04 - Heated printing surface (7/11/14)

Post date: Jul 11, 2014 5:15:14 PM

Generally when printing with PLA the results have been fine. However, when printing larger objects, I would have corner/edge curl and lift issues. This was very pronounced when I tried printing ABS. As long as I kept the object small and used a very thin raft, the results were decent. I tried the Hair Spray, the ABS/Acetone slurry, and glue sticks. I had the best results with glue sticks directly on the glass. This is messy and honestly a pain. I decided it was time to go with a heated print surface. It means going back to heavier materials, but I seem out of other options. I weighed everything with my kitchen scale in grams to try and find the lightest combination that would still give a strong, flat surface. The glass-on-glass solution I have been using totaled about 1100 grams. I think I can keep the new configuration around 2000 grams.

After reading up on many different approaches, I decided to try the following design (layers top to bottom):

------- Kapton tape

------- glass

------- Aluminum plate, 1/8" thick 6061-T6, 12" x 13" from Speedy Metals for ~$9

------ QU-BD 12" x 12" heating element attached to the underside of the top plate with Permatex RTV Red high temp gasket maker goop adhesive junk

| | Standoffs (flat-head stainless 8-32 bolts, counter sunk into the top plate with some pretty heavy duty compression springs)

------- Aluminum plate (1/8" thick 6061-T6, 10" x 13" from Speedy Metals for ~$9)

++++ bearing brackets and belt holder (bolted with custom standoffs), and wing-nuts

The wing-nuts let me adjust the table level in any direction but it's mostly for front to back level since there are individual Z axis adjustments for side to side. The idea is the aluminum will spread the heat evenly, so no hot spots and I want the top surface in direct contact with that, no gaps created by spacers (I had been using pieces of paper etc to level the glass front-to-back). If the glass is a problem (cracking etc.) I can go with custom tempered glass or I can try Kapton tape directly on the Aluminum which will probably involve polishing it up better.

The edges of the metal were filed to clean up burs, the holes drilled, and I used 600 emery paper on the top and bottom of the upper plate.

Pictures of the progress:

Here is the assembly (upside down), the belt bracket is not shown but I measured the front and rear offsets and got the belt height as close as possible (I think the rear offset was about 15 mm, while the front offset was about 5 mm) Note that I pre-drilled to switch the right side over to dual bearings to help square and distribute the load more evenly. I will print another bracket once it's up and running the heater then switch over to the 2-per-side configuration.

I had tried some other configurations, such as a 1/4" piece of plexi for the bottom plate, it was lighter but it flexed enough that it would sometimes bump into the rear motor bracket.

I was also concerned about the amount flex in the stainless guide rods but this config is actually lighter than when I had (2) 5/8" chunks of MDF on it.

I also realize I am losing about 35 mm of print height, but it's a fair trade.

I wanted to run a test print without the heater just to prove to myself the size and weight were still manageable, that I had all the clearances ok etc.

Note the gap between the upper plate and the fender washer, there is a nut in there. This helped stabilize the side-to-side wiggle because now the standoff bolt is held rigidly straight. Though not installed in the upper picture, a small steel washer between the wing-nut and Aluminum is recommended because of the softness of the Aluminum, it can get gouged when turning the wing-nut.

Once I was satisfied that I hadn't messed anything up, I cleaned the silicone pad with alcohol, cleaned the aluminum with acetone and applied a grid of dabs of the RTV to the aluminum and clamped everything. I had to be careful not to crush the thermistor, and there is another piece of wood on the underside to protect the aluminum. Also the amount of clamp force is not too heavy.

Overall I am very satisfied with the design, I will experiment with how fast I can print with the extra weight but I prefer the quality over the speed.

Next steps, re-install the top plate with the heater installed and connect it up and update the firmware/slicer as necessary. I realize I should have tested the heater before gluing it in place but it was late and I wanted to get it setting/drying overnight. I might end up paying the price for the impatience.

While waiting for the final curing, I applied the Kapton tape to the glass using the "soapy water technique" - was a little reluctant at first but saw only success stories with it.

A really big and up close picture of the relay. Because of it's current needs, the heater used shouldn't really be connected directly to RAMPS. The output (D8) that would normally drive the heater connects to the energizing coils, the power source runs through the contacts. Click, Click, Click and so on is now a regular part of printing until I go solid state. This is a simple 40A relay from Radio Shack designed for automotive applications.

The RAMPS power was moved over to the 11A input and a jumper made to the 5A input. Also, the thermistor from the heater is connected into T2 (green/black on top). As mentioned the power for the relay is on the bottom at D8

I found the RAMPS 1.4 Wiki to be of enormous help when figuring out how to connect things...

For power, I went with a Cooler Master EX2 725 because it has two 12V rails, they claim each rail can do 28A. Doing a little research I discovered the separate rails are identifiable externally because on one rail all the yellow wires are solid in color, the other rail have a black stripe on the yellow wires.

Partly to support local businesses I bought the power supply from Milwaukee PC, the proprietor was able to nearly match the NewEgg price and I didn't have to wait :) On that same theme, I have supported ACE hardware, Menards, Home Depot, Lowes, Speedy Metals, Radio Shack and probably a few more that are escaping my memory.

The relay will get mounted onto the channel eventually.

Once installed, spot-checking the temp seemed like a good idea, there is still calibrating to do for when I start printing with ABS but this is a promising start. I don't have a fancy IR thermometer, just a harbor freight multimeter. I Kapton taped the thermocouple to the print surface...

Also very promising are the results, the disc on the left was the print on the new bed but without the heater, on the right, well, you know...

It may not be obvious but these discs are flipped over so the left one shows the lift/curl I mentioned at the start of this post.

Here is the custom belt holder mentioned earlier, it's just 3 pieces of plexi, about 5 mm thick each the larger one is the base then 2 smaller ones stacked. You might ask "Why didn't you just print that?" The answer is, "because I had the printer torn apart and didn't plan that far ahead."

Time to make the bearing bracket so the right side doesn't feel inadequate, the STL file provided had 3 in it, I will have spares.

Here is a red bracket.

Isn't it funny how we make our 3D printers so we can make stuff for our 3D printers? From what I hear, building a CNC is similar.

Update 7/17/2014

The relay is mounted onto the frame

I decided it was time to see how well this heated surface would work with ABS. The first step was to switch the hot end. I have a .25 mm nozzle dedicated for the single spool of ABS I own - black. After reading about difficulty people had switching plastic types in the same hot end, I decided at the prices I could get them for at Ohio Plastics I could have one of each diameter for each type of plastic. I may be solving a non-existent problem but have a good wiring setup to make the switch over pretty painless. The only real hassle is the re-calibration of Z.

Next was to make sure the thermistor table was correct in the firmware. I had only measured the accuracy up to 60c for the PLA . A quick search revealed that a newer version of Marlin had an updated thermistortables.h that had the QU-BD as #11. Awesome. After updating the firmware, I started things heating while I updated the slic3r settings for the .25 hot end (the speeds were all wrong with the new heavier print bed).

After a short time I was printing the "Nina logo coin" that I personally use as a calibration print.

By far the best print surface I have seen on my device. The ridge is from the seam in the Kapton tape, I need to order a wider spool.

There are some potential future mods I would like to make or at least suggest to others.

• Insulate between the plates. There is significant heat loss radiating downward, it doesn't cause any damage, the bottom plate does not get noticeably warmer but it would be more efficient to keep the heat up top where it is intended.

• Lighten the aluminum plates by drilling holes or cutting in some pattern that retains strength. I had to turn up the current for the Y axis because of a couple slips. There is a fair amount of inertia in the 2200+ grams being flung around.

  • Move the stand-off position on the top plate to be more equidistant. The holes are drilled a little farther out to the edge than is optimal, this causes the outer edge of the aluminum to flex down a couple tenths of a millimeter.