hotbed design/build question

I am upgrading to a 371x305x6.35mm MIC6 bed with a 300x300mm 120v 600W silicon heater and PET layer.
I have a 3 point mount with 5mm screws & springs thru printed blocks to the V rail frame.
My concern is the printed blocks and how much heat will be transmitted to them via the screws for a very long print.
The screws are mounted 10mm from the long edges, so they are about 30mm from the pad. The screws are terminated into an imbedded nut in the blocks.

Will the PLA or ABS soften?

Edited 2 time(s). Last edit at 01/23/2018 11:47AM by ruggb.

I've done this sort of thing a few times. Here's how I do it. I use teflon or torlon blocks for the leveling screws. Teflon is easier to find, cheaper, and easier to drill and cut. Either one can withstand the high temperatures that the screws will be subjected to. There is no need to embed nuts in the blocks. Drill undersized holes in the plastic and let the screws roll their own threads. It will be more rigid that way.

A large bed will expand when heated- in your case, about 0.6 mm when heated to 105C for printing ABS. The support that the screws go into will not expand because it isn't getting hot, that means the bed plate is going to push against the screws. The plastic blocks the screws are driven into may flex enough that it won't matter, but there is a way to make sure the plate expansion won't be a problem. You can use a kinematic mount. In a kinematic mount the bed plate rests on top of the leveling screw heads, held down by springs. The reference screw sits in a chamfered hole, the pitch screw sits in a chamfered slot. The roll screw just contacts the flat underside of the bed. This arrangement allows the bed to be leveled while keeping it stable and allowing thermal expansion. Details here.

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Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]

If you go for metal screws make sure to get stainless steel, it transfers heat a lot less than cheaper steel screws.

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Thanks --
It would seem like the expansion would be mitigated by oversized holes in the bed. IE, if the screw is 5mm and the hole is 6mm, then the bed can move a total of 1.0mm in X or Y direction.
With the spring load, the bed would otherwise be stable in the XY directions during printing, especially since it only moves in Z direction.

Since the bed is connected via the screw and spring to a nut in the mount block my concern was how hot is the nut going to get. If it gets to 100°C also, then that will soften the block enough to change the bed level.
Maybe fiber washers would do the trick.

Quote
ruggb
Thanks --
It would seem like the expansion would be mitigated by oversized holes in the bed. IE, if the screw is 5mm and the hole is 6mm, then the bed can move a total of 1.0mm in X or Y direction.
With the spring load, the bed would otherwise be stable in the XY directions during printing, especially since it only moves in Z direction.

Since the bed is connected via the screw and spring to a nut in the mount block my concern was how hot is the nut going to get. If it gets to 100°C also, then that will soften the block enough to change the bed level.
Maybe fiber washers would do the trick.

Teflon and torlon don't care about 100C. You can get a couple small teflon blocks or one piece that you can cut up via ebay. [www.ebay.com]

The type of screw heads and their interaction with the bed plate will determine if the plate will be laterally stable while printing. If you rely entirely on friction you're gambling. It might work...

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Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]