How-To: Make a Recycled Plastic Watch

Step by Step Build Process

Step 1: Prepare the Mould

Clean the mould cavity thoroughly. Apply a light coat of mould release spray to both halves. This prevents the plastic from sticking and makes part removal much easier, especially with PP.

Heat the mould to 60°C. Use an oven, heat gun, or the mould heater if your setup includes one. A warm mould helps the plastic flow evenly and fill the cavity without leaving short shots or cold spots.

Step 2: Place the Glass Dome

Before closing the mould, place the 30.5 mm domed watch glass into the front cavity. The flat surface sits against the mould face, with the dome protruding into the cavity. The plastic will be injected around it, locking the glass permanently into the watch case.

Take your time here. If the glass shifts during injection, the finished case will be unusable. Press it gently into position and confirm it is seated flat before moving to the next step.

Step 3: Close and Bolt the Mould

Carefully close the mould halves, making sure the glass dome does not shift. Once aligned, bolt the mould together firmly. Even pressure across all bolts prevents flash (thin plastic leaking from the mould seam).

Step 4: Inject

Set your Injection Mini V2 or Arbour Injection Machine V2 to 220°C. Load your prepared recycled PP into the barrel and inject into the mould.

The first injection fills the cavity around the glass dome, forming the watch body with the crystal already embedded. Make sure you have enough material loaded before injecting.

Step 5: Open and Remove the Body

Once the injection is complete, unbolt the mould and carefully open it. Remove the moulded body gently. If the part sticks, check two things:

• Mould release: you may need a heavier coat on the next cycle.

• Mould temperature: if the mould has cooled below 60°C during injection, the plastic may grip the cavity walls. Reheat and try again.

Do not force the part out. Prying with sharp tools can scratch the mould cavity and damage future parts.

Step 6: Inject the Back Plate

Using the same mould, prepare for the second injection. The back plate does not require the glass dome, so this is a simpler shot. Clean and reapply mould release, confirm the mould is still at 60°C, and inject at 220°C as before.

Remove the back plate once the injection is complete.

Step 7: Let Both Parts Cool and Shrink

Leave both moulded parts to cool for at least 24 hours before any drilling or assembly work.

Polypropylene shrinks as it cools. While the parts are still warm, they have not reached their final dimensions. If you drill holes or fit components before the shrinkage is complete, the tolerances will be off and the assembly will not fit properly.

Set both parts aside on a flat surface at room temperature.

Step 8: Drill All Holes

Once both parts have fully cooled and shrunk to their final size, drill all the holes in one go.

Place the watch body face down on a soft, firm mat (a padded cushion or folded microfibre cloth works well). This protects the glass dome and the front surface from scratches while giving you stable control during drilling. A Dremel with the appropriate drill bits works well for this. A drill press is also suitable if you have one.

Crown hole. Drill a hole in the side of the body for the crown stem using a 2 mm drill bit. Drill from the outside in, keeping the bit perpendicular to the case surface. Mark the drilling point with a centre punch first to prevent the bit from wandering on the curved surface.

Screw holes. Drill the pilot holes in the back plate for the M2 x 6 mm case screws using a 1.2 mm drill bit.

Go slowly with both. Recycled PP drills cleanly, but rushing can crack the case wall.

Step 9: Clean the Case and Prepare Your Workspace

Before any assembly begins, clean the inside of the watch body thoroughly. Once the back plate is screwed on, anything trapped inside stays there permanently.

Use a lint free cloth to wipe down the inside of the case. Pay particular attention to the inside surface of the glass dome. Fingerprints, dust, and drilling debris will all be visible through the crystal once the watch is assembled. Work in a clean, dust free area and avoid touching the inside of the glass with bare fingers from this point on.

Step 10: Assemble the Movement

From this point, assembly follows standard watchmaking practice. The NH35 is one of the most widely documented movements available, so you will find detailed tutorials for each of these steps with a quick search.

Prepare the dial. The 28.5 mm dial has four small pins on the back. Two of these align with the locating holes on the NH35 movement and must stay intact. Snip the other two flush using side cutters or fine flush cut pliers. If you leave them, the dial will not sit flat against the movement.

Fit the dial. Place the prepared dial onto the NH35 movement, aligning the two remaining pins with the corresponding holes. The dial should sit flat and level against the movement plate.

Secure the movement in the holder. Place the NH35 into a movement holder. This is a metal cradle that grips the movement firmly so it does not shift while you press the hands on. Do not attempt hand fitting without one.

Attach the hour and minute hands. Press the hour hand onto the movement shaft first, then the minute hand. Use a watch hand press (also called a hand setting tool) for both. It applies even, downward pressure without scratching the dial or displacing hands you have already fitted.

Attach the second hand. The second hand fits onto a much thinner pinion at the centre of the shaft. Switch to a second hand pusher, which has a smaller diameter tip than the standard hand press. Using the full size press here will damage the other hands or the dial.

These tools are inexpensive and available from the same watch parts suppliers as the movement and dial. Some hand press kits include interchangeable tips for both sizes.

Seat the movement in the case. Place the assembled movement and dial into the front case. The glass dome is already embedded, so the dial faces outward through the crystal.

Fit the crown and stem. The NH35 has a stem release lever on the side of the movement. Press it to release the existing stem, then remove it. Screw your 7 mm crown onto the new stem. Insert the stem through the crown hole you drilled in the case and push it into the movement until it clicks into the winding position. At this stage the crown will sit too far from the case because the stem is longer than needed.

Measure and cut the stem. With the stem fully engaged in the movement, use a pair of callipers to measure the gap between the crown and the case wall. This tells you how much excess stem to remove. Pull the stem back out by pressing the release lever again, then cut it to length using stem cutters or fine side cutters. Be precise. If the stem is too short, the crown will not engage the winding mechanism properly and you will need a new stem. If it is slightly too long, the crown will sit proud of the case but will still function.

Reinsert and test. Push the trimmed stem back into the movement through the case. The crown should now sit flush against the case wall. Test all three positions: pushed in (winding), pulled to first click (date setting), and pulled to second click (time setting). The crown should move smoothly between positions without resistance or looseness.

Step 11: Close the Case

Place the back plate onto the body and secure it with the M2 x 6 mm screws through the pre drilled pilot holes. Tighten evenly in a cross pattern to avoid warping the case.

Step 12: Attach the Strap

Thread the 22 mm Velcro strap through the integrated strap slots on each side of the case. The mould produces slots designed for this strap width.

Your recycled plastic watch is complete.

Tips and Troubleshooting

Short shots or incomplete fill. The plastic is not reaching all areas of the cavity. Increase the mould temperature slightly (up to 70°C) or ensure the barrel is fully up to 220°C before injecting. Also check that you have loaded enough material.

Flash around the mould seam. Thin plastic leaking between the mould halves. Tighten the bolts more evenly, or check the mould faces for debris preventing a clean seal.

Glass dome shifting during injection. The injection pressure can move the glass if it is not seated firmly. Make sure the flat face is fully flush with the mould cavity before closing.

Part sticking in the mould. Apply more mould release spray. If the mould has cooled below 60°C, reheat it before the next cycle. PP generally releases well from aluminium moulds, so persistent sticking usually points to temperature or release issues.

Drill holes not aligned. Measure twice. Always use a centre punch to mark drilling points before starting. This prevents the bit from wandering on the curved surface of the case.

Suitable Plastics for Watch Cases

Polypropylene (PP) is the recommended material for this mould. It offers a good balance of toughness, chemical resistance, and mouldability at desktop scale.

HDPE works well and produces a slightly stiffer case, though with a more matte surface finish than PP.

Avoid PVC, ABS, and any plastic you cannot positively identify. PVC releases harmful fumes when heated and must never be used with any SDS machine. ABS can work in some moulds but has a higher shrinkage rate that may affect the glass dome fit in this design.

For a full breakdown of which plastics work best with SDS machines, see our Plastic Types guide.

Looking for a Different Watch Design?

The SDS Watch Mould produces one specific case shape with an L crow strap configuration. If you have a different design in mind, whether that is a different case profile, a different strap attachment, or a completely custom watch shape, get in touch. SDS offers custom mould design and manufacturing for projects exactly like this.