Gallery, Projects and General => How do I?? => Topic started by: Brass_Machine on July 20, 2021, 11:24:18 AM

Title: Machinable Wax??
Post by: Brass_Machine on July 20, 2021, 11:24:18 AM
Does anyone have a good recipe to make machinable wax? I have a couple things (molds) that I want to make out of silicone and was thinking machinable wax would make a good mold (for the mold!)

Anyone have experience with it?

Title: Re: Machinable Wax??
Post by: awemawson on July 20, 2021, 11:49:15 AM
I just heated normal candle wax to melting point and started mixing in MDPE pellets of the sort used for injection moulding plastic parts. As the pellets dissolve slowly raise the temperature to about 135 - 150 or so.

The more pellets you manage to dissolve the firmer the resulting m/w is but about half as much pellet as wax seems about right.

LDPE and HDPE can be used but obviously it changes the  machinability. I've successfully used short slices of blue MDPE pipe that we use here in the UK for water distribution underground. I've heard of people using rolls of plastic bags!
Title: Re: Machinable Wax??
Post by: Brass_Machine on July 20, 2021, 04:21:28 PM
Thanks Andrew!

I will obviously need to create a form to pour it into. Since this will be a large block, I may have to do 2 melts to fill it. Would you see that as a problem or should I just buy a large pot?

Title: Re: Machinable Wax??
Post by: awemawson on July 20, 2021, 04:40:52 PM
From Ade Vickers experience when he was casting a large block as a prototype of his dry sump (*) before machining the final aluminium, the machineable wax from the two pours doesn't really bond.

(* it was this project that lured me into the clutches of Mad Modder back in 2010 and I'm still here  :bugeye: )
Title: Re: Machinable Wax??
Post by: Brass_Machine on July 20, 2021, 06:16:00 PM
I have some of his wax blocks in the shop. Just haven't used them yet (10 years later)

Big pour it is. Will find an el cheapo lobster boil pot and that should work. Don't want to ruin my wifes pots... Remember, she owns more hammers than me and knows how to use them!

Just need figure out the best ratio/mixture/ingredients to get a smooth finish for a silicone pour.
Title: Re: Machinable Wax??
Post by: rklopp on July 21, 2021, 12:22:11 AM
Watch the shrinkage. I tried to use commercial machinable wax to support machining a thin-wall brass part sort of like a 4” canoe. The wax shrank like mad, so that idea was a flop. I ended up using Wood’s metal.

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Title: Re: Machinable Wax??
Post by: awemawson on July 21, 2021, 02:13:01 AM
When I cast the 170 mm diameter blank for my chuck adaptor prototype I was getting about 10 mm shrinkage, ie shrank to 160 diam.
Title: Re: Machinable Wax??
Post by: AdeV on July 22, 2021, 01:38:29 PM
Most of my wax was made with a 4:1 ratio (4 pats wax, 1 part plastic) by weight. It shrinks like a bugger so make your mould oversized.

Once it's set, assuming you're machining the final thing to dimension, you'll be OK (dimensional stability is good with temperature, as far as I know)

Yes - definitely try 1 pour at a time, it doesn't like to stick to itself (because of the shrinkage, I think) if you pour it in multiple goes.

My first wax was made with melted candles + freezer bags from the local supermarket. Later wax blocks were made from scrap candle wax & raw virgin LDPE pellets. Any LDPE source will do. I made a very small test sample using HDPE too, which didn't seem to be any different to the LDPE version - so I guess you can use whatever is cheapest.

Most instructions say to add the LDPE slowly to the molten candle wax. I found it made no difference if you added it slowly, or in one go. Stirring can be harder if you add the plastic in one go...

The key thing is temperature: Don't let it get above about 200C (IIRC), because it'll start burning around that temp. Candle wax will catch at much lower temperatures, so as Andrew said, it's better to bring the temperature up slowly after about 90C. That said, I did mine in a tar boiler (the thing roofers use when doing felt roofs); not much temperature control there, and I only had one fire (easily put out by turning off the heat & putting the lid on to exclude oxygen).

Once the batch is ready, pour *immediately*. If it solidifies in the pot, it's an absolute bear to re-melt, as it's an amazing thermal insulator. I once tried to re-melt a ~1 cubic foot block, and never managed it (there was still a solid lump left after 8 hours in the pot). To re-use solidified wax, you really need to grate it into swarf first.

You will never get the pot clean again, so buying a dedicated pot is definitely a wise move. Remember you need a volume at LEAST 10% bigger (I'd got for 20%) than the final volume of your pour, to account for expansion in the molten phase; and also because you need to stir it quite hard. Definitely use a drill mounted paint stirrer to make your life easier!

HTH, sorry about the brevity, I'm running late & have to go home now! Good luck!
Title: Re: Machinable Wax??
Post by: efrench on July 22, 2021, 02:56:04 PM
Would polymorph plastic work?
Title: Re: Machinable Wax??
Post by: Brass_Machine on July 27, 2021, 08:35:21 AM
Thanks Ade!

I appreciate the information.