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Gate Location
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The location and number of gates used in a mold design are integral to the strength and robustness of the molded part. Understanding the impact of incorrect gating on the molded part, consideration should be made when designing a part with features that would assist the mold designer in choosing the best location for and the correct number of gates. Correct gates should permit easy filling of the mold. If these parameters are not chosen correctly, the molded part will have quality issues during the molding process. Appearance issues such as weld lines, jetting, flow marks and gate-blush may be present. The part may also contain areas of incomplete fill and part deformation.
Appearance
Residual vestige of a gate is normally unacceptable on a visible surface. Gates should be positioned on non-visible dimensionally non-critical surfaces whenever possible. Consider which surfaces can have gate vestige.
Stress
Gates should not be placed near highly stressed areas. The gate itself and the degating of the part that may be required, result in high residual stresses near the gate area. Also, the rough surface left by the gate creates stress concentrators.
Pressure
Gates should be placed in the thickest section of a part. If a part with sections of varying thickness is gated in the thin section, the thin section freezes off too soon to allow the thick part to be sufficiently packed. Severe shrinkage occurs in the thick part and warpage and sinks result. Gating in the thick section allows both sections to be adequately packed before the gate freezes off, allowing for more uniform shrinkage (see figure 1).
Figure 1. Recommended Gate Locations on Varying Wall Thickness
Weld Lines
In general, gates are positioned in a mold so as to equalize the flow length throughout the cavity. Also, gates are located with an intent to minimize the number and length of weld lines (see figure 2). Weld lines may be somewhat weaker than the rest of the part and may cause surface defects. When weld lines are unavoidable, gates are placed close to the obstruction, providing a high melt temperature and ensuring a strong weld.
Figure 2. Positioning Gates to Eliminate Weld Lines
Filling
Gates are located so that the polymer impinges against a wall or a pin (see figure 3). These obstacles disperse the resin for a more uniform filling pattern and help prevent flow marks and gate-blush on the surface of the part.
Figure 3. Positioning Gates to Improve Polymer Flow
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