Select Page

PlasticsEdge™ microsite sponsored by...

Injection Molding Reference Guide

7 Basic Mold & Part Design Guidelines

Plastic parts and injection molds require many design consider-ations to meet the expectations of the consumer. The following plastic part and mold design rules should be taken into consider-ation when designing either a part or mold.

7.1 Uniform Wall Thickness

Design the part with the most uniform nominal wall thickness as possible. This will minimize sinking, warping, residual stresses, material usage, and improve cycle times. Ribs should be placed in areas of stress instead of increasing the wall thickness. By minimiz-ing the wall thickness you will reduce the cycle time, shot weight, and have quicker cooling.

Figure 7a: Suggested Wall Thicknesses for Injection Molded Thermoplastic Parts

minimum

average

maximum

Material

mm

in

mm

in

mm

in

Acetal

0.4

0.015

1.6

0.062

3.2

0.125

ABS

0.8

0.03

2.3

0.09

3.2

0.125

Acrylic

0.6

0.025

2.4

0.093

6.4

0.25

Cellulosics

0.6

0.025

1.9

0.075

4.7

0.187

FEP

0.3

0.01

0.9

0.035

12.7

0.5

Nylon

0.4

0.015

1.6

0.062

3.2

0.125

Polycarbonate

1.0

0.04

2.4

0.093

9.5

0.375

Polyester T.P

0.6

0.025

1.6

0.062

12.7

0.5

Low Density polyethylene

0.5

0.02

1.6

0.062

6.4

0.25

High Density Polyethylene

0.9

0.035

1.6

0.062

6.4

0.25

Ethylene Vinyl Acetate

0.5

0.02

1.6

0.062

3.2

0.125

Polypropylene

0.6

0.025

2.0

0.08

7.6

0.3

Polysulfone

1.0

0.04

2.5

0.1

9.5

0.375

PPO (Noryl)

0.8

0.03

2.0

0.08

9.5

0.375

Polystyrene

0.8

0.03

1.6

0.062

6.4

0.25

Styrene Acrylo Nitrile

0.8

0.03

1.6

0.062

6.4

0.25

Polyvinylchloride (Rigid)

1.0

0.04

2.4

0.093

9.5

0.375

Polyurethane

0.6

0.025

12.7

0.5

38.1

1.5

Ionomer (Surlyn)

0.6

0.025

1.6

0.062

19.1

0.75

7.2 Corner Radii

All corners should have generous radii to avoid excessive stress concentrations. Such stress concentrations can lead to unexpected part failure and should be minimized during all parts of the plastic part production. All features such as bosses, ribs, and shut offs should have generous radii to reduce stress concentrations. To maintain uniform wall thickness and reduce stress concentrations, the use of an inside radius of ½ the nominal wall thickness and an outside radius of 1½ of the nominal wall thickness is recommended.

7.3 Ribs and Gussets

These design features should be placed anywhere the part needs in-creased stiffness. These features will reduce the material consump-tion, decrease cycle time, as well as increase the structural integrity of the part. Always avoid excessively tall, thick, thin, and sharp cornered features when possible.

7.4 Draft Angle

An appropriate draft angle will aide in the removal of the part from the mold. Without the proper draft angle parts may become hung up or stuck upon ejection and mold damage may occur. Be sure there are appropriate draft angles on all bosses, ribs, and nominal walls. The draft angle is most often between 1° and 2° depending on the material and desired surface finish.

7.5 Undercuts

Undercuts are mold features that prevent part ejection or mold opening. Undercuts such as clips are commonly added to the part design to simplify post molding operations such as assembly. Unfortunately, the features increase the cost and complexity of the mold because of the additional mechanisms needed to free the part from the mold.

7.6 Surface Finish

Most plastic part designs call out a specific surface finish or texture. These features may significantly increase the cost of the tool and should be taken into consideration based upon the application. Highly polished molds require many hours of work to achieve a mirror finish and can be exponentially more expensive than the same tool with a rougher finish. The highest finishes require all lower grades of finishing to be performed before they can be achieved. Textured molds can be very difficult and time consuming to make and may require expensive equipment or a higher degree of skill than polishing.

SPI

Finish

Finishing Method

Visual Description

Roughness R.A.

A-1

Grade #3 Diamond

Mirror/Lens – 420 SS material

0-1

A-2

Grade #6 Diamond

Highly Polished

1-2

A-3

Grade #15 Diamond

High Polish

2-3

B-1

600 Grit paper

Medium – High Polish

2-3

B-2

400 Grit paper

Medium Polish

4-5

B-3

320 Grit paper

Med – Low Polish

9-10

C-1

600 Stone

Low – Med Polish

10-12

C-2

400 Stone

Low Polish

25-30

C-3

320 Stone

Lower Polish

35-45

D-1

Dry Blast Glass Bead

Satin Finish

High/Variable

D-2

Dry Blast #240 Oxide

Dull Finish

High/Variable

D-3

Dry Blast #24 Oxide

Coarse Finish

High/Variable

Figure 7c: Runner Shapes and Dimensions

A

Full Round

Modified Trapezoid

Modified Trapezoid

in

mm

in2

mm2

in2

mm2

in2

mm2

1/8

3.2

0.012

8.0

0.016

10.4

0.015

9.7

3/16

4.8

0.028

18.1

0.036

23.4

0.033

21.8

1/4

6.4

0.049

32.2

0.063

41.6

0.059

38.7

5/16

7.9

0.077

49.0

0.099

63.4

0.092

59.0

3/8

9.5

0.110

70.9

0.143

91.6

0.133

85.3

7/16

11.1

0.150

96.8

0.194

125.1

0.181

116.4

1/2

12.7

0.196

126.7

0.254

163.8

0.236

152.4

5/8

15.9

0.307

198.6

0.397

256.7

0.369

238.9

Figure 7d: Suggested Circular Runner Dimensions

Material

DIA. (in)

DIA. (mm)

ABS

.187-.375

4.7-9.5

Acetal

.125-.375

3.1-9.5

Acrylic

.321-.375

7.5-9.5

Ionomer

.093-.375

2.3-9.5

Nylon

.062-.375

1.5-9.5

Polycarbonate

.187-.375

4.7-9.5

Polyester

.187-.375

4.7-9.5

Polyethylene

.062-.375

1.5-9.5

Polypropylene

.187-.375

4.7-9.5

Polyphenylene Oxide

.250-.375

6.3-9.5

Polysulfone

.250-.375

6.3-9.5

Polystyrene

.125-.375

3.1-9.5

Polyvinylchloride

.125-.375

3.1-9.5

Styrene Acrylonitrile

.187-.375

4.7-9.5

Figure 7e: Suggested Vent Depths

Material

in

mm

in

mm

ABS

0.001

0.03

0.002

0.050

Acetal

0.0005

0.01

0.0015

0.040

Acrylic

0.0015

0.04

0.002

0.050

EVA

0.0005

0.01

0.0007

0.020

HDPE

0.0008

0.02

0.001

0.025

Ionomer

0.0005

0.01

0.0007

0.020

LDPE

0.0005

0.01

0.0007

0.020

Nylon 6/6

0.0005

0.01

0.002

0.050

PBT

0.0005

0.01

0.0015

0.040

PC

0.001

0.03

0.003

0.075

PC/ABS

0.0015

0.04

0.003

0.075

PC/SAN

0.0015

0.04

0.003

0.075

Polyester

0.0005

0.01

0.001

0.025

PEEK

0.0005

0.01

0.0007

0.020

PEI

0.001

0.03

0.0015

0.040

PES

0.0005

0.01

0.0007

0.020

PP

0.0005

0.01

0.002

0.050

PPO

0.001

0.03

0.002

0.050

PPS

0.0005

0.01

0.001

0.025

PS

0.001

0.03

0.002

0.050

PSU

0.001

0.03

0.0015

0.040

PVC (Flexible)

0.001

0.03

0.0015

0.035

PVC (Rigid)

0.0015

0.04

0.002

0.050

7.7 Tool Steels

Tool steels for a particular application are typically chosen based on their hardness, abrasion resistance, machining rate, resistance to deformation at elevated temperatures (red-hardness), and/or chemical resistance. Tool steel is generally heat-treated and an-nealed to produce the desired properties. The table below is a listing of common tool steels from DME Company LLC used for various structural and holder block applications.

Figure 7f: Steels for Structural and Holder Block Applications

#1 Steel

SAE 1030 Type easy to machine but is not “sticky”, permitting faster and smoother cuts.

#2 Steel

AISI 4130 Type pre-heat treated specified for durability and structural applications. High strength steel good for cavity and core retainer plates, clamping plates, and support plates.

#3 Steel

P-20 Type pre-heat treated is exceptionally clean and can be heat treated for additional hardness and polished. Good cavity steel. This steel is commonly used for insert molding applica- tions.

#5 Steel

Thermal Shock resistant AISI- H-13 type steel annealed for easy machinability and heat treated for hardness. Has good pol- ishability and hardness. Used for die casts and injection molds where hardness and a highly polished surface is necessary.

#6 Steel

Modified AISI 420 Stainless supplied fully annealed. Unlike No. 7 Steel this stainless steel is cavity grade and can be subsequently heat treated for a desired hardness, it also has exceptional polishability.

#7 Steel

Modified AISI 400 Stainless pre-heat treated for holder block applications. Corrosion resistant with great machinability but cannot be further hardened.

Figure 7g: Common Mold Steels & Important Characteristics

Steel

Hardness (Rockwell C)

Thermal Conductivity (BTU/hr/ft°F)

Description

1045

15-

16

29

High strength low cost carbon steel. Poor corro- sion and wear resistance but can be nitrided.

4140

31-

35

24.5

High strength with good fatigue, abrasion, and impact resistance. Common mold base steel.

410

Stainless

40-

45

14.4

Excellent polishability and corrosion resistance with good hardness. Commonly used for highly polished applications such as lenses and optics.

420

Stainless

54-

57

14.4

Excellent polishability & corrosion resistance with good hardness. Commonly used for cavity and core inserts, ejector pins, and ejector sleeves.

A6

61-

62

18.5

Heat-treatable, good wear resistance and tough- ness. Commonly used for ejection components.

D2

62-

64

12.1

High Carbon/Chromium heat treatable steel with good wear, shock, and abrasion resistance & high toughness. Used for gate blocks, high speed molding, and anything else needing great wear resistance.

H13

50-

54

15.5

Heavily alloyed versatile steel with excellent high temperature and wear resistance & excellent toughness. This steel is commonly used for high- speed machining of cavity and core plates.

P20

30-

32

18.5

Common mold steel with good fatigue abrasion and impact resistance. Typical uses include: mold bases, ejector plates, and larger core and cavity blocks which do not need heat treatment.

S7

57-

60

16.5

Excellent impact resistance, strength, heat trans- fer, and toughness with low wear resistance. Uses include cavity plates, core plates, laminates, and thin wall sections.

A2

60-

62

15

This steel has good corrosion resistance and excellent wear resistance. Occasionally used for core and cavity blocks where polishing is not re- quired. Most commonly used for high-wear mold components such as lifters and slide blocks.

M2

63-

65

12.1

This high hardness steel has excellent wear resistance which is very difficult to machine. Often used for pre-manufactured components such as ejector pins and ejector sleeves.

Figure 7h: Common Non-Ferrous Injection Mold Metals & Important Characteristics

Metal

Hardness (Brinell)

Thermal Conductivity (BTU/hr/ft°F)

Description

AL 7075-T6

(Aluminum)

150

75

High strength and corrosion resistance with good machinability. This aircraft grade

aluminum costs more than QC-7 and is used for mold cavities.

Al QC-7

(Aluminum)

167

82

High strength, hardness, and excellent textur- ing characteristics. Developed for molds and inserts.

Cu 940

(Copper)

210

150

Berrillium-free copper alloy with high strength and thermal conductivity. Used for cooling inserts and shut offs.

BeCu (Beryllium Copper)

371

100

BeCu has excellent fatigue resistance and thermal conductivity. Beryllium copper is used for core and cavity components.

7.8 Basic Heat Treatment

Heat-treating is the act of heating and cooling the steel at predeter-mined temperatures and times to adjust its’ hardness and strength. Steels that consist of 0.4% carbon or more can be heat-treated.

During the heat-treating process, the chemical structure and dimensions of the steel are changed. As the steel is hardened, it typically increases in size and may cause distortion. The amount of dimensional change is case dependent. For example; an increase in hardness of 20 Rockwell C during heat treatment will result in a more severe change than an increase of 10.

When heated, the steel reaches a specific hardening temperature which transforms it to an austenite state. In the austenite state, the carbon is dispersed within the steel and then must be cooled.

When cooled slowly, or annealed, the steel is changed into a soft-ened state, called pearlite. Annealing involves placing the steel into a medium, such as hot oil, warm air, or salt. This process is used to soften the steel before machining, or to significantly reduce the stresses within the steel after machining.

When the steel is cooled quickly from the austenite state, it changes to a very hard, yet brittle, state called martensite. Hardening is performed in a medium, such as cold oil, air, or water, which quickly removes the heat from the steel.

Tempering involves heating the steel to a temperature slightly below the hardening temperature. In this process the steel slowly cooled to reduce hardness and to increase durability.

As with tempering, stress-relieving involves heating to a temperature slightly below the hardening temperature and then slowly cooling the steel. This reduces stresses within the steel before hardening to help reduce distortion. Stress-relieving is performed after a large amount of steel is removed, or after welding.

7.9 Common Surface Treatments

Surface hardening heats the surface of the steel to above the hard-ening temperature through the use of lasers or electron beams. The surface cools quickly and a hardened layer of steel is formed. As with heat-treating, this procedure can be performed on steels having a carbon content equal to or greater than 0.4%.

Steels that cannot be surface hardened are sometimes carburized. This process heats the steel in the presence of a high-carbon medium, such as leather or charcoal. The carbon-rich surface is then cooled quickly for hardening.

When processing corrosive materials such as PVC and acetals; mold components are often nitrided. This process involves heating the steel in an ammonia-rich environment to incorporate nitrogen into the surface.

Boriding involves surrounding the steel with boron-rich metal plates. The boron migrates to the surface of the steel being treated and creates a thin, yet hard layer of iron boride. This process cannot be used for components having complex geometry, since the steel must come in contact with the boron-rich plates.

7.10 Common Surface Coatings

To increase the surface hardness of steel, a layer of titanium carbide or titanium nitride is often added. Titanium carbide is among the hardest coatings available and is applied to prolong the life of high-production cores and cavities. Titanium carbide or titanium nitride coatings are commonly used with abrasive additives, such as glass fibers, talc, and mica.

Nickel or nickel-teflon plating is used to improve the chemical resistance and to reduce the surface friction of the steel. These are commonly used in applications to improve the speed of part removal due to sticking to either the core or cavity.

Adding a layer of chrome to the surface of the steel increases both its’ hardness and its’ chemical resistance. Chrome plating is usually reserved for treating highly polished core and cavity surfaces, or for use with polymers which create residue, such as ABS, PVC and polyethylene.

7.11 Wear Considerations

Galling occurs when mold materials of similar hardness wear together though contact. This results in the two mating surfaces balling up and creating large grooves on either side. To prevent gall-ing, mating materials must have a difference of at least 6 Rockwell C hardness.

To help reduce wear on actions, wear plates made of softer materi-als should be used whenever possible. These should be checked frequently and replaced whenever excessive wear is detected. Brass is commonly used for high wear components since it is easy to machine and will wear away before the harder steel components. Brass plated steel components are even more effective, having the wear characteristics of brass with the increased strength of steel.

Although expensive, graphite impregnated materials should be con-sidered whenever possible. These materials contain graphite inserts imbedded into the metal, which lubricate the surface and provide superior wear resistance.

7.12 Porous Metals

Porous metals allow gas to vent through the metal and are available in both porous steel and aluminum. To prevent volatile build up, pressurized air should be forced through the porous metal between each cycle.

Porous metals must be machined using either wire EDM or die sink-ing EDM machines. Other machining methods, such as milling and grinding, damage the surface by covering the pores. Since porous metals tend to be expensive, they are commonly reserved for small mold cavities, or used as inserts where localized venting is desired.

PlasticsEdge™ microsite sponsored by...

Injection Molding Guide

Welcome to

A Better Way

It's time to upgrade your training approach with Routsis - the premier training provider for the plastics industry. Hands-on, online and ongoing, Routsis Training's RightStart™ program combines the best attributes of traditional one-to-one plastics training with a powerful, dedicated online portal that's accessible anywhere.

President's Message

ara_portrait
Click here for a word from Andy Routsis, founder and president.

Routsis has the ideal solution for you.

Workforce Training

injection-molding-workforce-training-85x84Continuous, convenient, practical programs for the skills your organization needs to succeed.

Individual Training

injection-molding-individual-training-85x84-flippedOur effective, accessible courses help plastics professionals advance their careers.

Online Training

injection-molding-online-training-85x84Our comprehensive array of online plastics courses cover a broad range of topics and skill levels.

Online Training Portals

injection-molding-portal-training-85x84-flippedGet maximum flexibility with your company's own dedicated online plastics training portal.

Online Training Packages

injection-molding-packages-training-85x84Our exclusive online training packages deliver essential training to you or your employees.

Multi-Language Training

injection-molding-multi-language-training-85x84-flippedWe offer online plastics courses in Spanish, Portuguese, Mandarin, and Bahasa Malaysia.

SmartTech™ On-Site Training

injection-molding-on-site-training-85x84Hands-on training to help your compay imrpove efficiency, profitability, and competitive position.

Scientific SkillSet™ Labs

injection-molding-skillset-training-85x84-flippedPerform the same type of labs found in expensive seminars at your own pace, in your own facility.

RightStart™ Process

injection-molding-right-start-training-85x84Our exclusive training implementation process kick-starts your mission-critical training.

Plastics Training Knowledge Center

plastics training videos

Course Previews

Click here for free preview videos of our online plastics training programs.

injection molding reference guide

Injection Molding Reference Guide

Our handy pocket reference is now available on Android™ devices.

injection molding frequently asked questionsFAQs

Need an answer quickly? Check out our Frequently Asked Questions page.

injection molding technical support

Technical Support

Click here for resolutions to the most common technical support issues.

Training Catalog

injection molding training catalogClick here to download the latest catalog of our plastics training products.

plastics training partners

Our Training Partners

For more information about our partnerships with plastics industry and educational leaders, click here.

request_060x120Request Information

Need a fast quote?
Looking for free training demonstrations?

Click here to fill in our request form.

Get in Touch

Contact Routsis Training

Routsis Headquarters

Routsis Training, Inc.
PO Box 894
Dracut, MA (U.S.A.)
01826

phone: (978) 957-0700
FAX: (978) 957-1860

General Info: [email protected]
Tech Support: [email protected]
Sales: [email protected]
Blog: [email protected]

Send a Message

PlasticsEdge™ microsite sponsored by...