Creating High Quality, Reusable Molds with CNC Machined Foam and Stretchelon Bagging Film – aka “The Method”
This post is really the culmination of a few years of research, research that I’ve decided to set loose into the world. We call this “The Method,” or sometimes “The Stretchelon Trick” and it’s my primary method of creating tooling for composite parts these days. As an example, I’m using the Giles 200 gear leg fairings that I describe in depth in this post here. You might read that post and think “well that’s all well and good, but what the heck are you going to make those molds out of that’s cost effective?” The answer is medium density polyurethane tooling foam, normally 15 lb. density foam. For these gear legs, the depth of the mold is about 1.5″, so I used 2″ thick foam. A 2″X48″X96″ block of tooling foam is about $500, and this particular mold is about 23″X40″, so the material cost is a fairly small fraction of that. Here’s the machined foam of the gear leg fairings, cut yesterday:
Okay, granted, it’s a bit hard with the light here to really see what’s going on, but if you look at the post I link to above, you’ll see this is just the CNC machined version of the gear leg tooling. Since one of these molds is two part, the next step is to join the two parts together:
We usually join them together with some fast setting epoxy and then drive some dowels into them for good measure. Now at this point you’re probably thinking “well that’s nice and all, but now you’ve gotta spray some sort of primer/sealer onto that thing, and hand finish it.” But, you would be wrong. You see, I hate sanding. Actually, that’s not really true, I like doing body work, but I hate sanding when there’s no point to it. And now let me show you why there’s no point to it:
This, my friends, is the heart of “The Method.” The green stuff over the mold is an elastic vacuum bagging material called Stretchelon. Stretchelon is a high elongation vacuum bagging film. I hate dealing with bagging tape, and so I’ve routered channels into my work table here, coated them with shelf paper and then used hardware store screen door spline to create the seal. This method works quite well, but you can just as easily make a tube out of the Stretchelon and put the mold in the tube. I pull my vacuum for the mold from the bottom of the table. The mold is sitting on a piece of breather so that the air gets evacuated from all around the mold. The slight porosity of the foam does wonders to allow the air to travel to the edge of the foam. Closeup, it’s looks like this:
The Stretchelon pulls down right onto the foam, sealing it for use. Pretty cool huh? Yes, there is a bit of a texture to the mold. If you want a smoother mold, just buy higher density foam. But what I’ve found is that since the first thing I do when I go to prime a part is to scuff it up, that texture simply vanishes with a few strokes of a sanding block. The final thing to do is put some mold release on it – the film is polyurethane and so epoxy will stick to it. I’ve used Frekote in the past, but I like to use another Airtech product called Safelease 20L. It can be dispensed from one of those hand squeeze spray bottles you get at the hardware store, so I like the fact I’m not using a bunch of aerosol cans. And it works, so that’s nice. Anyhow, once you spray your mold down with some release and then wipe off the excess, this mold is pretty much like any other – yes, it’s slightly more ding prone than standard hard tooling. If you want to make lots of parts (let’s say, more than 10) then I would simply go with a more dense foam, say 25 lbs. If you get any dings, you can simply patch them with spackle. When I first starting doing this, I was really concerned about mold wear, but frankly it just hasn’t been a real problem. Some of my molds have been used 10-12 times now, and for the most part they look just like they did the first time. So now, it’s just a matter of laying up the laminate, in this case 3 layers of 5.7 oz carbon:
And then, you vacuum bag another layer of Stretchelon (or regular bagging film) over on top of the part:
Your standard peel ply/perf/breather fabrics are on top of the carbon. The vacuum for the top bag comes from a standard bag tap. So we’ve got two separate pumps running here – one for the mold, and one for the part. They’re both around 25-26″ of mercury. You’ll get a few more inches of mercury with a bag tube made with bagging tape, but the cost and labor savings of this whole screen spline method are nice.
After the part has cured, you simply switch both vacuums off, and remove the part. The Stretchelon comes off the foam with the part, and as long as you haven’t gone and poked some holes in the first layer (which you’d pretty much have to try to do) the mold is just as pristine as when you started. Here’s some advantages of The Method:
1. It’s freakishly fast. On small parts, I’ve done a design, mold fabrication, part fabrication and demold cycle in under 24 hours.
2. It’s very accurate. With the EXAscan laser scanner, what you’ve got is a fully digital production workflow. The data goes in on the EXAscan, comes out on a CNC mill, and you do NO hand finishing of the mold. If you need a tight tolerance, hand finishing can quite easily take you out of that tolerance. Of course the same level of accuracy applies to designs that start off in the computer.
3. Far less sanding. The first time I pick up a piece of sandpaper with this method is when I go to put on primer.
There are a few drawbacks:
1. Certainly, if you want to make molds out of cheap boat cloth and polyester resin, The Method will cost more.
2. There is SOME limit to what the Stretchelon can do. Not a lot, but some. One case in particular – if you have a cylinder that you want to lay up over, the Stretchelon will have a seam somewhere along the length of the mold. So, occasionally you have to be creative.
So there you have it – this is “The Method.” I’ve heard rumors of other people doing something like this, but I’m not sure if anyone has taken it to the extremes that I have. I’m going to be outlining a few of the finer points in a few days, and I’m sure people will have some questions, but that’s really the gist of it. Happy 1st Birthday to Better Living Through CNC, and thanks for reading. Enjoy!