Practical 3D Printing: Printer Calibration

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Printer Calibration

08.55

00:00	Precision instruments like 3D printers require calibration to work accurately.
00:05	This is a crucial process that directly impacts the quality of our prints, but can also help extend the machine's life as well.
00:13	The main focus of calibration for SLA printers is around exposure time testing, which as you might have guessed, helps us find the ideal exposure time to use for our prints, and this is of course different for each resin.
00:27	It also helps us ensure the exposure is even over the entire build platform, which would indicate issues with the release film or LCD screen.
00:36	Let's look at how we'd run an exposure test on our Alegoo Saturn 4 Ultra printer using the ChiduBox basic slicer.
00:44	This is a very affordable printer, representative of a consumer grade product, at least at the time of filming this course anyway, and ChiduBox is a widely used slicer, with the basic version being completely free.
00:57	We're going to gloss over the details when it comes to slicing for this example, because we'll come back to cover it properly in a coming module.
01:04	The first thing we need to do is decide how many zones we're going to split our build plate up into.
01:10	This could be four, six or eight, and each zone allows us to test a different exposure time.
01:16	Let's go with four for this example to keep things simple.
01:19	We'll place the same part model in each zone.
01:22	This part is specifically designed for this purpose, and has high detail and many different features, like engravings or embossings, for testing the quality of the print.
01:32	There are many different designs for these parts, so let's look at one that is most commonly used, being the XP2 validation matrix.
01:40	You'll find a file of this part attached to this module so you can use it as well, although it's usually easy enough to find online or in the slicing software.
01:50	With the part dropped onto the build plate in the slicer, we can first set the orientation.
01:55	We'll often use a print orientation with the part on an angle to improve the quality, but for this test, the part should be parallel to the build platform, so we can understand any variations in the dimensions for each individual axis.
02:09	The part needs to be copied into each zone, and it's important that the part fits entirely inside its zone and doesn't break through the imaginary dividing lines into another zone.
02:19	If the part is too big, don't scale it down, because we'll be checking its dimensions soon.
02:24	Instead, we'll just need to use fewer and larger zones.
02:28	We can then slice the part.
02:30	With all the settings, we'll be normal for this, with nothing out of the ordinary.
02:34	We'll get to this special step in just a minute.
02:37	After that, we just send it to the printer via Wi-Fi or using a USB drive.
02:42	At this stage, as far as the printer is concerned, this is just a normal printing job, but we actually need to set up the exposure test in the printer.
02:50	To do this, we navigate to the tool menu and then find resin calibration.
02:54	The icon in the bottom middle of the screen shows the number of zones, so for our example, we just need to set this to four.
03:01	Then we hit the plus button at the bottom right and import our model with the four calibration matrix parts.
03:08	Next, we can set the exposure time for each zone.
03:11	This should be centred around the recommended value from the resin supplier, with changes up and down in increments of 0.1 or 0.2 of a second.
03:20	And saying that, we could start with half a second differences to really get a broad range, and then narrow it down to whatever level of precision we think we need.
03:29	But 0.2 or so tends to be a fairly effective and efficient increment.
03:34	The trick here is to follow some order so we can remember the times easily, like using the max time in zone 1 and then decreasing it with each zone.
03:43	But it's best to also write them down to keep track of things.
03:46	As an example, we'll calibrate for the frozen TR300 high temperature resin that we'll be using in the coming practical modules, which has a recommended exposure time of 1.7 seconds.
03:58	So let's use 2.1, 1.9, 1.7 and 1.5 seconds.
04:04	Now we're ready to start the print.
04:06	If we removed the VAT and viewed the screen, we'd be able to see the different exposure times for each zone.
04:12	Of course, after the bottom layer exposure times were complete.
04:16	After the parts are printed, we can then carefully remove them from the build plate.
04:21	From here, we can clean the parts as thoroughly as possible, but also very carefully.
04:26	This is best done in a bath of cleaning agent like isopropyl alcohol.
04:31	This and the post curing is something we're going to get into in a coming module.
04:35	For these calibration prints, we don't actually want to post cure them, as this can change the size of the features slightly, and therefore skew the results of what we're getting straight out of the printer.
04:46	So how do we read the results from the validation matrix prints? First off, in the centre of the part, we want the tips of the infinity symbol to be just touching. If they're separated, this indicates underexposure, and if they're overlapping, that would be overexposure.
05:02	A similar idea is carried over to the bars and void section down the bottom.
05:07	We want each bar to just fit into the corresponding void.
05:10	If there's too much space, that's underexposed, and again, overlap would be overexposure.
05:16	Next, looking at the pins and holes in the left, we want to make sure these are an equal number of both.
05:22	More holes than pins means underexposure, more pins than holes means overexposure.
05:27	Past this, the middle square should be 20 by 20 millimetres.
05:31	Measuring either dimension smaller would point to underexposure, and larger to overexposure.
05:37	However, while exposure time has a big impact over the dimensional accuracy, it isn't the only type of dimensional calibration that we can do.
05:45	What I mean here is that the exposure of the print could be ideal, but the dimensions still be slightly off.
05:52	This mostly comes down to the settings of the machine in the slicer software.
05:56	Essentially, there can be a slight discrepancy between what the slicer thinks the dimensions of the printer are, and what they really are, and we can tweak the scale settings to correct these.
06:07	So if we print something with a known dimension in each axis, like our XP2 matrix, or more ideally something larger, then we can compare the physical measurements of the part to the nominal or target dimension.
06:21	If we take a measured dimension, for example 132.6 millimetres in the X direction, and divide it by a target dimension of 130, then we get 1.02, which means the print is 2% larger in this dimension.
06:37	We might see the same variation on the other axis, or it could be lower at 1%, or higher at 3% for example.
06:44	With this information we can then go into our ChidooBox software and open up the slice settings.
06:50	Under the machine tab we'll find the resolution and size settings that were set by default when initially setting up the slicer.
06:57	But this is something that we'll be covering in the next module.
07:00	Moving on, the resolution needs to remain the same.
07:03	This is the pixel resolution of the screen.
07:06	The size area we can think of like the scale settings, and these X and Y values should match the size of our screen, while Z is the build height.
07:16	If we unlink the resolution and size fields, and also the X and Y size values, we can actually tweak these numbers to account for the size.
07:25	For example, we need to multiply the X value by 1.02, so the slicer now thinks the screen is 2% larger in that dimension with the same number of pixels.
07:37	This means if we print the same part, the dimension should now be 2% smaller.
07:41	The important thing to remember here is that this shouldn't be used as a band-aid for incorrect exposure times, and in reality it shouldn't be needed to be adjusted for most modern and commonly used slicers and printers, where the accurate size of the screen is a well-known thing.
07:57	However, there still might be some instances where it can be required, like using older machines, or if our settings have been changed for any reason.
08:05	With that covered, let's summarise this module before moving on.
08:09	Exposure time calibration is a critical step to achieving quality prints.
08:13	For this, we'll use one of the many models available with features that we can check for under or over exposure.
08:20	The setup for this test will depend on the slicer and the printer that we're using.
08:24	A typical approach involves placing a model in each zone in our slicing software, and then setting up the desired exposure times for each zone in the printer settings.
08:33	After printing and cleaning the part, we can read the results, looking for signs of under or over exposure.
08:40	While the dimensional accuracy of this part is impacted by the exposure times, it can also be affected by the machine settings, which can be tweaked in the slicer to correct scaling issues.