Slow Down Your First Layer (Do This Every Time)
Last lesson we went through the five settings that actually matter. This one's just about the first one of those that matters most, and it's not really a setting so much as a habit.
Slow down your first layer. Every time. No exceptions.
I'll give you the opinion straight, because it is one I hold as close to fact as I hold anything in this hobby: the first layer is the whole print. Everybody wants their print to go fast. They see the default speed, they see a time estimate of six hours, they bump the speed up to make it four, and the first thing to suffer is the first layer. That's backwards. Get the first layer stuck down right, flat, even, no gaps, and the other 300 layers on top of it are going to behave. Mess up the first layer and you can have perfect settings everywhere else and it won't matter. Nine out of ten problems people bring me, when I ask what the first layer looked like, that's where the problem was hiding.
Why the first layer is different from every other layer
Every layer after the first one is plastic bonding to plastic. Same material, similar temperature, it wants to stick. The first layer is plastic bonding to your bed, which is a totally different material, usually cooler right at the surface, and it only gets one shot to grab on. If it doesn't grab, the whole print is loose from the ground up. Corners lift. The nozzle can drag the piece around. You get a shipwreck. I've got mine on the shelf, my very first print, looking like a candle that melted in the sun, because my bed wasn't level and I didn't know enough yet to slow down and watch.
So we slow down. Specifically:
What to actually change in your slicer
First layer speed. Most slicers have a separate setting for this, sometimes buried under "speed" and labeled something like "initial layer speed" or "first layer." Cut it to somewhere around 20mm/s, even if your normal print speed is 60. Some slicers do this by default. Check it anyway. Don't assume.
First layer height. Slightly thicker than your normal layer height, not thinner. If you're printing at 0.2mm layers, set your first layer to something like 0.24 to 0.28mm. That gives the nozzle a little more room to squish the plastic down and get good contact with the bed. Counterintuitive for some people. Do it anyway.
Bed temperature, first layer only. A couple degrees warmer than the rest of the print, if your slicer lets you set it separately. Warmer bed for that first layer helps adhesion, then you can drop it slightly for the rest of the print so things don't get too soft up top.
Watch it happen. Not the whole print. Just the first layer, which usually takes somewhere around 5 to 10 minutes depending on the size of what you're printing. Stand there. If it's not going down clean and even across the whole plate, stop the print. Don't let it keep going hoping it fixes itself. It won't.
What "clean" actually looks like
You're looking for lines that lay down next to each other with no gaps, no see-through spots, and no little beads catching on the nozzle as it passes back over. Run your finger near it, don't touch it, just look close. If you see the plastic looking translucent or stringy in spots, your bed's too far away in that area, or too hot, or your first layer speed still isn't slow enough. If it looks squished flat with almost no visible lines at all, you're too close. You want something in between, lines you can see but no gaps between them.
When a student gets this right I'll tell them "that's a clean first layer, dang" and move on to the next thing. That's about as much praise as you're getting from me and it means something when you hear it.
A word on skipping this because you're in a hurry
I get why people skip it. You're excited, the print's queued up, you want to walk away. I'll tell you about Elijah, my nephew, who came to me at like 8 o'clock at night needing something printed for a school project due the next morning. Zero notice. Normally I've got no patience for that kind of last-minute scramble, and I told him so. But I printed it anyway, cranked the speed up to something like 7x what I'd normally run, first layer included, because there just wasn't time to babysit it properly. It came out rough. Held together exactly long enough to get turned in and graded. I brought up the deadline math to him for weeks after, because that's not how you do it when you've got the option to do it right.
That's the exception, not the rule. You had a printer sitting there and a deadline, so you made a call. Most of the time you don't have that excuse. Slow down the first layer, watch it, don't walk away until it's laid down clean.
Before next time
Run one print this week where you only change the first layer settings we talked about here, nothing else, and watch that first layer the whole way through. Tell me next class whether it looked different from what you'd normally get.