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Complete Plastic Part Design in Autodesk Inventor MA3060Ryan BullockMechanical Engineer

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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.

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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

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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

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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

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Materials Used by Injection Molding

Virtually any type of Thermoplastic can be used for injection molding

Some common materials:

ABS Acetal Acrylic Cellulose Acetate Nylon Polyimide Polycarbonate Polyethylene Polypropylene

Polystyrene Polysulfone Polyurethane PVC Polyvinyl Acetate TFE

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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

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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

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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

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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

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Ethylene Vinyl - EVA

Flexible (rubbery) Good Chemical resistance High friction coefficient Used for handle grips, beer tubing,

vacuum cleaner hose

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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

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Surface Texture

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

surface

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Glossy Surface

Levels of Gloss – Highest to lowest cost

SPI FINISH GUIDEA - 1 GRADE #3 DIAMOND BUFFA - 2 GRADE #6 DIAMOND BUFFA - 3 GRADE #15 DIAMOND BUFFB - 1 600 GRIT PAPERB - 2 400 GRIP PAPERB - 3 320 GRIP PAPERC - 1 600 STONEC - 2 400 STONEC - 3 320 STONED - 1 DRY BLAST GLASS BEAD #11D - 2 DRY BLAST #240 OXIDED - 3 DRY BLAST #24 OXIDE

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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:

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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.

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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

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Example 1 – Print Server

Learning objectives: Draft angles Ribs Bosses Shell tool Snap features Minimum wall thickness Multi-body & derived parts Slides & Lifters

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Example 2 – USB flash drive

Learning objectives: Small features Surface evaluation Closed loop loft Shell tool Snap features Minimum wall thickness Slides & Lifters

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Questions?

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More Information:

Recommended reading and websites:

Plastics 2 – Materials for Inspirational Design - by Chris Lefteri Industrial Design – Material and Manufacturing guide – by Jim Lesko www.matweb.com www.protomold.com/DesignTips.aspx

ryanb@lantronix.com

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