Design for Closed Molding

John Moore from JHM Technologies, Inc. goes over some of the key elements you need to pay attention to when designing tooling for RTM, LRTM, VARTM (RSB), & HP-RTM.

  • No “Die Locking”
  • Positive Drafts
  • Uniform Thickness

Also available on YouTube.

Video Transcription Below:

Hi this is John Moore with JHM Technologies. Here today we’re [going to] talk about designing for closed molding. Whether that be RTM, Light RTM, High Pressure RTM (HP-RTM), [or] even VARTM. We all have the same the same concerns for these processes regardless of the acronym or the method that we are actually going to be molding. We are talking here about the part design.

Design Closed Mold—RTM, LRTM, HP-RTM—Tooling Without “Die Locks”

The first thing is that we can’t have any “die locks”. Now what is a die lock? Well, an example of a die lock part would be this air duct. You can see this duct is [die locked] in this area here, but it also [has] this channel. If we were to mold it in this plane, (in other words if we were to put a parting line here on the mold the lower half here, the upper half) this area would be locked. The mold wouldn’t be able to be opened. [To make this part] we put it in this position here so that the line of draw is in this direction. [We are] able to have a male mold with a channel down below, and this channel area here but when the molds opened up, and the part came out it wasn’t locked into the mold. That’s critical when it’s in a closed mold.

Now here were talking RTM, Light RTM, High Pressure RTM (HP-RTM) we have two mold halves. Technically, in a VARTM process where we have a silicon bag or a nylon bag [we would] be able to mold this against a tool surface and then pop it off. But specifically for this discussion lets consider two ridged mold halves. So in that case, here is a similar part that has no die locks. Now in that we’re able to have a male tool that would be the shape as you see on the outside and a female tool. We can load it with fiber, covering all areas, close the mold and then open it straight up, with no die locking concerns. [That is] critical in our part design.

Positive Draft in Your Tool Design

The next is we have to have positive draft. And I am saying greater than one and a half degrees is preferred. So here, it’s obvious [that] we have more than enough draft. These vertical walls are close; they actually have a degree and a half. But by having that positive draft we’re able to open and close the mold and de-mold the part easily. If we had no draft, the mold would drag as we’re opening, and the part would drag on the molds’ surface. [Positive] draft is another element.

Controlling Thickness in Your Tool Design

Uniform thickness. Now in that case, you can take this part for an example. You can see the red has highlighted the edge of [the] part. [You can see] that part is uniform in all the radiuses and across all areas. That’s ideal. If we end up with thicker radiuses or, by our design intent we make thicker areas of the part. Well first lets talk about the radiuses. If those radiuses are thickened and they have the same amount of fiber as they do in the nominal area you will tend to trap air and have porosity throughout the part but specifically air in the radiuses. So to have a consistent thickness is critical to the part design.

Next, if we decide that we want thicker areas for strength, that can be done but what we are going to end up having to do is to put extra fiber. So we want to think about the operator and their technique. They will have to put extra pieces of glass strategically where the thickened area is. We have to keep uniformity in our glass to resin ratio. So in thicker areas we need more glass, thinner areas we would have lower glass content.

The other element though, we need thicker areas. We can use cores. Here is an example of using wood as a core. This is a cutaway of a seat to a boat. So we have uniform thickness, we have uniform thicknesses around the core, and in this case they used wood. It could be foam, it could be a variety of different materials that [could be] used for the core, but never the less we’re able to have fiber and resin follow around the core with the core in the center for strength.

So if we consider those values in our design [we will] have a uniform strong part that is easy to load and easy to repeat in the process.

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