Wednesday, March 17, 2010

Wires pt. 4: Benchtop wire-printing

We are operating the wire extruder manually to simulate the wire-printing process, in order to figure out the different variables to control the process, such as such as temperature, extruder height, heating/cooling time etc.

Wire extruder setup

The wire extruder re-heats the plastic surface to make it soft. There were two heat sources: the heat radiated by the screw (which is heated by nichrome wire, just as the plastic extruder) and the heat conducted through the wire. The wire is copper, with a 0.5mm diameter, so it is able to conduct a considerable abount of heat.

Previously we reported failure at attempting to heat the PLA by radiation alone; however, with tip number 2 (all-stainless steel), this is no longer the case - we can now melt the wire into the plastic without contacting the surface. Likely it is the heat conducting through wire that is responsible for most of this success.

==Bond the starting point==

The steps to fix the starting point are:
1 set the z-bed so that the bottom of the wire extruder is 5mm above the plastic surface;
2 click the mechanical pencil to extrude 5mm of wire, so that it touches the plastic;
3 heat up the head to 170 degrees Celsius;
4 wait for 10 seconds for the system to warm up;
5 raise the z-bed 4mm (the hot wire will pierce into the plastic);
6 turn the heat off and the fan on until the extruder temperature is about 40 degrees;
7 hold down the pencil button, so the wire can move freely through the pencil;8 lower the z-bed by a millimetre or two, to allow the wire a suitable bend radius;
9 move the wire extruder to the location of the next bond (in the test we were moving the plastic piece and holding the extruder fixed).

Wire is pierced into the plastic when z-bed is raised

Comments:
the above process makes an accurate, strong and clean bond, so we plan to keep this process for the future testing.

Although project goal is to print 2D wire patterns, we were surprised by how easy it was to "inject" wire vertically into the part. The fact that the wire can pierce through several layers makes 3D wire printing very feasible. We will need to test how deeply the wire can be injected by our apparatus.


== Bond the consecutive points==

1 turn the heat on to 170 degrees;
2 continue holding the pencil button;
3 raise the z-bed until the extruder bottom merges into the plastic;
4 lower the z-bed (now the wire is fixed to the plastic and follows the z-bed when the bed is lowered);
5 switch the heat off and activate the fan to cool the new bond;
5 move the wire extruder to the location of the next bond.


Wire is dragged out when extruder moves (in this test, extruder is fixed so the plastic moves instead)

Wire sticks to the plastic after the z-bed is raised and lowered once

Done!

Comment: the bond is strong (the strength testing is blogged below); however, it's not clean because in step 3 the extruder gets into the plastic surface.
(The good news is we managed to reduce the problem by printing a plastic guide for the metal wire. We will talk about it in the next post. Stay tuned! =) )


== Test the bonding strength ==

PLA impresses Reprappers easily.

It's bio-degradable. It's crystal-clear. It has a very small thermal expansion coefficient (so printed parts almost never warp after cooling).It smells like cotton candy during plastic printing. And plus, its bond with metal is super strong!

In the March 8's post, we showed that various kinds of wire were firmly bonded when they were "push-inserted" into the plastic. Today the wire almost sat on the surface, so we were surprised that the bond was still incredibly strong.

In the following picture, the surface-bonded wire was able to support a 24.5N force (2.5kg weight) without showing any signs of failure. It is now very clear that wire delamination will not be an issue.

Plastic-wire bond
Test weight: 2.5kg

In the near future, the wire extruder will be mounted to the Reprap machine (with its own Arduino extruder controller), sitting in parallel with the plastic extruder. The mechanical pencil will be accuated by a servo motor for now. After we finish writing G-code, the whole wire-printing process will be controlled by the computer. (It may be some quite time before Skeinforge-style software exists to create G-code automatically, because STL files are unfit for wire data).

5 comments:

  1. That's pretty cool, keep up the good work!

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  2. STL files are indeed unfit for wire data. Direct G-Code, or perhaps some sort of SVG? Some kind of intermediate format may or may not be needed.

    Ultimately, KiCad or a similar tool will have to be modified to produce the code that we need. Easiest to use something that already understands schematics.

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