Single-cavity mold

An alternative approach for injection molding is the use of a rotary table injection machine. This type of machine uses the multistation concept, each station having a single cavity mold generally using a point gating/ injection point as previously mentioned. Consider a rotary table machine with eight stations, station one would be the injection station and station seven would be the unload and load metal insert station. This type of process has been successfully used for different types of lip type rotary shaft seals where more expensive elastomers are used and waste needs to be kept to a minimum. 

The use of small injection molding machines with a single cavity mold. This approach removes the need for a runner system and, when using a dia-... 

The use of multinozzle injection machines. In general, this type of machine has a horizontal reciprocating screw that feeds four injection nozzles each connected to a single cavity mold thus minimizing waste. However, one concern with this approach is the need to have the mold accurately lined up (the top and bottom surfaces of the mold will be in the vertical plane) to eliminate compound leakage at the nozzle/mold interface. 

Sprue. (1) The main feed channel that runs from the outer face of an injection mold to the gate in a single-cavity mold or to the runners of a multiple-cavity mold. The hquid polymer is forced through this orifice from a nozzle till the mold is filled to capacity. Some polymer remains in the sprue after the mold is closed, leaving a projecting piece that must be removed after the product is ejected. The viscosity of the polymer must be low enough to permit it to pass through the sprue readily. 

Sprue gating (direct gating) n. In injection molding with single-cavity molds, particularly those with circular symmetry, filling from the center with the sprue connected directly to the gate and mold cavity. 

Two-shot injection molding n. Confusingly, this term has been used in the literature for two processes that are distinctly different. One is described under double-shot molding. In the other process, one first injects a metered amount of one material into a single-cavity mold. As this material just begins to chill against the cold mold surfaces, a second material is injected. This fills the interior and forces the first material outward to the cavity surfaces. The second polymer, usually a reclaimed material forms the interior of the finished article, while the virgin material first injected forms the outer shell and surface of the article. 

Molds are also distinguished by the number of cavities cut in the mold. Single-cavity molds contain one cavity, while multiple-cavity molds can mold several parts simultaneously. In family molds (Fig. 5.68 ), several different parts are molded at one time. 

Production molds are generally multicavity and have integral heating provisions and ejection systems. And when they follow single-cavity hand molds, cavity dimensions may be fine timed, and vents and gates can be repositioned, based on experience with the single-cavity molds. 

It is difficult to precisely determine the effect of shrinkage on the dimension of many parts and the amoimt of deformation that will result from the thinned wall. Therefore, it is wise to build a prototype single-cavity mold designed with steel safe fitment dimensions and close in on the final dimensions by removing metal from the mold based on results from initial trials. 

The early results were primitive at best. We were clamping a sample of high density polyethylene sheet about l/16th inch thick in a single cavity mold, then heating the entire assembly to just below the crystalline melt point. Finally we would remove it from the oven and blow it with compressed air. Invariably, the plastic sample would rupture. 

Figure 7-88. An example of TO stamping on GE s Azdel versus other processes in regard to production cycle time as opposed to production rates based on single-cavity molds.

Cavity. Depression in mold the space inside a mold wherein a resin is poured the female portion of a mold that portion of the mold which encloses the molded article which forms the outer surface of the molded article (often referred to as the die) also, the space between matched molds. (Depending on number of such depressions, molds are designated as Single-Cavity or Multiple-Cavity). The cavity plate accommodates the individual mold cavities, which can be separate assemblies mounted into the cavity plate or, in the case of a single-cavity mold, can be machined directly into the cavity plate itself. 

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