Product Design With Plastics

Plastic Product Design

SARATH BABU MADDUKURI

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Index

Over View of Plastic Product Design

Polymer Fundamentals

Plastic Product Design Steps

Plastic Material Selection Process

Plastic Product Design Guidelines

Plastic Manufacturing Process

Basics of Injection Mold

Product Design Environment

Product Design & Development Steps
End Use Requirement
a) Anticipated Structural Requirement

Loads- Stresses a material will be subjected

Rate of Loading

Duration of Loading

Impact Forces

Vibration

Foreseeable Misuse

b) Anticipated Environment

Temp Extremes

c) Assembly and Secondary Operation

d) Cost Limits

e) Regulation Standards compliances

Establish Preliminary Design( Preliminary Concept Sketch and Sections)Select the material( Expected End Use Requirement, Material Data Sheets)
a) Mechanical Properties used for essential component design calculations

b) Other Relevant Properties

3Modify Design as per the calculations results and desired function

a) Specific property balance of selected grade

b) Processing Limitation

c) Assembly Method

d) Cost of Modification

CAD/CAEFlow AnalysisStress Analysis
5Prototype and Testing

6End Use Testing

Polymer Fundamentals

INTRODUCTION

Plastics were considered as Replacing Materials

Todays world plastics are unreplacable materials on the same level as the classic materials:
Primarily due to special combination of properties (profiles & material combinations) Plastics offers solutions, that are not possible with classic materials (Electronics, Medical care, Automotive industries etc.) Low weight, allows high accelerations & decelerations. Weather resistance (Corrosion) is better than resistance of metallic materials. Good Electrical Isolation properties (Housings of Electrical devices) Low manufacturing costs, especially with injection moulding technology.

CLASSIFICATION :

Thermoplastics

High-Molecular

(Makromolecular)

materails

Organic

Metals

(as Ores)

Thermosets

Elastomers

PLASTICS

Thermoplastic

elastomers

Inorganic

e.g. Glasses

Crosslinkable

(vulcanisible)

elastomers

Crosslinked:rubber

Natural

e.g. Wood

Synthetic resp.

Modified material

MATERIALS

Thermoplastics :

They are thread-like molecules (Linear & Branched)

They are always Deformable Fusible Soluble.

As degree of polymerisation (molecule length) increases strength & toughness increases, but flowability decreases.

They are further classified as
Amorphous thermoplastics &Crystalline (Partially crystalline) thermoplastics

Amorphous Thermoplastics:
Bulky thread-like molecules, with unarranged interconnected macromolecular structures, similar to that of staples in a cotton pad. Transparent (Exception) : Styrol copolymers with Butatein like ABS. Lower degree of Shrinkage & high precision can be achieved with less cost. High elastic properties between melt & freezing (Glass transition) temperature makes it to be produced at low holding pressure to avoid demoulding problems & high internal stress. They are more sensitive against solvents & the parts are more suspectable to stress cracking.
Examples:

Polycarbonate (PC) , Polyvinylchloride (PVC), Acrylonitrile Butadiene Styrene Copolymer (ABS), etc.
1.bin

Structure : amorphous Density : 1,03 1,07 g/cm Elastic-Modulus : ~ 2400 N/mm

Properties :

High rigidity & toughness also at low temperature to 40 C,

High Scratch resistance, High impact resistance, High suspectability to stress cracking

Temperature limits:

Short-Term ~ 100C, Long Term ~ 85C

Surface Quality :

High gloss surface can be achieved.

Natural colour: opaque, non-transperant

Manufacturing related properties :

Low shrinkage & low tendency to wrap,

Good Paintability & electroplatability.

Applications :

Automotive panels - (Interior & Exterior parts), etc.

Acrylonitrile Butadiene Styrene Copolymer (ABS) :

Acrylonitrile Butadiene Styrene Copolymer (ABS) : Applications
2.bin3.bin


Properties :

High strength & Hardness, Toughness at low temperature.

High impact resistance, High suspectability to stress cracking

Temperature limits:


Surface Quality :


Natural colour: Transperant


Low shrinkage & low tendency to wrap,

Good Paintability & electroplatability.

Applications :

Automotive panels - (Interior & Exterior parts), Headlights, Helmets, etc.

Polycarbonate (PC) :

Polycarbonate (PC) : Applications


Properties :

High hardness & stiffness.

High impact resistance at low temperature till -5C, below this brittleness increases.

High suspectability to notch failure.

Temperature limits:


Surface Quality :


Natural colour: Transperant till Opaque


Low shrinkage

High chemical resistance

Applications :

Ducts, Ventilation Channels, tubes, etc.

Polyvinylchloride (PVC) :

Polyvinylchloride (PVC) : Applications

Crystalline Thermoplastics:
Bulky thread-like slim molecules, which are alligned or with each other. Non transparent (translucent), naturally coloured good slip properties. Higher degree of Shrinkage due to higher package of molecules. Are less compressible than amorphous during hardening & freezing temperatures, hardly faces any demoulding problems. Due to higher shrinkage may form voids during cooling.
Examples:

Polyethylene (PE), Polypropylene (PP), Polyamide (PA), Polyacetal (POM) etc.
4.bin

Structure : Semi crystalline Density : 0.91 0.96 g/cm Elastic-Modulus : ~ 1200 N/mm

Properties :

High stiffness & Hardness. Good elastic properties.

Practically unbreakable, ductile till -60C

Temperature limits:


Surface Quality :


Natural colour: milky white


No water absorption, High Shrinkage & tendency to warpage


Applications :

HR inserts, Ducts, Channels, etc.

Polyethylene (PE) :
5.bin

Polyethylene (PE) : Applications

Structure : Semi crystalline Density : 0.90 0.92 g/cm Elastic-Modulus : ~ 1450 N/mm

Properties :

High stiffness & Hardness. Stability higher than PE.

High flexural fatigue strength. Low impact strength at low temperature.

Temperature limits:


Surface Quality :


Natural colour: Colourless shining through


No water absorption, High Shrinkage & tendency to warpage


Applications :

Car Coverparts (Interior & Exteriors), etc.

Polypropylene (PP) :
6.bin

Polypropylene (PP) : Applications

Structure : Semi crystalline Density : 1.02 1.15 g/cm Elastic-Modulus : ~ 1300 - 2800 N/mm

Properties :

High stiffness & impact strength.

Good friction & wear resistance

Temperature limits:


Surface Quality :


Natural colour: Translucent white-yellow


Good flow properties & chemical resistance,

Not so good shrinkage. Tendency to warpage.

Applications :

Car (Inner, Outer), Bearings, Gear wheels, etc.

Polyamide (PA) :
7.bin

Polyamide (PA) : Applications
8.bin9.bin

Thermosets :

They are closely crosslinked, that is the reason they are non thermoplastic.

They are always Non - deformable Infusible Insoluble.

Examples:

Epoxy (EP), Phenol-formaldehyde (PF), etc.
10.bin

Elastomeres:

They are loosely crosslinked, highly elastic & show very low plastic deformation.

They are highly deformable Insoluble.

Examples:

Natural Rubber (NR), Ethylen-Propylen rubber (EOM, EPDM), etc.
11.bin

Design Guidelines

REQUIREMENT

(For what ?, strength, assy)

MATERIAL SELECTION

(Cost , Manuf Prosess,Temp conds, Strength, Safety)

PACKAGING DATA & KINEMATICS

( From customer)

DECIDING SNAP & SCREW FIXING LOCATIONS

FIX TOOLING DIRECTION

(Locking 6 deg. Of freedom, DFA )

DECIDING STRENGTHING RIBS,LOCATIONS & GEOMETRY

(Packaging data,

strength requirement)

DRAFT ANGLES,RIBS WALL THICKNESS RATIO

(Die-Draw direction, Minimum silders and aesthetic requirement )

(As per design guidelines)

Design Guidelines

TOOLING FEASIBILITY

( Minimum core thickness, Slider ejection space, Sharp corners etc.)

DRAFT ANALYSIS A & B SURFACES

( Tolerance issues)

SECTIONS WITH PACKAGING THROUGH SNAP & RIBS

Design Guidelines

Material Selection:

The wide variety of injection moldable thermoplastics often makes material selection a difficult task.

Factors governing material selection
Cost Functionality Assembly (Typically when bonded) Temperature Strength Government Regulations. Surface finish/aesthetic etc.

Design Guidelines

Wall thickness/ Base thickness:

Proper wall thickness determines success or demise of a product. Like metals injection molded plastics also have normal working ranges of wall thickness. This can be taken into consideration while deciding wall thickness.

Factors to be considered while deciding wall thickness.
Structural strength of the part to be designed plays important role in deciding wall thickness. Normal working ranges of wall from chart for particular material selected. As a thumb rule 2.5mm. Prior experience or bench mark parts can also be referred while deciding on wall thickness.

Design Guidelines

Wall thickness/ Base thickness:

Once nominal wall thickness is decided, following are some design rules which should be followed.
Maintain uniform wall thickness wherever possible which helps in material flow in mold, reduces risk of sink marks, Induced stresses & consideration of different shrinkage For non-uniform wall thickness change in thickness should not exceed 15% of nominal thickness & should transition gradually. At corner areas minimum fillet at inner side should be 50% of wall thickness.

Design Guidelines

Core-Cavity-Slider directions & Parting lines :

It is always recommended first to decide upon the core-cavity direction. Generally core-cavity direction & parting line depends upon following parameters
The shape & function of the component. Shape in turn is governed by A- Surface, packaging/environment data. Core-cavity & slider directions should be considered such that they do not appear on A-Surfaces, unless otherwise specified & accepted by the customer.

Design Guidelines

Draft Angles (On component walls):

Draft is necessary for ejection of part from the mold & are always Tooling (Die-Draw) & Slider direction.

Recommended draft angle is minimum 1deg.

Factors governing draft angle.
Surface finish Highly polished mold requires less draft than an unpolished mold. Surface Texture (Graining) Draft increases with texture depth,normally 1 deg draft for every 0.025mm depth recommended. Draw depth To keep the draft angle to minimum as thumb rule draft angle draw depth charts are followed & often design engineer should discuss with tool maker.

Design Guidelines

Ribs :

Ribs should be used when needed for stiffness & strength or to assist in filling difficult areas.

For structural parts where sink marks are no concerns -Rib base thickness can be 75%-80% of adjoining wall thickness

For appearance parts where sink marks are objectionable: With texture (Graining) - Rib base thickness should not exceed 50% of adjoining wall thickness for part. Without texture (Graining) - Rib base thickness should not exceed 30% of adjoining wall thickness.

Some important points to consider while rib design.
Draft angle on ribs should be minimum 0.5 deg per side Rib height should be 2.5 to 3 times of wall thickness for effective strength. Recommended to add multiple ribs instead of single large rib, Spacing between multiple ribs should be at least 2 times that of rib thickness. Fillets at base of ribs should be 0.5mm Minimum.

Design Guidelines

Bosses :

Usually designed to accept inserts, self tapping screws, drive pins etc for use in assembling or mounting parts.

Some important points to consider while Boss design:
The O.D of the boss should be ideally 2.5 times of screw diameter for self tapping screw applications. If O.D exceeds 50% of adjoining wall thickness, thinner wall boss of O.D 2 times or less of screw diameter can be considered with supported by ribs. Bosses should be attached to walls with ribs. Thickness at base of rib should not exceed 50% of adjoining wall thickness. Boss inside & outside diameters should have 0.5 deg draft per side.

Design Guidelines

Bosses :

Design Guidelines

Coring :

Coring in injection molding terms to addition of steel to mold for the purpose of removing plastic material in that area Coring is necessary to create Pocket or, Opening in the part or to reduce heavily walled section.

Design Guidelines

Openings :

Openings are desired in a part to eliminate sliders, cams, pullers, etc. to accommodate features like snaps. As general thumb rule 5deg angle in the area of mating of core & cavity is required.

Design Guidelines

Assemblies :

Types of assemblies :
Molded-in assembly Chemical bonding assembly Thermal welding assembly Assembly with fasteners.
Molded-in Assembly : (Snap fit, Press fit, molded in threads etc.)

This is generally the most economical method of assembly. Assembly is fast, inexpensive & does not require any additional part or substance. Minimizes changes of improper assembly. Some times tooling becomes complex & expensive.

Design Guidelines

Snap fit assembly :

Design Guidelines

Snap fit assembly :

Q values to be referred from Material graphs

Y = Deflection

Important points to remember :
Design for given assembly force or overlap length & material. Deflection required to assemble the part should always be less than maximum deflection(strain) for safe design. Snaps increase possibility of sliders wherever possible try to eliminate sliders by providing slot below snap or moving snap to outer edge of the part, if design permits.

Design Guidelines

Press fit assembly :
Press fit design is more critical in plastics (Thermoplastics as they creep (Stress or Relax).Good design should minimize stress on the plastic,by considering assembly tolerance between assembled parts & clamping force due to creep relaxation.

Design Guidelines

Adhesive joints assembly :
Two similar or dissimilar plastics can be assembled in a strong leak-tight bond by using adhesives.The choice of adhesive depends upon the application & the environment to which the part would be subjected.Some of adhesives are Polyurethanes, Epoxies, Cyanoacrylates, Silicones etc.

Design Guidelines

Bolts Nuts - Screws :
Certain precaution must be taken while designing to reduce excessive compressive stress on the plastic.Larger head screw or larger washer is preferred as that contact area increases & stress reduces.

Design Guidelines

Molded in threads :
Coarse threads are preferred due to higher strength & torque limits.Generally 0.8 0.9 mm relief should be provided to prevent high stress at the end of the threads. To reduce the stress concentration minimum 0.25mm radius should be applied to the threads roots.External threads should be as far as possible located on parting lines to avoid need of unscrewing mechanism.Internal threads are usually formed by an unscrewing or collapse core.

Design Guidelines

Self Tapping Screws :

Further classified in 2 types Thread cutting & Thread forming
Thread cutting screw is most used on brittle plastics such as thermosets & filled (50%) thermoplastics. They should not be reinstalled Thread forming screws is mostly used on thermoplastics. They can be reinstalled for 3 to 5 times.
General Guidelines while using self-tapping fasteners:
Thread engagement length 2.5 times screw diameterBoss diameter minimum 2 times of pilot hole diameter.Cored hole should have 0.25 to 0.5 draft.Holes should be counterbored or chamfered to a depth of 0.5mm to aid alignment & avoid cracking of boss. Sufficient clearance to be kept between screw end & bottom of the hole.

TOLERANCE RANGE TO BE GIVEN ON DWGS:

HOW

SLIDERS & LIFTERS

WORK ?

SLIDER FOR UNDERCUT :

Undercut

Horn Pin

Slide

Molded Part

Pulled Undercut


Slide core

Horn Pin

Undercut

Molded part

Cover tool

Spring

Locking Block


Lifter

Angled pin

Undercut

LIFTER FOR UNDERCUT :

Lifter

Horn pin

Undercut

Molded part

Lose core


Undercut

Hydraulic Cylinder

Core pin

HYDRAULIC CYLINDER FOR UNDERCUT :

HYDRAULIC CYLINDER FOR UNDERCUT :

FORCED EJECTION :

Hydraulic Cylinder

Slide

Molded Part

MULTIPLE UNDERCUTS

MULTIPLE UNDERCUTS

Slide

Horn Pin

Undercut

Molded part

Locking Block

Spring

Core Pin

MULTIPLE SLIDERS:

MULTIPLE SLIDERS:

REFERENCES:
Honeywell Injection Moulding Processing Guide (2002). Honeywell Design Soultions (2002). JCI Plastics Training Manual. Injection Moulding Design by Pye

THANK YOU

Design For Stiffness
Relation between load and deflection of the part is Stiffness

Determined by material and geometry of the part

Material Stress Strain Curves ( Young's Modulus)
Design For Strength
Max Load that can be applied to a part without resulting into part failure

Determined by Tensile stress strain curves( Tensile Strength etc)
Design for Behavior overtime
Creep : Time dependent Increasing Strain under constant stress

Stress Relaxation: Reduction of stress under constant strain

Design for Impact Performance
Ability of material to withstand impulsive loading

Factors: type of material, geometry, wall thickness, size of component,

operating temp, rate of loading etc
Design for appearance
Sink Marks, weld lines, air traps, voids, streaks, delamination, jetting, gate marks etc
Design for precisionDesign for moldabilityDesign for RecyclabilityDesign for automation

Part Application Requirement

Material Selection Process

Design Based Material Selection

Guidelines for Injection Molded Design

Plastic Processing

Plastic Processing-Injection Molding

Plastic Processing-IMD

Plastic Processing-Injection Molding

Assembly Techniques for Plastic parts

Snap fit cantilever beam type

Snap fit cylindrical Type

Assembly Techniques Snap Fits

Factors for calculating cantilever beam for Snap fit


Mold Design For Snap Fits


Assembly Techniques Spin Welding

Assembly Techniques Ultrasonic Welding

Assembly Techniques Hot Plate Welding

Assembly Techniques Adhesive Bonding

Assembly Techniques Ultrasonic Insertion

Assembly Techniques Screw and Bosses

Assembly Techniques for Plastic parts

Injection Mold

Injection Mold- Slider and Stripper Plate

Injection Mold- Stripper Plate

Injection Mold-Hot Runner System

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Tooling considerations for
product design.

1. Maintain a uniform wall section - 2.0mm is typical.
2. Utilize the appropriate radii where applicable:
3. Strive to use snap fit and thread forming screws whenever possible to eliminate hardware, maximize design for assembly (DFA), and achieve the lowest cost.

4. Draft is mandatory. 1.5 degrees per side, plus 1 degree per 0.001 depth of texture.

5. Eliminate side draws (slides) and undercuts (lifters) whenever possible. Use through wall openings.

6. Use the general tolerance box - tight tolerances drive up part and tooling cost.

7. Do not put datum on flexible walls or points in space.

Plastic Design Major Messages

Rib to Wall Ratio

Typical Rules for Rib Thickness

Conventional Thermoplastics - 0.7T some sink mark will come

- 0.4T for part which is visible.
Structural Foam - 1.0T

Uniform Wall Sections

It is important to use uniform walls to minimize warp age and maximize manufacturability potential.

Injection Molding : 2 to 4mm
Structural Foam : 5 mm
No thin areas less than 1.5mm
No thick areas - core for uniform sections.

Always try to core from the ejector side of part.

Draft Angles

Draft is needed to facilitate release of part from mold.

The draft to use, unless otherwise specified, is 1.5 degrees per side.

Indicate if draft is to be added or subtracted from nominal dimension.

Show draft on part whenever possible to avoid confusion as to direction.

The "No Draft Allowed" is not to be used. Even on critical areas allow 0.5 degrees.

Limits of Undercuts

Eliminate undercuts by alternative redesign.

A minimum of 5 degree shut-off is required for all areas around a through opening. A 7 degree angle is even better.

See "Bad" steel conditions for steel limitations

"Bad" Steel Conditions

Generally, "Bad" steel conditions can be avoided if all standing steel has a height to width ratio of 1:1 or better.

Undercut

Horn Pin

Slide

Molded Part

Slide Core

Slide Core

Slide Core

Pulled Undercut

Slide Core

Slide Core

Excessive travel

Slide Core

Slide Core

Slide core

Horn Pin

Undercut

Molded part

Cover tool

Spring

Locking Block

Slide Core

Slide Core

Slide core

Horn Pin

Undercut

Molded part

Locking Block

Spring

Core pin

Slide Core

Accelerated Lifter

Lifter

Angled pin

Undercut

Accelerated Lifter

Accelerated Lifter

Crash condition

Hydraulic cylinder

Undercut

Hydraulic Cylinder

Core pin

Hydraulic cylinder

Hydraulic pin

Ejecting molded part

Actuating Core pin

Ejection of undercut part

Undercut

Slide Core

Hydraulic Cylinder

Ejection of undercut part

Pendulum Core Pin

Center Rib with Undercut

Undercut

Center Rib with Undercut

Forced Ejection

Hydraulic Cylinder

Slide

Molded Part

Multiple Undercut

Die Opening

Lifter Ejection

Part Ejection

Lifter Return

Slide Return

Die Closing

Slide

Horn Pin

Undercut

Molded part

Locking Block

Spring

Core Pin

Multiple External Slides

Multiple Undercuts

Slide

Horn Pin

Under

Molded part

Locking block

Spring

Core pin


Angled Lifter

Lifter

Horn pin

Undercut

Molded part

Lose core

Angled Lifter

Ejection

Die closing

Impossible lifter condition

A

B

A

B

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Thanks

Injection Mold-Hot Runner System

Documents

Product Design With Plastics