1. Complete Plastic Part Design in Autodesk Inventor MA3060
Ryan Bullock
Mechanical Engineer

2. Class Summary
Learn how to design plastic parts formed by injection molding. This is a beginner level class, assumes no / little experience designing plastic parts.
Inventor tools used in the process
Material selection
Surface finishes
Design rules
Injection molding vocabulary
*Inventor steps not specific to injection molding design will be used, but not explained in detail.

3. What is Injection Molding?
Mother of all plastic processing techniques
Plastic pellets fed from a hopper
Pushed by a screw through a heated cylinder
Injected at high pressure into a series of gates and runners into a steel mold
Plastic cools in the mold
Mold opens and ejector pins release the finished part from the mold

4. Why Use Injection Molding?
High volume production
Inexpensive unit price
Very accurate – tolerances +-0.1mm
Create complex shapes
Variety of surface treatments
Many options for materials, blended materials, over-molding, co-injection, reaction injection, etc.
Relative long tool life

5. Common Vocabulary Terms
Core - side of the tool where the plastic part will stick to and is ejected from
Cavity - upper half of the injection mold usually the show surface of the finished product
Lifter - used to create undercuts that cannot be accessed from the outside
Slide - Portion of Custom plastic injection molds that is used for creating undercuts that can be accessed from outside the part
Direction of Pull - the axis that the cavity and core separate on
Shrinkage - how much the plastic material will shrink after cooled
Undercut - feature that cannot be created by the cavity nor core because other features are in the way
Draft Angle – angle of the taper on the part / tooling

6. Materials Used by Injection Molding
Virtually any type of Thermoplastic can be used for injection molding
Some common materials:
ABS
Polystyrene
Acetal
Polysulfone
Acrylic
Polyurethane
Cellulose Acetate
PVC
Nylon
Polyvinyl Acetate
Polyimide
TFE
Polycarbonate
Polyethylene
Polypropylene

7. Acrylonitrile Butadiene Styrene - ABS
Commodity Thermoplastic
Low cost
Rigid
Can achieve hi gloss
Scratch & flame resistant
Good dimensional stability
Strong & stiff
Often blended with higher cost plastics like PC for improved material characteristics

8. Acrylic- PMMA Hard and stiff Good weatherability Glass clear Glossy
Scratch resistant
Chemical resistant
Used for aircraft cockpit covers in WWII
Can be fragile unless blended with other plastics

9. Polyamides (Nylon)- PA
Rigid, tough, hard-wearing
Fatigue and creep resistant
Resistant to fuels and solvents
Can be sterilized by steam
Slippery
Good for gears
Able to achieve high gloss
Used on the Swiss Army Knife
Can be drawn into fibers to make clothing or rope

10. Polycarbonate - PC Rigid, stiff, tough Outstanding impact resistance
Good weather and flame resistance
Water-like transparency
Can be made glossy
UV stable
Non-Toxic
Excellent dimensional stability – even at high temperatures
Used on cellphone housings, helmets, automotive headlamps, DVDs

11. Ethylene Vinyl - EVA Flexible (rubbery) Good Chemical resistance
High friction coefficient
Used for handle grips, beer tubing, vacuum cleaner hose

12. Polyvinyl Chloride – PVC
Cheap
Easy to form
Easy to color
Water and Chemical resistant
One of the first widely available plastics
Considered environmentally unfriendly
Produces harmful dioxins
Use of Chlorine
Stabilizers and plasticizers that impede degradation contain lead, barium and hormone disrupters

13. Surface Texture MoldTech / Yick Sang Hides surface imperfections
Requires increased draft angle
Cheaper than polishing mold for glossy surface

14. Glossy Surface Levels of Gloss – Highest to lowest cost
SPI FINISH GUIDE
A - 1 GRADE #3 DIAMOND BUFF
A - 2 GRADE #6 DIAMOND BUFF
A - 3 GRADE #15 DIAMOND BUFF
B GRIT PAPER
B GRIP PAPER
B GRIP PAPER
C STONE
C STONE
C STONE
D - 1 DRY BLAST GLASS BEAD #11
D - 2 DRY BLAST #240 OXIDE
D - 3 DRY BLAST #24 OXIDE

15. Design Rules – Wall thickness
Depending on the material type, there is a range for the wall thickness that will provide the best results. Here are some examples:

16. Design Rules – Ribs & Bosses
Ribs and bosses should have a wall thickness of ~60% the thickness of the external walls and a maximum height of 5x the exterior wall thickness.

17. Design Rules – Draft Angle
To allow the part to come out of the mold, a taper in the direction of pull is preferred, this is called the draft angle. Typically, 1.5 degree is the minimum required, but different finishes and materials can change this number

18. Example 1 – Print Server Learning objectives: Draft angles Ribs Bosses
Shell tool
Snap features
Minimum wall thickness
Multi-body & derived parts
Slides & Lifters

19. Example 2 – USB flash drive
Learning objectives:
Small features
Surface evaluation
Closed loop loft
Shell tool
Snap features
Minimum wall thickness
Slides & Lifters

20. Questions?

21. More Information: Recommended reading and websites:
Plastics 2 – Materials for Inspirational Design - by Chris Lefteri
Industrial Design – Material and Manufacturing guide – by Jim Lesko

22. Autodesk, AutoCAD* [*if/when mentioned in the pertinent material, followed by an alphabetical list of all other trademarks mentioned in the material] are registered trademarks or trademarks of Autodesk, Inc., and/or its subsidiaries and/or affiliates in the USA and/or other countries. All other brand names, product names, or trademarks belong to their respective holders. Autodesk reserves the right to alter product and services offerings, and specifications and pricing at any time without notice, and is not responsible for typographical or graphical errors that may appear in this document. © 2011 Autodesk, Inc. All rights reserved.