Build Your Own SLA/SLS

During printing the resin appears to become progressively more reactive. Small positive features can start to thicken by +5mm Z. Some larger prints may fail after +40mm Z and progressively adhere to the PDMS.

Any advice on how to minimize this effect?

Or how to taper exposure duration with gcode as the print progresses?

Nb I'm printing at 33um xy and 30um Z using 2.8s exposures with heavily pigmented (black makerjuice at 1.3%) in B9 black resin on fresh pdms. Prints are shelled and tilted as much as possible. The resin increases in temperature from 20C to about 27C during a typical print.

Using a flex fep vat has solved all this kind of problems I was having with the standard PDMS

Thanks - I've got an FEP flex vat and will test it. I'd found on a projet 1200 that the flexible vat is great for tall prints but tends to wrinkle and generate 'wobble' in the prints compared to the pdms.

Does this happen with the littlerp flex vat?

I know I'm drifting off-topic, but minimaker could you expand on your last reply? I'm using a flexvat in my LittleRP and I also see some waviness or wrinkling in my prints, probably the same as what cew is talking about. I assumed it was due to the force needed to peel the print off the vat causing the print to flex a little bit. Does the tensioning of the vat really make a difference in that? (I've found that with MakerJuice G+, prints generally peel off within 1-2 mm of lift; for B9 Emerald it can take as much as 4-5 mm but is usually less.)

Have made some useful progress to fix the problem of increasing resin reactivity during a print.

To tackle this I created a tall test part with graduated small cylindrical holes to monitor bleed at different heights (see below). I tilted this at about 15 degrees from vertical and ran some test prints:

Decreasing exposure during a print: To do this I extended the bottom layers from 4 to 180 layers (about 6mm). Bottom exposure was lowered to just enable printing the littlerp peg calibration file across the build platform (about 3s). The exposure time was reduced by about 15% (~2.5s) for the rest of the print. This helped but needs more effective tapering of exposure.

Increasing retraction speed: Surface bleed increased rapidly at about the level where the build platform is above the resin. To improve resin mixing I tried increasing the retraction speed from 70mm/m to 140mm /m, 200mm/m and 300mm/m. Lift was increased from 3 to 5mm. Settling time was 2s. 140mm/m reduced the surface bleed by about %50 and the higher rates 200 - 300 mm/m almost eliminated the bleed increase leaving clear build lines on both sides of the test print.

Together these prevented most of the increase in bleed and we have not had any pdms adhesion print failures over the last ~30h print jobs (~5-30mm tall) on the same petri dish.

Below is an image of the test piece I used for assessing changes in reactivity and z bleed at different heights. The bleed is seen filling the smaller holes.

A followup note after a few more weeks use.

The problem of increasing reactivity has largely been fixed with prints up to 50mm high. The PDMS coated dishes are lasting more than 40h of use without failures due to print adhesion to the silicone. The progressive clouding of the PDMs is similar to http://www.instructables.com/id/Window- ... /?ALLSTEPS

In hindsight the problem developed because I'd dropped the retraction rate from the default 200mm/m to 70mm/m for some delicate prints <5mm tall. It appears that the build platform mixes the resin adequately when low but higher retraction rates are necessary when it is moving above the resin.

The only compromise appears to be that a few more supports are necessary to ensure mechanical stability of the prints.

Another update after a few more months use:

High retract rates have helped; improve surface quality, reduce print failures plus 'flaking' on taller prints and improved pdms life. Presumably these help mix the resin similar to wipers used on other printers.

Am currently using 5.5 mm lift with 300mm/m retract that slows to 200mm/m for the last mm. Then a ~2s settle time.