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Only one way to find out re: pressure – use a gauge. It also very much depends on which plastic – hdpe is like chewing gum but polypropylene is fairly runny.
Plastic has a fairly low thermal conductivity so freezing is not a real concern in practical purposes.
As a matter of fact, cooling rapidly results in less crystallization and therefore a tougher/less brittle result. The goal should be to have rapid cooling while minimizing warpage – a low pressure clamp or other mechanism, as you suggested, would work
Sort to see this! I don’t have any ideas for te mould, but have you guys considered adding a pressure gauge somewhere in the system? Not only would it let you know if there’s adequate pressure for proper pressing and cutting, but it would actually let you know how much pressure is necessary for different plastics and thicknesses, and therefore to design a press that is perhaps lighter weight, thus cheaper to make and easier to transport. I’ve always suspected this press is significantly over built.
Now that I think of it, perhaps the gauge on your air compressor would suffice? Or does that have little relation to the tonnage imposed by the jacks? Probably. Its possible to tap a gauge into jacks – I’m sure you guys have the tools and skills to do so!
Nice results and info! I have a few questions though.
What temperatures and times did you use for those sheets?
I notice that you’ve made a few different sized sheets in the hot press. Are you using the same mould design as Mark’s cold press or something else?
Also, the sheets are sometimes larger than the cold press, yet it only has one hydraulic jack, which presumably has considerably less force than the 4 jacks on the cold press. What is the force of this jack and do you find that it is sufficient? Does it make completely flat sheets or is there some deflection given the central location of the jack. If the sheets are satisfactory, then perhaps the cold press could be made considerably smaller/lighter weight/lower cost – my suspicion has always been that it’s significantly over-built and most of the force just rests on the steel depth spacers rather than on the plastic.
Finally, do you have a build thread for the hot press somewhere?
I found a slew of articles/forums about building a press like this for making skis and snowboards, which is a much more difficult process, given the shape etc… They are heated presses and seem to use more pressure too.
One example: http://www.happymonkeysnowboards.com/MonkeyWiki/index.php?title=Monkey_Press_Construction
You’ve definitely got me thinking a bunch! Thanks again!
If I understand correctly, that is an extremely good and creative idea with effectively creating a large air bag/pneumatic press… As you say, it would result in an even distribution of pressure which would surely significantly lessen the required frame rigidity and thus weight and cost.
Or would it? You’d still more or less have a box frame to hold the panel/hose in place, and you’d have the same pressure requirements – the only difference would be that the pressure would be evenly distributed along the beam vs concentrated at the hydraulic jacks. Both would span the same distance and be secured at the ends in the same manner. I’ll have to look into beam deflection calculations for uniform vs point distribution of force…
I know nothing about compressors, so how would you size the compressor? Surely the total surface area/volume matters in some way…? What would differ between a 20cm x 20cm and a 2m x 2m press? With the same pressure requirements for the plastic, is it just a difference in the time required to fill it up?
I suppose that’s the case – pressure is pressure .. It’s easy to pump a road bike tire to 120psi with a floor pump, but getting a truck tire to 40psi with the same pump takes a long time (ive done it out of curiosity despite having a small compressor…)
On that note – and I’m just brainstorming – could you perhaps use a few truck tire inner tubes all connected to the same compressor? Maybe its necessary to be inside of a tire to keep it from exploding?
Any thoughts you have would be appreciated. And if you can find a link to the video you mentioned, that would be great as well.
@btmetz, this is how I’ve been experimenting and it works just fine. However, these sheets are far too small for what they’re trying to achieve. Also, the teflon will eventually wear off, so something else is necessary. Vaseline doesn’t really work well. Some form of silicone is probably the way to go.
Probably depends on a lot of factors – type of plastic, dimensions and shape of mould, etc…
I know for HDPE and PP, and probably others, the faster you cool, the less crystallization that occurs. This results in slightly weaker, but less brittle plastic.
If cooling is uneven, you can end up with warpage. Even if the mould remains in pressure to keep shape, the side that cools slower will be more crystalline than the other side, which can result in different internal stresses.
Only one way to really find out – just start tinkering.
Have you guys tested the strength properties of these yet? I’ve been working on my own sheet press and can make some very attractive, uniform, void-less HDPE and PP sheets, but they tend to be brittle. They have a lot of strength for bearing weight, but sharp impact with a hammer shatters them like glass – especially the PP. My most recent HDPE was pretty tough though.
According to academic literature, cooling speed seems to be the biggest factor – the faster you cool them, the less crystallization that happens, resulting in lower strength, but higher toughness.
I’ll keep experimenting with it all and eventually start my own thread.
I think the assumption that it has to be slow is somewhat faulty. If you browse the websites of industrial recycling equipment suppliers, they have a variety of models.
Some are slow, high torque “shredders”, others are higher speed grinders or granulators. The tend to be used in series – with the slow, high torque as an initial size reduction, then the high speed ones doing the final granulation.
Since we are unlikely to have massive chunks of trash to pre-crush before final size reduction, the high speed ones are more likely to work better. IN that regard, surely the plentiful agricultural shredders available cheaply on the market are sufficient.
I did way more research months ago, and cant find where I stored all the links, but here’s a couple to check out.
I’ve seen people successfully do this in videos (I don’t think I saved them anywhere), so it probably depends on the exact sort of machine you’ve got as well as the plastic being used. If you go down this route, let us know!
I’ve thought about this and plan to build a large gas-fired oven for my compression moulding project. You can get some inspiration here: https://www.powdercoatingforum.net/forum/ovens-and-booths/2877-my-propane-fired-oven-build
I’ll be making a lot of changes to it, and will eventually want to heat it with a rocket stove.
The suggestion by @brettc for using oil would work as well, and is probably better suited to injection and extrusion, where you could run pipes of hot oil along the cylinder. Obviously some oils would be more appropriate than others – you’d have to find ones with high smoke point, high heat transfer coefficient, etc…
@bluesbrother, yeah its a shame that they and other innvoative companies are closed. But I, and other people, are working on similar things that are intended to end up open-sourced. Funny enough, I expect this sort of collaborative, generous, altruistic model would end up putting any for-profit out of business.
@btmetz have you had any more experience/success with making fiberglass molds? I just thought of using it to make large rectangular sheet molds that I’d then press afterwards. Seems like it would be a lot cheaper than sheet metal. Not sure about temperature resistance, brittleness etc…
Very interesting. I haven’t made anything quite yet, but am planning to try to build houses out of recycled plastic here in Guatemala. There’s plastic everywhere and it can be bought pre-sorted by type fairly cheaply in any quantity at the “recicladoras”, but I don’t have any high volume shredding capability as of yet. So, I can buy it well shredded for about 3-4x the price, and a bit more for washed (I’m skeptical of how clean it will be given the marginal price increase – I’ll have to test it when I buy some soon).
Anyway, if I can build things of reasonable quality with dirty plastic, it would lower the cost. And would also make it more feasible to source raw plastic if all that’s required is to shred and melt it – i just assumed that it wouldn’t work very well if dirty. I suppose truly dirty stuff – e.g. motor oil – could be pre-screened, and only use plastic that is simply dirty or has other residues and adhesives on it. After all, most of what I’ll make won’t really be “structural” so much as just sheathing or furniture. The important pieces could be made with more “pure” plastic.
Plenty to keep experimenting with! I should be starting in the next couple weeks once another project comes to a close.
I look forward to following your progress!
This is wonderful, however I’m having trouble interpreting the results.
It seems that once the temperature is sufficiently above the melting temp, the inside is too runny and oozes out when laid on its side, rather than keeping its shape.
I suppose the goal, in order to conserve energy (and $$), is to use a temperature that is minimally sufficient to melt the plastic completely. However, especially in compression molding of larger parts, air pockets are a risk.
So, my question is: is there any problem with melting plastic above its melting temperature in order to make it less viscous, so long as you don’t burn the plastic?
To be specific, HDPE seems to form a smooth block around 150. Is there any harm in going to, say, 170?
A side note: a document in the Precious Plastic Kit lists HDPE’s melting temp from 130-137, and PP at 168-175. But those temperatures seem too low when looking at these visuals – again, HDPE seems better around 150 and PP at 190.
Given this, I suppose the best practice is to just use this as a guideline and then experiment with each plastic that we have to find what works for our process?
@pauldufour Fantastic work! I’m also starting to think and tinker with mixed, dirty plastics as it is much harder for me to find sufficient clean plastic that is affordable where I am.
“I’m curious to learn how much of the material’s binding success is due to melted plastic, how much is due to pressure, and how much is due to the glues mentioned above”
Have you been able to draw any (even speculative) conclusions about strength/quality vs pressure, adhesives, and melted plastic “glue”?
I’m very much looking forward to keeping up with your progress!
@andyn – would wiring in series halve the power for each one in series? Or, more accurately, 1/n ? If so, perhaps wiring in series in pairs, or whatever combination is necessary to account for the heat differential from center to edge to corner.
Aluminum plate would work well too, but likely expensive and perhaps require redesign of the press
@flo-2 another question – how long does it take to melt a sheet? Also, do you leave it to cool in the press? If so, I wonder if it would make sense to make a removable mould that, once pressed, could be secured down and removed to cool elsewhere. If on a flat surface, I don’t see how it would deform – it would simply shrink relatively predictably (which could be accounted for in advance). This would surely increase output volume tremendously.
What about adding some sort of resistors or other device into the circuits for the central heaters or just make the outer ones more powerful to compensate? Or wrapping the outside edge of the frame with some sort of heat tape/heating wire to give extra heating specifically to the edges?
Or, depending on temperature variation, you could have the probe on an edge which would allow the edges to get hotter but risk overheating the center.
As for extra PIDs, you could presumably have just 3 or 4 – 1 controls the corners, 1 the edges, 1 the center.
One other thought – I wonder what would happen if only one large heating element was used? You could make a big oven-style element using nichrome wire – I don’t actually know how to do this, but know you can buy it and other types of heating wire and bend it as necessary. For that matter, given the size of your press, I wonder if you could literally just use a couple electric oven heating elements?
The idea is that if the whole thing was heated from the same source, perhaps it would regulate/distribute heat better on its own rather than create hot spots? I have nothing to base that on other than a hunch, and can see how it would suffer the same issues as the current design.
@flo-2 thanks for the follow up on this project! However, I don’t think your pictures got uploaded. Could you try again?
Yes, it seems to me that without the ability to control all of the heating elements separately with PIDs or some other controller ($$$), putting the sheets in an oven at a relatively constant temp would be best. Having a fan for convection would help things a lot.
Another approach for a heated press could be to use a hot liquid inside of pipes but you’d probably need some specialized oil for it, or high pressure. Would have to heat it in a heat exchanger too. Totally feasible, and someday I’d like to try it and heat with a rocket stove, but its a pretty big project.
As for pressure, do you think it flows poorly because of the 5 tons being insufficient, or because the plastic isn’t completely melted due to the heating issues?
Also, similarly, in your previous comment you said that heating and pressing from both sides causes air bubbles – why is this the case? Does your design allow for excess plastic to flow out the sides (flashing)? This would require filling it with extra plastic and having a depth gauge/stopper to make sure it gets to the right depth.
I would love to see a video of the process if you have time!
Thanks again for all of the information! It’s very helpful.
Effective washing of glues, etc… is now the biggest (conceptual, since I haven’t actually built anything yet…) impediment in my project to turn plastic into building materials in the developing world. As it stands, I’ll have to purchase plastic that has already been collected, washed, shredded etc… But once I can wash it in an environmentally friendly and efficient manner, collecting and shredding will be easy and reduce input costs by a factor of 4-5x.
Like the previous response said, it seems like the plastic is not fully melted yet. Perhaps melt for longer? Or use higher temperature to make it flow easier?
* What is the size of plastic chunks that you are using? The larger they are, the more air pockets you will get
* Perhaps compress it slower? If you compress very rapidly, the mould might overflow before the air voids have time to fill?
* Perhaps design a mould that has a tighter seal to make it more difficult for excess plastic to escape. This would encourage it to fill voids rather than seep outside. Getting enough pressure might be a problem however – I suppose different sized car jacks and a solid mould/frame would solve this problem.
Yes, this is why I am thinking it is better to look at this in reverse. Rather than recreating the wheel – something people are having tremendous difficulty doing, even at great expense – why not use well-tested, more powerful machines that are readily available (since they can turn things bigger than branches into powder)?
Has anyone made progress on a gas-powered shredder? I’m starting to look into doing this in Guatemala.
I’ve got a few questions:
* What are the dimensions of your mould?
* How many tons is that jack?
* If you’ve successfully produced some sheets, does the pressure from the single jack seem to be adequate (no voids, square/flush edges, etc…)?
* Is the frame rigid enough to maintain its shape, and thus a uniform thickness of the sheet?