Protomold: Rapid Injection Moulding
November Design Tip

 Hot Tip Solves Gating Problems

Consider how much simpler life would be if we could simply teleport liquid resin into a mould. Of course, that wouldn't be injection moulding. 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 moulding is still some time off.

That leaves us with gates as a means of getting liquid resin from the barrel and screw of the moulding press into the mould. Unfortunately, gates interrupt the mould'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.

Tab gates are effective and most common, but not necessarily pretty. (See Figure 1.)

Fig. 1: Tab Gate

Fig. 1: Tab Gate

While they are a simple way of getting resin into the mould cavity, tab gates have several drawbacks.

  • First, they carry resin to the mould via a runner. Because this allows some cooling and thickening of the resin, tab gates require a relatively large opening into the mould 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 mould footprint. This can be a problem if the part pushes the limits of allowable mould size.
  • Third, because resin cools somewhat on its way to the gate, there are potential mould fill considerations like uniformity, concentricity, knit line formation, thin feature challenges, etc.
  • Finally, tab gates must be located at the parting line of the mould.

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 mould 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 mould 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 moulding 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 mould 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.

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.

Fig. 2: Spherical Depression Hot Tip Dimple

Fig. 2: Spherical Depression Hot Tip Dimple

Fig. 3: Cylindrical Depression Hot Tip Dimple

Fig. 3: Cylindrical Depression Hot Tip Dimple

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.

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 mould half. In addition, once a hot tip gate is used, it is much more costly to modify the mould. For example, there is no simple way to move a hot tip without completely re-making the "A" side of a mould.

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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Plastics Trivia Question

In 1992, a storm at sea caused 29,000 Friendly Floatees™ plastic bath toys to be washed overboard from the deck of a container ship in the North Pacific:

 A. causing an unprecedented "feeding frenzy" among local sharks.

 B. providing oceanographers with a means of tracking Pacific ocean currents.

 C. forming a bright yellow "spill" that could be seen from space.

 D. disrupting U.S. Navy activities in the Pacific for over six months.

 E. eventually washing up by the thousands on Antarctic beaches.

(Honor System: No Googling, Yahooing, or Dogpiling until after you've submitted your guess.)

Last month's question/results:
According to Smithsonian Magazine, plastics made from polylactic acid (PLA) resin:

A. can be derived from algae.

B. are being embraced by the military to counter accusations of environmental irresponsibility.

C. are easily biodegradable in a home compost heap.

D. were developed by chemists working at a kitchen stove.

E. are ideal for use in beverage bottles.

The correct answer is D. were developed by chemists working at a kitchen stove.

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