The folks at Leeds Hackspace have built themselves a shiny new C-beam based CNC mill. As you might expect everyone wants to try the machine out, but there’s a problem. A CNC machine presents a steep learning curve, and a lot of raw materials (not to mention cutting bits) can be used in a very short time. Their solution is simple: mix themselves some machinable wax from LDPE pellets and paraffin wax, then easily recycle their swarf and failed objects back into fresh machinable wax stock.
Making the wax recipe is not for the faint-hearted, and involves melting the LDPE pellets and wax to 130 degrees Celcius in a cheap deep-fat fryer. They bought the cheapest fryer they could find at the British catalogue retailer Argos, you really wouldn’t want to risk an appliance you cared about in this exercise.
Colouring came from an orange wax crayon, though they note recycling of mixed colours will inevitably result in a muddy brown. The finished mixture was poured into Tupperware lunchboxes to set, and the resulting blocks were trimmed to square on a bandsaw. The Tupperware proved not to have a flat bottom, so later batches were cast in a loaf tin which proved much more suitable.
We’ve mentioned the machinable wax recipe before here at Hackaday, but it’s worth returning to the topic here with a description of it being used in the wild. Having watched other environments get through learning materials at an alarming rate with very little to show for their effort, we can see it makes a lot of sense as a training material.
Cool, well actually at 130C it’s hot, but my question is why dump the LDPE and wax in at the same time, why not melt the wax, get it to the proper temperature and then add the LDPE bit by bit, that way you keep a higher temp and should make the process go all the smother.
No particular reason to not add it gradually, I just didn’t think of it. Will try it next time and see if it’s a bit faster.
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Dumping it in all together works fine. The paraffin melts first, then the LDPE dissolves over a bit of time.
How much LDPE to wax do you mix, is there a recommend ratio?
From the link way above:
“We’re using the machinable wax recipe from instructables, which calls for 4 parts paraffin wax to 1 part plastic. In this case, we are using LDPE pellets from eBay and paraffin wax from…”
But I would make several samples of different composition. Every mix will be unique especially if someone is using random candle-wax and cheap Walmart bags.
The consensus seems to be 25% by weight for LDPE. Some recipes call for a lower ratio (15%) using HDPE or PP, but at least one recipe I’ve found goes the opposite way, and another recipe says use 25% for LDPE or HDPE, but run hotter for HDPE. To be on the safe side I’ve kept it around 15%, which gives me a nice hard wax using polypropylene bottle caps, that doesn’t chip like paraffin, but isn’t too flexible like polyethylene. I wasn’t all that happy with HDPE grocery bags. These had red branding on them that turned the final wax into a sort of disgusting fleshy-pink color that was too flexible even at 15% HDPE by weight.
The bottom line is that more plastic -> thicker consistency in the liquid state, which translates into bubbles within the block that never rise to the surface. More plastic -> less brittle and more flexible, so you can customize the blend depending on whether you want something that acts more like wax (hard but brittle) or more like plastic (tough but flexy). One article tells me that there’s no upper limit to the amount of PE – it just gets thicker and thicker as you add more plastic. Of course the melting point goes up as well. It’s possible that when people say “keep adding PE until it stops dissolving”, what this really means is that they’ve found the point where the melting point has risen to the temperature of their mixture.
Now that I’ve spouted off like an expert long enough, let me say that I’m NOT an expert at this – I’ve only made four batches, which isn’t enough for controlling one variable at a time. These are just the observations of an experimenter looking for materials to make models from for sand casting in aluminum and zinc.
This would make excellent lost-wax castings.
As for ‘not for the faint-hearted’, making candles or just using the deep fryer as intended (which involves putting water-containing foodstuffs into fat at 185 degrees not 130!) requires pretty much the same precautions.
I’m not all that sure about lost wax. Where waxes tend to melt to a water-thin liquid and then vaporize completely, when you re-melt this you have a much more viscous substance and I’m not sure it will vaporize as quickly or cleanly. Worth a try, though – it’s much stronger than paraffin alone and you can carve, file, and sand it better, and can even “weld” pieces of it together using a soldering iron. If there’s a problem with vaporization, maybe less PE can be used – it’s not an all-or-nothing recipe. I DO wonder though, if this can be extruded into filament to use in 3D printers, then used in a lost-wax process used to make a metal part. Of course, it CAN be used with sand casting, and that doesn’t ruin the model and doesn’t depend on vaporizing it.
I agree that making machining/carving wax is safer and easier than deep frying. Generally food products contain water, which is what causes the spitting and popping hazard that comes with deep frying. None of that here – it just melts quietly. There IS one hazard: paraffin is very flammable, so never do this over a flame – use an electrically-heated pot. I use an electric fondue pot I found at a thrift store for $7. Not sure what to do with the forks now, though.
This was such a great article. I want to try some lost-wax(+sand) casting in concrete. If normal wax fails my project I may try this.
How is the LPDE better than the wax hardener that you can buy anywhere that you can buy hobby paraffin?
And as for the deep fryer – well you will need a thermometer as the hysteresis of the normal el-cheapo temperature control may be too wide – you would have to sit there and manually switch the heater on and off. Don’t try a large electric frying pan as the hysteresis is even wider.
And also +10 for resource recycling!!! This would be invaluable for a class situation.
Sounds like a project – making a PID controller for your Fry Daddy or electric skillet. I KNEW there was a proper application for Arduino here.
That’s actually something I have considered just for the purpose of cooking.
Deep fryers aren’t that bad because the temp sensor is in the oil / wax / whatever so the hysteresis is just the thermostat.
Electric frying pans are a different story as the sensor is far from the heating elements and has very poor thermal contact and consequently, a very high hysteresis which would make it totally unsuitable or even out right dangerous for wax / plastic.
*sigh*
Now I want a cheapo deep fryer! I think I will get one. Melting wax in a boiler-pot sucks.
I keep a mini ABC extinguisher handy for this kind of potentially flammable project. Just throwing some safety out here!
P.S. The Arduino is there to blink the lights! How else would we know the fryer is on? [trollgrin]
Yes, that’s right. Arduino is all about blinkenlites. Advanced users can use 3-color LEDs for continuous color change proportional to temperature.
If you don’t have to exceed 100C (say, just melting wax alone) then use a double boiler for safety
Idea for even more recycling: use LDPE disposable shopping bags instead of LDPE pellets.
That’s how I make it. Cheap, easy to find and it recycles. Saves the some of the wear on the bits and less wear and tear on the CNC while trying things out.
Yep – that’s how I do it as well, and it works.
Getting enough shopping bags to make 15% – 25% of the total mass is sometimes a problem.
Also, while the bags will dissolve, the *lettering* on the bags usually will not, resulting in a ‘kinda cheap looking homemade wax.
I wasn’t aware you could get LDPE pellets on eBay, so now I’m going to try that. It sounds like that will result in a much more even and professional-looking product.
It’s great stuff for trying out toolpaths. You can run very high chip loadings.
I ran this http://www.caswa.com/images/forums/mould_small.jpg at 50KRPM 3mm, 2 flutes, 3mm DOC, 2m/min…
Btw the reason you don’t just use wax hardener is that wax on its own will just melt. The LDPE helps it form chips.
A couple of points:
1) HDPE also works, as does polypropylene (PP). PP is found in a lot of things, like soda pop bottle caps, and in many areas isn’t recycled. PP makes a slightly harder wax. Just make sure it’s really PP – I melted down a bunch of fruit cups because some of them had the PP 5 recycling mark but others didn’t. The ones without the mark turned out to be laminated with another plastic that didn’t dissolve in paraffin, which made a heckova mess. You CAN mix PP and LDPE, which means the LDPE seals in soda bottle caps won’t foul it.
2) Although the orange color is very pretty, if you want a consistent color when using recycled plastics, use a black crayon. Although if you use blue and yellow and green bottle caps you can get a cool olive green – just keep the red ones out.
You can test an unknown plastic by melting a sliver of it with a lighter. If it smells kind of like a candle, it’s PE or PP and will work. If it smells nasty and/or gives off black smoke, it’s something else and won’t work. Do this outside!
I tried this out with a few old candles and plastic bags, melting them in my toaster/reflow oven. Result is pretty nicely machinable, though the vanilla smell is pretty strong :)
Most annoying part is flattening them afterwards, as the wax will shrink when cooling resulting in a concave top surface.
Is there any chance that this machinable wax is food safe?
I would like to cnc mill positives for silicone chocolate molds and then sell the chocolate bars.
Or am I worried that any dangerous stuff might get into the silicon?
If you make it yourself, yes. Polyethylene and polypropylene are definitely food safe – they’re used for food packaging. And paraffin has long been used in canning, to seal jars, and for a waterproof coating on paper cups. If you buy machinable wax, though, you’re buying somebody’s “secret formula”, which could contain anything.