Printing or Casting for 100 copies? Material price looks the same

This is a little off-topic, but I’ve seen informed opinions on here about 3D prints and mold making so I thought I would try asking!

I’m printing these little drawer organizers (~50 mm cubes), and realized it would take 3.5 kg of filament to print enough for a full drawer. Cost (CDN) = 3.5 rolls * \$25/roll = \$105 for 3D printing . That was a little steep for a fun side project, so I looked into alternatives.

I’ve seen videos and articles about molding/casting in polyurethane, and I assumed that was for both speed of production and lower price. However, when I sketched out the costs, it was almost identical for the polyurethane option.

Printed volume = 2.8 liters, and cost/liter of polyurethane \$36:

Cost (CDN) 2.8 * \$36 = \$100.80 for polyurethane molding (or more, due to losses during mixing and pouring)

So the costs between 3D printing and polyurethane casting look almost identical. Am I missing something?

(If the costs are the same, I’d rather just run the printer for a week almost no manual intervention, instead of making a mess with two-part mixes and molds!)

That sounds about right, cost-wise. Molding only wins when the quantities get really large.

Are you able to optimize that print at all to save some weight? 35g for a 50mm cube seems a little bulky. I can slice a 3 wall open top box and PrusaSlicer estimates only 20g of filament (16.5c^3).

Thanks: glad I wasn’t missing obvious in the calculations! So the volume discount for urethanes must kick in when you get past the 5 gallon sizes and into the barrels?

For the volume/mass of the product: good eye! The calculations and the photo were from two different models, for different drawer depths.

• The photo was a 50 mm cube because it was on my desk, and you’re right: it comes in on the slicer at 19 g.
• The 35 g in the calculations came from actually weighing a similar design, but for a deeper (90 mm tall) drawer.

I’ve never seen casting being cheaper than printing (but I haven’t looked). I know injection molding is cheaper.

There is also the wear and cost of the printer too though.

But at some point, you should look at the fact that Ryan and even Prusa are printing their parts, even at much higher volumes than we are. So the economics must be hard to get around.

But at some point, you should look at the fact that Ryan and even Prusa are printing their parts, even at much higher volumes than we are. So the economics must be hard to get around.

Excellent point! I was thinking about Prusa’s model as a comparison, but figured they had different needs: allowing incremental design changes, plus a very long unique parts list, which would be reasons to avoid molds and casting. I like your economics argument though: if Prusa can still be competitive on final product price while using 3D printing instead of casting, at their high volume of production, then it points to there being only small price advantages to any comparable manufacturing alternatives.

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Depending on the complexity of the mold, your first part could cost you about \$10,000+!

It looks like you have overhangs that would not mold nicely, at least in a rigid 2-part mold. Is this critical to the design or can it be modified to separate cleanly into two halves (with draft).

Polymer concrete might be similar (or the same) as urethane with a cheap volumetric filler like sand. I’m not sure how much filler you could use but I would think more than 50% would be possible. It might be a lot of hassle to mix in small batches and/or have a large number of molds, so you wouldn’t get high efficiency unless you are making a ton of them. Maybe you can clone some molds by 3D printing the positive (part) shape and casting negative (mold) shapes out of something cheap, and then after you have a good collection of molds, use the molds in turn to cast the boxes.

But the 1Mth part is basically free

You’ve highlighted the exact issue for me with all the complex mold processes! Aside from the lag time in printing, it’s so much easier to just press ‘print’ on the Prusa a couple of times a day for a week and walk away, compared to all the manual futzing that would be required to make clean molds and then use them.

The only reason spending all the labour in using molds would make sense is if (a) molding material costs were significantly cheaper, but that doesn’t seem to be the case, or (b) I needed production speed that couldn’t be achieved with a single 3D printer.