Consider how much simpler life would be if we could simply teleport liquid resin into a mold. Of course, that wouldn't be injection molding. And since we presume that the developers of practical teleportation will come to us for their plastic prototypes (and they haven't), we're prepared to state categorically that teleportation molding is still some time off.
That leaves us with gates as a means of getting liquid resin from the barrel and screw of the molding press into the mold. Unfortunately, gates interrupt the mold's surface, and that interruption, unavoidably, produces a cosmetic defect on the surface of the part. Depending on the function of the part and the location of the gate, this may or may not be a problem. For example, if the part or surface will be hidden from view, any vestiges left by the gate probably won't be a problem. But if the gate is located on a visible surface, you'll want to consider its cosmetic impact in designing the part.
Fig. 1: Tab Gate
Tab gates are effective and most common, but not necessarily pretty. (See Figure 1.)
While they are a simple way of getting resin into the mold cavity, tab gates have several drawbacks.
First, they carry resin to the mold via a runner. Because this allows some cooling and thickening of the resin, tab gates require a relatively large opening into the mold cavity. This, in turn, leaves a large tab to be trimmed, a process that can mar the finished surface.
Second, the runner leading to a tab gate takes up real estate within the allowable mold footprint. This can be a problem if the part pushes the limits of allowable mold size.
Third, because resin cools somewhat on its way to the gate, there are potential mold fill considerations like uniformity, concentricity, knit line formation, thin feature challenges, etc.
Finally, tab gates must be located at the parting line of the mold.
In many cases, the solution may be a "hot tip" gate. A hot tip gate has a small circular gate opening in the "A" side of the mold that lets plastic into the cavity. It's called a hot tip gate because there is a thermostatically-controlled heater bolted to the back of the mold to keep the resin hot enough (and thus fluid enough) to pass through the small gate hole.
The hot tip can be thought of as a direct extension of the molding press's barrel and screw. Resin is hotter at the point of injection, so the opening can be smaller. Because no runner is required, the part can use virtually all of the allowable mold X-Y real estate. Hotter resin also means material may be pushed further into a thin feature.
Hot tip gates are typically located at the top center of a part (as opposed to on the parting line, as is the case with a tab gate) and are ideal for round or conical shapes where uniform flow can improve concentricity. The hot tip gate leaves a small raised nub on the surface of the part. Adding a hot tip dimple to your design may help shift the nub below the surface of the part, which might allow something like a decal to be applied over it with little or no need for trimming beforehand.
Fig. 2: Spherical Depression Hot Tip Dimple
Hot tip dimples are usually 0.125 to 0.375 inches in diameter and 0.010 to 0.030 inches in depth. This can take the shape of a spherical or cylindrical depression. (See Figures 2 and 3). To maintain the always desirable uniform wall thickness, you could add material to the opposite side of the part so the material is not restricted in flow.
Be sure to consider that the "A" side of a part is usually the externally-facing cosmetic side, which means if you use a hot tip, the gate vestige ("nub") is likely to be visible in an assembly. As noted above, a hot tip dimple is a common method of trying to help hide the nub and should be considered when selecting decal locations on high cosmetic parts. Also be aware that some materials such as acetal and glass filled resins are not compatible with hot tip gates, and that small volume parts may be problematic because of the tendency of resin to "cook" in the hot tip longer, potentially degrading its properties.
Fig. 3: Cylindrical Depression Hot Tip Dimple
When asking for a hot tip gate, you should also consider the extra expense of installing a hot tip gate, such as the features that need to be machined into the back of the "A" side mold half. In addition, once a hot tip gate is used, it is much more costly to modify the mold. For example, there is no simple way to move a hot tip without completely re-making the "A" side of a mold.
As with any plastic part, design and resin both have a great impact on the success of your molded part. A well-designed part with a carefully chosen gate type and location that are compatible with the resin you select will allow us to achieve the best possible results.
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