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polyester resin versus epoxy resin

S

squirrel

Joined
Sep 27, 2007
Messages
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Location
Belgium
Hello,

A friend gave me some walnut wood shortly before he died. The pieces are rather small. I want to mould them in resin and then turn a platter from the walnut-resin piece to give it as a gift to his wife.
But do I need epoxy resin or polyester resin to fill in the mould? Which of the two can be turned?

And how shall I finish it? With polyester lacker?? or what else?

Best regards - Squirrel
 
Polyester resin is usually less dear than epoxy. But, please, try a practice piece first, perhaps without any embedment, except for mounting.

The cured resin can be its own finish. After finish sanding to about 600, try something like Scotchbrite, then softer polishing compounds such as toothpaste, and finally Brasso (yes, Brasso), or a similar metal polish.
 
I use cast-n-craft which is crystal clear. I thought it was a polyester resin but I would have to check. I have also used Clear Inlace. both are expensive but very clear. They turn very well and polish to look like glass.
The epoxy I have used turns well but is is a little harder to control. It's called System 3 mirror coat. It is designed as a bar top finish. I've only used it on small areas filled to 1/4" thick. It becomes very viscous so it must be contained religiously. I've had it seem through the end grain in some woods.
 
I always thought that polyester was much clearer than epoxy which seems to have a cloudy yellow appearance. There is also a danger of epoxy getting too hot when mixed in large batches -- the curing heat can get rather intense and will result in the epoxy getting even darker in color (orange or even brown). This caution mainly applies to fast curing epoxy, but don't ask how I came about this bit of wisdom. 😀
 
Are you wanting to use the casting mix to encase or impregnate the wood? Basically, do you want it to soak all the way into the wood to harden it?

As for the heat generated, the longer the cure time of the product, the less heat (generally).
 
I've done some playing with both, in addition to some of the above comments I feel that the Poly is more brittle and chips easier (ie tear-out issues)
 
I find turning either one can be brittle. You have to let the tool do the cutting. Any forcing at all will chip.
 
Hi ,
I want to use the casting mix to encase the wood - a lot of wood and very little resin around. Is there not a famous turner who became famous with this technique?

I fear not to be successfull. Yes, I think the resins are very brittle. And certainly at the edges. I think the use of very sharp scapers will be the best solution, don't you think so?

Squirrel
 
epoxy hardness

Just like metal alloys, all epoxy mixtures are not the same. You can get them where they are soft or extremely brittle. When I worked for an electronics firm many, many years ago we potted battery replacements with a soft flexible mix and capped it with a hard rigid cap. Check with a vendor like system 3 and maybe they'll help you find the right one for your use.

Vernon
 
I'm no expert, but here's what I think I know:

Whether you use expoxy, polyester or urethane you want clear casting resin if you are not coloring the resin. Polyester is the cheapest, but don't overlook something like Alumilite (urethane). For a colored opague resin, my choice would be epoxy. Fiberglass dealers will sell the resins for less than most art / boat places.

Polyester uses a small amount of catalyst (usually included free with the resin, but not always) which generates heat which cures the resin. Polyester is nasty stinky stuff, but it's relatively cheap. The more catalyst, the more heat and the harder / more brittle the result. You can even bake the cured resin and harden it up if it's too soft right out of the cast. None of this should be done in the house / shop or anywhere people / animals you care about will be.

Epoxy and urethane are essentially two part mixes that rely on a chemical reaction between the hardener and resin. If you don't get the ratios just right, there isn't a lot you can do to fix the problem.

Check out the work of Gianfranco Angelino for some inspiration:

http://www.delmano.com/exhibitions/2005/mainExhibitions/TurnedWoodampScu/artists/gAngelino.htm

Ed
 
I have used a number of different epoxies over the years in model building and mold making. I don't really care for ployester as it gets brittle with time and more prone to cracks than epoxy.

There are a number of epoxies that would work, but you will not find them at your local box store. They sell mostly glues and never any casting resins. You might want to check a local foundry supply. They carry a number of casting resins. I also bought some of what I use from a manufactor, it was one that you don't find locally. MArine supply normally carry West Systems or System 3, which are very good epoxies.

There is a wide variety available. There are different mixing ratios, working times, set times and cure times. I use epoxies that have 1:1, 1:3, 1:5, and 2:7 mixing ratios. The 1:1 is pretty fleixable, can be 60:40 or 55:45 and still work, basicly get it in the ball park. these are what you find at the home centers. The others that have the 1:3 and 1:5 are more exacting, but not a problem to measure using a simple balance beam.

Since epoxy generates heat as part of the curing process, you will need one that has a long working time to cast something the size of a platter. There are some resins made for this (I have seen up to 8 hours set times), where the typical home use 30/60 min epoxy will generate too much heat mixed in that volume and gel within a few minutes.

The other thing with epxoy is the cure time before machining. While the "set time" may be a few hours (4, 8, 24), the cure time is seven days for most that you will find. Some of the casting resins used in foundries can be cured quicker at 300-500 degrees f and there are some that will only cure at those temps. These would definately be outside your wants, unless you want Walnut charcoal. Once you do get it machined, you can wet sand it down to 2200 grit for a "glass" finish. I use my normal wood turning tools when I turn it and treat it the same as wood. I do keep the tools sharp on my Tormek and can get very clean surfaces.

The best thing would be to contact a resin supplier or specialty house and see what they have available. The prices are also better than buying small quantities from the local box stores too.

Good luck in your search,
Fog
 
Hey here is a crazy idea,

Turn your molds to as close to the final shape as you can, cast it, then you have minimal turning, maybe even just mounting to sand and polish.

To turn the molds cut 2 pieces of say 4/4 a little over diameter you want finished. mount one on the lathe turn athe mold for the top of the plate. then mount the other blank and turn the mold for the bottom making sure the rim are is same diameter. On either piece there should be NO UNDERCUTTING. once you are happy (here is the fun part) with the lathe running slow with the bottom form still mounted, part out the bottom (we have all done this just never intentionally).

now that you have the 2 halves use some type of release agent to coat the insides of the forms, good coat of parrafin will prolly do. clamp them to gether then fill with resin throught the hole in the bottom. after it has cured un-mold it mount it on lathe in cole jaws CA a scrap piece of wood on bottom, turn scrap block round, reverse finish top, the remount on cole jaws and finish bottom.

Sounds crazy but i think it could be doable.
 
Ed_McDonnell said:
Epoxy and urethane are essentially two part mixes that rely on a chemical reaction between the hardener and resin. If you don't get the ratios just right, there isn't a lot you can do to fix the problem.
Click to expand...

I can certainly vouch for the truth in that statement. I have an epoxy casting that was not mixed well enough so it had some small pockets of uncured resin even after more than five years and the worst part is there are tiny seepage paths in the casting that allow the uncured part of the resin to very slowly seep out and create a perpetual gummy mess.
 
I agree with most of what's been added, especially the need for clear material.

The polyester resins available at the big box stores is very likely unsuitable. Some that I've used is more of a patching material; it has dark pigment, akin to Bondo - might actually be made by Bondo. Bondo itself is fine for foundry patterns, though, according to the late Dave Gingery.

Craft shops will probably have the highest-quality resins. A close second should be boat repair sources.

Tony's two-part mold would be satisfactory for castings without embedments. But with embedments, there would likely be a lot of air bubbles entrapped.

It's been almost 30 years since I worked with the clear casting resin, with embedments. I had best results by placing a thin layer in the mold, then coating each embedment with resin and placing them, and then pouring a last layer for the top/bottom. This was all before I took up turning, and I shaped the pieces with a wood rasp, taking very light cuts. Even so, there was a little chipping.

Except for the material waste, the mold doesn't need to be particularly close to the final shape, and the waste part can actually be beneficial, as a sink for bubbles or other accidents.

I repeat my suggestion to try some practice pieces, to assure proper material selection and your cutting and finishing procedures, before committing these family heirlooms to encasement.
 
I do some of this with putting wood or similar items into resins -- things like cones from conifers and different woods like those with shipworm channels (marine borers). There are those who do miracles with resins - and I am thinking of the back cover of American Woodturning of a few years back having some inspiring examples.

System 3 (Auburn Washington) and West Systems (Michigan) both make good two part epoxy products. Some are clear and some tend to be amber hued. Some cure fast, and some such as products designed to penetrate rotted wood are very thin and set very very slowly.

West Systems 205 is lightly amber, works easily, sets with moderate time, and looks pretty good with dark items like a spruce cone.

I find that small bubbles can be an issue. And while they can be treated with additional epoxy or with ca glues, avoidance and minimization is better.

Some like Ed Davidson (YoYo Spin) have taken resins to far limits beyond what I have done, and pen turners have moved this towards an art form.

Moulds of cardboard and very good tape can be constructed. Plywood also works. I am envisioning a shallow disk with a plywood base that will be turned away as a likely way to go. Extra wood that extends upward will also be turned away.

I use 5 minute clear System 3 epoxy to recover small pieces of wood that have great character but not much continuity like ring shake in pacific yew. This turns easily, but tends to be softer than the yew wood, so some care in using backed abrasive is needed to keep fair curves smooth.

Great idea for a tribute -- look forward to seeing pictures as you develop your technique
 
For casting materials, I would suggest

Polytek

or

Smooth-on

Both have a variety of casting and tooling products. I have used some of it, but have used both of their mold making products. Since Squirrel is in Belgium, I would think one of these companies has overseas distribution that would work for them.
 
OK!
A lot of very usefull information! Thank you!
I think it must be possible to put a kind of varnish on the finished piece to get the wood as shiny as the epoxy is, is it not?
What would your suggestion be?
Squirrel
 
Marilyn Campbell has been using epoxy and very fine sawdust filler, bonded to wood for some really outstanding sculpture effects, for many years. She happens to be one of this year's featured demonstrators at the AAW's Albuquerque symposium. Here's a listing of the demo's she'll be giving:

- Getting Creative with Epoxy
- Making Halfmoon vessels with Epoxy

This link goes to her gallery where several photos of epoxy-wood sculpture are posted. And here's a view of the epoxy/wood piece she's donated to this year's EOG auction:
 

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It would be great to be able to go one year to the symposium. Unfortunately, it will not be this year.
Maybe I can get a grant in maybe two, three years.
It would be great to follow some workshops in America. You are so well organised!
Here in Belgium, we don't have so much different techniques- a pity!
Squirrel
 
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