Interlayer adhesion

I started this thread to comment on the cookie extruder and of course had three ideas come into my head while composing it. The cookie extruder is a interesting application, I could see a basic food reprap being an interesting sideline to help push the bulk of the research. I suggest you go have a look at it, but onto the main point of this post.

Nophead, I dont think its the conductivity of the metal surface you are having problems with its the surface tension. Have you tried extruding the material over a primed surface. Primer is specifically designed to reduce the surface tension and to give a better "gripping" surface to the next layer. Also I suspect that a lot of your shrinkage is due to molecular relaxation that occurs at high tempatures coupled with the stretching of the material by the weight of the extuded bead. Thats particularly likley if minimizing the distance between the nozzle and the work surface seems to help.

That brings an interesting possibility to mind, I wonder if it would be possible to design an extruder that could both push and pull a filiment at various levels. You could then sandwish a "pulled" layer (that would deform and curl) inbetween two "pushed" layers to form a simple captured leaf spring assemble. Extrude a very thin layer of support material on the areas you dont want to stick and you could assemble alternating layers of "pushed" and "pulled" material and form the equivalent of a foam. Thats similar to the theroy behind the "twheel" which is a new concept for car tires.

In fact it is potentially a solution to a lot of performance issues that will arise out of balancing plasticity against durabilty in the future. One middle of the road material can do a lot more if you can control the macromolecular structure even if its only crudely.

Mike

Mike,
No I haven't tried extruding onto primed metal, I will give it a go.

I think the shrinkage is purely due to thermal expansion / contraction. If you look up the figures for HDPE you will find it shrinks 2% between its melting point and room temperature. That is also the figure given for "Mold Shrinkage" in this injection molding data [www.ides.com]

I am not sure what you mean by "pushed" material. Do you mean extruded with a flow rate faster than the head movement? I started with that scenario but have moved to the head movement much faster than flow rate to get a finer filament. I don't think I observe any difference in the amount of shrinkage or curling, as I say I think it is just 2% worst case,less if you can hold it while it sets. I.e. I think building on a raft and building from the inside out helps.

I will know more when I get my machine back up and running and start to make solid objects.

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[www.hydraraptor.blogspot.com]

I haven't got hold of any primer yet but I did try some mat painted aluminium and that did not stick at all.

I have been using white shiny foamboard to extrude onto. I tried some mat black foamboard for a change as I thought it would be easier to photograph. It does not stick to that at all either. There is something magic about the white glossy surface I think.

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[www.hydraraptor.blogspot.com]

I tried copper clad PCB material with metal primer and MDF with MDF primer. Neither would stick.

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[www.hydraraptor.blogspot.com]

Random thoughts
*a waxed surface
*Stainless steel or other low(er) thermal conductive metal

Not sure about waxed as it would just melt and provide lubrication but I will give steel a go, thanks.

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[www.hydraraptor.blogspot.com]

One thing that does look promising though:

A thin layer of HDPE cut from a milk bottle stuck to a piece of primed MDF with double sided sticky tape.

It's late here now for doing a full test but I will try one tomorrow.

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[www.hydraraptor.blogspot.com]

IT works very well I tried it last weekend.. I used a glue stick on a piece of foam board... and it stuck very well so well it is now a part of the cup I was trying to make..

Bruce W

The double sided tape let go when it got hot, I should have seen that one coming!

So the filament welds perfectly to the HDPE sheet, as expected, but now I have the problem of making the HDPE sheet stick to the MDF, or whatever.

What glue did you use Bruce?

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[www.hydraraptor.blogspot.com]

How about replacing your MDF with a sheet of corkboard, and using thumbtacks, or even upholstery tacks, to secure the base sheet?

The base sheet needs to be held down all over to prevent the object being made on top of it from warping upwards. It might be easier just to use Forrest's idea of extruding on to a solid board of HDPE. That has the disadvantage of needing to be sliced off whereas the thin sheet becomes the base of the object and you just cut round it with scissors, but all I seem to have done is swap one problem for another. I now need a high temperature strong glue that will stick HDPE to something hard and will not give.

I think the ideal base material is something porous that the HDPE can bind to, with low surface tension, low specific heat capacity, low thermal conductivity, high strength and high melting point. I just can't think of anything with all those properties. In addition it would be nice if the object could just be snapped of afterwards but held down enough to stop it curling. That may be an impossible dream with HDPE.

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[www.hydraraptor.blogspot.com]

How about something water soluble, with a high temperature... like salt? a work surface made out of/covered with a slab of "saltlick" or something? I can't think of anything else water soluble that's got a high enough temperature...

I couldn't find how they make the synthetic stuff, but maybe we could figure out how to make lightweight sheets of it to extrude onto?

Maybe there's some other combination of stuff that can withstand the temperature and be dissolved?

Take plaster of paris and mix with water, pour into a form made of 1/2 inch plexi strips any size you want. Let dry for a good day and then put in the oven at 175f degrees for around two hours. You can tell when plaster is good and dry because it rings when thumped with a finger.

Always add the plaster to the water not water to plaster, keep adding plaster until a small valcano forms above the water line and stops wicking water up the valcano, mix well and try to keep the spoon below the top while mixing to prevent introducing air while mixing. Don't mix pancake batter this way because you want the air in pancakes.

Wood at least is a good insulator. I think it has a lot of thermal inertia, however.

Does the HDPE sheet increase in surface area when material is applied, or just warp, but maintain the same surface area?
If it just warps, but doesn't stretch, you could hold the material tight with clamps, or with an oversized needlepoint frame. If it lenghtens too, that won't work.

How about cloth? That'd definitely be porous enough, and you could stretch it over some other material, one that could satisfy the other two requirements. You would have to accept the fabric would become part of the bottom surface, however. You could trim/sand it off, but you would have to remove it from the board before you could remove it from the part.

Thanks, I will put plaster, salt and sugar on my list of things to try.

Sean,
I have not tried ordinary wood, only MDF, which I can't get to stick. The reverse side of hardboard might be worth a try, I don't think it will stick to the shiny side. I might try plywood as well.

The problem with cloth and thin sheets of HDPE is that the object being extruded puts an upwards force on the base material at its corners. You could never stretch anything tight enough horizontally to resist that vertical movement, there is too much mechanical advantage.

When you lay down a raft of HDPE under the object it is strong enough to resist the horizontal contraction force but when you add layers above they pull inwards creating a turning force which drags the corners upwards. There is quite some leverage so the force required to hold the corners down is pretty high. That means the base material has to be very rigid and the bond the HDPE makes to it very strong.

The larger the object you are making is, the more force is required to hold it down. I.e, there is probably an upper limit to the size of an object.

Edited 1 time(s). Last edit at 11/17/2007 07:17AM by nophead.

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[www.hydraraptor.blogspot.com]

Oh well, I've got several pounds of ABS filament and the high torque motors that I need to pump it. I guess it's time to build another extruder and see if low expansion ABS is going to do the job. Shouldn't be a big deal now that Zach is casting extruder parts kits.

How about a simple vacuum table to hold down the thin HDPE sheet, MDF is porous and will allow a vacuum to suck through it. Using some tape to seal the edges and exposed areas will give you good holding force even with a moderate suction.

My Henry vacuum cleaner pulls about 0.3 Bar (4.35 PSI) the HDPE will have no mechanical advantage and I would suspect to could not produce this kind of force (but I could be surprised on this one!).

Use a 3mm MDF top sheet on a thicker MDF base board that has been grooved with air passages Hydraptor could easily make this, it will also do basic work holding for other milling work .

Just an idea but i have used Vacuum tables and they have amazing holding force if the item sits flat + bigger the area bigger the force.

Hi Ian,
I had thought about a vacuum but I think although it will hold down a solid sheet, the force on a small area is not that big so with a flexible sheet of HDPE the corners might still be able to lift. I tried double sided tape on 2mm aluminium which kept it cool so the glue did not melt but the corners were still able to lift, slightly less than when using foam board.

I had also heard MDF is porous before but it's hard to believe, although water can certainly get through it.

Something else to try and as you say, a vacuum table would definitely be good for milling which so far has been far more successful than FDM.

I only have one day to try things before I go on holiday for three weeks.

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[www.hydraraptor.blogspot.com]

Ian,
I am struggling with the concept of MDF being porous. You are talking about medium density fiber board which is wood fiber and resin?

I just got a small piece 6mm thick and covered one face and all four edges with duct tape. I then cut a 20 mm hole in the duct tape and placed a vacuum cleaner pipe over it. I then place a piece of paper on the other side but there was no evidence of any suction at all.

Is there another type of MDF, perhaps without the resin?

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[www.hydraraptor.blogspot.com]

How about pegboard, and clamp the sheet through the holes? It requires you know where to cut the sheet before laying down filament, though.

You could use simple bolts, with washers and wingnuts, or even thumbscrews, washers and wingnuts, to clamp the edges of a thin sheet down, close to where the corners would be.

Well, maybe not real pegboard. That stuff is a bit too flexible. Perforated sheet aluminum?

I got a sheet of perforated steel with something like that in mind. I wasn't thinking about putting a vacuum table together, though, since my xy platform moves.

So does mine but that only needs a flexible vacuum hose to solve.

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[www.hydraraptor.blogspot.com]

I found this on the web:

The "anecdotal" evidence says that if MDF is used then the hard "crusts" must be skimmed off to expose the porous inner core.

So another experiment due when my wife allows me.

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[www.hydraraptor.blogspot.com]

Hi Nophead

I have a fully vacuum pump (1 Bar) and this will suck through 18mm MDF! I was certain (but not tested) that a Vacuum Cleaner would work through 3mm but maybe it won't.

Have a good holiday and I will try and test how best to achieve a home brew Vacuum table (I need one for milling myself anyway) and I'll report the results, fortunately I have a vacuum gauge so should be able determine testable results.

Ian

PS yes just normal MDF resin and all.

Make has or had an article on making a small vacuum forming table out of a vacuum cleaner. I once found detailed plans in a book on making stage props, but I don't have the ISBN.

It all hinges on whether 1 bar is enough pressure to hold down the corners of HDPE. If it is then this would be a nice solution. You cut a sheet from a piece of milk bottle as big as the base of your object, put tape around the edge, vacuum it down, extrude the object and let it cool. Release the vacuum and trim the sheet to the object's perimeter.

The other unknown is whether once the object is set and you release the vacuum if it curls up anyway. That can be tested independently by extruding onto an HDPE chopping board and then slicing the object off.

Edited 1 time(s). Last edit at 11/18/2007 10:25AM by nophead.

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[www.hydraraptor.blogspot.com]

"It all hinges on whether 1 bar is enough pressure to hold down the corners of HDPE."

I don't think that it will even get close.

I just did the experiment with the HDPE chopping board. I laid down a raft at 240C and then built a 10mm by 40mm by 20mm high thin walled box on top. While it was stuck to the board it was curved slightly the other way because the board was domed a bit. The raft held down very well but surprisingly I was ably to prise it off afterwards with a penknife so the board is reusable.

After it was removed it did curl upwards a bit, perhaps 0.5mm over 40mm, but that is far less than I have achieved before on this shape. The base is only the raft which is one layer thick. A thicker base, or a solid object may warp less, or perhaps more.

Have to go now, got a plane to catch.

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[www.hydraraptor.blogspot.com]

'Make has or had an article on making a small vacuum forming table out of a vacuum cleaner. I once found detailed plans in a book on making stage props, but I don't have the ISBN.'

That was probably:
The Prop Builder's Molding & Casting Handbook (Paperback)
by Thurston James
ISBN 1558701281
[www.amazon.com]
[www.abebooks.com]

Not as good as
The Mouldmaker's Handbook
by Jean-Pierre Delpech et. al.
[www.amazon.com]
but the latter has nothing on vacuum-forming.

I believe you are right. The store where I work had it in stock several years ago. The fish looks familiar. If the example on Amazon showed making a fake line of books, vacuum formed, I'd know it was the one I remember.