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I want to make Polyurethane molds for concrete using 3D printed PLA or ABS master object. like this video:

(this video is not about concrete of course!)

I'm not sure if it will stick to PLA or ABS master or not! if it does stick, whick wax material can solve this problem... Do I need to print my masters with another filament?

enter image description here

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  • $\begingroup$ The process is the same for concrete molds just need a sealer then an agent release. youtube.com/watch?v=LZbikO7HUrE here is a video for concrete process. $\endgroup$
    – Fernando Baltazar
    Oct 9, 2019 at 19:28
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    $\begingroup$ I'm concerned about the interaction between Polyurethane and PLA! $\endgroup$
    – 2012User
    Oct 9, 2019 at 19:39
  • $\begingroup$ Check the curing temperature of Polyurethane if the temperature is lower than the softeting PLA temperature won't be problems. softening temperature is between 10 and 15 lower than melting temperature. However I´ve seen other creaing molds from PLA, just google a little more to dissolve your concerns. $\endgroup$
    – Fernando Baltazar
    Oct 9, 2019 at 19:47
  • $\begingroup$ I was told that this type of polyurethane for making molds, needs a hardener and no melting is involved in the process. $\endgroup$
    – 2012User
    Oct 9, 2019 at 20:09
  • $\begingroup$ @FernandoBaltazar PLA starts to soften and deform as low as 60 °C $\endgroup$
    – Trish
    Oct 10, 2019 at 18:40

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In my experience, polyurethane sticks to PLA like super glue, not good. But silicone and alginate doesn't stick at all.

What I do is print the model of the mold with PLA or ABS, no matter. Then, cast a mold of the PLA model of the mold with alginate, then you have the negative of your mold.

Now with this alginate mold of the mold cast your actual mold with silicone. And then you can cast your part on polyurethane in the silicone mold.

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Temperature

As polyurethane cures (or hardens), it undergoes a chemical bonding reaction, linking the mono- and oligomer strings in the components into long polyurethane chains. The chemical reaction is exothermic, it creates heat.

So, we have a process that heats up the polyurethane mixture as it hardens, but how much? Well, it's hard to find numbers for it, but I suspect it can easily reach 30 to 40 °C, depending on the mixture (fast curing) it could easily go higher. To combat the effects of heat softening of the PLA/ABS model inside the mold, I strongly suggest printing with extra shells and extra infill. While most items can get away with 10 %, in this case, I suggest 20-30 %. ABS would be the superior choice above PLA as it starts to deform at a higher temperature. PLA can start to deform at around 60 °C, ABS only at about 80 °C.

The temperature of the PU curing depends on the speed of the curing process - it is safer for the masters to choose a slower curing mix as the heat is generated over a longer time and the maximum temperature is thus lower as a result (as excess heat is lost to the room)

Surface

To reduce the sticking to the surface from the material creeping into the gaps of the model, it has to be as smooth as possible and best also sealed. If you choose ABS, a quick acetone vapor bath would do the trick in this case. PLA should be lacquer sealed as it doesn't like to stick to most waxes.

Adding a mold release agent isn't necessarily needed, but could help in removing the masters from the mold.

Conclusion

ABS might be the better choice in this application. It is advisable to use extra-thick walls (3+), a lot of infill (20-30 %) and a vapor smoothed surface.

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