Fighting with computers

Computers are not always friendly.

Sunday, March 03, 2013

Heated bed blues

Reprap 3D printers can benefit of having a heated bed because that helps preventing part's warping. When a part is printed, it is done depositing layer after layer of fused plastic filament. Either PLA or ABS, these fused filaments will experience some contraction when the melted filament solidifies and cools down. This effect will be more dramatic the faster it happens. And the end result is that some of your printed parts will have a round bottom instead of a flat one. And that is if you are lucky, as some parts will completely detach from the print bed mid print and they will be totally ruined.

So we use a heated bed so the print bed is kept at a relatively high temperature. This way the filament will cool down not so fast and hopefully the parts will stay well grabbed to the print bed even in the corners.

There are several ways of creating a heated bed: a printed circuit board with a long copper track zig-zagging the surface that will create a resistor.

An aluminium plate with some power resistors attached at  the bottom. A heat mat, Peltier cells, and I am sure I am missing others. So I recently created a heated bed for a new printer with four 6ohm/10W power resistors as shown in the picture below. I assumed that though the total power will be around 100W, the resistors will handle that extra power given the aluminium plate would act as a big heat sink and they never will get too hot to break.

The system work nicely for a while achieving both 60C and 110C without a problem, which are the two temps used when printing either PLA or ABS plastic. Unfortunately, this bed would work only for more than a day. Next attempt was taking no chances, so I moved up to 50W power resistors, almost same price but quite larger.

This time the bed warmed happily every day, but the resultant bed was a bit taller than 30mm. This means that the tallest object you can print is now 30mm lower than without the bed. This is not a big deal most of the time but it needs to be taken into account.

So I was looking for a way to reduce the resistor's footprint and after checking the heated bed I have in my Prusa that is working nicely for almost a year I realized I was using 25W resistors. I bought some one eBay and I noticed they can fit nicely in the bed holder cutouts of the Prusa i3 as shown below.

Because the resistors can attached to the bottom of the bed's aluminium plate in a pattern that matches these cutout, the distance between the top plate of the bed and the one shown in the picture can be reduced to just a few millimeters (instead of more than 25 mm of my other bed). So this is what I am going to do for my next bed for my aluminium frame i3.

While a PCB-based heated bed is definitely thinner, these beds do need something like a glass on to as the PCB is not very flat and it warps when heated, which adds cost and parts to the bed (maybe some bulldog clips will be needed for keeping the glass fixed to the PCB too). An aluminium bed can be very flat and strong with the need of an additional glass. Kapton tape will make both glass or aluminium surface something hot plastic can attach too. Some plastics may bond well to some glasses without the need of kapton tape, but none to aluminium.