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Can we melt PLA pellets to use it for molding? Maybe microwave it?

Posted by realthor 
Can we melt PLA pellets to use it for molding? Maybe microwave it?
February 18, 2016 08:59AM
I've looked in the Fabrication section but that looks deserted so I'll post this here. I am thinking about mold making with the use of a 3D printer and if we're doing that why not cast in PLA? We can use ABS molds or Nylon to replicate our parts if we need many of the same one.

this way we could use PLA pellets instead of filament, which would be much cheaper overall.

I don't know though if we can safely melt PLA pellets before they burn, say in a microwave, and then pour it in a mold. I don't know what viscosity we can achieve and how "pourable" can we get it and how much time do we have to finish pouring.

Any experience or ideas?


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Re: Can we melt PLA pellets to use it for molding? Maybe microwave it?
February 18, 2016 09:20AM
Ive seen people use a normal Extruder/nozzle to fill a mold with PLA. Basicly they make their mold with a hole that fits the nozzle, another hole for air, and they just secure or hold the mold to the nozzle and extrude until full. I don't see why you couldn't rig something up that heats and uses pellets. Though the microwave idea seems unrealistic.
Re: Can we melt PLA pellets to use it for molding? Maybe microwave it?
February 18, 2016 09:44AM
Quote
McSquid
Though the microwave idea seems unrealistic.

Why? They melted aluminum in microwaves and others have been firing ceramic clay in microwave ovens so why not melting plastic?


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Re: Can we melt PLA pellets to use it for molding? Maybe microwave it?
February 18, 2016 09:54AM
I'm far from an expert but I just see many issues getting in the way. Uneven heating/cooling, Pouring not creating enough pressure (without a vacuum to fill out your mold) etc. Don't let my fairly uninformed skepticism stop you from experimenting!
Re: Can we melt PLA pellets to use it for molding? Maybe microwave it?
February 18, 2016 10:50AM
Quote
McSquid
I'm far from an expert but I just see many issues getting in the way. Uneven heating/cooling, Pouring not creating enough pressure (without a vacuum to fill out your mold) etc. Don't let my fairly uninformed skepticism stop you from experimenting!

Oh no experimentation until I learn more about it. This is so far just an idea that popped up into my head while researching epoxies for the same purpose. But I don't want to use epoxies. And I realized, hey we already have PLA that softens at 60C and melts at 210-250C ... wouldn't it be cool to buy in bags in pellet form and use it for molds? ...

I don't think pouring melted plastic into molds creates poorer adhesion than extruding it via a nozzle. The problems I see now are getting to viscosity necessary for pouring, keeping the recipient (most likely a pyrex cup) hot long enough to finish what's inside and researching about harmful fumes PLA might outgas while doing it.

The 3d printing toxicity studies show PLA as the least harmful so maybe a Pyrex glass with a lid would be possible to use in the same microwave you are heating your food in. Otherwise a different one placed in a well ventilated area, a fume-retaining mask and a pyrex glass should be everything needed.

I am talking about simple molds here not complex ones that need vacuum bags.


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Re: Can we melt PLA pellets to use it for molding? Maybe microwave it?
February 18, 2016 11:28AM
If molding plastic were as simple as you seem to think it is, of what use is the decades old, proven process of injection molding for? Sometimes (most times) people in the past aren't stupid morons who do things because they like more work for less return. Might be worth reading and trying to understand what and why they did what they did before perpetually inventing "problems" to find "solutions" for.
What you propose is dangerous and foolish (pouring molten thermoplastic into a thermoplastic mold). Please don't do this or encourage others to do so.
Re: Can we melt PLA pellets to use it for molding? Maybe microwave it?
February 18, 2016 11:34AM
youtube is choke full of videos about molding fishing baits out of some sort of plastic. I am not encouraging anyone to do anything and I am not dismissing injection molding, where do you get that fromsmiling smiley

If you don't agree with the discussion you shouldn't make part of it, this isn't about injection molding vs diy mold making. And injection molding is dangerous too if you want to be on that side... should we stop doing anything?

Your only argument that gets me thinking is the pouring thermoplastic in thermoplastic ... I see where the foolishness is in that one. Maybe use PLA print to make a silicone mold to pour molten PLA into. Or plaster instead of silicone.

BTW: I was thinking at an application where I would need to pour molten plastic to a an already created plastic part that is watertight and that will end up fusing with the molten volume. In that regard that isn't foolishsmiling smiley ... maybe dangerous? why? the temperatures? the fumes? some arguments please if you insist...

Edited 2 time(s). Last edit at 02/18/2016 11:51AM by realthor.


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Re: Can we melt PLA pellets to use it for molding? Maybe microwave it?
February 18, 2016 11:54AM
Again, my suggestion is that you research injection molding techniques and methods as there is a wealth of knowledge built over decades. Or you could just watch YouTube and dismiss what I said. Terribly sorry to spoil your thread with actual experience in doing what you are talking about, I won't chime in any further in your threads.
Re: Can we melt PLA pellets to use it for molding? Maybe microwave it?
February 18, 2016 11:59AM
If that's you're field than you haven't been very helpful by chiming in.

If we all could just study everything then we wouldn't need to ask anybody else. I am asking exactly to skip doing another degree in thermoplastics by getting suggestions, opinions and constructive discussion about this. I could advise many people to just go and study whatever they are asking me but that's just playing smart.

Edited 1 time(s). Last edit at 02/18/2016 02:26PM by realthor.


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Re: Can we melt PLA pellets to use it for molding? Maybe microwave it?
February 18, 2016 01:10PM
Pretty sure you'll have a hard time reaching temps where PLA has a low enough viscosity to pour like a liquid.


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Re: Can we melt PLA pellets to use it for molding? Maybe microwave it?
February 18, 2016 08:41PM
My intention is not to have it as a liquid that is easily pourable. A paste/plaster-like viscosity would be enough to begin with.

I am reading some scientific literature on PLA and a few things are very interesting:

Quote
http://naldc.nal.usda.gov/download/4048/PDF
- "PLA undergoes thermal degradation at temperatures above 200C (392F) by hydrolysis, lactide reformation, oxidative main chain scission, and inter- or intramolecular transesterification reactions. PLA degradation is dependent on time, temperature, low-molecular-weight impurities, and catalyst concentration."

Why are we then using 200-230C for extruding PLA? At least this is the temp the Ultimaker in my local maker space uses.

Quote
http://naldc.nal.usda.gov/download/4048/PDF
- "Poly(lactic acid) homopolymers have a glass-transition and melt temperature of about 55C and 175C, respectively. They require processing temperatures in excess of 185–190C. At these temperatures, unzipping and chain scission reactions leading to loss of molecular weight, as well as thermal degradations, are known to occur. Consequently, PLA homopolymers have a very narrow processing window."

" Pure poly(D-Lactide) or poly(L-Lactide) has an equilibrium crystalline melting point of 207C but typical melting points are in the 170C–180C range. This is due to small and imperfect crystallites, slight racemization, and impurities. It has also been observed that a 1:1 mixture of pure poly(L-lactide) with pure poly(D-lactide) will yield an insoluble gel formeby the stereocomplexation (racemic crystallite) of the two polymers during crystallization or polymerization. This pure stereocomplex has a melting point of 230C and mechanical properties greater than either pure polymer"

I might have answered my own question here, but not sure what kind of PLA are we using in 3D printers.

Quote
http://naldc.nal.usda.gov/download/4048/PDF
- " Poly(lactic acid) quickly loses its thermal stability when heated above its melting point. A significant level of molecular degradation occurred when PLA was held 10C above its melting point (160C) for a sustained period of time." ... here the "sustained" periods were in the range of 30 minutes, which is much more than I expect for a quick processing.

Quote
http://naldc.nal.usda.gov/download/4048/PDF
-"However, brittleness and other properties such as low viscosity, low thermal stability, high moisture sensitivity, medium gas barrier properties, high cost (comparing with PE, PP, PS…) and low solvents resistance (e.g., against water) of the pure polymer are often insufficient for a lot of applications."

The keyword here is low viscosity, which indeed can be a deal breaker for using PLA for any casting. However, the same study compares it to LDPE and they conclude that: "The true viscosity of PLA decreases sharply with increasing temperature, whereas the true viscosity of LDPE varies slightly only with temperature. Also it was found that the true viscosity of the blends decreases with increasing PLA content which was attributed to PLA low viscosity."

Quote
http://www.wseas.us/e-library/conferences/2011/Corfu/GEMESED/GEMESED-70.pdf
- "The experiments were performed at two different temperatures 170, and 190C keeping constant along the reservoir and capillary zone. [...] The resin melting time of three minutes was kept in order to prevent polymer degradation from long residue time. "

Quote
http://www.wseas.us/e-library/conferences/2011/Corfu/GEMESED/GEMESED-70.pdf
"[...] low data set at temperature of 190C has significantly lower values of viscosity in the whole range of shear rate compared to the flow behaviour of tested PLA at 170C. Furthermore, shear viscosity of tested PLA decline with increasing shear rate for the each mean pressure indicating a preservation of shear thinning behaviour typical for a common thermoplastic polymer. Thus it can be concluded that the pressure increase in the PLA melt did not influence the shear thinning behaviour while it effects only on the magnitude of viscosity. "

I don't quite get this :-/. What i understand is that temp is much more imp than pressure for PLA viscosity. This is somehow confirmed by their conclusion: "[...] it can be concluded that tested PLA
has relatively high sensitivity to both pressure and, especially, temperature."

Quote
http://www.plastemart.com/upload/Literature/ProcessingofPLA%28biopolymer%29.asp
- "PLA picks up ambient moisture very rapidly and most of the problems experienced on PLA arise from inadequate drying. Hence, PLA is required to be properly dried before processing."
- "PLA viscosity is also very sensitive to temperature and moisture, so PLA web tends to stretch and sag unevenly. PLA also shrinks 9% in the transition from melt to solid, going from a melt density of 1.13 g/cc to a solid density of 1.24 g/cc."

So in my situation PLA has to be brought to extruder (melting) temperatures for it to adhere to another part, like I need to but for self-contained part that is press-molded it could be doable. Definitely doable as I am researching this field. Key would be to find the lowest temperature that this can be done with least pressure.

Anyway my head spins from reading such scientific lingo smiling smiley so I'll pause here, hoping that I've selected easily readable fragments from all that data. Hopefully someone who has some experience or better understands the scientific literature will contribute to the thread.

Edited 3 time(s). Last edit at 02/18/2016 08:48PM by realthor.


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Re: Can we melt PLA pellets to use it for molding? Maybe microwave it?
February 19, 2016 01:05AM
One possible issue is that is may not heat up at all. Microwaves heat food by vibrating water molecules so fast they create friction and thus heat, not sure if PLA would have enough moisture in it to heat up.

I know I have had microwave safe plastics before that do not even get warm in the microwave. Just a thought.
Re: Can we melt PLA pellets to use it for molding? Maybe microwave it?
February 19, 2016 02:10AM
Interesting... I've had some success with modifying failed PLA prints with the help of boiling water... pour on boiling water and you can adjust the size or shape of a hole in a print. So 100C might be good enough to make an extrudable paste or plaster.
Re: Can we melt PLA pellets to use it for molding? Maybe microwave it?
February 19, 2016 03:34AM
I suspect you would need something like these microwave kilns, which is basicly metal inside a ceramic holder. The metal gets very hot and heats anything else around it...

I’m not sure how you would keep it cool enough not to destroy the PLA though, maybe a much smaller metal area in the ceramic bowl, maybe a mesh pattern, for even heating... and or a temp control circuit, that turns off the magnetron
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Re: Can we melt PLA pellets to use it for molding? Maybe microwave it?
February 19, 2016 04:10AM
Orbi-tech is doing this sort of injection-moulding with a 'hand-gun' and up to 1000bar pressure ... but only with small moulds, made from metal:

[www.orbi-tech.de]


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Re: Can we melt PLA pellets to use it for molding? Maybe microwave it?
February 19, 2016 06:00AM
Indeed PLA would have to be melted inside a sort of kiln or coated with a susceptor that heats at contact with the magnetron waves or even mixed with filler material that can absorb microwaves..

It is though difficult to control the temperature in a microwave because one would have to maintain say 190 deg and make sure it's consistent throughout the PLA mass inside. Is it even possible to maintain the temperature in a microwave? My kitchen microwave doesn't have a temperature setting, not temp control, just time and wattage. I know of no microwave (at least the ones that are consumer-grade) to have temp control as it would involve a feedback mechanism to be interfaced with the on-off switch of the magnetron. PLA needs a very narrow temp band in which it doesn't loose properties and that temp limits should somehow be controlled. Microwaves have many advantages and I have found literature that they are researched for forming thermoplastics:

There's an interesting paper on the use of fillers to enable the microwave processing of polyethylene.

But having to create a control mechanism for a microwave oven is something out of scope for the most of us. I haven't found any decent article on this online. There are some research papers that I need some sort of subscription to access so I will conclude that microwaving with the existing equipment is not doable in a controlled manner.

I'll fall back to less heat-intensive methods for softening PLA to a formable goo.
The boiling water is interesting but isn't PLA absorbing moisture if kept in water? Or is this contact too brief in this case that it doesn't affect the PLA in a significant manner?
I am curious about the malleability of a 100deg PLA. It should be ok for press molds like tose used in ceramic/clay molding.

Edit: forgot about the orbi-tech gun. It's interesting for people doing it for small batches of commercial stuff but for the home hobbyist it might not justify the investment. I see it uses metal moulds so all in all it's still too expensive for home use. It reminds me of the manymaker i've seen lately in news.

Edited 3 time(s). Last edit at 02/19/2016 08:17AM by realthor.


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Re: Can we melt PLA pellets to use it for molding? Maybe microwave it?
February 19, 2016 08:15AM
imho pouring plastic into a mold is not an issue, your selection of the plastic to pour is the problem grinning smiley Why do you want to use PLA and not another plastic? There are multiple plastics available that are beter for this.

Never tried it but maybe just dissolving ABS in aceton and then pour it into a mold, slightly heat it to force evaporate the acetone faster then normal might work, you would need to ventilate that room cool smiley and be ridiculously careful with any heat source in that vicinity

Edited 1 time(s). Last edit at 02/19/2016 08:16AM by [email protected]
Re: Can we melt PLA pellets to use it for molding? Maybe microwave it?
February 19, 2016 08:24AM
I am holding on PLA for its low toxicity level, bio-based source and low temp melting point. Enough to try to dig some unexplored path for mold pressing.
I can always go for bi-component resins and the such but I wanted something rather simple (which it didn't prove to be smiling smiley ).

Strange things one can find when researching stuff that he doesn't know much about... These guys use a deep fryer to melt store plastig bags into lumps/ingots of plastic.

360F is 182C which would be perfect for PLA, considering that PLA doesn't like oil so it will not combine with it. Those are kinda high temps to work with with plastic and oil so I am quite doubtful about the validity of this method. But I can't see a better use for a deep fryer tongue sticking out smiley

Edited 2 time(s). Last edit at 02/19/2016 08:36AM by realthor.


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Re: Can we melt PLA pellets to use it for molding? Maybe microwave it?
February 19, 2016 09:00AM
What is wrong with normal electricity? AFAIK magnetrons aren't very efficient. Normal screw extruder could make the filament quite nicely.
Re: Can we melt PLA pellets to use it for molding? Maybe microwave it?
February 19, 2016 09:46AM
Are you suggesting building a whole filament extruder? Because that is something I was trying to avoid by going the "melt-in-a-pot" path. I am not trying to make filament but have a PLA mass that is hot enough to be able to take a mold-press treatment and conform to that shape. Now because of viscosity issues specific to PLA I can't expect intricate molds to work but simple 2 side molds that one presses together can to the trick.

What you suggest is very close to injection molding, where a specific plastic mass is heated to injection-ready temperatures and then pushed with an auger-type screw into the mold. That uses normal electricity to heat the plastic.


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Re: Can we melt PLA pellets to use it for molding? Maybe microwave it?
February 19, 2016 10:13AM
I'm no way specialist in plastics. I have minor in paper converting and I thought about the PE coating extruder which is heated screw and its mechanics are rather simple. It is basically screwing in heated barrel with some nozzle.

But yes. Injection molding would be possible with screw extruder approach. And I thought about the bowl also. Would gravity be enough if you opened valve from bottom of it?

Totally wild idea: Could there be so short valve that it could fit in bottom of frying pan and stove?

To me there are two problems. Control the temperature of molten plastic and getting it to flow right way.

One interesting way to heat the plastic would have some steel part coated with PTFE and then heat it inductively. Pot could be then some other material and the melting would happen only in near use point.
Re: Can we melt PLA pellets to use it for molding? Maybe microwave it?
February 19, 2016 10:40AM
I am skeptic that the frying pot can actually do more than to make a goo out of PLA, which can suffice for mold-press applications. I wouldn't use it to extrude filament. The induction heating would do a better job there but I am not going the filament extruding route. We have plenty of filament options ready-made nowadays and those care much better than anything we can do DIY-style.

I need something to fill the gap between 3d printing (slow) and injection molding (specialty equipment needed). Something I've seen in clay/ceramics, which is mold-pressing. I need the consistency of a putty or slightly thinner. I want to use it as a filler material in PLA parts and also in mold-pressing of decorative items. I want it to be PLA for the reasons I mentioned before (compostable, less toxic, low temp). Otherwise I'd go for other types of resins.


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Re: Can we melt PLA pellets to use it for molding? Maybe microwave it?
February 20, 2016 02:59AM
Have you had a look at Polycaprolactone (PCL)? Its listed as being biodegradable, non-toxic and melting temp of 60 deg C.

I remember it from a few years ago and was called "Polymorph", comes in little pellets and just put them into boiling water. Not sure how well it would go in a detailed molds, shrinkages etc. There is a video on youtube of someone using it in a large mold.
Re: Can we melt PLA pellets to use it for molding? Maybe microwave it?
February 20, 2016 07:15PM
That is actually a very cool find, thanks for the suggestion. I have researched a bit on the thing and watched several videos about it.

On one hand, I wonder why we can't find filament made of this thing but even as is, it can have so many applications. I will definitely order some to test.

On the other hand, the way it turns to a malleable mass after dipping it in boiled water is exactly what I was thinking about when I was playing with the idea of using PLA. As users of 3d printers, we get a lot of failed ones, either because of our printers or because of our design iterations and it would have been cool to be able to reuse the failed parts once the final prototype is done and actually re-embed them into a usable product. So for that reason the deep fryer method could work for PLA. Why throw something that has a lot of embodied energy when one can reuse it.

Well @sdavi you might have just offered a good alternative. I thank you again for that.


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Re: Can we melt PLA pellets to use it for molding? Maybe microwave it?
February 21, 2016 11:55AM
I think logically you would have to make the mold out of something other than PLA. So make it out of ABS which has a higher melting point. Then u can use a 3D pen to use and "inject" plastic into the mold. This might work.
Re: Can we melt PLA pellets to use it for molding? Maybe microwave it?
February 21, 2016 12:15PM
With PCL "the specific heat and conductivity are low enough that it is not hard to handle by hand at this temperature." -as Wikipedia has it- so I believe that PLA will easily take the contact at this temperature without deforming. I find PLA pretty stubborn to even take a metal insert that I kept on the electric stove for a while.

With PLA on PLA it's a different story and you might be right about having ABS or PC/nylon molds.


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Re: Can we melt PLA pellets to use it for molding? Maybe microwave it?
February 21, 2016 12:31PM
Why not print your part in PLA or ABS, make a silicone mold, or print the mold in nylon and cast using polyester resin? The viscosity is very low so it will fill small voids, and you can always use a vibrator to ensure minimal air bubbling. I think polyester will withstand higher temperatures than PLA. You may have to scale the original part to compensate for shrinkage as the polyester sets.


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Re: Can we melt PLA pellets to use it for molding? Maybe microwave it?
February 21, 2016 01:11PM
The idea was to use low-toxicity/compostable materials and also to use the cheaper pellet form of PLA/PCL.

I am ok with making nylon molds or even cement ones smiling smiley but would like to see the failed prints be able to be reused in some form before throwing them away and also to use the pellets instead of the filament for faster production times.

There are lots of ways to achieve parts using molds, I am after a particular set of requirements, which I have explained in more than one post.

Edit: I have mixed feelings about silicone molds because I basically need to turn a malleable mass of goo into a pressed part so I think the mold itself has to be quite rigid to be able to take the pressing force.

Edited 1 time(s). Last edit at 02/21/2016 01:26PM by realthor.


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