Monday, January 19, 2015

Prusa i3 Rework: Build Log #8- Heated Bed Assembly

Finally, moving on to the heated bed! This step was shorter than the others, and rather easy to complete. However, it required some special supplies, which were not used otherwise in the i3 Rework.

Above, you can see the main supplies for this step. This includes the wired bed, the thermistor, the binder clips, and the screws, nuts, and washers (missing springs). One of the big differences between my heated bed and Replikeo's is that my bed has a glass build plate, instead of the fiberglass plate provided with the kit. Beyond the standard i3 Rework parts, I also needed fire block sealant and Kapton tape.

Why did I choose to order a glass plate? The fiberglass plate has been known to bend before when people have used it. After I read about that, I decided to buy a glass plate, which is stronger and tougher. It pretty much won't bend at all.

Here, I placed the borosilicate glass plate on top of the heated bed. My glass plate is 8 inches square, which fits perfectly on the PCB. The reason that it is not standard glass is because borosilicate glass has improved strength against heat-induced cracking. After all, Pyrex test tubes and kitchen products are also made of the same material, and they hold up quite well when exposed to flame.

Here is the glass plate, attached to the PCB heatbed with binder clips. Although the clips look unprofessional, they are necessary to deal with the thermal expansion of the glass plate. If the plate was screwed in, it could crack due to it not being able to expand. 

Here is a closeup shot of the binder clip, attaching the glass to the build plate.

This next step is a bit tricky (and wasteful): I bought a whole tube of 3M's Fire Block Sealant and used only a drop. I pressed the tiny amount of sealant into the hole in the middle of the PCB (the bottom side) and fed the thermistor into it. This will keep the thermistor from falling out of the bed while the printer is running, which can lead to disastrous consequences. In the end, the fire block sealant provides a glue for the thermistor that will stand up to the high temperatures of the heatbed.

I also used Kapton tape to secure the thermistor wires strongly to the heatbed underside.

You can now see that the Kapton tape job is complete.

Now the heated bed can be attached to the rest of the frame. It took a long time to get the screws and springs screwed in for bed leveling (at the corners of the heated bed), but it was worth it. My heated bed was set up.

Above is a shot of the entire printer at that point- looking good! The next thing to do is to assemble the extruder.

After I started to work with the printer, I found out that I made one small mistake- the binder clips were on the wrong edges of the bed. If you look closely at this image and the one above it, you can see that the binder clips switched sides- they used to be on the sides of the bed in the upper picture, but in the lower picture they are in the front and back.

It's an easy fix as long as you move one at a time. Moving all four clips off the bed at once could cause issues with how your thermistor is glued onto the glass, because the glass could move off the heated bed while being unsecured. Also, all of the binder clips used afterwards were the large ones- the smaller ones just couldn't grip on tightly enough.

This post is the eighth of ten that goes over my Prusa i3 Rework build. These posts are in the order given on RepRap's wiki page for the Prusa i3 ReworkAll of the images from the build are in this Flickr photo album.

My build log: