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Mains Powered Heated Bed

A project log for Delta 3D Printer

A custom scratch built delta printer of my own design.

jasonJason 03/31/2017 at 12:560 Comments

The initial build used the MK2B heated bed that came with the electronics kit. There were a few round beds available but they didn’t fit in my budget at the time. I’d had a lot of success printing PLA with diluted PVA glue on glass. I wanted to increase the size of the bed to take advantage of my printer’s reach and also wanted to try something less messy. I’d started hearing about people using PEI (Polyetherimide) sheets with great success.

My plan was to get some aluminium sheet, laminate PEI on top and glue a silicone heater to the underside.

I called up my friend who helped me manufacture the main printer frame. He had access to a water jet cutter at his work and was willing to cut a round bed from aluminium. I designed a circular bed to be cut from 5mm aluminium with a radius of 280mm. I figured this would be thick enough not to warp when heated.

Water jet cutting

Water jet cutting

The next problem was heating. It becomes quite inefficient to heat up such a large area using a 12V supply. A considerable amount of current is required that can be difficult to control efficiently. A few people online had started using mains powered silicone heater mats driven by solid state relays (SSRs). This solution seemed pretty good for me. There’s less load on the printer power supply as the printer is more efficient using the mains power directly. I did some googling and found an eBay store (Keenovo) that manufactures custom silicone heater mats. They even include an adhesive backing and built-in thermistor.

New heated bed

New heated bed

SSR and printed end cover

SSR and printed end cover

For thermal runaway protection, I decided to use a 128°C (262°F) non-resettable thermal fuse. There were some self-resetting fuses available but I didn’t want to risk getting into some type of temperature cycle. Once the bed temperature exceeds the fixed limit these fuses will open permanently. The only way to reset is to replace it entirely.

In-line fuse

In-line fuse

For the PEI surface, I bought some 0.03" x 12" x 12" sheets from Amazon. I also purchased some 12" x 12" 3M 468MP adhesive transfer tape. The tape was first applied to the aluminium.

Transfer tape applied

Transfer tape applied

I then peeled off the backing from both the transfer tape and PEI sheet. The PEI sheet was rolled down onto the transfer tape. PEI sheets come with a matte surface finish on one side and a gloss finish on the other. I decided to go with the matte surface for printing on.

Overhanging edges

Overhanging edges

The bed was flipped over and I carefully scored the PEI using the aluminium edge as a guide. I used some small files and sandpaper to clean up the rough edges.

One third done!

One third done!

I reused the same 3mm bolts and springs from the old bed. A cork tile was cut to size and placed underneath for insulation.

Spring mounted

Spring mounted

Cork insulating tile

Cork insulating tile

The control board required no modification as the SSR is driven from the existing heated bed MOSFET. I had to lower the PWM frequency to 5Hz so as not to switch the SSR too frequently. This works fine with PID control as heating the bed is still a very slow process. After a PID tune, the bed now takes about 30 seconds to get from ambient temperature to 60°C (140°F) for printing PLA.

I initially had a lot of trouble getting PLA to stick. It was quite disheartening after all that effort. I did a lot of reading and tuning to try and get better adhesion but saw no improvement. Eventually, the solution was to print the first layer about 10°C degrees hotter than normal. Now I have no first layer adhesion issues. I use some spray on isopropyl alcohol to clean the bed every few prints.

I’m really happy how the new bed turned out. Its larger, more efficient and cleaner to use.

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