Dot printing rather than Extruding?

Has any one tried messing around with dot (or blob) printing of objects? along the lines of a inkjet printer? shotting small dots of plastic or possibly something else to form layers of the object instead of using a stream. I have a few ideas on some fairly low tech and easyish to make methods of doing this.

How do comercial high cost units do it? dots or streams?

What is the molten temp of say HDPE? not when it gets soft but gets about the consistancy of say motor oil or even water? can it even do this? (my designs should be able to cope with fairly high temps)

The basic design i've got in my head needs to use a liquid that can harden, dry or cure fairly quickly. but i have some ideas on how to cool or heat it fairly quickly once shot.

anyway i better get back into bed an try and sleep again. but i got a feeling i'm gunna lay awake for a few more hours thinking about this

One of my possible designs should work with the current extruder controls and software. thats if dot printing even works and my ideas work...

Now all i gotta do is build a standard working machine first.

On that note i've been fooling around with templates to make wooden molds for pouring resin to make Darwin's parts. with little woodworking skills. Would other people be interested in these? other wise i'll do then for myself only without writing instructions (quicker for me) but i'm happy to help if it's going to be useful. i would love to comtribute to this possibly historic project
i might post a what i've done for making the extruder parts tomorrow night after work(if i'm not completely buggered from staying awake later every night brainstorming ideas since i discovered this project...)

Edited 2 time(s). Last edit at 10/01/2007 12:41PM by moonspud.

Most polymers exhibit very high viscosity (thickness) in the molten state. HDPE is no exception to that rule. Off-hand I don't know of any that would have the consistency of water or motor oil at 1 atm.

what temp do they start to go to a molten state?

HDPE's melting point runs in the vicinity of 135C. I extrude it at about 180-190C. In that range it has about the consistency of white toothpaste.

Forrest Higgs Wrote:
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> HDPE's melting point runs in the vicinity of 135C.
> I extrude it at about 180-190C. In that range
> it has about the consistency of white toothpaste.


Does it keep getting softer the hotter it gets? at what point does it break down or burn?

Edited 2 time(s). Last edit at 10/01/2007 12:57PM by moonspud.

Autoignition temperature for HDPE runs about 390C in ordinary air.

Listen, getting these numbers and this kind of data isn't rocket science. I've been using Google to answer your questions. You could, too. There's a lot of experiential data I have picked up actually trying to extrude HDPE into useful things that you won't easily find on the web and I'll be happy to share all that with you. The sort of information you've been inquiring after, otoh, is well within your grasp with a minimum of search engine skills.

I did some tests that seem to show there is very little change in the viscosity of HDPE between 150C and 220C.

---

[www.hydraraptor.blogspot.com]

nophead Wrote:
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> I did some tests that seem to show there is very
> little change in the viscosity of HDPE between
> 150C and 220C.
>
I've had a similar experience but only in the range of 150-190C. Mind, down at 150C it tends not to want to adhere to previous layers of HDPE very solidly. I guess that there simply isn't enough energy in the extrusion to heat the base layer sufficiently for a solid weld.

... for ink-jet-printing you need viscosities equal or lower then water!

There are very expensive hot-wax-ink-jets, which print 3D, but the actual powder-3D-printers use normal ink-jet-heads with a glue-liquid, which is thinner then water for koagulating the powder.

I think the 'objet'-printers use a very low-viscose epoxyde wit UV-curing to hardening the wet droplets on the surface ...

Old thermotransfer-printers with a thin slice of meltable coloured plastic-powder on a transfer-ribbon could be used for 3D-printing or here - [forums.reprap.org] i discussed about micro-droplets of polymers or solvents for adding or subtracting OLED-like structures ...

And here - [forums.reprap.org] i asked, if a 3D-LOM-building with an old needle-printhead as mikro-holes-penetrator would be an interesting approach ...

I think, with a cheap high-power-laserdiode (maybe from an outweared DVD-writer) and the LOM-principle i can easily and faster build 3D-objects, then with an extruder, because i have only to perform the outlines of the slices and not the inner areas.

Viktor

You might dig through these forums for posts by Fernando. Check the blogs too.
He was experimenting with photo-cured plastics. This might be suitable to your ideas.

Last I remember reading, he was trying to find something that would cure the plastic fast enough, and a cheap enough, and potent enough, UV source to do so with.

k thanks for the info guys. more food for thought.

sorry forest didn't the p you off just thought it would be easier to get info from ppl who have hands on experience rather than technical data specs. but i will google first next time

Moon,

Not to worry, I'm not pissed off with you. I've just found over the years that when you've got the technical specs under your belt you tend to ask much better questions. I doubt that HDPE is going to work for the approach you're exploring. That's not to say, however, that some other polymer might not.

Edited 1 time(s). Last edit at 10/02/2007 10:37AM by Forrest Higgs.

How about lay out the powdered metal with the printer head then shoot it with a friggin laser to melt it.

... it's the principle of SLS (SelectiveLaserSintering) but it's heavy time-consumpting, to lay down the single trays with the extruder and melt them to solid (but i'll do it too for higher accuracy in the micron-area).

Mostly you lay down a complete slice of powder and draw your paths with the laser as in a pen-plotter.

Another concept is to heat a spot on the surface of your object and blow metall-dust in the glowing spot, so it can melt additive - this is suitable for lathe-fabbing too ...

Viktor

One thing I suspect you forgot to consider with SLS. When you lay down the first layer, you're fine, but you immediately lose any overhangs starting with the second layer as your metal dust falls through to the layer below before you get a chance to irradiate it.

Personally, I'm wondering how much candlepower you'd need to use a video projector, and sinter a frameshot at a time, but it still leaves the question, how do you get the working material to hang in the air until you fuse it to the rest of the object.

Apparently you hold it in place by using the powder itself. This link will explain it better than I can [www.rapidsolutions.com].

Hi Sean,

... the powder-bed-SLS is 'selfsupporting' with not melted powder, as shown in sillyarms' link (like in the ZPrinters, where you ink-jet on a plain powder-surface).

With paste- or dustblowing SLS you work only on a single spot, so you sinter/melt the material just in time ...

With the lathe-method you have a rotating axis, on which you spotwise blow dust in the hot laserspot and only the melted dust stays in the spot and adds material, the other will be blown away.

But you can sculpt soft overhangs and embedded cavities too, as the melting area with the cooling down solid body beneath is selfsupporting with the inherent stiffness of the already sintered or solidified material ...

Viktor

Oh. Sorry. I'd known that about the dustbed, but managed to forget.

Sounds like a dustbed would work wonderfully. I still see problems with "just in time" sintering, as you couldn't form components that were suspended from an overhang not yet made.

Hi Sean,

... with overhangs you have the same limitations as with the FDM-extruder, what resulted in the 'teardrop-shape' of fabbed cavities ...

With powder-melting you have a 'sandy' surface with the accuracy of the powder-granuity plus random coagulation-/adhesion-effects on the border between melted and untouched powder.

For finer surface-finishing and much faster fabbing i try to realize a LOM-approach (=Laminated-Object-Modelling/Manufacturing), where i stack thin slices of a material (actually plastic or dark coloured paper, as the laser isn't so poerfull) and cut the very top sheet with my diode-laser - then i can achieve an accuracy of the thickness of the sheets and in the sheet-plane of the accuracy of my positioning ...

Viktor

Hi Moonspud,

I will try to type in my thoughts about "dot-printing" in the next few lines.

I work for an injection moulding company at the moment where my work mainly consists of flow analysis with a CAE software. So far, I have analized and processed a lot of different polymers with different characteristics, but I do not know any kind of polymer which would be useful for dot printing.
My thinking is very simple: polymers are non-newtonian fluids which means their viscosity value changes in function of temperature. For example, the viscosity of water and motor oil remains the same (well, let's say nearly constant) under different heat loads until a point where the molecular structure changes (at vapouring point for example). In case of polymers the viscosity value rapidly cahnges with temperature which is due to thier molecular structure: they are bulit up of long chainmolecules that do not let eachother go past. This effect can be examined when you heat up the polymer and start to pull a sting of it. It comes and comes and gets thinner and thinner but does not want to break. To break it you would need a mechanical force such as a knife but it would make the process unreasonably slow... So, I would not experiemnt with dot printing of polymers at the moment. However, as there are about 10.000.000 kinds of sinthetic polymers around the world, a material may be introduced tomorrow which know the charasteristics you want, Moonspud. I suggest, keep walking with open eyes and keep visiting the biggest polymer manufacturer websites to be up to date.

Anyway, SLS sounds a better way with low melting point alloys. Some of these melt around 250 degrees, so you should build a machine with higher heat resistance... If you have a RepRap already, this should not be a big task. Remove the paste extruder, and build a roller which rolls the powder after each layer...

But, let it be the future of RepRap! :-))

Hope, this helped!

Adam

Hi Adam,

... with 'dot-printing', 'polymer' and 'heating' i think on a print-head, which didn't extrude drops from a continuous stream, but place little polymer-balls, which are hard when picked up, then heated up and placed as melted droplets ...

Think on a support of small polymer-balls, from which a pick'n'place vacuum-dispenser takes ball after ball, heat and insert them in the next point-position ...

With different sizes (and maybe heads) you can build in different speeds and accuracies.

With a real fast system there should be same building-time as with continuous extruding, but the finishing and edge-accuracy should be much better ...

Viktor

It continually amazes me the diversity of people we get here. Very handy.

Demented

Hey Viktor,

The idea sounds fantastic to me!

However, as far as I know this is technology of the EOS and Concept machines. Is that right? The only exeption is that they work with low melting point alloys, but they really melt up the alloy balls and thus stick them together.

Anyway, I have not heard of anyone trying a similar technology with polymer balls. This sounds a really good idea to me, and how simple it is to realize as all polymers can be easily ordered as granulates. Nowadays, you can even choose the size of granulates in many cases. (This would really drop the cost of rapid protoyping with RepRap...)

I think this idea is worth thinking on...

Adam

It's like 3d pixels!

Could you elaborate on this vacuum dispenser? How would you a) only pick up one granule, b) heat the granule without sticking it to the picker-upper, and c) keep it fast (surely the process of picking up a granule, moving to the correct place while heating it, depositing the granule, then moving back to the tray, would be very slow)?

... i would use a head with more vacuum-dispensers (like a star with 4, 6 or more dispenser-needles)

Gripping a ball: - the actual dispenser activate vacuum an dive into the ball-support, until a pressure-sensor measure that the dispenser is blocked.
With small balls the vacuum can be switched off, as the balls stick with adhesion to the dispenser-tip.

For the heating: - either heat the dispenser-needle until the ball melts completelly, or heat with IR-heating, diode-laser or by touching a hot point only the bottom-half of the ball, so the upper part stays hard enough, to handle it.

When the dispenser-tip is coated with PTFE, so the melted plastic would stick to other plastic better then to the tip, so releasing should be no problem ...

Viktor

Along with using a head with more vacuum-dispensers, I would use a very light head so that you can swing it around fast. Less weight==less momentum==faster traversal times.

Demented

Hi Demented,

... it's your decision: - (very!) fast'n'easy or slower with a complexer toolhead.

The main problem in 3D-printing is the fabbing-time.

If it's no problem for you, wait many hours to days antil your part is finished, it's OK ...

I think around how i can reduce the fabbing-time drastically but enhance the accuracy too!

Most of the 3D-printers have to process every 3D-point of the object, so you can simply calculate the estimated time by the 3D-point-count and the time to process a single point ...

I'll try to reduce the processed points only to the outlines, so i would have to process much lower points and can go in enhancing accuracy.

The only way to this seems to be processing sheetwise - either with LOM (LaminatedObjectManufacturing) by stacking slices and laser-cutting the outlines and separation-lines - or even faster: by Ink-jet-printing with saltwater the outlines and separation and melt the complete slice at once with an IR-heater!

Viktor

viktor,

Why process each point in a slice? You only actually need to process points on the line you are tracing because it isn't zero width. If we account for the width of the filament in code more completely, it will reduce the number of points we have to process at a given time.

Also, are we experiencing a bottle-neck with respect to bandwidth between the RepRap and the computer? If so, lets increase it so that processing goes faster. I sort of doubt that this is the case, however. Has anyone really nailed down why we have such slow print speeds? Something to look into.

Demented

Hi Demented,

... my '3D-points' are the representation of the smallest units of the object - with a 1mm thick tray it schould be something like a 1x1x0,7mm cubicle (length x width x height) ...

So it's obviously, that with a 0,5mm thick tray you have 0,3 to 0,4mm thick slices with 0,5 mm distance between the trays, so you can calculate the amount of '3D-points' per slice.

If i want to reduce the tray-diameter and the slice-height to 0,1 mm, the amount rise drastically to 5x5x(4)=100times!

With 0,05 then 8 times more, and so on.

And then remember, that my primary goal is achieving submicron accuracy!

The easiest way to rise accuracy without slowing down the fabbing to death is to change from processing all the body-points to only the contour-outline-points (plus some separation-lines).

This could be made with already predefined slices (as plastic-sheets or other materials) or with melting the complete surface of a powder-slice at once with an IR-heater (i have two IR-quartz-tubes from a toaster).

With the ready sheets i have to stack/glue the slices and cut the lines - either with a cutting-plotter-knife or with a diode-laser.

With laying down powder-slices and melt them with IR i have to process only the lines too, here ink-jetting saltwater-droplets as separation is the best way - for this i ordered the book from you

Viktor