SMOOTHING 3D PRINTS

My interest in 3D printing is deliberately limited (so little time, so many potential areas of interest) to buying in 3D prints that are used as “waxes” for lost wax casting in stainless steel. I use the stainless castings to make a range of handlebar levers etc for vintage and veteran motorcycles.

As bought, the prints often have minor defects and visible “layer lines” all of which can result in a failed or poor quality casting. To get round this I go over the castings with a wax product I get from the foundry. I think this is a locally-made product here in Australia and the foundry uses it to repair blow holes etc in conventionally produced wax masters before casting. This process is slow and time consuming and I want to find an alternative.

What I need is a spray-on wax that gives a thickish build on the 3D print, which I can then smooth off to give a good surface to the print.

My question is – can anyone think of such a product, maybe designed for a totally different purpose that would do this job? Aerosols of car polish wax give too thin a build to be of much use.

Some alternatives that I have already looked at are:

1. Print more finely – raises cost per wax significantly
2. Use acetone vapour to smooth the prints – as a retired chemist I view this method as an easy way to die. Good luck to anyone who boils acetone to use the vapour at home, but I have no wish to feature in the Darwin Awards.
3. Methylene chloride brushed on works but only after a fashion and is quite toxic and probably a cancer causative

Chas

4th option is to print in another material and spend time smoothing and then use that as a master to make a silicon mould to cast waxes in. That way you only do the smoothing once assuming these are not one offs

Hi Chas

The commercial prints for investment casting are typically done using a 0.6 nozzle, I would check with the printers what size nozzle they’re using. As you say the layer height and to a lesser degree the nozzle size will improve print quality at a cost. Acetone works on ABS prints but this is not suitable for investment casting as the ABS expands and cracks the shells. You need to check with the printers they are using the correct printing media.

Prints can be smoothed with steel wool, and you get a special silicon spray used in the investment foundry industry that seals the prints, but it is unlikely you’ll find this anywhere other than a specialised foundry supplier. I have used this on a large investment pattern we had sealing issues with.

I’m sorry to say that with once-offs there is no substitute for meticulous checks and repairs of the patterns. For multiple parts, as Jason mentioned, you can’t go wrong with silicon moulds. I wrote a short article on 3D printing for pattern making (including flexible moulds) in ME that might be of interest to you.

Hope this helps…

Spray wit diluted nail varnish?

Most certainly don’t go boiling 2-propanone in domestic surroundings. - do the job safely elsewhere!

https://www.smooth-on.com/product-line/xtc-3d/

To smooth with acetone you just put a bit of acetone and the part in a sealed container - which could be outside. You don't need to boil.

High build spray on primer/filler

heat gun

resin printer? Better detail. 

Edited By jaCK Hobson on 14/05/2021 08:14:02

Spray putty as used to fill rough surfaces (small imperfections) prior to undercoating when painting a car. Available from auto parts suppliers in aerosol cans. (sold by AutoOne, Repco, etc in Australia). Easy to sand smooth after it has dried but gets harder in time.

Paul

I'm not sure some of these spray on primer fillers will work on the wax prints Chas is talking about, OK for plastic or masters for the silicon moulds.

I agree, the bulking agents for these automotive fillers is talc. It absorbs water during the investment dipping process and creates ash when the shell is fired...

Hi Charles, I think you have misunderstood the technique.

Just get a suitably-sized container. Pour an adequate amount of acetone in the bottom, give it a while to reach equilibrium, then place an ABS print inside on a stand or suspended and fit a lid.

Keep a close eye on it, as it will start smoothing very rapidly.

Your exposure to fumes should be no more than someone painting their nails; bear in mind acetone's toxicity is very low.

Neil

There's a great guide here:

rigid.ink/blogs/news/acetone-vapor-smoothing

Neil

This may not apply since I think you are buying prints and don't have the CAD model, but an alternative approach is to use a resin printer. I run the Gauge 1 3D printing forum and there is a thread there from Markus Neeser which describes the use of special resins to print waxes. Now, the resin is not cheap, and a brake lever might need a midrange printer to fit in, but the quality will be outstanding. Take a look at

https://gaugeone3dcircle.groups.io/g/home/topic/lost_wax_castings_from_3d/71397306?p=,,,20,0,0,0::recentpostdate%2Fsticky,,,20,2,20,71397306

**LINK**

Adrian

Edited By Adrian Johnstone on 14/05/2021 10:47:33

And on smoothing, I certainly agree with Neil's approach: hot acetone apart from being an obvious H&S bomb waiting to go off is far tooo active for useful smoothing. You need it to be at room temperature to give the necessary control. My approach is even more tentative that Neil's: I spot acetone onto kitchen paper that is draped over the walls and bottom of one of those polypropelene boxes that have a seal. I then put the print in and seal the lid. Thus very little in the way of escaped volatiles. After I've finished I dispose of the paper sheets in the brazier..

Adrian

Edited By Adrian Johnstone on 14/05/2021 10:47:59

You could try this stuff

regards

Steve

You can thin beeswax blocks by warming till it melts, then add a little white spirit (for a slow dry) to make a paste. I used it to fairly runny to make a thick mould release for GRP.

If you are using the prints in a lost wax process you also need to test what happens to the filler in the burn out process. Metal casting fillers probably have the emphasis on permanence. I wonder about PVA glue which is sold very diluted as a sanding sealer (big con).

XTC is very good but expensive and goes off very very quickly so you can't hang around applying it.

I have used Araldite or Gorilla twin pack glue cut with meths. It goes runny but coats well and does go off OK.

With either it is much easier to get a good finish if you use the wide V shaped foam brushes.

Amazing! such a quick response and so many very helpful replies

Jason: I do use exactly the technique you describe, except that the foundry I use will not accept silicone moulds. I have to get Aluminium/Epoxy moulds made. I find 3D printing just about indispensible for prototypes before committing myself to the expense of hard moulds. To avoid confusion what I do is hobby stuff to keep me mentally and physically fit at 81 years old. Any negligeable profit is quickly absorbed by new tool purchases!

Luker: Thanks very much for your reply, is Acetone only useful for ABS or does it work on other print materials? I will look into the silicone spray question but my problem is more the quality of the print surface rather than porosity. I would very much like to see a copy of your article but I don't get ME - any chance I could get a copy of the article from you? The tip of using steel wool sounds interesting!

Paul Lousick: I don't think this would work because the fillers in the primer would probably not be carried out completely with the wax when that is melted and lead to incomplete castings.

Neil: You are quite correct, it's all a question of terminology. As a retired chemist, vapour implies concentrated matter to me while what you describe would be fumes. No matter, your terminology is every bit as good as mine I shall study your link in detail as soon as I finish this post!

Adrian Johnstone: Thanks for your link. Again I will study it in detail as soon as I finish this post. I have seen prints from resin printers and they are truely of amazing quality. However the one I know about use "acrylic resins" which may not be suitable as "waxes", although they would be excellent for making silicone moulds. For me I suspect costs would be prohibitive.

Steve F: I shall certainly investigate this

Bazyle: Unfortunately the foundry are no more co-operative than they need to be but I will try to get them to run a few prints coated with PVA just to see what happens. "Just to see what happens" is the curse of the scientific mind. It can lead one into all sorts of trouble DAHIK. The choice of foundries in Australia is very limited.

Chas

Hi Chas,

I honestly don’t know if acetone works on other printer filaments, never tried. In my humble opinion and from my personal experiments fume polishing has less effect on the vertical edges of a print which is where it’s needed most.

The spray is not for porosity it’s to seal the print. If you have hundreds of lines thermally joined there might be places where they don’t join properly. When dipping the pattern the slurry fills these areas causing poor surface quality. If you’re using printed patters for investment moulding the foundry should do a vacuum test. An easy way for you to check is to submerge the pattern in some warm water and check for any bubbles as the air inside the pattern expands.

I think you misunderstand Jason and I RE the flexible (silicone) moulds. The end result is a wax pattern you give to the foundry. A positive is printed using 3D printing (or a sample), the silicone is poured around this (a negative mould), and finally you can pour wax into this mould. I unfortunately don’t have pictures of this exactly but I have used this process to make badges for one of my vintages, the underlining concept should be clear from the pics… Incidentally you don’t need to buy potting silicone, calking silicone with corn starch mixed in and a little terps as thinning agent will also work if you want to experiment a little. Tricks for mixing and releasing agents are in the article…

Chas what you say is a little confusing. In the opening post you say you are having waxes printed and want to improve the WAX surface. Yet in the second reply you say you are taking hard moulds off of presumably solid prints and then casting waxes from those. The two surface treatments will differ depending on if it it wax or a plastic you are wanting to improve the surface of.

Also how do your foundry handle the waxes? if they are being made into a "tree" with wax runners than any non wax surface treatment may affect how they are melted onto the tree.

The other advantage of casting the waxes in a silicon mould is you can pull complex shapes from the silicon that would otherwise need draft angles etc on a solid mould.

My mistake Chas with suggesting spray putty. I missed the point that you were doing lost wax casting and was thinking solid pattern.