Technique for creating custom plug for new part - is this okay?

General Composites Discussions Composites Talk
#1

I just wanted to run this by everyone here to see what feedback I would get, and possibly any tips that could help…

The photos below are from the first plug & mould that I made. I used an expanding polyurethane foam to shape the part, which worked well because I could form the shape quickly by cutting and shaping the foam. Then I used body filler to help get a smooth finish and build up any areas that needed it.

I think the big mistake was that I used a rattle can gloss black paint… Maybe I didn’t let the paint dry long enough, or didn’t use enough wax either… The mould came off the plug okay, but the plug was in quite bad shape…

I am going to be making some more parts from scratch, as well as modifying some existing parts which are made from ABS plastic. So, the questions are:

[i][u]Should I use polyurethane foam and body filler to build up & shape the plug? Or is there a better way?

Is a 2 pack paint necessary for the plug, or would a chemical resistant paint suffice?[/u][/i]

I think that body filler is okay as long as I don’t try and use it in a thin application, all edges of that plug are body filler (the most is on that lower front area, about half is bondo), it’s just the thin areas where I had troubles (I used thin applications on the top surface to help smooth out the shape).

I suppose I could just pour resin over the foam to fill the little holes, sanding it flat and using more resin to basically flow coat it if I got the foam shape close enough to what I wanted?

On the subject of paint I came across this today: KBS Diamond Finish which is chemical & fuel resistant, and they say harder and tougher than 2 pack clears. I also know about Spray Max 2k, but at the moment I’m not sure if I can get that into Australia or source it here… so I’m just looking for other options that might work.

If a 2 pack paint is necessary, I will just go that route, buy & mix my paint then use a Preval Sprayer. (That’s another thing that I just learnt about today, seems like the ideal solution for small jobs!)

I also saw a suggestion on another forum to cover the foam in latex house paint? They were specific that it must be latex - would that work?? Seems like it would still need a clear coat to get a good polish?

Another small question - you can probably notice the fibreglass texture visible through the gel coat on the mould. I think this is because I just applied the gel coat and didn’t put it on thick enough, and may not have waited long enough before proceeding with the job. The gel coat was tacky, could the texture also have come through from pushing too hard with a roller?

#2

If you are using a single component paint then you need to use PVA to block the chemical reactivity between the resin and the paint. I like to use PPG Deltron DC3000 (2K) and the Frekote FMS sealer and Frekote 770-nc release on my plugs.

In the end you want to test any plug surface (paint), release system, and molding resin to insure that everything is compatible. Not everything plays nice together.

The print-thru of the fabric onto the mold surface is from resin shrinkage. To reduce the problem you want to:

  1. Use low shrinkage molding resins (epoxies or zero shrinkage VE systems)
  2. Use several very light fabric/fibers first before moving to heavier materials
  3. Allow the mold to cure adequately before demolding it. I often let epoxy room temp systems cure for 4 to 6 days before demolding them. I often heat treat the mold while it’s still on the master/plug if they can tolerate the heat.
#3

Ok, thanks for that info. I wasn’t sure if PVA would completely block any chemical reactions.

I used a polyester resin on that mold, and I’m fairly sure that I took it out within the next day or two (it was at least a couple of years ago when I actually did this!). It had just been curing at room temperature (about 25ºC). I think I will have to invest in some heat lamps… if they would be hot enough to make a difference? If it’s just a room temp cure I wouldn’t think you’d need much additional heat?

That all gives me some good things to test out. Lots of testing coming up for me…

#4

Polyester resin will shrink more than any other laminating resin out there. Print-thru will be a problem. With polyester you want to cure it as slowly as possible. The faster you cure the polyester the more it will shrink.

#5

What other resin would you recomend than poly??

#6

Ahhh… okay, gotcha. Slow cure for polyester resin…

Brainstorm2 - epoxy resins (generally) have zero shrinkage. But there are so many different resin systems out there, I don’t think you can automatically assume that it won’t shrink just because it is epoxy… Check the manufacturers website, they will usually state any shrinkage rates, or whether it has zero shrinkage.

#7

One other thing I just thought about… you can see the thickness of my mould on the bottom of the picture - thinking about poly resin ideally being cured at lower temperatures, and more layers on the mould generating more heat from the curing reaction… I suppose laying up all those layers at the same time would have also contributed?

So maybe the better way to use poly resin is to get the first layer or two of cloth/CSM onto the gel coat, then let it full cure before adding addition layers, going only 2 layers at a time? More time consuming, but I suppose that is the trade off if you want to use poly resin…

#8

Just use epoxy tooling gelcoat and resin. It will save you the trouble.

#9

If your mold is small them the price difference between epoxy and polyester isn’t that great. Polyester will often add to the labor because you need to sand and polish the mold. If you make the mold correctly from epoxy resins then you should be able to skip the sanding and polishing.

#10

Im looking to swap to a low shrinkage resin system, ill look in to a few systems. I will also have to change matting aswell so its something i will look into

#11

The advantage of polyester moulds is that these withstand the chemical attack from styrene (polyester) when making polyester products in the mould. Epoxy is not 100% styrene resistant.

Shrinkage can be split in 2 types:
thermal shrinkage
chemical shrinkage

Thermal shrinkage is easy: no heat, no shrinkage.
Chemical shrinkage is some 7% for polyester, and 2% for epoxy. Most of the shrinkage is in the liquid stage.

Always test the system you plan to use, as wyowindworks suggested, to prevent nasty surprises.

A trick to use PVA without the hazy finish is to wax the plug, apply PVA, and wax again. This provides a thicker wax film (the second waxing procedure does not dissolve the previous wax application, as the PVA protects that). After applying PVA most paints are protected sufficiently. If you do not like PVA, go for a 2-pack pu paint.

Other options for tooling construction:
-Rapid Tooling Systems (polyester based). These contain a polymer which expands when heated. A certain thickness needs to be achieved to ensure the right exotherm.
-Polyester tooling gelcoat (high gloss!) with a skincoat behind it (VE). After cure apply epoxy and fabric.

#12

BTW, epoxy resins are not “zero” shrinkage resins. They just shrink less.

#13

Herman, I have recently read serveral studies that show that the shrinkage rate is higher during the later part of the cure and has very little in the early gel stage.

Here is one online study. I cant’ release the others. :o

http://www.4spepro.org/pdf/002583/002583.pdf

#14

Just a few Qs if you all could.

So does epoxy cure chemically to VE or are we going strictly for some mechanical bonding?

Does the VE need to be fully cured or tacky?

It’s been a long time since I even touched poly’s and ve’s, would you not add the wax(air dry… cant remember the name) which keeps the cured exposed surface from being tacky?

If you had the option wouldn’t you still opt to have an all epoxy mold?

Wouldn’t you get differential expansion etc with a poly/VE/epoxy combination mold with heating/postcure of the parts?

#15

True about minimal price difference. I have also found a local supplier who has West System resin/hardener at very good prices, so I am thinking that I will try that out. wyowindworks, I have seen your posts about mixing your own gel coat using those two types of West fillers, that sounds good, I’m thinking I will give that a try also.

But is it not possible to create a good mould if the poly resin process is carried out correctly to avoid excess shrinkage? Or is that always going to be an issue that requites extra labor on the mould?

The reason I was trying to solve the problem with polyester was just for my own knowledge. I figure that I am doing something wrong, I would prefer to get it right, so then at least I know what the problem was rather than just move to using epoxy (regardless of whether or not I do use epoxy in the end).

I sure am going to have a bunch of testing to do once I get this all clear in my head…! :eek:

Let’s say that I make a good mould, for the final parts I am thinking of making fairly thin parts with a clear finish. If the mould is good, do I still have to worry about shrinkage if I use polyester resin for the part? I was thinking 1 layer of 200g carbon fibre, a back layer of 400g carbon (with maybe 300g woven glass in between if the parts seem too thin/flexible). They will be used on a 2 stroke motorcycle, which vibrates a lot. So I was thinking that polyester resin would be better for the part because it will be more flexible and more UV resistant? Or have I mislead myself on those couple of points?

Thanks very much to everyone for your input thus far. :slight_smile:

#16

Mugget, you can reduce shrinkage but you typically can’t eliminate it. Some the of the new zero shrink tooling claims to be a net zero shrinkage system. It does require that everything in the system can handle the higher exotherm temps to cause the “swelling” filler to swell with the curing temps. A normal polyester, regardless of how it’s cured, will shrink more than most epoxy systems.

Polyester is prone to developing micro-fracturing due to vibration and internal stress (shirnkage). I would use epoxy over a polyester is a high vibration scenario.