Sunday, November 15, 2009

More printing large objects with HDPE

After I achieved a 145x5x20 mm beam in HDPE, I decided to go for the limits of what my Rapman 3.0 could achieve. Rapman 3.0 has a printing area of about 200x200 mm. My thought was that if I rotated my beam around the z-axis by 45 degrees I ought to be able to push my largest dimension out to 280 mm or so.

If I were printing a line that would be true. Unfortunately, by the time I took the width of the raft that was necessary to keep the beam tacked down during printing was taken into account, I was only able to achieve a touch less than 180 mm, a length that was about the same as I could have managed if I'd printed the beam either along the x or y axis.

If you look at the tension pads you can see that they began to separate just as the print finished. This put about 1-1.5 mm of bowing into the beam over its length.

The printing parameters for the print were:
  • printer: Rapman 3.0
  • firmware version: 1.0.6
  • slice and dice app: Skeinforge, 2009-10-31 build
  • material: HDPE {no additives}
  • print speed: 16 mm/sec
  • extrusion speed: 65 rpm
  • fill: 40% using hexagonal pattern
  • raft temperature: 225 C
  • first layer temperature: 225 C
  • beam print temperature: 230 C
  • lab temperature: 18.3 C
  • lab humidity: 50%
  • raft perimeter width: 12 mm
  • beam dimensions: 5x20x180 mm
  • print time: ~100 minutes
  • bowing: 1-1.5 mm over length

I am going to attempt this print again with larger tension pads to see if I can get rid of the bowing observed in this print.


nophead said...

You say HDPE, no additives but on the pics it looks whiter and more opaque than mine.

Forrest Higgs said...

Yes, I wondered about that, too. I asked Jim Waring at New Image plastics, however, and he said that he hadn't added anything, pigments or otherwise after the consignment was delivered.

I can say this. It is transparent when it comes out of the extruder head when I am running the filament in before printing. ABS, otoh, stays opaque.

AKA47 said...

Out of curiosity.

Do you think it would help any to try and devise a heat treatment strategy (domestic oven) to relieve stress in the object. As a post printing thing.

I have been recently reading a book on knife making and the author does a lot of heat treatment of the blanks etc at various stages in the making process to relieve the stresses set up in the material as part and parcel of the making. Not just annealing and hardening.

Forrest Higgs said...

I tried heat treating HDPE prints with crosshatched infill some years ago with poor results. My parts just sort of collapsed in that I was using sparse infills.

I think Nophead did some work like that, too. I can't recall what he found out, though.

AKA47 said...

The reason I was thinking this route is because the guying technique (as far as my understanding goes, and could be wrong) does not actually eliminate warping.

As I understand it the forces that create the warping are still embodied in the block. (Perhaps more so) A bit like a compressed spring.

You are holding it in place until it has cooled sufficiently to resist the warping forces. Which are still embodied in the material.

A bit like pre-stressed concrete the tension is maintained until the concrete sets enough to resist the stresses in the reinforcing elements.

Interestingly enough they might actually be greater, as the components warping relieves the stresses that it embodies.

To my limited understanding this has two effects.

1. under certain circumstances if the component is used in a way that the uses the embodied tension to good effect it will give the part greater strength. But only in that mode.

2. failure modes of the part may be other than as expected. IE it may appear to randomly delaminate and spring into it's own preferred warped state at a later time.

I have got to admit that pre-stressed concrete is an overly dramatic analogy. Sorry. (Particularly as the failure modes of pre-stressed concrete can be spectacular)

Thinking about the above is what prompted the original question.

I guess ultimately without knowing what the magnitude of the captured forces are in real terms my thoughts could turn out to be irrelevant.

Forrest Higgs said...

Bogdan, who posts in the forums at BitsFromBytes, has said much the same thing. It makes sense to me.

Mike Fassbind said...

Residual Stess is the proper term as I recall from school

Forrest Higgs said...

Yup, that's the term. You see the effects when you try to cut polypropylene sheet. Cutting releases the residual stress and the cuts can warp into some very weird shapes over a few hours to a few days.