What can cause these grooves?

In the picture below there a 2 truck bodies about 1/2 inch long. One is a duplicate of the other created by preform and they were printed on the same job. They both have some grooves on the top surface of the hood but the one on the left is much worse. I didn’t have this problem last week, but has been quite noticeable on the last 2 prints. The groves always seem to run parallel to the wiper. I also notice they get worse at the front of the build platform.

Here is the positioning of the parts:

This job also had groves at the spot I marked with red lines.

The only thing I can think would do this is something moving. Either the build platform is not locked down tight, or the resin tank is not locked down tight, or the PDMS in your resin tank is moving around somehow… But maybe it’s just a worn out tank. How many liters have you printed through this tank and what does the heatmap look like? Does the darkest area of the heatmap correspond with the print defects?

Can you include a picture of the support structures? From this angle it looks like you have very few support structures, not enough to hold the part fixed during printing.

Here’s the supports. I must keep as many points as possible on the bottom so I don’t mar the body of the cab. On this job I was using .4mm points.

Based on these pictures, I suspect insufficient supporting. One easy way to test (if you can spare the time and resin) is to turn up the touch point size and density and see if the problem disappears. If it does, then you can work to find the sweet spot of minimal supports vs part quality. If not, then it would appear you have a different problem.

I did another print with with 4 similar truck cabs placed along the back and front edges of the platform and they came out OK. Was using the same density of supports but the point size was left at the default of .6mm so maybe Craig is right about the point size being a problem.

Yeah, it looks to me like the thin structure was likely flexing too much as it separates each layer and that makes them misaligned.

I exclusively use 0.3mm point size for these kind of small parts. Your problem is probably the (lack of) strategic placement of supports allowing the part to flex and move a bit.

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Would rotating the part so it isn’t parallel to the wiper help?

Rotating and adding a couple more supports on the back of the cab seems to have fixed it.

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