1. Basic Concept of Polymer Materials

Polymer materials are composed of elements such as carbon, hydrogen, oxygen, nitrogen, fluorine, chlorine, and silicon, which are connected by covalent bonds to form long molecular chains.

The basic structure of a polymer material generally includes:

• Monomers

• Repeating units

• Polymer chains

• Intermolecular bonding

• Crystalline and amorphous regions

• Additives

• Fillers or reinforcements

A monomer is the starting substance used to form a polymer, while a repeating unit is the fundamental structure that is repeated along the polymer chain.

For example, polyethylene is formed by the repeated bonding of ethylene monomers.

• Monomer: Ethylene

• Chemical formula: C₂H₄

• Repeating unit: –CH₂–CH₂–

• Principal constituent elements: Carbon and hydrogen

Compared with metals, polymer materials generally have the following characteristics:

• Low density

• Good corrosion resistance

• Good electrical insulation

• Ease of forming complex shapes

• Low coefficients of friction in many grades

• Ability to absorb vibration and noise

• Low thermal conductivity

• Low elastic modulus

• Increasing deformation with temperature and time

• Susceptibility to creep and stress relaxation

• Possible degradation due to ultraviolet radiation, heat, moisture, and chemicals

Therefore, polymers should not be regarded merely as lightweight substitutes for metals. They are design materials used for electrical insulation, corrosion resistance, low friction, moldability, vibration damping, and noise reduction.

2. How to Understand the Composition of Polymer Materials

2.1 Difference Between Metal Alloys and Polymer Compounds

Metal alloys are generally defined by the mass percentages of elements such as iron, carbon, chromium, and nickel.

Polymer materials are more appropriately defined by a combination of the following factors:

1. Base resin

2. Monomer and repeating unit

3. Molecular weight

4. Copolymer composition

5. Degree of crystallinity

6. Reinforcement

7. Inorganic filler

8. Plasticizer

9. Flame retardant

10. Heat and light stabilizers

11. Colorants

12. Lubricants and mold-release agents

For example, “PA66-GF30” is not simply nylon 66.

• Base resin: PA66

• Reinforcement: Glass fiber

• Glass-fiber content: Generally approximately 30 wt.%

• Other ingredients: Heat stabilizers, lubricants, colorants, and similar additives

Therefore, unreinforced PA66, GF15, GF30, and GF50 grades have significantly different mechanical properties even though their base resin is the same.

2.2 Typical Composition Ranges of Polymer Materials

These values represent general trends. Actual formulations vary depending on the manufacturer and product grade.

3. Basic Classification of Polymer Materials

Polymer materials can be classified according to their response to heat and their molecular structure.

3.1 Thermoplastics

Thermoplastics soften or melt when heated and solidify again when cooled.

Representative materials:

• PE

• PP

• PVC

• PS

• ABS

• PA

• POM

• PET

• PBT

• PC

• PMMA

• PPS

• PEEK

• PTFE

Typical processing methods:

• Injection molding

• Extrusion

• Blow molding

• Thermoforming

• Rotational molding

• Machining

• Additive manufacturing

3.2 Thermosetting Resins

Thermosetting resins form a three-dimensional network structure through crosslinking during molding or curing.

Once cured, they do not melt again when reheated.

Representative materials:

• Epoxy

• Phenolic resin

• Unsaturated polyester

• Vinyl ester

• Melamine resin

• Thermosetting polyurethane

Main characteristics:

• High heat resistance

• High dimensional stability

• Good adhesion

• Low softening deformation

• Cannot be remelted after curing

• May exhibit impact brittleness

3.3 Elastomers

Elastomers undergo large deformation under relatively low force and recover close to their original shape after the load is removed.

Representative materials:

• Natural rubber

• NBR

• EPDM

• Silicone rubber

• FKM

• CR

• Polyurethane rubber

• TPE

• TPU

Elastomers are used for sealing, vibration isolation, shock absorption, and friction control.

4. Relationship Between Polymer Structure and Mechanical Properties

4.1 Molecular Chain Structure

Polymer chains may have the following structures:

• Linear

• Branched

• Crosslinked

• Networked

Linear polymers generally allow greater chain mobility and often exhibit thermoplastic behavior.

As crosslink density increases, the following tendencies appear:

• Increased stiffness

• Increased heat resistance

• Increased solvent resistance

• Reduced creep

• Reduced elongation

• Loss of remeltability

4.2 Crystalline and Amorphous Structures

Crystalline Polymers

These materials contain regions in which polymer chains are arranged in an ordered structure.

Representative materials:

• PE

• PP

• PA

• POM

• PET

• PBT

• PPS

• PEEK

• PTFE

General characteristics:

• Good chemical resistance

• Good fatigue resistance

• Relatively high molding shrinkage

• Usually opaque

• Distinct melting point

• Often good friction and wear properties

Amorphous Polymers

These materials have polymer chains arranged in a disordered structure.

Representative materials:

• PC

• PMMA

• PS

• ABS

• PVC

General characteristics:

• Relatively low molding shrinkage

• Good dimensional accuracy

• Potential for transparency

• Governed mainly by glass-transition temperature rather than a distinct melting point

• Susceptibility to solvent-induced stress cracking

4.3 Glass-Transition Temperature

The glass-transition temperature is the temperature range in which the amorphous portion of a polymer changes from a rigid glassy state to a flexible rubber-like state.

Below the glass-transition temperature:

• Stiffness and hardness are high.

• Deformation is relatively small.

• Impact brittleness may increase.

Above the glass-transition temperature:

• Stiffness decreases.

• Creep increases.

• Deformation becomes larger.

• Rubber-like behavior may appear.

4.4 Molecular Weight

As molecular weight increases, the following properties may generally improve:

• Tensile strength

• Impact strength

• Fatigue resistance

• Wear resistance

• Melt strength

However, higher molecular weight also increases melt viscosity and may make injection molding and extrusion more difficult.

5. Commodity Thermoplastics

5.1 Polyethylene — PE

Polyethylene is a representative hydrocarbon polymer produced by polymerizing ethylene.

Basic Composition

The theoretical elemental mass ratio of the repeating unit is approximately:

• Carbon: 85.7%

• Hydrogen: 14.3%

Commercial products may also contain antioxidants, ultraviolet stabilizers, pigments, and fillers.

Structure and Properties

Because PE consists of nonpolar hydrocarbon chains, it offers good resistance to water and many chemicals.

HDPE has fewer branches and a higher degree of crystallinity, resulting in:

• Higher stiffness

• Higher density

• Good chemical resistance

• Low moisture absorption

• Relatively high molding shrinkage

LDPE has more branching and exhibits:

• High flexibility

• Low stiffness

• Good impact absorption

• Excellent film-forming capability

UHMWPE

Ultra-high-molecular-weight polyethylene has extremely long molecular chains.

Main characteristics:

• Very high wear resistance

• Low coefficient of friction

• High impact strength

• Excellent chemical stability

• Very low moisture absorption

• Difficult adhesion and painting

• Limited precision dimensional stability

Main Applications

• Sliding guides

• Chain guides

• Liners

• Conveyor components

• Wear plates in food machinery

• Tanks

• Piping

• Insulation parts

5.2 Polypropylene — PP

Basic Composition

The theoretical elemental mass ratio is:

• Carbon: Approximately 85.7%

• Hydrogen: Approximately 14.3%

Structure and Properties

The methyl side group gives PP greater stiffness and heat resistance than PE.

Main characteristics:

• Low density

• Excellent chemical resistance

• Low moisture absorption

• Good living-hinge fatigue performance

• Good electrical insulation

• Higher heat resistance than PE

• Limited low-temperature impact resistance

• Possible degradation under long-term ultraviolet exposure

Common Additives

• Antioxidants

• Ultraviolet stabilizers

• Nucleating agents

• Talc

• Glass fibers

• Impact modifiers

• Flame retardants

Talc-filled PP has higher stiffness and dimensional stability, but impact strength and elongation may decrease.

Main Applications

• Chemical tanks

• Fans and ducts

• Hinges

• Food containers

• Pump components

• Machine covers

• Piping

• Electrical insulation components

5.3 Polyvinyl Chloride — PVC

Basic Composition

The theoretical elemental mass ratio of the repeating unit is approximately:

• Carbon: 38.4%

• Hydrogen: 4.8%

• Chlorine: 56.8%

Composition and Properties

The high chlorine content affects the following characteristics:

• Improved flame resistance

• Increased polarity

• Improved chemical resistance

• Increased density

• Limited thermal stability

PVC is inherently rigid, but it can be made flexible through the addition of plasticizers.

Typical Composition of Rigid PVC

• PVC resin

• Heat stabilizers

• Impact modifiers

• Processing aids

• Lubricants

• Pigments

• Optional inorganic fillers

Typical Composition of Flexible PVC

• PVC resin

• Plasticizers

• Heat stabilizers

• Lubricants

• Pigments

• Fillers

Plasticizer content can be relatively high. As plasticizer content increases, hardness and stiffness decrease while flexibility increases.

Main Applications

• Piping

• Ducts

• Wire insulation

• Hoses

• Covers

• Flooring

• Chemical-equipment linings

5.4 Polystyrene — PS

Basic Composition

Structure and Properties

The bulky aromatic phenyl group restricts chain mobility and provides high stiffness and transparency.

However, limited chain mobility also makes the material relatively brittle under impact.

Main characteristics:

• High transparency

• Good dimensional stability

• High stiffness

• Low cost

• Low impact strength

• Sensitivity to solvents

• Limited high-temperature use

HIPS contains a dispersed rubber phase to improve impact strength.

Main Applications

• Equipment housings

• Trays

• Packaging

• Transparent covers

• Thermal insulation

• Low-load components

5.5 ABS

ABS is a multiphase copolymer composed of acrylonitrile, butadiene, and styrene.

Basic Composition

The proportions of the three components vary by manufacturer and grade to balance stiffness, heat resistance, impact strength, and flow behavior.

Principal Elements

• Carbon

• Hydrogen

• Nitrogen

Characteristics

• High impact strength

• Good surface appearance

• Excellent injection moldability

• Can be plated and painted

• Good dimensional stability

• Limited weather resistance

• Sensitive to certain solvents

• Limited continuous high-temperature capability

Main Applications

• Equipment housings

• Control handles

• Electrical enclosures

• Automotive interior components

• Covers

• Appliance components

6. Engineering Plastics

6.1 Polyamide — PA

Polyamides contain amide bonds in the polymer backbone.

Representative grades:

• PA6

• PA66

• PA11

• PA12

• PA46

• Aromatic polyamides

PA6 Composition

PA66 Composition

Amide groups form strong hydrogen bonds between molecular chains.

This results in:

• High strength

• High toughness

• Good wear resistance

• Excellent fatigue performance

• Good heat resistance

• Good frictional properties

However, the amide groups interact readily with water, causing moisture absorption.

Effects of moisture absorption:

• Dimensional expansion

• Reduced stiffness

• Increased toughness

• Reduced electrical insulation

• Changes in assembly clearances

Typical PA Compounds

Actual composition varies by product grade.

Main Applications

• Gears

• Bearing cages

• Rollers

• Chain guides

• Pulleys

• Connectors

• Fans

• Housings

• Cable-carrier components

6.2 Polyoxymethylene — POM

POM is also known as acetal or polyacetal.

Basic Composition

The theoretical elemental mass ratio is approximately:

• Carbon: 40.0%

• Hydrogen: 6.7%

• Oxygen: 53.3%

Structure and Properties

Its regular chain structure and high crystallinity provide:

• High stiffness and strength

• Low coefficient of friction

• Excellent wear resistance

• Low moisture absorption

• High dimensional stability

• Good fatigue performance

• Excellent spring-back behavior

Limitations:

• Limited resistance to strong acids and strong alkalis

• Possible thermal decomposition at high temperature

• Difficult adhesion and painting

• Decomposition-gas control required during overheating or combustion

Common Modified Grades

• Glass-fiber-reinforced POM

• PTFE-filled POM

• Silicone-lubricated POM

• Antistatic POM

• Conductive POM

• Food-contact-compliant POM

Main Applications

• Precision gears

• Racks

• Bearings

• Cams

• Valve components

• Rollers

• Clips

• Snap-fit parts

• Precision housings

POM is widely used in precision parts requiring high stiffness, low friction, and good dimensional stability.

6.3 Polycarbonate — PC

Basic Composition

Representative bisphenol-A-based PC is formed from bisphenol-A-related raw materials and carbonate linkages.

Structure and Properties

Aromatic rings and carbonate bonds provide high stiffness and heat resistance.

Main characteristics:

• Very high impact strength

• Transparency

• High dimensional stability

• Good heat resistance

• Good electrical insulation

• Low molding shrinkage

• Excellent low-temperature impact performance

Limitations:

• Sensitive to certain organic solvents

• Susceptible to stress cracking

• Relatively low scratch resistance

• Possible yellowing under long-term ultraviolet exposure

• Requires drying before injection molding

Polycarbonate is widely used where transparency and high impact strength, including at low temperature, are required.

Representative Compounds

• Transparent PC

• Flame-retardant PC

• PC-GF10 to GF30

• PC/ABS

• Weather-resistant PC

• Optical-grade PC

• Food-contact PC

Main Applications

• Transparent safety covers

• Machine guards

• Sensor windows

• Electrical housings

• Lighting covers

• Gears and structural parts

• Helmets and impact-protection parts

6.4 Polyesters — PET and PBT

PET and PBT are thermoplastic polyesters containing ester bonds.

PET Composition

PBT Composition

Properties

PET:

• High strength

• Good gas-barrier properties

• Excellent wear resistance

• Good dimensional stability

• Can be made transparent

PBT:

• Rapid crystallization

• Excellent injection moldability

• Good electrical properties

• Low moisture absorption

• Excellent dimensional stability

Representative Compounds

• PET-GF30

• PBT-GF15

• PBT-GF30

• Flame-retardant PBT

• Impact-modified PBT

• PET/PBT blends

Main Applications

• Electrical connectors

• Sensor housings

• Relay components

• Precision gears

• Automotive electrical components

• Films

• Bottles and containers

6.5 PMMA

PMMA is a transparent polymer commonly known as acrylic or acrylic resin.

Basic Composition

Main Characteristics

• High light transmission

• Excellent weather resistance

• Good surface hardness

• Low density

• Can be machined and polished

• Lower impact strength than PC

• Sensitive to notches and cracking

Main Applications

• Transparent windows

• Lighting covers

• Instrument panels

• Display windows

• Optical components

• Decorative parts

7. High-Performance Engineering Plastics

7.1 PPS

PPS contains aromatic rings and sulfur linkages in its molecular backbone.

Basic Composition

Characteristics

• High heat resistance

• Very low moisture absorption

• Excellent dimensional stability

• High chemical resistance

• Flame resistance

• High electrical insulation

• Unreinforced grades may be brittle

Typical Formulations

Main Applications

• Pump components

• Valves

• Electrical insulation parts

• Sensor housings

• High-temperature connectors

• Automotive engine-compartment components

• Precision injection-molded parts

7.2 PEEK

PEEK is the abbreviation for polyether ether ketone.

Basic Composition

Aromatic rings provide stiffness and heat resistance, ether bonds provide a degree of toughness and processability, and ketone bonds contribute to heat and chemical stability.

Main Characteristics

• High heat resistance

• High mechanical strength

• Excellent chemical resistance

• High creep resistance

• Low smoke generation

• Excellent wear resistance

• Resistance to hot water, steam, and hydrolysis

• Radiation resistance

• High cost

• High processing temperature

Unreinforced PEEK offers excellent high-temperature stability, chemical resistance, creep resistance, and friction and wear performance.

Representative Grades

Effects of Reinforcement

Glass Fiber

• Increases stiffness

• Reduces thermal deformation

• Reduces creep

• Reduces thermal expansion

• Can retain electrical insulation

• May increase wear of mating surfaces

Carbon Fiber

• Provides very high stiffness

• Improves dimensional stability

• Reduces thermal expansion

• Increases thermal and electrical conductivity

• Can improve wear behavior

• Increases cost

Main Applications

• High-temperature bearings

• Seals

• Valve seats

• Semiconductor-equipment components

• Medical devices

• Aircraft components

• High-temperature electrical insulation

• High-performance food and pharmaceutical equipment parts

7.3 PTFE

PTFE is polytetrafluoroethylene.

Basic Composition

The theoretical elemental mass ratio is approximately:

• Carbon: 24%

• Fluorine: 76%

Composition and Properties

Fluorine atoms surrounding the carbon backbone form extremely stable carbon-fluorine bonds.

This results in:

• Extremely high chemical resistance

• Very low coefficient of friction

• Non-stick behavior

• Excellent electrical insulation

• Wide temperature capability

• Low surface energy

• Almost no moisture absorption

Limitations:

• High creep

• Low stiffness and strength

• High thermal expansion

• Difficult conventional melt injection molding

• Very difficult adhesion

• Possible deformation under repeated loading

Common Filled PTFE Grades

Main Applications

• Gaskets

• Seals

• Sliding bearings

• Valve seats

• Linings

• Electrical insulation parts

• Chemical equipment

• Non-stick components in food machinery

7.4 Polyimide — PI

Polyimide is a high-temperature polymer containing imide rings in its molecular backbone.

Main Composition

• Aromatic carbon structure

• Imide bonds

• Carbon

• Hydrogen

• Oxygen

• Nitrogen

Characteristics

• High heat resistance

• High mechanical strength

• Excellent creep resistance

• High electrical insulation

• Suitable for vacuum and radiation environments

• Good friction and wear properties

• High cost

• Difficult molding and machining

Polyimide grades may contain graphite, PTFE, carbon fiber, or other fillers to tailor bearing performance.

Main Applications

• High-temperature bearings

• Vacuum equipment

• Semiconductor equipment

• Aerospace components

• Electrical insulation parts

• High-temperature seals and spacers

8. Thermosetting Polymers

8.1 Epoxy Resin

Epoxy resin forms a three-dimensional crosslinked structure through reaction between an epoxy resin and a curing agent.

Basic Composition

Representative Curing Agents

• Amine-based

• Anhydride-based

• Phenolic

• Latent curing agents

Characteristics

• High adhesion

• Low curing shrinkage

• High dimensional stability

• Excellent electrical insulation

• High chemical resistance

• Wide formulation flexibility

• Possible impact brittleness

• Requires careful curing-condition control

Representative Formulations

General Adhesive Epoxy

• Epoxy resin

• Amine curing agent

• Fillers

• Additives

Electrical Epoxy

• Epoxy resin

• Anhydride curing agent

• Silica filler

• Flame retardant

• Pigment

Carbon-Fiber Composite

• Epoxy resin as matrix

• Carbon fiber as primary reinforcement

• Actual fiber content varies with manufacturing method

Main Applications

• Structural adhesives

• Electrical encapsulation

• Composite matrices

• Molds

• Coatings

• Flooring

• Potting materials

• Repair materials

8.2 Phenolic Resin

Phenolic resin is a thermosetting polymer produced by reacting phenol-based and formaldehyde-based materials.

Main Composition

• Phenolic raw material

• Formaldehyde-based raw material

• Curing agent or catalyst

• Wood flour

• Glass fiber

• Mineral filler

• Lubricant

• Pigment

Characteristics

• High heat resistance

• Good flame resistance

• Low smoke generation

• Good electrical insulation

• High dimensional stability

• Relatively high brittleness

• Limited color options

Main Applications

• Electrical switches

• Brake-friction materials

• Heat-resistant handles

• Insulation parts

• Bearing cages

• High-temperature structural parts

8.3 Unsaturated Polyester

Unsaturated polyester resin is generally used with a styrene-based reactive diluent and crosslinked using an initiator.

Composition

• Unsaturated polyester resin

• Reactive diluent

• Initiator

• Accelerator

• Glass fiber

• Calcium carbonate or other fillers

• Low-shrink additives

• Pigments

Characteristics

• Relatively low cost

• Easy manufacture of glass-fiber composites

• Good corrosion resistance

• Suitable for large components

• Exhibits curing shrinkage

• Requires control of odor and volatile substances

• Adhesion and fatigue performance may be lower than epoxy

Main Applications

• FRP tanks

• Ducts

• Covers

• Boats

• Chemical equipment

• Large machine enclosures

9. Elastomers

9.1 NBR

NBR is a synthetic rubber composed of acrylonitrile and butadiene.

Main Composition

As acrylonitrile content increases:

• Oil resistance increases.

• Gas-barrier performance increases.

• Strength may increase.

• Low-temperature flexibility decreases.

• Elasticity may decrease.

Main Applications

• Oil seals

• O-rings

• Hoses

• Gaskets

• Hydraulic and pneumatic seals

• Fuel-system components

9.2 EPDM

EPDM is composed of ethylene, propylene, and a small amount of diene monomer.

Main Composition

• Ethylene

• Propylene

• Small amount of diene monomer

• Carbon black or inorganic filler

• Vulcanizing agent

• Plasticizer

• Antioxidant

Characteristics

• Excellent weather resistance

• Excellent ozone resistance

• Good resistance to hot water and steam

• Good electrical insulation

• Relatively good resistance to acids and alkalis

• Unsuitable for mineral oils and fuels

Main Applications

• Water seals

• Steam gaskets

• Outdoor seals

• Hoses

• Door seals

• Hygienic seals for food machinery

9.3 Silicone Rubber

Silicone rubber has a silicon-oxygen backbone rather than a carbon-carbon backbone.

Basic Composition

Characteristics

• Wide service-temperature range

• Excellent low-temperature flexibility

• Good weather and ozone resistance

• Good electrical insulation

• Food- and medical-grade formulations available

• Potentially low tear strength

• Limited wear and oil resistance

• Relatively high gas permeability

Main Applications

• Food-machinery gaskets

• Hygienic hoses

• Oven seals

• Electrical insulation parts

• Medical tubing

• High-temperature packings

9.4 FKM

FKM is a high-performance fluorinated elastomer.

Main Composition

• Fluorinated vinyl monomers

• Carbon

• Fluorine

• Hydrogen

• Curing agents

• Fillers

• Antioxidants

Characteristics

• High heat resistance

• Excellent oil resistance

• Good resistance to fuels and solvents

• Low gas permeability

• Excellent weather resistance

• Limited low-temperature flexibility

• High cost

• Steam and some chemical environments require grade-specific review

Main Applications

• High-temperature O-rings

• Chemical seals

• Fuel-system gaskets

• Vacuum seals

• High-temperature oil seals

9.5 TPU

TPU is a thermoplastic polyurethane elastomer.

Main Composition

• Polyol-based soft segments

• Isocyanate-based hard segments

• Chain extenders

• Stabilizers

• Lubricants

• Pigments

Characteristics

• High wear resistance

• High tensile strength

• Excellent elasticity

• Suitable for injection molding and extrusion

• Good resistance to oils and greases

• Possible hydrolysis in moisture and high temperature

• Polyester- and polyether-based grades are available

Main Applications

• Rollers

• Belts

• Seals

• Cable jackets

• Shock-absorbing components

• Wheels

• Flexible couplings

10. Reinforcements and Fillers

10.1 Glass Fiber

Typical composition:

• Silica-based glass

• Alumina

• Calcium oxide

• Other oxides

• Surface sizing agent

Effects:

• Increased tensile strength

• Increased elastic modulus

• Reduced thermal deformation

• Reduced creep

• Reduced molding shrinkage

Limitations:

• Reduced elongation

• Possible reduction in impact performance

• Increased mold and cutting-tool wear

• Anisotropy caused by fiber orientation

• Increased surface roughness

10.2 Carbon Fiber

The primary constituent element is carbon.

Effects:

• High stiffness and strength

• Low density

• Low thermal expansion

• Increased electrical conductivity

• Possible increase in thermal conductivity

• Reduced creep

Limitations:

• High cost

• Anisotropy

• Potential galvanic corrosion with metals

• Possible brittle fracture

• Dust-control requirements during machining

10.3 Mineral Fillers

Representative fillers:

• Talc

• Calcium carbonate

• Mica

• Silica

• Wollastonite

• Glass beads

Main effects:

• Reduced cost

• Reduced molding shrinkage

• Improved dimensional stability

• Increased stiffness

• Reduced thermal expansion

• Modified surface properties

Limitations:

• Possible reduction in impact strength

• Increased density

• Reduced elongation

• Increased abrasiveness

10.4 PTFE, Graphite, and MoS₂

These additives are used to improve friction and wear behavior.

PTFE

• Reduces friction coefficient

• Reduces initial breakaway resistance

• Increases non-stick behavior

Graphite

• Provides dry lubrication

• Improves thermal conductivity

• Stabilizes wear behavior

MoS₂

• Improves lubrication

• Adjusts stiffness and hardness

• Reduces wear

11. Mechanical Properties of Polymer Materials

11.1 Elastic Modulus

The elastic modulus of polymers is generally much lower than that of metals.

Therefore, under the same load and geometry, polymers exhibit greater deformation and deflection.

Design measures include:

• Increasing wall thickness

• Adding ribs

• Reducing support spacing

• Using curved or box-shaped sections

• Adding glass-fiber reinforcement

• Using metal inserts

11.2 Tensile Strength

Tensile strength is influenced not only by the polymer type but also by:

• Temperature

• Moisture

• Strain rate

• Fiber orientation

• Weld lines

• Notches

• Molding conditions

• Recycled-material content

Therefore, a single tensile-strength value is insufficient for design in the same way as it may be for metals.

11.3 Impact Strength

Impact strength varies greatly with:

• Temperature

• Notch geometry

• Thickness

• Molecular weight

• Rubber-phase content

• Moisture absorption

• Fiber reinforcement

PC, ABS, and impact-modified PP are generally suitable for impact-loaded parts.

PS, PMMA, and highly filled PPS require caution under notched impact loading.

11.4 Creep

Creep is the gradual increase in deformation under a constant load over time.

Polymers may exhibit creep even at room temperature.

Factors affecting creep:

• Load

• Temperature

• Time

• Moisture

• Degree of crystallinity

• Fiber content

• Stress concentration

• Fastening method

For fastened joints, spring elements, supports, and compressed gaskets, long-term creep may be more important than short-term strength.

11.5 Stress Relaxation

Stress relaxation is the gradual reduction in internal stress under constant deformation.

Typical design problems:

• Loss of bolt preload

• Reduction in snap-fit holding force

• Loss of gasket sealing force

• Reduction in clip or spring force

11.6 Fatigue

Polymer fatigue life is strongly affected by temperature rise and self-heating.

Repeated deformation can generate heat through internal hysteresis.

Design considerations:

• Minimize stress concentration.

• Avoid abrupt section changes.

• Provide sufficient radii.

• Limit repeated strain.

• Consider cooling and heat dissipation.

• Control weld-line position.

12. Thermal Properties

12.1 Thermal Expansion

Polymers generally have higher coefficients of thermal expansion than metals.

Possible issues include:

• Changes in assembly clearance

• Changes in bearing fits

• Stress around metal inserts

• Warpage due to temperature change

• Sensor-position errors

• Changes in seal compression

Glass fiber and carbon fiber can reduce thermal expansion, but anisotropy may occur depending on fiber orientation.

12.2 Thermal Conductivity

Most polymers have low thermal conductivity.

Advantages:

• Thermal insulation

• Reduced risk of burns

• Electrical insulation

• Thermal barriers

Limitations:

• Accumulation of frictional heat

• Difficulty dissipating heat from electronic components

• Local overheating

• Thermal deformation

Thermally conductive plastics can be produced using conductive fillers, but electrical insulation, density, and processability may change.

12.3 Service Temperature

Polymer service temperature is not determined solely by melting point.

The following should be reviewed:

• Glass-transition temperature

• Melting point

• Heat-deflection temperature

• Continuous-use temperature

• Short-term peak temperature

• Load condition

• Duration of exposure

• Oxygen and chemical environment

• Thermal cycling

13. Chemical Properties and Environmental Degradation

13.1 Moisture Absorption

Materials with relatively high moisture absorption:

• PA

• Some polyesters

• Some thermosetting resins

Materials with relatively low moisture absorption:

• PE

• PP

• POM

• PTFE

• PEEK

• PPS

Moisture absorption affects dimensions, stiffness, electrical insulation, and frictional behavior.

13.2 Hydrolysis

Hydrolysis is the breaking of molecular chains by hot water or steam.

Materials requiring attention:

• Some polyesters

• Some TPU grades

• Some PC grades

• Some PA grades

Because behavior varies greatly by grade and stabilizer system, dedicated hydrolysis-resistant grades should be used for hot-water cleaning or steam sterilization.

13.3 Ultraviolet Degradation

Ultraviolet radiation can break polymer chains and accelerate oxidation.

Possible effects:

• Color change

• Surface cracking

• Chalking

• Loss of strength

• Increased brittleness

Countermeasures:

• Ultraviolet absorbers

• HALS stabilizers

• Carbon black

• Coatings

• Shading structures

• Weather-resistant grades

13.4 Environmental Stress Cracking

Cracking may occur when mechanical stress and chemicals act simultaneously.

Materials commonly requiring review:

• PC

• PMMA

• ABS

• PS

• Some PE grades

• Some POM grades

Problems may arise from combinations of cleaning agents, oils, adhesives, and screw-tightening stress.

14. Friction, Wear, and Bearing Design

Advantages of polymer bearing materials:

• Possible oil-free operation

• Low noise

• Low inertia

• Corrosion resistance

• Reduced damage to mating shafts

• Suitability for dusty environments

Design considerations:

• Surface pressure

• Sliding speed

• PV value

• Temperature

• Shaft surface roughness

• Mating-material hardness

• Lubrication

• Moisture

• Particle contamination

• Thermal expansion

Representative materials:

• POM

• PA

• UHMWPE

• Filled PTFE

• Bearing-grade PEEK

• Bearing-grade PI

15. Polymer Materials in Food Machinery

15.1 Commonly Used Materials

• UHMWPE

• HDPE

• PP

• POM

• PA

• PTFE

• PEEK

• Silicone rubber

• EPDM

• TPU

However, food-contact suitability cannot be determined from the generic resin name alone.

Even within the same base resin, the following may differ:

• Pigments

• Plasticizers

• Flame retardants

• Lubricants

• Mold-release agents

• Reinforcement fibers

• Recycled content

• Antimicrobial additives

Therefore, the food-contact compliance of the exact grade and the supplier’s certification should be confirmed.

15.2 UHMWPE

Typical applications:

• Conveyor guides

• Chain guides

• Slide plates

• Wear liners

• Product-transfer surfaces

Advantages:

• Low friction

• High wear resistance

• Low moisture absorption

• Ease of cleaning

• High impact strength

Considerations:

• Creep

• Thermal expansion

• Low stiffness

• Limited precision fastening

• High-temperature washing conditions

15.3 POM

Typical applications:

• Gears

• Rollers

• Cams

• Sprockets

• Precision guides

• Valve components

Advantages:

• High dimensional stability

• Low friction

• Good fatigue resistance

• Low moisture absorption

Considerations:

• Strong acids and alkalis

• High-temperature steam

• High-temperature decomposition

• Confirmation of a suitable grade

15.4 PTFE

Typical applications:

• Gaskets

• Seals

• Valve seats

• Non-stick liners

• Sliding components

Advantages:

• Excellent chemical resistance

• Non-stick behavior

• Low friction

• Wide temperature capability

Considerations:

• Creep

• Deformation

• Low stiffness

• Reduction in clamping pressure

• Food compatibility of fillers

15.5 PEEK

Typical applications:

• High-temperature bearings

• High-pressure valve seats

• Steam-cleaning components

• Precision guides

• High-performance seals

• Pharmaceutical and food-equipment components

Advantages:

• High-temperature stability

• Chemical resistance

• High strength

• Creep resistance

• Resistance to steam and hydrolysis

Limitations:

• High cost

• High processing temperature

• Demanding mold and molding-equipment requirements

• Need to verify food-contact compliance of the exact grade

15.6 Metal-Detectable Plastics

Food machinery may use metal-detectable or X-ray-detectable plastics to identify fragments from damaged components.

Typical composition:

• Base resin

• Metallic or high-density inorganic detectable additive

• Blue pigment

• Stabilizers

Considerations:

• Detectability depends on fragment size and equipment settings.

• Food-contact compliance must be confirmed.

• Additives may change strength and wear performance.

16. Design Principles for Polymer Components

16.1 Wall Thickness

Wall thickness should be as uniform as possible.

Abrupt thickness changes can cause:

• Differential shrinkage

• Sink marks

• Internal voids

• Warpage

• Residual stress

• Longer cooling time

16.2 Ribs

Ribs can increase stiffness without greatly increasing wall thickness.

Considerations:

• Avoid excessively thick ribs.

• Provide radii at rib roots.

• Review shrinkage and sink marks.

• Consider resin-flow direction.

• Consider glass-fiber orientation.

16.3 Corners

Sharp corners cause stress concentration and poor resin flow.

Adequate fillets should be applied to internal corners.

16.4 Screw Fastening

Direct metal-bolt fastening into polymers can lose clamping force due to creep and stress relaxation.

Countermeasures:

• Metal inserts

• Wide washers

• Compression-limiting sleeves

• Reduced tightening torque

• Thread-forming designs for plastics

• Inserts for repeated assembly

16.5 Fits

Thermal expansion, moisture absorption, and creep must be considered.

Applying metal-like tolerances directly can lead to:

• Increased interference at high temperature

• Looseness at low temperature

• Moisture-induced expansion

• Long-term deformation

• Cracking

16.6 Snap Fits

Snap fits are representative assembly features that use polymer elasticity.

Design considerations:

• Allowable strain

• Number of assembly cycles

• Notch radius

• Material creep

• Temperature

• Fiber orientation

• Weld lines

17. Molding Processes and Material Characteristics

17.1 Injection Molding

Advantages:

• Complex geometry

• High-volume production

• Good repeatability

• Insert molding

• Integrated part design

Considerations:

• Shrinkage

• Warpage

• Weld lines

• Voids

• Jetting

• Fiber orientation

• Residual stress

17.2 Extrusion

Typical products:

• Pipes

• Profiles

• Films

• Sheets

• Cable jackets

• Hoses

17.3 Blow Molding

Typical products:

• Tanks

• Bottles

• Hollow ducts

• Storage containers

17.4 Machining

Engineering plastics are available as rods and plates for machining.

Considerations:

• Cutting heat

• Low thermal conductivity

• Residual stress

• Clamping deformation

• Burr formation

• Moisture absorption

• Post-machining dimensional change

17.5 Additive Manufacturing

Representative materials:

• PLA

• ABS

• PA

• PC

• TPU

• PEEK

• PEI

• Fiber-reinforced filaments

Strength and fracture behavior depend strongly on print orientation, and injection-molded isotropy should not be assumed.

18. Comparison of Major Polymer Materials

19. Drawing and Material-Specification Designation

It is insufficient to specify only “nylon,” “acetal,” or “Teflon.”

The following should be indicated whenever possible.

19.1 Base Resin

Examples:

• PA66

• POM-C

• PEEK

• PTFE

• PC

19.2 Reinforcement and Content

Examples:

• PA66-GF30

• PEEK-CF30

• PPS-GF40

• PBT-GF30

19.3 Functional Additives

Examples:

• PTFE-filled

• Antistatic

• Flame-retardant

• UV-stabilized

• Metal-detectable

• Food-contact compliant

19.4 Color

Colorants and carbon-black content may affect weather resistance, laser transmission, and detectability.

19.5 Manufacturing Process

• Injection-molding grade

• Extrusion grade

• Machining plate stock

• Compression-molded stock

• Additive-manufacturing filament

19.6 Supplier Grade

For high-performance parts, specifying the supplier and exact trade grade is recommended.

The reason is that the same base resin may differ in:

• Molecular weight

• Viscosity

• Reinforcement-fiber length

• Additive package

• Heat stability

• Food compliance

• Flame resistance

• Color

• Recycled content

20. Polymer Material Selection Procedure

20.1 Confirm the Function

• Does the part carry structural loads?

• Is it a friction component?

• Is electrical insulation required?

• Is transparency required?

• Is sealing required?

• Must it absorb impact?

• Does it contact food?

20.2 Confirm the Load

• Tension

• Compression

• Bending

• Torsion

• Impact

• Fatigue

• Contact pressure

• Long-term static load

20.3 Confirm the Temperature

• Minimum service temperature

• Continuous-use temperature

• Short-term peak temperature

• Steam-cleaning temperature

• Thermal cycling

• Frictional heating

20.4 Confirm the Environment

• Water

• Humidity

• Acids

• Alkalis

• Oils

• Fuels

• Solvents

• Ultraviolet radiation

• Radiation

• Vacuum

• Cleaning chemicals

20.5 Confirm Dimensional Stability

• Moisture absorption

• Molding shrinkage

• Thermal expansion

• Creep

• Fiber orientation

• Post-shrinkage

• Residual stress

20.6 Confirm the Manufacturing Method

• Injection molding

• Extrusion

• Machining

• Blow molding

• Thermoforming

• Additive manufacturing

• Adhesive bonding

• Welding

20.7 Confirm Regulations and Safety

• Food contact

• Medical use

• Flame resistance

• Electrical safety

• Hazardous substances

• Recyclability

• Combustion gases

• Dust safety

21. Final Summary

Polymer materials are not simply plastics. Their performance is determined by molecular structure, crystallinity, molecular weight, copolymer composition, reinforcements, and additives.

Polyethylene and polypropylene are nonpolar polymers composed of carbon and hydrogen, offering low moisture absorption and excellent chemical resistance.

PVC contains chlorine, which provides flame resistance and chemical resistance, but its properties vary greatly with heat stabilizers and plasticizers.

ABS balances heat resistance, impact strength, and moldability through the combination of acrylonitrile, butadiene, and styrene.

Polyamides obtain high strength and wear resistance from hydrogen bonding between amide groups, but dimensional changes due to moisture absorption must be considered.

POM achieves high dimensional precision, low friction, and good fatigue performance through its regular oxymethylene structure and high crystallinity.

PC provides high impact strength and transparency through aromatic structures and carbonate bonds.

PPS and PEEK achieve high heat resistance, chemical resistance, and dimensional stability through aromatic structures and strong chemical bonds.

PTFE has extremely high chemical resistance and a very low coefficient of friction due to strong carbon-fluorine bonds, but its stiffness and creep performance are limited.

Thermosetting resins provide high heat resistance and dimensional stability through crosslinked structures, but they cannot be remelted or reshaped.

Elastomers provide large elastic deformation and recovery through flexible molecular chains and controlled crosslinking.

The actual performance of a polymer component is not determined by the base-resin name alone.

The following must be considered together:

• Base polymer

• Monomer and repeating unit

• Molecular weight

• Degree of crystallinity

• Type and content of reinforcement

• Inorganic fillers

• Plasticizers

• Flame retardants

• Heat and light stabilizers

• Lubricants

• Colorants

• Recycled content

• Molding conditions

• Service temperature

• Moisture and chemical environment

Therefore, polymer selection is not merely the process of choosing a lighter alternative to metal.

It is a design process that combines the appropriate molecular structure and compound formulation according to load, temperature, friction, moisture, chemical exposure, dimensional stability, manufacturing process, and regulatory requirements.