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I recently bought a Kelant s400 LCD DLP 3D printer. I am using Wanhao resin (green) to print, using the recommended settings from Wanhao. I was printing a hollowed out cube as a test and it printed fairly ok, except one corner didn't print. Please see the attached photo. Turns out the corner stuck to the transparent sheet and not the rest of the print (base layers printed fine). Why would this happen and what can I do to prevent it?

Thanks

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The first thing to check is the transparent sheet itself - are there any defects (rough patches, creases, grazes) in the area in which the print was stuck to the "transparent sheet"[1]. This will cause the newly cured layers to cling better to that sheet than they should in that area, and likely caused the failure. If the sheet is marked in any way in that area, you likely need to replace it.

This kind of failure happens when the forces holding the newly cured layer to the transparent sheet are weaker than the forces holding the part together/to the build platform. These issues are usually to do with exposure settings - underexposed layers are less likely to bind together well and more likely to fail.

Since the majority of your part printed correctly, it looks as though your exposure settings are probably about right. If there are no defects in your transparent film though, there may be unevenness to the spread of light across your LCD that means some areas on the build area cure less well than others. You don't mention what slicing software you are using, but many slicers allow for a 'greyscale mask' that can compensate for this by making the image displayed on the LCD screen less transparent in the areas that the UV output is stronger so that the weaker areas can catch up. Note that after applying such a mask, you may need to alter your layer cure times to compensate.

[1] This transparent sheet is likely FEP film - essentially transparent flexible teflon, though I can't actually find that specified in the product details for this model of printer

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