Print, assemble, and test

This is the payoff chapter. You designed a model in Chapter 13: a PETG bowl with a central ridged reamer cone, a strainer floor, and a pour spout, saved out as an STL file. Now you turn that file into a real object you can hold, juice a lemon with, and judge honestly.

Treat this as a lab. Read it once all the way through so nothing surprises you, then work the numbered steps in order, with the slicer open and the printer warmed up. At the end you fill in a results table and keep it, because the next chapter, Iterate and improve, starts from exactly those notes.

A quick map of where we are going. First you set up the slicer for this specific part. Then you print it and watch the critical first layer. Then you do the post-processing that food contact demands. Then you assemble it (if your design is more than one piece), and finally you run five plain tests to find out whether the thing actually works.

Outsourcing instead of printing it yourself? If you are sending the model to an online print service (Chapter 2) rather than running your own machine, the service does the slicing and printing for you, so skip step groups A and B below. Tell them to use FDM in PETG, then rejoin this chapter at step group C, the post-processing, and run the same five tests. The finishing and the food-safety care are still yours to do.

Step group A: slice it

Slicing is the act of turning your STL into G-code, the line-by-line instructions the printer follows. We covered the general idea in Chapter 5; here we tune those settings for a juice-contact bowl. Open your slicer, import the STL, and set the following.

1. Material: PETG. Select a PETG profile, not PLA. We chose PETG back in Chapter 3 because it shrugs off the mild acid in lemon juice and tolerates a wash in warm water better than PLA does. If your slicer has a generic PETG profile, start there and adjust as below.

2. Nozzle: stainless steel. This is a hardware check, not a slicer setting, but do it now before the print starts. Chapter 14 made the case: a brass nozzle can contain a trace of lead, and this part touches food you eat. A stainless-steel nozzle removes that worry. If your nozzle is brass and you cannot swap it, reread Chapter 14 and make an informed choice; do not let it slide by forgetting it exists.

3. Layer height: 0.2 mm. Each printed layer will be 0.2 mm tall. This is the sensible middle setting: fine enough for a smooth-ish juicing surface, coarse enough that the print does not take all day. Finer (0.12 mm) looks nicer but takes far longer; we do not need it here.

4. Walls (perimeters): 3 to 4. A wall, also called a perimeter, is a solid outline the printer traces around the edge of each layer. More walls means thicker, stronger sides and a denser, less porous surface where juice will sit. Set 3 walls as a minimum, 4 if your slicer allows it without ballooning the print time. The bowl takes pressure when you press a lemon onto the cone, so this strength matters.

5. Top and bottom layers: generous. These are the fully solid layers that cap the bottom and (where relevant) the top of the print. Bump them up: roughly 5 to 6 top layers and 5 to 6 bottom layers, or in millimetres about 1 mm of solid skin. A thicker solid skin gives you a less porous, easier-to-clean juice-contact floor. Porous means full of tiny gaps; we want as few of those as we can get where juice pools.

6. Infill: 20 to 30 percent. Infill is the internal honeycomb that fills the hollow space between the walls. The juice never touches it (the walls and skins seal it off), so its only job is structural support. 20 percent is plenty for a small bowl; go to 30 percent if you want extra stiffness under the reamer cone.

7. Supports: only if the spout needs them. Supports are temporary scaffolding the printer builds under steep overhangs, which you snap off afterward. They leave a rough scar, which you do not want on a food surface. The cure is orientation, not supports: Chapter 9 explained that a printer can bridge and overhang a fair amount on its own. If the slicer's overhang preview shows the pour spout drooping, first try rotating the part a few degrees rather than reaching for supports. Add supports only if the spout genuinely cannot print clean, and keep them off the inner juicing surfaces.

8. Brim: on. A brim is a flat skirt of extra plastic the printer lays down around the base, fused to the first layer, that you peel off later. PETG plus a round base is a classic recipe for a corner lifting off the bed mid-print (called warping). A brim of about 5 mm of width grips the bed and holds the part down. The round base of our bowl has no corners to anchor it, so the brim earns its keep.

9. Temperatures (approximate): nozzle around 230 to 245 C, bed around 70 to 85 C. These numbers are approximate on purpose. PETG from different makers wants different heat, and the spool's own label is the authority. Find the temperature range printed on the spool or its box and use that. If there is no label, start near the middle (around 240 C nozzle, 80 C bed) and adjust using the symptoms in Chapter 7.

10. Orientation: bowl open-side up, reamer point up. This is the single most important slicer choice for this part, so give it a moment. Sit the bowl on the bed the way it sits on a table: opening facing the sky, reamer cone pointing up out of the bowl. Printed this way, the juicing surfaces (the inside of the bowl and the cone) face up and print as smooth top surfaces rather than against the textured build plate. Smooth juice surfaces are easier to clean and hold less in their crevices. Upside down would print the cone tip into thin air and ruin it.

Don't be confused. "Orientation" and "position" are not the same thing. Position is where on the bed the part sits (center is fine). Orientation is which way up it is tilted, and that is what decides which surfaces come out smooth and whether you need supports. When we say "bowl open-side up," we mean orientation.

Slice, then look at the slicer's print-time and material estimate so you are not surprised, and check the layer preview: scrub through it and confirm the cone, the strainer holes, and the spout all appear and look solid. Export the G-code to your SD card or send it to the printer.

Step group B: print it

11. Start the print and watch the first layer. The first layer is where most prints succeed or fail. Stay and watch it. You want each line of plastic squished gently onto the bed and stuck to its neighbor, forming a continuous sheet with no gaps and no plastic balling up behind the nozzle. The brim should lay down flat. If the first layer looks wrong (lines not sticking, or the nozzle gouging the bed), stop, and turn to Chapter 7: first-layer problems are almost always bed level or nozzle height, and they are quick to fix.

12. Let it run. Once a good first layer is down, the rest usually takes care of itself. Glance at it now and then. PETG can leave fine hairs of plastic strung between features (called stringing); that is cosmetic and we deal with it in post.

13. Let it cool before you remove it. When the print finishes, wait. Pulling a warm PETG part off the bed can deform it or tear chunks of the bed surface. Let the bed cool to room temperature and most parts pop off with a gentle flex of the plate. Do not pry with metal near the juicing surface.

Step group C: post-process for food contact

The raw print is not finished. These steps tie directly to Chapter 14, so keep its reasoning in mind: we are reducing the rough, crevice-filled texture where gunk and bacteria could lodge.

14. Remove the brim and any supports; trim strings. Peel the brim off the base. Snap off any supports you added. Trim stray PETG hairs with flush cutters or a sharp blade. Take your time around the spout and the strainer holes so you do not nick the shape.

15. Sand the juice-contact surfaces, dry, with a mask. Lightly sand the inside of the bowl, the cone, and the spout to knock down the layer ridges and any roughness. Two safety rules here are not optional. Sand dry (wet-sanding makes a plastic slurry that is hard to manage), and wear a dust mask and work in a ventilated spot. Do not breathe plastic dust; fine particles are not something you want in your lungs. Start around 200 grit to remove ridges, then 400 grit to smooth. You are after a uniform, matte, snag-free surface, not a mirror.

16. Optional: apply a food-safe coating, and let it cure fully. Some people seal the juice surfaces with a coating sold as food-safe to fill the micro-gaps between layers. This is optional. If you do it, use a product actually labeled for food contact, follow its instructions exactly, and let it cure for the full time the label states before any food touches it. A half-cured coating is worse than none. We keep this honest: a coating helps cleanability but is not a magic certification.

17. Wash it well in warm soapy water. Sanding leaves dust; printing and handling leave who-knows-what. Before the first real use, wash the whole part in warm water with dish soap, rinse, and dry. This is also a good dress rehearsal for test 4 below.

Step group D: assemble (if needed)

18. Seat the reamer in the bowl. Many versions of this project print as a single piece, cone and bowl together, and if yours did, you can skip this step. If you split the design into a separate reamer and bowl (perhaps to print the cone in a different orientation), now is when you press or twist the reamer into its seat in the bowl floor and check it sits square and does not wobble. A wobbling cone will juice poorly and is the first thing to fix in the next chapter.

Step group E: test it (the five checks)

Here is the real exam. Back in Chapter 11 we wrote down what success means for this squeezer. Now we measure it. Gather a couple of medium lemons, a small kitchen scale, your lunch box, and a tap. Cut the lemons in half across the middle (equator), not pole to pole; that exposes more juice channels.

Test 1: juice yield. Put a lemon half on the scale and note its weight in grams. Juice it over a cup using your printed squeezer, then weigh the spent half again. The difference is roughly how much juice you extracted. Here is the handy trick: 1 millilitre of lemon juice weighs very close to 1 gram, so a drop of, say, 18 grams on the scale means you got about 18 ml of juice. Target: roughly 15 ml or more from a medium half. Less than that and the cone shape or ridges probably need work.

Test 2: seed catching. Pick a visibly seedy half and juice it. Then look in the cup and count how many seeds made it through to the juice. Zero or one is a pass; the strainer floor is doing its job. A handful of seeds means the holes are too big or too few.

Test 3: mess. Repeat a squeeze held over your open lunch box. Did the juice run down your wrist, or did it stay in the bowl and leave by the spout? You pass if you can juice a half without juice escaping down your hand.

Test 4: cleaning. Take the just-used, sticky squeezer to the tap and rinse it. You pass if it comes clean under running water in under a minute with nothing trapped in the strainer holes or the cone ridges. If pulp clogs the holes, note it; that is a classic next-chapter fix.

Test 5: size. Set the squeezer inside your actual lunch box and close the lid. It either fits or it does not. No partial credit. If it is a hair too tall or wide, write down by how much.

Your results table

Fill this in as you go. Be honest; a failed check now is a free improvement later.

Keep this table. The next chapter turns each failed or marginal row into a specific design change, and you cannot improve what you did not measure.

Don't be confused. A successful print and a successful product are two different wins. A successful print means the part came out clean: layers stuck, no warping, the cone is crisp, the surface is smooth. A successful product means it actually juices a lemon well. A part can print flawlessly and still juice badly, because juicing depends on the shape you designed, the ridge depth, the cone angle, the hole size, not on print quality. If your print looks great but test 1 gives you 8 ml, the print succeeded and the design needs work. Judge them separately so you fix the right thing.

Takeaways

• Slice this part as PETG, 0.2 mm layers, 3 to 4 walls, generous top and bottom layers, 20 to 30 percent infill, with a brim, and orient it bowl open-side up so the juice surfaces print smooth.
• Use a stainless-steel nozzle for a food-contact part, and use supports only if the spout truly needs them; prefer fixing overhangs by rotating the part.
• PETG temperatures (nozzle around 230 to 245 C, bed around 70 to 85 C) are approximate; the spool's own label is the real authority.
• Watch the first layer, let the part cool fully before removing it, then post-process for food contact: debrim, trim, sand dry with a mask, optionally seal with a fully cured food-safe coating, and wash before use.
• Run all five tests and write the numbers in the results table. A clean print is not the same as a working squeezer; measure the product, not just the part.

👉 You now have a finished squeezer and a table full of honest results, some of them probably less than perfect. That is exactly what we want. Head to Iterate and improve, where we read those numbers and turn each weak spot into a concrete change to the design.