BIG print bed advice wanted.

Hi.
I have a heated bed 700mm x 400mm made up of two layers of 6mm glass with nichrome wire in between.
Its printing ’normal’ size multiple parts very well but larger ones always pull up at the ends. Its not entirely unexpected as my workshop is cold and draughty. The plan is to eventually enclose the entire printer. The bed (1.3kw 240v) will keep the inside warm so I will probably need an extractor fan to keep the temp reasonable.
I have used kapton tape to good effect on my mendel but putting 2x 200mm widths on the new bed is quite a task, only to get ripped up by a bad Z home setting so currently Im using ABS slurry, bed 80deg and extruder 240deg Its not that good. Has anyone tried scratching or etching the glass?
The other big problem with a bed this big is flatness. There is are highs and lows in the toughened glass which differ by about .3mm. This also causes problems with the first layer stick.
Z probing may help but I believe so far it can only cope with planar surfaces and will not map contours?
Are there other large bed machines out there working ok?

What polymer are you testing, ABS?

To help me trouble shoot, I'm writing a guide for printing large objects, a few notes for you to consider:

Notes:
Dry your filament.
For tall parts you will have to adjust your Z axis to account for the shrinkage.

Temperature:
Use a heated enclosure.
Higher extrusion, and Substrate/Bed temperatures = more shrinkage = greater force to delaminate marginal fusion welds.
Use the lowest practical melt temperature possible.

Guying and Shrink-Wells/Pads:
Print a substructure to the object to constrain the geometry:
1. The object is guyed to pads onto a raft.
2. Guying long prints [blog.reprap.org]
3. Continued guying [blog.reprap.org]

Cooling:
Non-uniform cooling of surfaces, (the part warps toward warm surface).
Prevent local drafts with an enclosure, (oven bag might work).
Rapid cooling causes warping, (use a slow cooling rate).

Design/Geometry of the object effects shrinkage:
Uniform/Even/Equal wall thickness reduces differential shrinkage.
Geometric asymmetry causes warping, (e.g. ribs on one side, planer surface on opposing side).
Use Geometric manifolds to reduce/hide warping due to shrinkage, (e.g. corrugated walls, Sin walls, ZigZag walls.).


ABS, (Acrylonitrile butadiene styrene, Type: extruded):

Drying Temperature:
Range: 70°C to 93.3°C, (158ºF to 200ºF)
Average: 80.9°C, (178ºF ).

Dry time:
Range: 2 to 4 hours.
Average: 3.2 hours.

Glass-transition temp:
Range: 108°C to 109°C.
Average: 108°C, (226.4ºF ).
Wikipedia: 105°C, (221ºF ).

Begin testing around this temp:
Substrate/bed temperature: 105°C, (221ºF ).
Enclosure temperature: 105°C, (221ºF ).

Material properties are generally stable between:
−20 and 80 °C (−4 and 176 °F).

Linear Mold Shrinkage (extruded):
0.002 – 0.008 cm/cm.
Average value: 0.00532 cm/cm.

Machine parameters:
Extrude temp (RepRap): 210 C ~ 240 C

Edited 2 time(s). Last edit at 12/26/2013 08:51PM by A2.

@Vince,

It sounds like you are printing ABS.

I was using ABS slurry on Kapton tape, but found this gave far too much adhesion, and I'm now virtually printing just on plain glass, normal window glass.
If I need a bit more adhesion I use ABS juice, but on the whole if I can get the bed height correct, the first layer seems to adhere very well with or without ABS juice.

One quick thing you can try to reduce drafts, is just drape an old sheet over the whole printer. However be carefull your motor still get enough ventilation, as mine got hot if they were in close proximity to the sheet

I ended up building a wooden enclosure with plexyglass front, and its working really well.

Cheers

Roger

I have a 12"x12" that I made from a large granite tile, its perfectly flat, I plan on expanding it out to 18"x18" using more of the granite tile. its perfectly flat and wont crack due to heat differential. as for the surface, I found that 5mil kapton works great, however i havent found a good solution for gluing it to the bed, perhaps a very thin epoxy with work well.

Quote
aduy
I have a 12"x12" that I made from a large granite tile, its perfectly flat, I plan on expanding it out to 18"x18" using more of the granite tile. its perfectly flat and wont crack due to heat differential. as for the surface, I found that 5mil kapton works great, however i havent found a good solution for gluing it to the bed, perhaps a very thin epoxy with work well.

How do you heat that thing? Or do you only print pla?

I etched a 12" pcb with two sets of 5mm traces. if you can get 5mm electrical tape its quite easy to make at home, however I used a toner transfer.

Quote
aduy
I etched a 12" pcb with two sets of 5mm traces. if you can get 5mm electrical tape its quite easy to make at home, however I used a toner transfer.

I'm collecting information on how to make large heaters.
Can you post a picture with some instructions?

Tks!

Good info thanks.
The 105deg enclosure temp is a little concerning as all the motors and most of the electrics will have to cook as well, also many of the ABS parts on the printer will begin to soften but I will experiment once the sides are on.
I have just done a little experiment with CA, superglue, on part of the glass. I spread it about with a drop or two of water to set it off. Once dry it forms a rough surface that sticks very well. I will try to print on it later.

Quick vid of the machine
[www.youtube.com]

Edited 1 time(s). Last edit at 12/27/2013 09:56AM by Vince.

You can always water cool the motors.