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Coefficient of Expansion

Posted by Forrest Higgs 
Coefficient of Expansion
April 08, 2007 12:51AM
I'm grinding through how to handle thermal contraction during the fabrication of HDPE artifacts on the Tommelise printer right now. The fact that plastics have a substantial coefficient of expansion means that it's really hard to extrude them on a flat surface and have them not peel up at the perimeter.

For HDPE, for example, the coefficient of expansion runs something like ~4 x 10^-5 mm/mm/deg C. This means when you extrude HDPE at about 152 degrees Celsius you can expect a 30 mm dimension to shrink about 0.5 mm.

This factor makes rather a mockery of talking about printing at an accuracy of "accuracy" of 0.1 mm.

I'm not criticising, mind, but rather pointing out that the whole issue of thermal contraction of printed parts is one which will be facing us all and is going to have to be solved. So far we haven't done a lot of talking about the larger issue but have confined ourselves to trying to get parts to adhere to printing surfaces.
Re: Coefficient of Expansion
April 08, 2007 05:33AM
Totally agree.
Thermoplasts have one big advantage and on big disadvantage.
The advantage is that as soon as the polymer leaves the nozzle, it immediately starts to set, which ensures the stability of the printed material in very short time frames.
The disadvantage is the inherent volume changes that a heated up material undergoes.
As far as i know, the case Forrest mentions above is probably one of the most extreme, as HDPE is very prone to volume changes, because it's long polymer chains untangle when heated up, which makes it melt and flow but also expand. Once the temperature sinks, the long chains start creating bonds between each other and tangling up like cold spaghetti. But the volume reduces drastically. Polymers with shorter chain lengths would behave better.

What solutions are there? Just thinking: could this volume change be accounted for by the rendering software? The longer the printed out lengths, the more importance this effect will take, so dividing the printout surfaces in smaller bits and letting them cool down before returning to that part could help?
Those are random ideas, i think you guys have more experience on the extruder front.
Re: Coefficient of Expansion
April 08, 2007 11:55AM
this is indeed a problem we will have to overcome. how do materials such as ABS and PP stack up against it in this regard?

perhaps this is why the Strat keeps the entire build chamber at a high temp.

i wonder what would happen if you had a heat gun blowing on the build area to keep things relatively toasty during build?
Re: Coefficient of Expansion
April 08, 2007 12:22PM
***how do materials such as ABS and PP stack up against it in this regard? ***

ABS and HPP have slightly lower coefficients of expansion, not enough to be significant, though.

***perhaps this is why the Strat keeps the entire build chamber at a high temp.***

Given that ABS melts at about 105 I'm left to wonder just how hot that build chamber is being kept, especially since I understand from both Adrian and two other Stratasys users that I've become acquainted with that Stratasys' printouts tend to delaminate on the z-axis. That would say to me that it isn't very hot in there at all, maybe not over 100 or so.

***i wonder what would happen if you had a heat gun blowing on the build area to keep things relatively toasty during build?***

That would be an easy enough fix for using polycaprolactone. It sounds like a real energy pig for working with HPP and HDPE, though. Can you imagine how much electricity a heat gun would burn up if you kept it on for days at a time? :-0
Re: Coefficient of Expansion
April 08, 2007 12:34PM
well, that all depends on the heat gun. we dont need 1,000 deg temps... but rather just enough to keep the thermoplastic in the build area from cooling off *too much* and shrinking.

most guns range from 350 - 1500 watts. lets choose one in the middle range, say 600watts. electricity companies charge per kilowatt/hour.

running it continuously 24/7 would be 600 watts * 24 hours = 14.4 Kwh
total for an average month is: 30 * 14.4 = 432 Kwh for the month.

price per Kwh ranges from $0.08 to $0.15, depending on usage time, etc. Lets go with the large one, just to get a worst-case scenario:

432 * $0.15 = $64.80

how about best-case:

432 * $0.08 = $34.56

Keep in mind that this is for continuous 24/7 operation for a whole month... which I would not recommend as you're likely to start a fire. If it does come down to keeping the build area heated, this isnt *that* much of a cost. You're basically paying $0.05 per hour.

Edited 1 time(s). Last edit at 04/08/2007 12:40PM by Zach Hoeken.
Re: Coefficient of Expansion
April 08, 2007 01:35PM
One little mistake in your calculation. You don't pay an average rate for electricity for additional usage, you pay the marginal rate. Right now my marginal rate for electricity set by the state utilities board is $0.224/kwhr thanks to the huge sense of environmental awareness that this state is afflicted with.

432 x 0.224 = $96.76 :-s
Re: Coefficient of Expansion
April 08, 2007 02:30PM
...which is still not too bad. You'd be paying $0.14 / hr to build an object.

hopefully we can find a way around the shrinkage stuff in hardware, but if we cant, we'll have to come up with a solution like this.

on the plus side... this stuff may come out in our favor. if things shrink after we print them, that means we get HIGHER resolution in things after they have shrunk down to proper size. we just have to make sure that we get all the shrinkage stuff integrated properly with the software, or find a way to keep it from shrinking til the end. =)
Re: Coefficient of Expansion
April 09, 2007 02:42AM
I don't have much experience with this sort of thing, but I'm eager to see RepRap succeed (I REALLY want to play with one), so I'll toss this out in case it is helpful, or perhaps triggers a helpful thought in someone else:

Assuming we can compensate for thermal contraction in the control software so that the end product is accurate after cooling, the main problem I see is keeping the items being printed at an even temperature to prevent warping/delamination/etc. Since this is caused (I'm assuming) by different parts of the item being at different temperatures, keeping the whole part at the same temperature is important. So far you have discussed keeping the entire part very warm, but have you considered rapid cooling as well? Perhaps one of you can try printing out an item onto a heatsink, such as a large slab of aluminum at room temperature. I'm thinking it might help keep the temperatures more even and prevent warping. If it works better than without the heatsink, perhaps we can add a temperature sensor to the heatsink and 'pause' extrusion when the heatsink gets too hot?

Another possibility might be to somehow scan the surface temperature after each layer, and if the center is warmer than the edges, 'pause' extrusion until the temperature is more uniform.

Would this work?
Re: Coefficient of Expansion
April 09, 2007 05:08AM
Like Eric, I think we should be cooling the work piece, not warming it. Doesn't Vic use a fan to cool down the object being made? I know he is using lower temperature CAPA, but rather than trying to keep the work warm perhaps we should do the opposite. If the deposited layer has cooled fully before the next layer is placed on top of it then it might be stronger and able to resist the contraction of the layer above. This theory could be easily tested by leaving a big delay between each layer.
Re: Coefficient of Expansion
April 09, 2007 10:09AM
Have you considered the possibiliy of using filler in the thermoplast? Adding a little sand to the plastic would reduce the flexibility of the final product (maybe good maybe bad) but would dramatically reduce the dilation of the material. Polyester, nylon, or fiberglass fiber would also be a possibilty depending on if you wanted wear resistance, rather than stiffness. Either sand or fibers would reduce the contraction, although sand would do more so. A ceramic like sand or fiberglass would also likley speed the transfer of heat to the air.

Other possibilites would include sawdust or even refractory materials like


They want $12/lbs with is robbery, but I havent found another small quanity supplier YET.


The thoughts and ideas expressed in this post do not reflect those of my employer and are intended only as communications between individuals. Any attempts at implement are at your own risk

Edited 1 time(s). Last edit at 09/12/2007 09:47PM by ohiomike.
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