Picture this: it's a holiday weekend. Three lanes of traffic are on the motorway headed out of town and suddenly everything backs up. The problem, as you will discover in a couple of miles, is a jackknifed truck narrowing the road to one usable lane. With not much else to do for the next few miles, you start watching your fellow drivers and realise that a substantial part of the movement that is occurring is drivers who've gotten frustrated with the slow pace and are now taking the exits and driving parallel routes. Not only that, but a couple of cars, crawling along on a scorching hot day, have overheated and are now blocking traffic.
It's that narrow section on the road that's causing all the trouble, and that's pretty much what happens when you configure a thin spot in an injection moulded part. Molten resin, driven by pressure at the injection gate, travels quickly and smoothly until it reaches that thin spot, and that's where the trouble starts. First, as long as there are thicker areas for the resin to flow into, the pressure driving it into the thin region goes down, slowing its flow. See figure 1. (It's like those cars pulling off onto routes parallel to the bottlenecked motorway.) Second, because the surface-to-volume ratio is higher in the thin area, resin there cools faster. Cooling makes the resin more viscous, slowing flow even more. In the worst case, the resin actually solidifies in the thin area, blocking flow there entirely (like those overheated cars stalled on the motorway).
Figure 1 - Example of thin and thick area resin flow problems
But wait! That's just the start of your problems. If the resin has to travel through the thin area to fill a thick area behind it, there may not be enough flow to fill the thick area and you'll end up with voids (like that light traffic downstream of a blockage on the motorway). Or resin may flow into thick areas surrounding the thin area first (it's called race-tracking) and then into the thin area from its edges. This traps air in the thin area, resisting the entry of the resin, and you end up with voids. Or cooling flows of resin may meet in the thin area and form knit lines, causing cosmetic or even structural problems. See Figure 2.
Figure 2 - Issues caused by cooling resin flow
The obvious solution is to avoid areas of significantly different thickness, but sometimes you need a thin area. It could be for translucence, flexibility, to allow room for something behind the wall, or for a knockout (as in an electrical box). If you must have a thin wall, you'll have to do some planning. You can't put a gate in a thin area, but you may be able to gate in a nearby thick area. You may have to vent the thin area to keep air from being trapped and blocking the entry of resin. Tough decisions, but you can always call Protomold; we're here to help.