What a 3D print really looks like: layer lines, tolerances and fits
A print is not an injection-moulded part and it is not trying to be. What the surface looks like up close, how exact the dimensions are, how to get a snug fit or a working thread, and how strong it is.
The best moment in this job is handing over a part and watching someone turn it in their hands. The second best is when nobody is surprised by what they see, because they knew what a print looks like before they ordered. This post is for that second moment: the honest tour of a finished print, from the surface down to the millimetre, so you can design for it and judge the result fairly.
The surface: layers you can see
A printer builds a part from the bottom up in layers, usually 0.2 mm tall. On vertical and sloped surfaces those layers show as fine horizontal lines, like the contours on a map. Run a fingernail down the side and you feel them. On a matte PLA they are subtle; on glossy PETG they catch the light. Thinner layers (0.12 mm) make them finer and more than double the print time, which is worth it for a figurine and wasted on a bracket. Shallow slopes show the lines most, since each layer is a visible step; vertical walls and flat tops hide them best.
Three other marks are normal. A faint seam runs up the part where each layer starts and ends; we hide it in a corner when the shape allows. The bottom face is very smooth and flat, with slightly rounded edges where the first layer was pressed onto the bed. And wherever the model overhangs, there were supports: temporary scaffolding we snap off afterwards, leaving a slightly rougher patch underneath. The top surface is the one that looks most like a moulded part.
Dimensions: about a third of a millimetre
Printed dimensions typically land within about plus or minus 0.3 mm of the model, a bit tighter on small parts and a bit looser on large ones. Two predictable biases help you plan: holes come out slightly smaller than drawn (by around 0.2 to 0.4 mm on the diameter), and the first layer spreads a touch, giving the very bottom edge a tiny flare. If a dimension matters, say so in the quote and we compensate for it, or print a quick test piece.
| You want | Draw it with | Why |
|---|---|---|
| A loose fit (a lid, a cap) | 0.4 to 0.5 mm clearance per side | Slides on and off without force |
| A snug fit (a holder, a sleeve) | 0.2 to 0.3 mm clearance per side | Holds with light friction |
| A press fit (a pin, a bearing) | 0.0 to 0.1 mm clearance, tell us | We tune it and test it first |
| A hole for an M3 screw | 3.4 mm diameter | Holes shrink; the screw still passes |
| A hole for an M3 to self-thread | 2.8 mm diameter | The screw cuts its own thread into the plastic |
Threads and screws
Printed threads work from about M8 upwards; below that the printer cannot shape the thread profile well. For small screws there are three good options: let a self-tapping or a normal machine screw cut into a slightly undersized plastic hole (fine for a part that is assembled once), print a hexagonal pocket that traps an ordinary nut, or use a heat-set brass insert that we melt into the plastic and that takes a machine screw hundreds of times. Tell us how often the part will be taken apart, and we pick.
Strength: good in two directions, weaker in the third
A print is strongest along its layers and weakest across them, where one layer is welded to the next. Pull on a part sideways and it is impressively tough; pull it apart layer from layer and it gives up sooner. So orientation is a design decision: a hook is printed lying down so the load runs along the layers, not stacked up so it peels them apart. We make that call for every part. Inside, prints are rarely solid: a grid pattern at 15 to 40 per cent fills the volume, which is plenty for most things and keeps the price down. Parts that take real force get more walls and more infill, and PETG rather than PLA.
Colour and finish
The colour is the colour of the filament all the way through, so a scratch does not show a different colour underneath. Different spools of "the same" colour can differ slightly, so for a set of parts that must match, order them together. If you want a smoother or painted look, PLA sands well (start with 200 grit, work up), takes filler primer and then any spray paint; PETG is harder to sand but can be primed and painted too. We deliver parts cleaned of supports and stray strands, not sanded or painted.
Designing for the printer
- Keep walls at 1.2 mm or more; 2 mm is comfortable.
- Avoid overhangs steeper than about 45 degrees where you can, or accept support marks there.
- Add a 45-degree chamfer instead of a sharp bottom edge; it prints cleaner and hides the first-layer flare.
- Round inside corners. Sharp ones are where cracks start.
- Give text at least 5 mm height and 0.8 mm depth.
- Split a tall thin part into two that glue together, or lay it down.
The post on which file to send has the checks to run on the model itself.
If something is wrong
Check your parts at handover. Layer lines, a seam and support marks are the nature of the process, not defects. A crack, a missing feature, a warped base or a part that does not match what we confirmed is a defect: you do not pay for it and we print it again at no cost. If the print matches your file but the design itself does not fit, a corrected version is a new order, which is exactly why we are happy to look at a file with you before printing. The terms spell this out.
Have a fit, a thread or a tolerance to get right? Mention it in the quote and we plan for it.
Get a quoteCommon questions
- How accurate is 3D printing?
- Our prints typically land within about plus or minus 0.3 mm of the model, tighter on small parts and looser on large ones. Holes come out slightly smaller than drawn, by roughly 0.2 to 0.4 mm on the diameter. For a dimension that matters, tell us in the quote and we compensate or print a quick test piece first.
- Can you see the layers on a 3D print?
- Yes. A print is built in layers, usually 0.2 mm tall, and they show as fine horizontal lines on vertical and sloped surfaces. They are subtle on matte PLA and more visible on glossy PETG. Thinner 0.12 mm layers make them finer at more than double the print time, which is worth it for display pieces and not for functional parts.
- Can 3D printed parts have threads?
- Printed threads work from about M8 upwards. For smaller screws we either let the screw cut into a slightly undersized hole, print a pocket that traps a standard nut, or melt a brass heat-set insert into the plastic, which takes a machine screw many times. Tell us how often the part will be opened and we choose the right one.
- How strong is a 3D printed part?
- Strong along its layers and weaker across them, where layers are welded together, so we orient every part so the load runs along the layers. With thicker walls, more infill and PETG, a printed bracket or clip handles everyday forces well. Prints are not load-bearing safety parts, and are not food safe or medical, unless agreed in writing.
- Can a 3D print be sanded and painted?
- Yes. PLA sands well, starting around 200 grit and working up, then takes filler primer and ordinary spray paint. PETG is harder to sand but can be primed and painted too. We deliver parts cleaned of supports and strands; sanding and painting are up to you, or ask and we will tell you what the part needs.