see Fig. 4.14). 

Compression molding is a relatively simple and inexpensive procedure for the manufacture of moderate piece counts. For the production of bipolar plates compression molds with surface structured inserts were designed and manufactured. For the molding, the granulate can directly be molded in a heated mold under high pressure. Another way is the direct extmsion of a profile (semifinished material) into the structured mold, forming the plates from this plasticized polymer. In this variant the tool temperature is kept constant. With a multicavity-mold the cycle time can accordingly be shortened, and the output quantity of plates per cycle increased (Fig. 12). 

Usually multicavity molds made of steel, up to 50000 shots... 

Other stations can be included, such as decorating. Multicavity molds are used extensively. As is typical of injection molding and extrusion machines (Chapters 2 and 3), they can have sophisticated computer controls to ensure proper operation of all machine and material functions. 

When a multicavity mold designed for several parts is used, the ejected product is complex, consisting of runners, a spruce, and flashing that needs to... 

Heated steel molds, preferably hard chrome plated, are used, which may be of multicavity design. Tooling costs are higher than for compression moulding since appropriate gates and runners must be included in the mould. 

Injection molding Low-viscosity resin Multicavity molds and parts with thin sections difficult to fill... 

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