What wood is best for making mold patterns?

[Art Bianconi]

I am about to mill about a dozen hexagon shaped pockets in a substrate. The hexes are 1.625" across the flats. All the inside radii are 0.125" The pockets are all the same depth 1.285".

I intend to use a 2 axis CNC program and a 1 degree tapered end mill with a .25" round nose.

All along I figured on using a 1.75" block of machining wax 12" square. I've used this material before and know how to adjust the spindle speeds and material feed to keep things cool and get a smooth finish.

However, at the last minute I realized that even if I machine it properly without melting it, I run the risk of softening the wax and perhaps even melting it when I pour an exothermic material into it. Plaster or Paris, Dental Plaster, Hydrocal all generate some heat. 125 degrees maybe.

It occurs to me that I may have made a mistake choosing machining wax for this pattern and so I am looking to make the same pattern using wood instead.

With the finished pattern only 1.66" thick, I figure I'll lay up 12" square boards, planed to 0.25" with alternate grains perpendicular to each other until it's built up to the necessary thickness. Then one final pass on the plane and it's ready for pocketing on the mill.

When I need help is choosing the wood. Surface finish is important. It will NOT be sealed with polyurethane as I need the absorptive properties of naked wood to help dry the plaster molds. The pattern will not be used that often except to make new plaster molds so frequent exposure to moisture is unlikely.

Can you offer some suggestions of a wood that is well suited to this process? What kind of wood do pattern makers use?

Thanks

Art

[Michael Mezalick]

Art,
You will need to seal the wood or the plaster will stick to it. You really want the plaster to cure on its own. Having the wood draw out the water will force the plaster to cure too quickly and self destruct.
I would suggest a good ridged foam and sealed to a high gloss paint or epoxy.
Michael

[Rob Cunningham]

Art,
Have you considered a plastic material such as PVC, LDPE, or Teflon? I don't know if they would draw enough moisture, but the casting material should not stick to it.
Pattern makers use a variety of woods, however, the wood is only used to make an impression into a sand mold, not as the mold itself.
Machinable wax has a softening temperature of 226*F, so you might be okay with it. Try one cavity and see what happens.

[Jamie Buxton]

Any wood is going to need protection from the water in plaster. It will absorb water and expand. And most wood is going to need a coating to prevent the plaster from bonding to the wood.

How 'bout Corian? It is readily available, isn't affected by water, can be machined with carbide router bits, and plaster will not adhere to it.

[Marty Paulus]

Most patternmakers use a product callen renboard. Basically it is a block of bondo. From there they machine the shape and finish to a very smooth surface. They then use a parting wax.

Try to google Ren Board.

[Chip Lindley]

I've heard that mold pattern makers used mahogany or cherry for making patterns. I bought some excellent cherry at an auction that were said to be *blanks* from a lead foundry. Not a defect in them! All FAS!

[Walter Plummer]

Hello Art. If I understand correctly you are going to cast 1.625" x 1.285" hexagonal "pucks"? May I suggest another option? Make your patterns positive and seal them. Then use latex mold rubber to make the mold itself. Latex works well with plaster and will flex to de-mold. This is a big help if your patterns do not have any draft angle to aid in releasing from the mold.

[Art Bianconi]

Art,
You will need to seal the wood or the plaster will stick to it. You really want the plaster to cure on its own. Having the wood draw out the water will force the plaster to cure too quickly and self destruct.
I would suggest a good ridged foam and sealed to a high gloss paint or epoxy.
Michael

I realize that the wood must be sealed. I am anxious for the plaster to cure properly throughout and was concerned that while a sealant would give a better finish to the plaster part, it would inhibit it's drying and delay things.

I did a lot of plaster casting of plasteline clay models many years ago and made molds put of plaster supported latex for casting resin parts so I'm a little familiar with some of the things that must be done.

This part is big however, almost 12 inches across.

Thanks Michael

[Art Bianconi]

Art,
Machinable wax has a softening temperature of 226*F, so you might be okay with it. Try one cavity and see what happens.

That temperature was not known to me Rob. That's important information. Thanks!

I used a one inch wide blade on the band saw to slice a thick block of this wax in half or this project. While the wax did melt and coat the blade a little, it did not get sloppy wet which is consistent with your information. It further supports the idea that I may be fretting over something unlikely to happen.

Thanks

Art

[Art Bianconi]

How 'bout Corian? It is readily available, isn't affected by water, can be machined with carbide router bits, and plaster will not adhere to it.

I've a buddy in Ohio who used to be in the kitchen renovation business (until he got injured). That guy builds more things out of Corian than I can count!

It is quite a novel material. It's dimensionally stable and works beautifully for fixturing some of the devices on my optical bench. I hadn't thought of using it because I need material almost two inches thick. Can two pieces be bonded together with suitable adhesive?

Thanks

Art

[Art Bianconi]

Most pattern makers use a product called renboard. Basically it is a block of bondo. From there they machine the shape and finish to a very smooth surface. They then use a parting wax.

Try to google Ren Board.

I've never heard of it but it sounds a lot better than wood. I'll check it out.

Thanks Marty

Art

[Art Bianconi]

I bought some excellent cherry at an auction that were said to be *blanks* from a lead foundry. Not a defect in them! All FAS!

I've a buddy who is a project manager for a big construction firm that puts up warehouses. When they deliver all those welded up steel trusses and monster "I" beams, they keep the metal parts from causing damage to each other by stacking them on thick mahogany planks that get thrown away!

I took one home; ran it through the planing machine and got a lovely piece of rich looking material. Especially so after a coat of satin polyurethane.

I called him "What are you going to do with all that mahogany?"

"It's stacked on the wood pile for the fireplace. Why?"

[Jamie Buxton]

... Corian... I hadn't thought of using it because I need material almost two inches thick. Can two pieces be bonded together with suitable adhesive?

Yes, it can be glued. Corian countertop edges are glued on. And countertop subassemblies are glued together on-site to make a full countertop. And sink basins are glued into countertop sheets to make fully-integrated sinks. I think epoxy is the most common adhesive.

[Art Bianconi]

Hello Art. May I suggest another option? Make your patterns positive and seal them. Then use latex mold rubber to make the mold itself.

Hi Walt!

Thank you! This pattern IS the positive. It's scaled up 15% to allow for the shrinkage expected during the firing process. The pattern will be used to cast a mold from Hydrocal, plaster or Dental plaster. The actual slip used to make the ceramic part will then be poured into the mold made from the process.

Latex works very well as a mold, especially when backed up by rigid plaster "mother mold". On big ones, I interweave strips of burlap or cheese cloth. I may go with silicone rubber molds as an alternative.

However, the refractory operator suggests that because I do have a one degree draft and no undercuts, that plaster molds will dry the slip to a green state more uniformly.

Thanks for the suggestion

Art

[Art Bianconi]

Yes, it can be glued. Corian countertop edges are glued on. And countertop sub-assemblies are glued together on-site to make a full counter top. And sink basins are glued into counter top sheets to make fully-integrated sinks. I think epoxy is the most common adhesive.

I did as you suggested Jamie. It cost me more than I would have liked but the benefits of using Corian as a pattern (or a mold) outweigh the costs.

Three 12" square panels, each a half inch thick, cost me $75 plus sales tax.

I'll use two part Safe-T-Epoxyas the lamination adhesive and vacuum bag the lamination until cured.

Safe-T-Epoxy is an old Hexcel formula I've worked with for years.

Thanks!

Art