October 2011 Design Tip

Mould Finish vs. Part Finish

Resin performance can be measured in many ways, and many characteristics can be found in data sheets. One of the more difficult traits to reduce to numbers is finish, which often takes a backseat to more functional characteristics.

Many factors contribute to a finished part’s appearance—e.g., resin colour or tendency to sink or “orange peel” (texture resembles the bumpy surface of the skin of an orange)—but the finish of the mould itself can be a key factor. The problem is that mould finish doesn’t always equal part finish. Resins like polycarbonate and ABS will typically match the mould finish quite faithfully. Conversely, the finish of glass filled nylon is more influenced by the glass content than by the mould surface, though most users consider its unattractive appearance a small price to pay for its high strength. Similarly, the thermoplastic elastomer Santoprene will show more-or-less the same matte finish no matter how you finish the mould, while providing flexibility and impact resistance that few other resins can match. Between these two extremes, resins like acetal and HDPE have both strengths and weaknesses when it comes to appearance and are reasonable choices when appearance is of some value but is not the primary consideration in resin selection. In short, if appearance counts, don’t just assume that mould finish will determine exactly what you get. If a specific finish is important, choose a resin that will accurately reproduce that finish.

To give some sense of how widely finish fidelity can vary, we’ve developed a new design aid we call Protogami. This consists of six linked tetrahedral pyramids that can be rotated through four different configurations.

Proto Labs' newest design aid, Protogami.

Proto Labs' newest design aid, Protogami.

Each tetrahedron shows four different finishes on a single resin:

  • SPI-A2: Grade #2 Diamond Buff, 1-2 Ra
  • PM-F0: Non-cosmetic—finish to Protomold discretion
  • SPI-C1: 600 grit stone, 10-12 Ra
  • PM-T2: Protomold texture, SPI-C1 followed by medium bead blast

Each of the Protogami’s four configurations shows six different resins in a single finish. The resins are:

  • ABS
  • Acetal
  • Polycarbonate (PC)
  • Thermoplastic elastomer (TPE)
  • Glass-filled nylon (PA-GF)
  • High-density polyethylene (HDPE)

In addition to showcasing resins and finishes, the Protogami highlights clip design, living hinges, the challenges of working in multiple resins, and design for ease of assembly. Request your own (no cost) Protogami, ready for assembly.

Check out this blog about the development of the Protogami.