Need some advice on a one-off part

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#1

I’ve got a fairly simple project on the bench that I could use some advice on. I’ve done similar parts in the past, but not using the same materials.

What I’m doing is molding a part over a cheap beadboard (Styrofoam) form. I’ve done this several ways in the past. I’ve covered the foam with packing tape and layed up the part with epoxy resins - no problem with epoxy and foam. I’ve glassed the foam with epoxy and layed up the part with epoxy/fiberglass - again, epoxy and foam play nice together.

This time though, I need to lay up the part with polyester gelcoat and glass/poly resin. I do need a fair surface finish, so I’ll glass the foam with epoxy. For a disposable mold that only needs a fair surface finish and little dimensional accuracy, what materials would be suitable to finish the mold surface with as little cost/effort as possible? I’ll wax and pva the surface prior to layup.

Is a thin epoxy/glass surface going to be enough to keep the polyesters from dissolving the foam?

Thanks for any suggestions.

#2

so long as the epoxy surface coat and plys cover the foam and form a protective barrier, you should be fine. I’ve worked with molds like that. I’ve also used sealers to coat eps foam for use with polyester, but that route can be a little more risky.

If your’e worried about cost, i’d say that time would be more expensive than just switching from polyester to epoxy. Is there some reason it must be polyester?

#3

I’ve given the time consideration some thought. I’m not entirely sure my plan is the most cost effective from that standpoint.

I’m using polyesters (which I have very little experience with) simply because I want to use tooling gelcoat for the finished surface of the part. I’ve used tooling epoxies in the past, but I’ve got a pretty large area I’ll be working with and would rather just shoot gelcoat and proceed. The tooling epoxies I use are a bit of a pain and the cost for the area I’m coating would be significantly more than gelcoat. I also wanted to build laminate thickness in the part with chopped strand mat.

#4

oh yes… that is a good reason.

Gelcoat is good for that, easy to apply and cheap.

A new user, Snowmixer (aka charlie) was just mentioning a smooth on product to seal foam with. never used it so i don’t know if it’s good? Seems like it could be good in an eps foam mold application like your project.

http://www.smooth-on.com/Epoxy-Coatings-EPS/c1397_1400/index.html

#5

I’ve got two suggestions. The first would be if you are going to use foam, use urethane foam board. It is relatively inexpensive (in the lower densities) and is not affected by polyester resin. You can get it in many different densities from 2 lb up to over 30 lb per cubic foot. The higher density material is more expensive but holds finer details.

I’ve used 2lb for patterns with very simple geometry. A word of caution - the low density stuff will require you to be very careful when handling. It is pretty brittle in relative terms. I’ve found that 15 lb is a pretty good compromise when making patterns that need detail. We use a 5 axis cnc router to cut some of our patterns and the 15lb leaves a good surface to work with. Pattern accuracy is very good.

The second method, if you use styrofoam is to coat it with epoxy. 2-3 coats of a non-blushing epoxy will give you a good barrier. After the epoxy, you can spray with Duratec primer to do the finish work.

I’ve used a couple of products like Styro-Safe and have not had good results. If you are not familiar, Styrosafe is a polyester based product that is supposed to create a shell over the stryofoam so you can follow with regular polyester resins. I’ve found it to distort the surface. It slightly dissolves the surface of the styrofoam. Like I said before, we cut some patterns on the cnc router. Styrofoam should be within maybe .015-.020" of the design. I’ve measured surfaces off by as much as .120" after applying Stryro-safe. Not acceptable in my book.

#6

Roger,

Yeah, I’ve never used EPS for anything where I needed to hold tolerance. This project is just a coolant basin for one of my machine tools. It has some features constraining it, but I’ve got at least +/-0.1" of tolerance in the important areas. Which I’ll have no problem holding.

I’d love to have a 5-axis machine to carve this thing. I will have a 3-axis router one of these days.

I looked at some of the foam coating products you guys have suggested. Definitely something I’ll want to experiment with in the future. I’ve gotta stick with proven methods for this project though. I’m gonna go ahead and epoxy/glass/primer the foam, and proceed from there

Thanks again!

#7

The Smooth-On “Epsilon” epoxy coating will not eat into the foam, but does seem to adhere to it well. I have found that it will protect the foam from the gelcoat, even when applied in (or sanded down to) very thin layers. The down side would be the extra time spend applying it, as well as the cost. By the time you factor in the cost of the coating, would it still be less expensive to go that route, or would you be time and money ahead to have either gone with the correct foam or just use the epoxy gelcoat on the Styrofoam?

The “Epsilon” coating is pretty easy to work with, but does tend to flow continuously until it is fully cured. The directions state to ‘keep and use at 73 degrees (F)’ and that is pretty accurate…not 68, not 71.6, but 73. Okay, so it may not be THAT precise, but it takes significantly longer to cure to a sandable hardness with the temperature even a few degrees cooler, at least that has been my experience.

I would suggest either making sure whatever surface you apply it to is fairly level (to minimize flowing), or let the epoxy start to cure and become thicker, then spread it. Even after it starts to become very thick (similar to the consistency of caramel, maybe even a little thicker), you can still spread it nicely with a plastic spreader/squeegee. This way it will cure shortly after you get it where you want it, and your chances of ending up with thin spots will be minimized. This may not matter at all if you do not intend to sand it, but obviously you will want to coat a little thicker than you need if you do want to do some final sanding and shaping. So again, this would add even more time to the process of making your part, but could possibly be worth it to get it the way you want, using the Styrofoam.

Some time in the future, I intend to try machining some fairly simple shapes and patterns into some EPS (I’ve been using the pink insulation foam), making the initial cuts oversize/deeper than necessary. Then I will apply the Epsilon coating thicker than necessary, and run another final machining program to cut the hardened epoxy to the size/shape desired. I don’t know that this will be any easier than machining a mold out of another substance, but certainly will be inexpensive.

Good luck with your project, and I hope this helps! :slight_smile:

#8

5-axis is big money but if you can keep it busy, it will pay for itself.

One things I forgot to mention, epoxy over EPS works but the surface is a bit “delicate”. EPS has a little give to it. Glassing and then priming, like you mentioned will go a long way towards stiffening the surface.

The pattern guys I’ve dealt with in the past do something similar to what you are proposing. Overcut by 1/4" to 1/2" (depending on the size of the part) and then epoxy coat the foam. On really large parts, like boat hulls, they may even glass the surface. After that cures, they spray on a polyester based syntactic foam. When the syntactic cures, they cut to the final shape.

The thickness of the coating creates a fairly stiff surface that can be faired quickly to remove tool marks and then the finish coat can be applied.

You can trowel on a filled resin to create a part in a similar fashion to the way the pros do it but without the expense of the $15,000 spray rig.

#9

Great tips on this topic, thanks!

I went with a process I’m familiar with and glassed the foam with epoxy. A little body work and some epoxy primer finished the surface. None of these were done with great care as the part I need to produce is non-structural and has a pretty wide dimensional tolerance.

I had some problems with the gelcoat on the first go at building the laminate, so I’m prepping for the second attempt. I’ve never been a fan of polyester products and really should have just stuck with an epoxy based process for this part. Oh well, I was looking for a good project to brush up on some lesser used skills so I shouldn’t complain. I’ll get a few photos up before too long.

Thanks for all the info, I’m definitely gonna experiment with some of these suggestions in the near future.

#10

BTW, now I remember why I never use beadboard for these kinds of things. I didn’t have access to a large enough piece of rigid foam at the moment and it’s cost me a lot of time.

#11

Here’s a few photos of what I was actually trying to build - a new basin for my milling machine enclosure.

I’m happy enough with the result, but I could have done a few things better. Been a long time since I worked with some of these materials so it was good refresher.

#12

Started with a foam form. Glassed with epoxy, a little body work then some epoxy primer. Sanded, waxed and pva’d before layup.

#13

laminate built with gelcoat, then glass cloth and chopped strand mat (polyester resin).

#14

I didn’t maintain relief angles so I knew removing the mold was gonna be fun. I could have done it in a way that the mold would release without destroying it, but the added labor in mold construction wasn’t worth it to me. I only need one of these parts and had no plans to keep the mold in storage.

Only a little coaxing was required to separate the part once the foam was removed from under the mold surface.

#15

I ended up with some gelcoat alligator skin in a few places, but I had some problems when spraying the gelcoat that I wasn’t able to remedy in time. Sprayed too thin cause I was low on material and my gun was giving me fits.

Would be easy enough to resurface a few of the bad spots, but I’m not worried about it.

Now the fun stuff, trimming, bonding it to the base and plumbing in all the coolant system parts. I’ve got chip trays yet to build that will sit in each basin. I’m anxious to see how it works and I’ve got my fingers crossed that coolant/machine oils/metal chips won’t be too hard on the gelcoat over time.