Monday 26 August 2013

Plastic Primer and ABS adhesion to glass bed

(Summary for the busy - plastic primer does not help ABS adhesion to glass heatbeds!, for those interested in more detail -read on)

I have been using "ABS juice" applied to glass sheets on top of a heated bed for a long time now as the only reliable method I have found to get parts to stick well. The one thing that continues to annoy me is having to reapply the coating between prints. The issue is that the film of ABS does not stay stuck to the glass when it's cool, rather it stays stuck to the part printed on it (which makes perfect sense). What I have been looking for is something that will stick really well to the glass and ABS when hot but only to glass when cold.

This is a quick report on Plastic Primer, designed to get "Plastic" style paints to stick to anything, including glass.

It's the best thing I have found so far but it still does not work that well! The problem is the primer sticks really well at low temperatures <50C but becomes tackier the warmer it gets and by the time it's up to ABS bed temperature it does not stick to the glass very well.

The best results I got were after repeated applications of a thin coat of primer, followed by a wipe with ABS juice. The acetone dissolves the primer, making an ABS and primer mixture. This mixture sticks to the bed better than pure ABS Juice and sticks to the object better than pure primer.

Setting the heated bed to the right temperature is also key: too hot and the primer/juice mixture lifts off with the part, too cold and the abs warps and lifts off the bed. In the picture below the marks towards the centre are from when the bed was too hot (100C) - almost clean glass left. The marks towards the front are with the bed too cold (60C).

The parts I was printing are not normally a problem for warping: the Y bearing mounts for a Mendel90. They have rounded corners which helps to reduce warping as well. An example of the warped parts:

Setting the bed to 80C, no fan and first layer 200% extrusion, I managed to get non-warped prints that stuck to the bed:

That said, this method was far from ideal, and appears to only work with parts that are not very prone to warping.

So..... if anyone knows of an adhesive/primer/something else that sticks really well to glass at 20C to >100C, sticks really well to ABS at >100C but does not stick to ABS at room temperature then let me know!

Thursday 15 August 2013

Stepper Driver Thermal Testing

As mentioned in my previous post I am working on an improvement on the original Stepstick design. I arranged for a test to be carried out on the original Pololu (green, A4988), a Pololu "Black Edition" (A4988 on a 4 layer board), A4982 Stepstick (bought from eBay, assumed thin copper and 2 layer board), and the prototype of my new design, provisionally called the "Ice Blue Stepstick". Each one was subjected to the same current and stepped through the same sequence with thermal images to see how efficient they are at heat dissipation. The result: the new design is more than 10% better than the Pololu Black Edition!

The test results in full are below:

Comparison of Stepper Driver boards

Thermal imaging tests carried out for Think3DPrint3D.

Set up
Thermal testing of various different stepper driver “shields” carried out using a test sketch.
       Sketch set to run 10,000 x 100us steps clockwise followed by 15,000 x 50us steps counterclockwise, with a 200ms delay between direction changes.
       Drivers were set to 1/16th microstepping with the current limit set to 1.25A.
       Drivers were run for 5 minutes and a thermal image of the driver shield was taken at the end of that time, while the motor was still running.
       No additional heatsinking was added to the stepper driver ICs.
       The motors were all the same model, unloaded, 4.4kg/cm NEMA 17 motors used on many 3d printers.

Drivers tested were as follows:
1.    Stepstick - A4982 - TSSOP IC - 2 Layer PCB - 1oz copper 
2.    Pololu 1182 - A4988 - QFN IC - 2 Layer PCB - 2oz copper
3.    Pololu 2128 - A4988 - QFN IC - 4 Layer PCB - 2oz copper
4.    T3P3 Ice Blue Stepstick - A4982 - TSSOP IC - 4 Layer PCB - 2oz copper


1.     Stepstick - A4982 - Max temp = 133.8C, thermal shutdown was occurring during the test. 

2.     Pololu Green - A4988 - 2 Layer - Max temp = 110.6C

3.     Pololu Black - A4988 - 4 Layer - Max temp = 88.8C

4.     T3P3  Stepstick – A4982 – Max temp = 79.0C


The old Stepstick tested as number 1 as almost certainly built using 1Oz rather than 2Oz copper and is dual layered rather than 4 layered hence the worst thermal performance. In addition the supplier has not published their design changes from the original Stepstick so it is hard to see how thermally efficient (or not!) that design is.

Pololu claim a 20% improvement in thermal efficiency between the 2oz 2 layer board (no. 2) and the 2oz 4 layer board (no. 3) for the A4988 which these test results support.

The "Ice-Blue" Stepstick which is similar to the A4988 4-layer Black Edition Pololu runs slightly cooler than the Pololu, probably due to the larger heat dissipation area of the A4982 TSSOP package.

The 1.25A test current is higher than that used for most RepRap type 3D printers and heatsinking & a cooling fan would be recommended to further reduce the temperatures, even for the Black Pololu/Ice Blue Stepstick.

Tuesday 6 August 2013

Stepstick re-design

The Pololu stepper motor driver carrier and the open source Stepstick are pretty ubiquitous now as stepper driver daughter boards for a wide range of 3D Printer controllers. The Pololu driver has gone through a few iterations and the latest Allegro based version is the "Black Edition" which we have been supplying for some time now. It runs significantly cooler than the older version but is more expensive.

At the same time there have been a number of variations on the Stepstick theme but these have either had a bad reputation for poor quality assembly or have not had the designs published and so were not open source.

In the quest to continue to reduce prices while keeping high quality I decided to redesign the Stepstick to use an A4982 on a 4 Layer, 2Oz copper PCB (same as the Pololu Black Edition), aiming in the design to maximise heat dissipation. At the same time I have incorporated the design changes recommended by Nophead to fix low current microstepping issues and also increased the range of current supported by the Stepstick to the full 2A that can theoretically be supplied by the A4982 chip. Schematic is below:

I have had some prototypes made and initial testing is very promising: They run a mendel 90 fine with no heatsinks, where the original Pololus on that machine needed heatsinking.

There is further testing to do and I will post an update on that shortly.

Update:  Thermal test results are in! : Stepper Driver Thermal Testing

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