Guide

How to prepare an STL file for jewellery wax printing

By ZCAD. Published .

The short answer

Before you send an STL for wax printing, confirm the size and units, check that every part meant to print as one piece is a single closed solid, look for duplicate or stray geometry, make sure fine details suit the wax and the casting process, and agree the sprue plan with whoever will cast the piece. Do these checks on the CAD file and again on the exported STL, because problems can appear during export.

1. Confirm the size and the units

An STL file describes a shape as triangles. It does not carry a dependable statement of the units it was drawn in. Some programs write units into the file and some do not, so a ring modelled in millimetres can open ten or about twenty-five times too large or too small in another program. Rhino's STL import options, for example, ask which units the model uses and only show units when the file has them saved (McNeel: STL import and export).

When you send a file, state the intended units and one or two key dimensions, such as the ring size, the inside diameter or the overall height. ZCAD's 3D viewer assumes millimetres unless you switch it, so a size that looks wrong there is a useful early warning.

2. Check the CAD file and the STL separately

A clean, closed polysurface can still produce a poor mesh if the export settings are too coarse, and Rhino will save an open mesh if you choose to export anyway (McNeel: STL import and export).

3. Open edges, watertight geometry and separate parts

For printing and for any volume or weight estimate, a part needs a clear inside and outside. A closed, or watertight, model has every edge shared by two faces. An open model has naked edges or gaps, so the software cannot tell what is inside. McNeel notes that some machines require completely closed meshes (McNeel: STL import and export), and that a mesh made from a solid in Rhino is watertight (McNeel: Mesh command).

Open geometry A box-shaped part with its top face missing, so the edges around the opening are naked and the inside is undefined.

Open: missing top face

Naked edges, no defined inside.

Closed geometry The same part with every face present, so every edge is shared by two faces and the inside is defined.

Closed (watertight)

Every edge shared, inside is defined.

Open versus closed geometry. A closed part has every edge shared by two faces; an open part has naked edges.

Separate parts are a different question. A file can hold several pieces on purpose, such as components that will be printed apart and assembled later. It can also hold pieces that only look connected: a head resting on a shank, or a gallery touching a band, without being united. In the exported mesh these remain separate shells, and slicers may handle touching or overlapping shells differently. Tell us which parts must print as one piece. We would rather ask than merge everything automatically.

Intentionally separate parts A ring band and a separate component drawn apart, with a clear gap between them. They are separate on purpose, listed and agreed in advance.

Separate on purpose

Listed and agreed in advance.

Accidentally disconnected parts A head sitting on a ring band, touching and overlapping it without being united. It looks connected, but the exported mesh holds two separate shells. Slicers and print preparation software may handle touching or overlapping shells differently, so check the exported mesh.

Touches or overlaps, not united

Looks connected; check the exported mesh.

Separate parts can be intentional. Parts that only look connected are separate shells in the mesh, and slicers may handle touching or overlapping shells differently, so join or unite them in the CAD model and check the exported mesh.

4. Join and Boolean are not the same as Weld

In NURBS modelling, Join connects surfaces that share edges into a polysurface, and Boolean operations such as union combine solids into one. When a closed, joined solid is meshed, Rhino merges the vertices along the joined edges so the mesh has no gaps (McNeel: Mesh command).

Weld works on meshes. It merges vertices that already sit in the same place, which is useful for exposing real openings: McNeel's watertightness check welds the mesh and then looks for naked edges (McNeel: STL import and export). Welding alone does not fill a hole, and it does not turn two overlapping solids into one. If a part should be a single piece, fix it in the CAD model with Join or a Boolean union before exporting.

5. Duplicate geometry, mesh validity, orientation and resolution

  • Duplicates and internal geometry. Copies stacked in the same place, hidden construction objects or faces left inside a solid can confuse printing software and volume estimates. Delete what should not print.
  • Mesh validity. Look for flipped normals, degenerate faces and naked edges in the exported mesh, not only in the CAD.
  • Orientation. Print orientation is usually decided when the print job is prepared. Tell us if any face, such as an engraved surface or a stone seat, matters most, and whether your caster has a preference.
  • Resolution. Export tolerance controls how closely the triangles follow the curved surfaces. Smaller values give more accurate but larger files (McNeel: Mesh command). The right value depends on the piece, so check fine details after export rather than relying on one universal setting.

6. Thin walls, claws, engraving and stone seats

Delicate features are where a model that looks fine on screen can fail. Check thin walls, claws and prongs, fine engraving or text, and stone seats against the wax, the printer and the casting process that will actually be used. There is no single minimum thickness that suits every material and caster, so if you are unsure, ask your caster and tell us what they expect. We can review or adjust the model through CAD amendments.

7. Sprues: agree a plan with your casting house

A sprue is the channel that carries molten metal into the piece. Before printing, agree with the casting house where metal will feed in, how the wax will attach to the tree, and how the sprue will be removed and the area finished. Some casters prefer to attach wax sprues themselves; others ask for feed sprues modelled into the file. An STL without a sprue is not defective, and adding one does not make a successful cast certain.

As one example, Formlabs' casting guide for its own castable resins describes printing feed sprues as part of the model for small details and placing thicker parts lower on the tree (Formlabs: casting with Formlabs resins). That guidance is written for those resins and that process. Treat it as an example of the questions to ask, not as a rule for every wax or caster, and not as ZCAD policy.

Illustrative sprue planning An illustrative ring attached by one feed sprue to a main sprue. Numbered markers show 1, the main sprue or tree; 2, the feed sprue; 3, the attachment and removal area. Placement, size and number of sprues depend on the piece, the material and the casting process. 1 2 3
  1. Main sprue or tree. Usually the caster's.
  2. Feed sprue. Modelled into the file or added by hand.
  3. Attachment and removal. Plan the finishing here.

Illustrative only.

Placement, size and number of sprues depend on the process.

Illustrative sprue plan. Where metal feeds in, how the wax attaches and how the sprue is removed are agreed with your casting house; the right answer depends on the piece, the material and the process.

8. What the ZCAD viewer can and cannot tell you

The 3D viewer on our pricing page opens your STL on your own device, without uploading it, and shows the model in metal with an estimated weight. It is a visual aid. It does not confirm that a file will print or cast successfully, and the weight is only meaningful for a closed model in the right units.

Checklist before you send

  1. Units stated, and one or two key dimensions given.
  2. Source CAD checked: parts that should be one piece are joined or united.
  3. Exported STL checked again: closed, no naked edges, no stray or duplicate geometry.
  4. Intentionally separate parts listed, with how each should be printed.
  5. Thin walls, claws, engraving and stone seats checked for the wax and casting process.
  6. Sprue plan agreed with your casting house, or a note saying who will add sprues.
  7. The purpose of the print stated: casting, checking proportions or a master for a mould.

Not sure about a file? Tell us about it before anything is printed. See jewellery wax printing for wax options, or contact us to discuss your file.

Have an STL ready for wax printing?

Tell us what the print is for, the units and any requirements from your caster. We will reply with what we see in the file and the best way to send it.

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