Notes injection moulded plastic

Design

1. More or less required:
   1. All measures & tolerances nominal
   2. Dont use asymetric tolerances (higher cost for tool design and risk for errors)
   3. Specify roughness for all surfaces. Avoid highly smooth surfaces. Surfaces are created when the tool cavity is finished, usually with Milling or Spark erosion (EDM), that might be followed by additional treatments like Polishing, Texture, Etching, or Blasting. Specify as coarse tolerances as possible. Better roughness than the one provided by an ordinary milled surface in the tool adds cost for the tool.
   4. Proper draft angles (typically 1° to 3°) so the plastic part comes out of the tool.
      1. Larger angles for surfaces with texture, and for visible surfaces
      2. Where tool steel meets tool steel typically 5° is used
         • Avoids flash and tool damages
   5. Part material thickness even and reasonable (aim for 1 - 3 mm)
      1. Shorter cycle time in injection moulding unit (i.e. lower unit cost)
      2. Better plastic parts
   6. Plastic ribbons should be thinner than the thickness of the plastic (t)
      1. Rule of thumb 0,5 to 0,7*t (in order to avoid shrink marks)
   7. Unbroken surfaces
2. Advice and improvements:
   1. Avoid sharp inner corners on the plastic part (more difficult to get out the injection moulded plastic part of the tool)
   2. Avoid using ribbons:
      • Significant additional tool cost
      • Makes it more difficult to get out the plastic part of the tool
      • If possible, mould samples without ribbons, and add them afterwards if they turn out to be needed
   3. Avoid high thin towers on the plastic part (give lower strength of plastic part, more expensive tools, and problems with trapped air when moulding)
   4. The Tolerance standard DIN 16901 (since 2011 being revised) is better suited for plastic than ordinary ISO-tolerances (intended for metal), even though level C sometimes can be too demanding. Observe that the standard says nothing about the plastic parts shrinking and changes with temperature, swelling with moisture, chemical impact, etc. Neither does it say anything about deformations when the part is charged.
3. Tool manufacturing:
   1. Discuss with the tool manufacturer where the inmoulds can be placed (the place on the cavity where plastic material is injected) and how much these points are allowed to be visible on the ready part. You can accept the small mark that a conventional inmould gives, or use hot runners (more expensive tool), or post treating it manually or by robot (expensive for each part).
   2. Send the tool manufacturer files with 3D-solids in format according to the following priority order: (IGES common, BUT often gives the manufacturer unnecessary practical problems)
      • 1. Parasolid (*.x_t), 2. STEP, 3. ASIS, 4. IGES
   3. Discuss the intended production volumes with the tool manufacturer. Up to a couple of thousand plastic parts prototype tools usually provide the best economy. The number of cavities in the tool (in a moulding cycle with the tool each cavity gives a copy of the part) might strongly affect the production economy. Depending on the production volume you can choose tool materials (such as quenched and tempered steel) and finishes (eg surface hardening) for different tool life (injection mould cycles) and tool cost (manufacturing and maintenance).

Reading

1. Articles and introductions for designers:
   1. "Konstruktionstipsens 10 i Topp", Plastforum nr 9, del 2, 2002 (Plastnet)
   2. "PIR Plastskola", Plastinformationsrådet
   3. "Design Guides for Plastics", Clive Maier, Econology Ltd (PRW)
   4. "Mycket på en gång - Formsprutning med flera färger och funktioner", från "Bearbetning från A till Ö", Plastforum nr 2, 2005 (Plastnet)
2. Books design plastic and injection moulding:
   1. "Konstruera i plast", Berggren (Adlibris)
   2. "Producera i plast", Strömvall (Adlibris)
   3. "Arburg Practical Guide to Injection Moulding", Goodship (Amazon)
   4. "Plastic Part Design for Injection Molding: An Introduction", Malloy (Amazon)
   5. "Injection Molding Alternatives: A Guide for Designers and Product Engineers", Avery (Amazon)
   6. "Designing Plastic Parts for Assembly", Tres (Amazon)
3. Books multiple components injection moulding:
   1. "Multi-Material Injection Moulding", Goodship & Love (Amazon)
   2. "In-Mould Decoration of Plastics", Goodship & Love (Amazon)
4. Reference:
   1. "SS-ISO 12165 / Tools for moulding -- Components of compression and injection moulds and diecasting dies -- Terms and symbols (Svenska, English, Deutsch) / Formverktyg - Termer och symboler", SIS Industriteknik (SIS Förlag)