April 2006 Design Tip

Hot Tips on Gates

Gates are a necessary evil. Once resin has been injected into a mould, they serve no purpose but to mar the design you’ve created with the vestiges they leave. But unless someone comes up with a way to “beam” resin into a mould, there will always be gates, and their design and placement will affect the appearance and performance of your finished part. Most Protomold customers don’t specify the type, shape, or placement of gates for the parts they design, but there are still good reasons to understand how they work, if only because you will be asked to approve a final mould design that includes gates.

Gates are more than just openings in a mould through which resin enters. Their design and placement is largely determined by the behavior of resin as it is heated, as it moves through a mould, and as it cools.

  • Resins are heated and pressurised at the point of injection into the mould. Contact with the mould material draws heat from the resin, which begins to thicken and eventually solidifies.
  • Resins shrink as they cool. The amount of shrinkage varies with the type of resin.
  • Some resins shrink differently in the direction of flow than they do in the “cross-flow” direction. This may be particularly pronounced in resins filled with a linear material like glass fibre, but it can also occur in non-filled materials.
  • The pressure under which resins are injected—up to 20,000 psi—can force gases into solution in the liquid resin. As the resin cools and shrinks, these gases come out of solution in the form of bubbles.

These factors all drive the placement of gates in a mould. Injected resin travels through the mould, displacing air, which leaves the mould through tiny vents at the ends of the mould farthest from the gates. A gate must be placed such that the resin it carries does not solidify before filling the mould (or its segment of the mould) on its way to the vent. If the path from a gate to a vent is such that resin would harden before reaching the vent, one or more additional gates must be added to shorten the distance.

As in any body, heat loss is determined by the ratio of volume to surface area; in other words, thin areas lose heat faster than thick areas. For this reason, gates in the mould for a part with both thick and thin areas should be located in the thick areas. Resin flowing through the thick area retains heat long enough to continue flowing and fill the adjoining thin areas. If it had to travel through the thin area first, the faster heat loss might cause it to freeze before it could reach the thick area. This premature freezing of resin in the mould can also trap bubbles.

After the mould is filled with resin, the moulding press maintains “hold pressure” at the gate. This forces additional molten resin into the mould to fill any gaps left as the resin in the mould cools and shrinks or as bubbles form. Like a thin area in the mould, a narrow gate is at risk of losing heat and freezing before the mould segment it feeds can be fully filled. In other words, we don’t want the gate to be a “thin” feature compared to the mould segment it serves. This consideration helps determine the size of the gate.

Warp is affected by the type of resin and thinness of the part being moulded. Whether caused by filler in the resin or by the resin itself, it can be minimised by controlling the direction of resin flow within the mould. Proper placement of gates can provide multiple, counterbalancing flows, reducing the tendency to warp.

Then there is the problem of “knit lines.” These occur when resin flow is split, for example by a core in the mould. As with water flowing around an island in a river, the two flows rejoin on the other side of the core, but if they have cooled enough, the two leading edges may have developed “skins” that keep them from fully blending. The result is a visible line that can be either cosmetically or structurally problematic. Expert gate placement can manage flows to minimise negative effects of knit lines.

Finally, there are cosmetic considerations. Most types of gate will leave some sort of vestige. Sometimes these can be located in a non-cosmetic area of the part. If you have any questions about Protomold’s proposed gate layouts, please feel free to ask your Protomold customer service representative for an explanation. If necessary, we will work with you to revise gate layouts to meet your requirements.

Figure 1: Gate—red square end. 4 yellow vents—
0.0254mm thick.

Figure 2: Bubbles/voids inside thick ABS part.