Acetals, like Delrin, struggle to be cosmetic and have a tendency to crinkle as they cool, leaving an "orange peel" finish. HDPE is not ideal if polish is critical, but it can provide a reasonable finish, though not as true as polycarbonate or ABS. In short, if appearance counts, don't just assume that mold finish will determine what you get. If a specific finish is important, choose a resin that will accurately reproduce it.
What comes out of a mold obviously depends on the design of the mold itself, but it can also be affected by what goes into the mold. In complex molds, injected resin may travel a long way, cooling as it goes. As it cools, flow may be impaired, making it difficult to fill small features. If flows are split by a through-hole or similar features, knit lines will form where flows meet on the other side. Knit lines can even form on solid bodies if there is a thin wall beyond which the resin flow has to back-fill.
Resins with a greater tendency to shrink can cause sink or leave voids as they cool. Resins that don't flow particularly well can require higher injection pressure, which may result in more noticeable cosmetic or structural flaws such as flash, sink, porosity, and warp. Basically, if the geometry and resin combination cause difficulty in processing, quality will suffer.
With plastics being so widely used and deployed in such varied applications, material choice can be as important to your final results as any other aspect of your design. The options available to today's designer are greater than they have ever been. Standard resins alone offer a dizzying range of capabilities, and with custom compounding, the possibilities are virtually endless. In some ways, more choice means more complexity, but sites like ides.com, matweb.com, rtpcompany.com, and polyone.com offer resources that simplify the selection process.
Material data sheets are just the first step in resin selection. How various resin characteristics manifest in the real world often depends on their interaction with the design in which they are used. Integration of resin data into your design may begin with finite element analysis (FEA) software, either of the standalone variety or within a CAD program. This can be a significant step forward, but it cannot tell you all you need to know about the part that will come out of a mold. Prototyping can.
Gus Breiland is a customer service engineer manager at Proto Labs Inc.