Thread: Moulding machinist wax in sand issue

How have you gone with this?

I am casting some polyurethane rubber at the moment, it is suitable for everything from mold making to floor mats, depending on the hardness.
Ew

Originally Posted by Ueee

How have you gone with this?

I am casting some polyurethane rubber at the moment, it is suitable for everything from mold making to floor mats, depending on the hardness.
Ew

Ew, I ended up getting a bit further than the thread detailed. I poured molds, mixed wax and plastic, cast wax and did a bunch of fettling. I ended up getting stopped dead in my tracks due to three reasons. The first being that the Smooth On "OOmOO" was rediculiously expensive ($50 per liter). If I recall correctly, I needed 8 liters to do the mold pictured below. That was just to make a mold to copy the original. I made this mold as a 3 piece mold. Somewhat impressive, and fortunately it came out usable. The next issue that I had after taking a serious whack to the back pocket, was that the wax was developing porosity/air bubbles/voids when I was casting it. Sometimes parts of the mold would have air bubbles removing important features. I ended up shaking it, drilling holes, changing the wax/plastic composition and I could not nail it. I think I needed to do vacuum casting. So stuff started getting complicated and I hardly wanted to fork out another $400...

The third and final reason was that I got very lucky (rare!) and happened upon a supercharger from one of these.. Which meant that the mount needed further modifications. In the end I decided to stop stuffing around and get on with my CNC. As it stands today I am one piece (rather trivial bit) away from having all the mounts done for my RF45 CNC. All the hard work is done, ballscrews are purchased, power supply and NEMA34 Servos are here from the US, and the mounts all bar one piece of AL are done. The CNC will be used to cut the template. Possibly out of wax...

IMG_4964.jpg IMG_4966.jpg mould_section.jpg IMG_4969.jpg IMG_2161.jpg IMG_5025.jpg

The high grade polyurethane cold castable i am using is about $26+gst a kilo. There is a lower grade that is cheaper. The killer for me is the bonding agent, it is over $100 a liter.

Ew

Originally Posted by Ueee

The high grade polyurethane cold castable i am using is about $26+gst a kilo. There is a lower grade that is cheaper. The killer for me is the bonding agent, it is over $100 a liter.

Ew

The only alternative I know is buying direct from China. I made some inquiries and It was really cheap. Unfortunately the volumes required would only work if there was a group that needed it. It really surprised me how much I needed to do the mount mold. I thought I could do it with 4 liters, but turned out that I needed 8. It was really good stuff to work with though. For small parts it would be pretty darn good. Especially plastic parts that can be poured straight in. I can see how it is really useful for plastic trim parts in cars etc. The result with wax on my large part was hit and miss due to the fact it kept developing air bubbles. It did not matter if I relieved the section with a hole, just migrated the problem elsewhere. I tried different pouring positions in the mold, high, low etc. Different wax temperatures. Different wax/plastic mixes. No matter what I tried I could not get it consistent. I reckon the way is to do it under vacuum.

How much bonding agent do you need per kilo of PU?

Originally Posted by variant22

How much bonding agent do you need per kilo of PU?

Bonding agent is for sticking it to another surface, not to be mixed in, so it depends on the application at to how much. I reckon i'm going to go through 50+ kg of poly before i even start to look like using up the bonding agent.

Ew