General guidelines
The dimensions of the sprue depend primarily on the dimensions of the molded part and
especially its wall thickness. Here are some general guidelines you should consider:
The sprue must not freeze before any other cross section. This is necessary to permit
sufficient transmission of holding pressure.
The sprue must de-mold easily and reliably.
Example
Figure 1 below illustrates a sprue designed under these guidelines. Use the following formulas to
design the sprue. A radius r2 is suggested at the root of the sprue to avoid the sharp corner between
the sprue and molding and to facilitate the flow of material.
FIGURE 1. Radius at sprue base improves processing
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