DRAGON INNOVATION, INC. !

DESIGN  FOR  MANUFACTURING  !

COURSE 5: INJECTION MOLDING !!!!!!!

SCOTT N. MILLER | CEO | @DRAGONINNOVATE | WWW.DRAGONINNOVATION.COM

•Overview •Molding Process •Part Design •Materials •Techniques •Shot Nomenclature •Tooling Fabrication

AGENDA

Overview

Recommended References

www.matweb.com

Typical Engineering Molded Parts (Examples)

• Housings / Brackets

• Transmissions

• Joints

Injection Molding Advantages

• Supports highly integrated design - Complex geometry and fine details possible

• Low part cost • High volume production • Wide range of thermoplastic materials and fillers • Wide range of sizes • Low scrap rate • High reproducibility & Reasonable tolerances • Little post production required • Good surface finish • Can be fully automated • Reduces piece count • Simplifies assembly

Injection Molding Disadvantages

• High tooling costs and long lead-times

• Difficult to make changes

• Large undercuts are difficult

• Requires nearly uniform wall thickness

• Limited to Thermoplastic materials

• Cannot produce very large parts as a single piece.

Molding Process

Molding Cycle

1. Melt Shot

2. Clamp and Fill

3. Pack (Gate Freeze)

4. Cool

5. Eject

Molding Process

Three Plate Mold

Part Design

Temp and Pressure

12k psi @ 440F

Shrinkage

Design Guidelines

• Design for 2mm uniform wall thickness (0.5mm – 6mm).

• Ribs 80% wall thickness. • Draft depends on texture and resin. Assume 0.5 deg. • Use rounds and fillets. No sharp edges except parting

line. • Shoot for planar parting line if possible. • Tolerance is a function of the resin, part geometry and

mold construction. • ABS: +/- 0.1mm • ABS Hole: +/- 0.05mm up to 6mm diameter.

• Surface Finish: SPI A-1 Grade 3: 6000 Grit Diamond

Reference: Product Design for Manufacture and Assembly. Geoffrey Boothroyd,, Peter Dewhurst and Winston Knight.

Part Features

Use 2mm Uniform Wall Thickness

Reference: Plastic Part Design for Injection Molding. Robert A. Malloy.

Uniform Wall Thickness, con’t

Reference: Plastic Part Design for Injection Molding. Robert A. Malloy.

Gusset Bosses & Avoid Shrink Marks

Reference: Plastic Part Design for Injection Molding. Robert A. Malloy.

Draft Overview

Reference: http://www.diecastingdesign.org/design/basic/draft/

Use at 0.5 - 1.0 Degree Draft Angle (depends on texture)

Avoid Penetrations and Weld Lines

???

Reference: Plastic Part Design for Injection Molding. Robert A. Malloy.

Avoid Undercuts if Possible

Reference: Plastic Part Design for Injection Molding. Robert A. Malloy.

Avoid Undercuts if Possible

Lifter (Undercut)

Reference: Plastic Part Design for Injection Molding. Robert A. Malloy.

Screw Bosses

No Flatheads!

Reference: Plastic Part Design for Injection Molding. Robert A. Malloy.

Use Shut-Offs Instead of Slides

Reference: Plastic Part Design for Injection Molding. Robert A. Malloy.

Use Shut-Offs con’t

Reference: Plastic Part Design for Injection Molding. Robert A. Malloy.

Integrate Joint into Parts

Reference: Plastic Part Design for Injection Molding. Robert A. Malloy.

Living Hinge

Hide Rib Shrink Marks

Reference: Plastic Part Design for Injection Molding. Robert A. Malloy.

Materials

Materials

•Thermoplastic polymers (95%). Can go through molding cycle many times (i.e. similar to ice). No chemical bonds between strands.

•Thermosets (5%). Chemical react during process to form cross linked polymer chains. Irreversible. Excellent creep, dimensional stability, temperate and chemical resistance. Ex: phenolics and epoxies.

Materials, con’t

•Additives - Enhance a specific property (UV, stiffness, color,

flame retardants, etc). All have side effects. - Add chopped fiber (up to 30% volume). Strength

and stiffness, lose on impact, abrasion and tool live (cut down to 1/3). Properties approaching metal.

•Need to operate below 250C (400C max) •Low specific gravity (0.8 - 1.8) Steel = 7. Strength to weight ratio good. Automotive. •Some materials are transparent. •Very sensitive to changes in temperature (vs steel) •Good electrical insulation

Example: Chi Mei Polylac PA-747

http://www.matweb.com/

Resin Comparison

Resin Use Yield (MN/m2)

Modulus (MN/m2)

Heat Deflect (1.8 MPa : C)

Cost ($/kg)

Shrink (cm/cm)

ABS Housings 39 1,800 86 $2.09 0.004

PC Lenses 65 2,400 128 $2.81 0.007

POM Gears 65 2,800 105 $1.48 0.02

Polyamide Strength 123 2,750 65 $3.63 0.013

PP Food 36 1,380 93* $1.76 0.015

Aluminum 276 68,900 $1.75

Steel 435 205,000 $0.39

http://www.matweb.com/ http://www.nhh.com.hk/eng/trading/price_trend.asp

Techniques

Use Moldflow Simulation

Use a clear shot to see mechanism (ABS -> PC)

Other Techniques

• Steel Safe Design

• Add additional ribs after first shots

• Use Inserts around change areas

• Review mold drawings carefully. Watch out for cooling lines.

Other Types of Molding

• Co-Injection / Overmolding (toothbrush)

• Gas Assist

• Low pressure (EE)

Calc Molding Size

• Assume holding 2 - 5 tons / sq. in

• Assume 50% cavity pressure lost due to friction

• Calc surface area of combined parts and runners

• Look up injection molding pressure (ABS = 1k bar)

• Calculate cavity pressure (Molding Pressure * 0.5)

• Calculate Tonnage: F = P * A

Shot Nomenclature

Shot

Shot Nomenclature

Shot Nomenclature

Reference: Plastic Part Design for Injection Molding. Robert A. Malloy.

Shot Nomenclature, con’t

Tooling Fabrication

Tool Shop

EDM

Open Tools

Polishing

Finished Tools

Questions?

dragon@dragoninnovation.com