HDPE, cool after base layer

Hi, im using a rapman, and am trying to get hdpe to print reliably. The results so far are promising, however hdpe holds its temperature for so long (above the glass transition temp) that as the much finer (therefore quicker cooling) intermediate layers are printed onto the base, they shrink, and pull up the base. The settings so far give a good, well adhered base, but as Bogdan mentioned on the bits from bytes forum, the internal forces during cooling are very strong.

Could someone advise me how to implement a 2 minuite or so cooling period after the base layer is printed. As mentioned in nopheads post; [objects.reprap.org]
I can manually pause the print on the rapman, however the heater turns off during this period and molten plastic drips from the extruder, meaning it really needs to do the whole startup sequence again.
I imagine this could be done by directly writing gcode, im not sure how to add a pause in gcode for a specified amount of time?
However i am trying to print off many 100's of test pieces, small 3cmx3cm rafts, and so an automated way of adding in the cooling time would be much better.

The aim is to ultimately publish the results, including on the arxiv.org. As it seems that so far, all the materials research on fused deposition modelling has been done internally by the engineering companies who design the commercial 3d printers.
Ill be making another post about this in more detail soon- as id like to upload some .gts files of some tensile test strips (ASTM D638-IV) and get the community to produce some to there best ability and mail them to me, i will then have an info on the average ability of the reprap design as a whole. By the way this is with the polymer physics department at Leeds University.
Thanks
John Mooney

Edited 1 time(s). Last edit at 07/07/2009 06:27AM by QIX_Leeds.

Have a look for Hydraraptor videos - Hydraraptor is Nophead's machine. When cooling, yes the nozzle oozes, but he has set up a little brush in the corner and programmes the extruder head to go there before continuing so that the excess ooze is brushed off. Very neat, very simple. Cooling is effected by having a fan directing over the build area.

I'm not far enough along my own build etc to know anything (yet) about programming to do this, but I'm sure others will help with that.

Hazel.

Take a look at the Blogs, I remember a post about a hairdryer (I guess it was Adrian who tested "warming" the print)
But the result was that warming everything a bit reduced warping a lot.

Just search around, you should find it quite easily

'sid

To insert a pause: First, print to a GCode file. Open the file and find the first z movement that matches your desired pause time. After that, add "G4 D1000" to delay for 1 second. The D number is the number of milliseconds. You can turn the fan on with M106 and off with M107. Then, load GCode file instead of STL file and print.

So, for a 2 minute cool down with a fan after you move to z=0.8 mm, would look like this:

G1 Z0.8
M106
G4 D120000
M107
G1 X100 Y100

I believe the heater will stay on during this time. If not, send "M104 S140" where 140 is the temperature you want. Send "M104 S0" to turn the heater off.