From wearables to electric vehicles, from medical devices to smart homes, plastic parts are ubiquitous. For a successful product design engineer, plastic part design is a critical skill, material selection can be difference between success and failure.
A single incorrect choice can lead to mold failure or a massive component recall – a lesson I've learned firsthand. This guide will break down the key details from catastrophic failure to proactive strategies. Let's dive in.

 

Plastic Performance Chart

Plastic materials impact quality, performance, reliability, and cost of plastic parts. A wrong choice can easily lead to failure of the entire product development process. Take my own experience as an example:
Case Study: Connector Assembly Failure
Background: A client designed a complex, one-piece molded connector assembly and specified a black PA66 + 30% GF material from their connector supplier. Customer evaluated material and determined it met product requirements. They requested connector continue using this material, also customizing it in a green color. We initially warned customer that green color scheme could lead to fiber floating and color drift, but customer insisted on using this material for a trial run.
Problem: Connector's product structure and mold structure are complex, and exterior molding surface relies entirely on slide mechanism. Severe sintering occurred when mold temperature exceeded 100℃. During trial run, even with slide mold temperature controlled at 95℃, two major issues persisted:

 

Fiber Floating
Severe fiber floating and color drift on exterior surface resulted in product's appearance not meeting customer requirements;
Buckle joint was prone to breakage and lacked toughness.
Initial Solution: Customer initially determined that excessive glass fiber content was causing fiber floating and insufficient toughness, so they used a black PA66 + 15% GF material for a second trial run. While improvements were achieved, fiber floating persisted, buckle joint breakage issue remained, and exterior quality did not meet required standards. Project stalled, and customer faced significant delivery pressure.

 

Broken buckle
Final Solution: Therefore, we innovatively proposed replacing PA66 with PC, a material that combines excellent appearance and mechanical toughness. Although customer initially expressed skepticism about material change, they ultimately agreed to a trial due to lack of a better solution. After selecting PC, we optimized mold flow path and process parameters, successfully resolving fiber float and color bleaching issues while also ensuring buckle toughness met functional requirements.
This case study clearly demonstrates that material selection can determine success or failure of a project.

Complexity of material selection stems from two main challenges:
Vast material ecosystem
There are over 45 major plastic types (thermoplastics and thermosets) and approximately 85,000 commercial grades.
Even for same resin, performance can vary significantly between different suppliers (or even between different grades from same supplier).

 

Vast material ecosystem
Therefore, selecting the most suitable plastic material from this vast ocean of available plastics is a challenging task.
Many engineers are accustomed to using materials from previous product generations, believing that "what worked in the past will work now." However, as shown in above case, when product structure changes, previous generation of materials is no longer suitable.
Complex Performance Parameters
Performance parameters of plastic materials include:

Plastic material performance parameters are numerous and complex, making them difficult to remember and understand. Understanding practical significance of each parameter and its impact on quality, reliability, and performance of plastic parts is even more challenging. Low-cost, small-brand plastic materials may not even have some performance parameters, particularly chemical resistance.
Even when data is available, differing testing standards (ISO vs. ASTM) and units (metric vs. imperial) hinder material comparison.

Selection of plastic materials must be determined early in product design phase, as differences in plastic material properties can lead to completely different part designs. This includes: wall thickness; rib thickness; screw hole diameter; assembly process selection.
In addition, due to varying shrinkage rates, processing different plastic materials in same injection mold can result in dimensional deviations. Once mold production begins, changing plastic materials is unlikely or prohibitively expensive.
Basic Principle: Never wait until product is molded to reveal material issues during testing. Material selection should be completed during product design phase, ensuring that it meets requirements for part structure, manufacturing process, and performance. This proactive approach can significantly reduce subsequent risks and ensure cost-effective product development.

These four steps will help you choose the best plastic material for your needs!

2.2.1.1 Product Usage Environment:
Product's usage environment includes ambient temperature and contact media.
Plastic materials can only maintain their properties within a certain temperature range. Below or above this temperature range, plastic parts are susceptible to brittle failure due to mechanical stress or chemical attack. Furthermore, different plastic materials have different operating temperature ranges.
For example, as shown in figure below, tensile strength of Polyplastics POM M90-44 at 80℃ is significantly lower than that at 23℃. Therefore, it is important to clearly define product's operating environment, including maximum, minimum, and long-term operating temperatures. Temperature conditions during assembly, post-processing, and transportation must also be considered.

 

DURACON M90-44 SS Curve
When selecting plastic materials, it is also important to consider various chemical media (such as release agents, oils, degreasers, lubricants, detergents, dyes, and adhesives) that product may be exposed to during production and use, as well as effects of rain, UV rays, and sand, to ensure compatibility with environment.

 

 

▲ Failure Cases Caused by Chemical Exposure
2.2.1.2 Plastic Part Loading Conditions
When selecting plastic materials, it is important to carefully analyze load conditions part will experience in actual application, including magnitude, type, and duration of load. Then, physical properties of plastic materials must be compared to select appropriate material. Table below lists five typical loading conditions and plastic material properties that should be considered.

▲ Typical Loading Conditions and Plastic Material Properties to Consider
2.2.1.3 Functional Requirements
Appearance: Consider appearance requirements such as transparency, texture, plating, and surface gloss. These factors will influence choice of plastic material.
Assembly Requirements: Selection of plastic materials must consider how parts will be assembled into final product, as different materials are suitable for different assembly methods. Some plastic materials are suitable for gluing, others for ultrasonic welding. Hook-and-loop connections require sufficient strength, elasticity, and dimensional stability.
Dimensional Stability: Amorphous plastics such as PC (polycarbonate) offer excellent dimensional stability, while semi-crystalline plastics such as PBT (polybutylene terephthalate) and PA66 (nylon 66) have lower stability due to their high shrinkage and tendency to warp. Therefore, appropriate plastic material must be selected based on dimensional stability requirements of part and product.
Safety: When selecting a plastic material, check required safety certifications (such as 3C, FDA, USDA, UL) and UL94 flame retardancy ratings (5VA, 5VB, V-0, V-1, V-2, HB). Flame retardancy varies by plastic type and grade.
2.2.1.4 Cost
Cost is crucial in product design. When selecting a plastic, consider both price and density, choosing a more affordable plastic material that meets requirements of product application.

2.2.2.1 Physical Properties
2.2.2.1.1 Density
Density significantly impacts cost of plastic parts. Higher density means heavier parts per volume. Therefore, when evaluating material costs, consider not only price but also density. For example, a material with a density of 1.09 and a price of 25 yuan/kg is more cost-effective than one with a density of 1.57 and a price of 20 yuan/kg.

 

Density of Major Plastics
2.2.2.1.2 Melt Flow Rate
Melt flow rate (MFR), also known as melt index, is weight of a plastic melt passing through a melt flow meter in 10 minutes at a specific temperature and pressure. It is measured in grams per 10 minutes.

 

Meaning of MFR:
Significant differences in melt flow rate between batches of same plastic indicate changes in formulation or process, indicating inconsistent quality. During injection molding process, significant changes in melt flow rate indicate possible high-temperature degradation of plastic.
Significant changes in melt flow rate during use indicate possible degradation due to exposure to chemical media.
2.2.2.1.3 Shrinkage
Plastic parts shrink as they cool after molding, and their dimensions will inevitably be smaller than cavity size. Shrinkage refers to difference between mold cavity size and part size as a percentage of cavity size. Shrinkage in flow direction and perpendicular direction differs and is also affected by factors such as plastic part structure, part wall thickness, injection molding process, type of additives, and gate location. For example:
Features such as holes and ribs restrict movement of plastic part, reducing shrinkage.
Shrinkage generally increases with increasing wall thickness.
Shrinkage decreases with increasing injection and holding pressures.
The farther from gate, the greater shrinkage.
Carbon fiber and glass fiber fillers reduce shrinkage in flow direction. Carbon fiber and glass fiber reinforced plastic parts shrink two to three times more perpendicular to flow direction than in direction of flow.

 

Importance of Shrinkage in Product Design:
Semi-crystalline plastics typically shrink more than amorphous plastics, which shrink more uniformly in all directions. High shrinkage reduces dimensional stability and increases risk of warpage. Therefore, amorphous plastics with low shrinkage are more suitable for precision applications.
When designing products, dimensional instability should be considered for plastic parts with high shrinkage. Optimization methods such as adding holes, ribs, mold compensation, or adjusting injection molding parameters can improve dimensional stability and reduce warpage.
Different plastics have different shrinkage rates, so for parts of same size, corresponding mold cavity dimensions vary. Therefore, it is important to determine material before mold production; otherwise, changing material after mold is built will result in significant dimensional deviations.
2.2.2.2 Mechanical Properties
Mechanical properties of plastics, such as tensile strength, flexural strength, impact resistance, creep, and fatigue resistance, are crucial for material selection, performance evaluation, and stress behavior prediction.

 

It is important to emphasize that mechanical property data for plastics is derived from laboratory testing and cannot be directly used to evaluate actual plastic parts. These data should be used for comparative reference only; true mechanical properties of plastic parts must be verified by testing in actual application environments.
2.2.2.2.1 Tensile Strength
Tensile testing involves applying a tensile load along longitudinal axis of a specimen until fracture under specific temperature, humidity, and tensile speed. Load and deformation are recorded and converted into a stress-strain curve.
Stress (σ): Tensile load applied to the specimen per unit initial cross-section.
Strain (ε): Ratio of dimensional change of specimen under stress to its original dimensions. σ = F/A₀, ε = (L-L₀)/L₀(F = tensile load, A₀ = initial cross-sectional area, L₀ = original measured length, L = deformed length).
Key Points of Tensile Stress-Strain Curve
Proportional Limit Point: In region from origin to proportional limit point, stress and strain are linearly related. Curve deviates from linear behavior at proportional limit.
Yield Point: Plastics with good ductility will continue to deform beyond yield point, but stress will no longer increase.
Fracture Point: Test specimen breaks at this point.
Key Tensile Properties
Tensile Modulus: Slope of tensile stress-strain curve from origin to proportional limit is tensile modulus. Tensile modulus indicates material's stiffness (its resistance to deformation). A higher tensile modulus indicates less deformation and greater stiffness.
Note: Due to viscoelastic behavior, tensile modulus of plastics is less accurate than that of metals.
Tensile Yield Strength (σ_Y): Stress at yield point represents maximum stress at which a plastic can withstand permanent deformation. A higher σ_Y indicates greater strength.
Tensile Elongation at Yield (ε_Y): Elongation at yield is percentage of material's length change relative to its initial length when yield strength is reached. A higher ε_Y indicates greater toughness and elasticity in plastic.
Tensile Break Strength (σ_B): Maximum stress a plastic can withstand before breaking. A higher σ_B indicates greater plastic strength.
Elongation at break (ε_B): Elongation at break is ratio of maximum extension before breaking when stretched to initial length. A larger ε_B indicates greater toughness and elasticity in plastic.
Ultimate tensile strength: Maximum stress a specimen can withstand before breaking during a tensile test:
For brittle plastics, this is equal to σ_B (breaking strength).
For ductile materials, this may be equivalent to σ_Y (yield strength) or maximum strength before breaking.

▲ Tensile Properties of Common Plastics
2.2.2.2.2 Flexural Strength
Flexural properties test applies a bending load to a standard specimen under specified test conditions until specimen breaks or strain reaches 5%.
Main Flexural Properties
Flexural Modulus:Flexural modulus is ratio of stress to strain in elastic region of a plastic's flexural stress-strain curve. It measures plastic's resistance to flexural deformation (stiffness).
Flexural Strength: Maximum stress experienced during a flexural test usually corresponds to breaking point or maximum allowable strain.

 

▲ Flexural Strength and Flexural Modulus of Common Plastics
2.2.2.2.3 Impact Resistance
Impact strength is a key factor in selecting plastic materials. It is defined as energy absorbed per unit cross-sectional area when a specimen is subjected to an impact load and is a measure of material's toughness.
Because factors such as specimen shape, thickness, stress concentration points, injection pressure, ambient temperature, and impact velocity affect impact performance, test data often deviate from actual part performance. Therefore, different impact testing methods are used for different application environments.

 

Two Main Impact Testing Methods
Izod Impact Test(ASTM D256, D4812, or ISO 180):
Notched specimen is fixed on one side, with notch facing the pendulum.
Energy dissipated is calculated.
Key variables are recorded: specimen thickness, notch presence, and test temperature.
Charpy Impact Test(ISO 179):
Specimen is fixed at both ends, with notch facing away from pendulum.
Difference from Izod test method lies in specimen support and placement.
Design Implications: When designing plastic parts, especially those in consumer electronics that are susceptible to impact, avoid sharp corners or grooves. Using rounded corners and smooth transitions can minimize stress concentrations, improve impact resistance. For example, smartphone cases with rounded corners are more resistant to drops and reduce stress point cracking compared to cases with sharp corners.
Correlation with Tensile Modulus
Impact strength and tensile modulus together determine fundamental properties of plastics:
High impact strength + high tensile modulus: Indicates greater toughness (e.g., PC).
High impact strength + low tensile modulus: Indicates high ductility and flexibility (e.g., rubber-modified plastics).
Low impact strength + high tensile modulus: Indicates greater brittleness (e.g., rigid PS).
These correlations highlight importance of balancing multiple properties when selecting materials to simultaneously meet functional performance and durability requirements.
2.2.2.2.4 Creep
Creep is process by which a plastic's deformation increases over time under a constant load. Even under low loads, plastics will continue to deform as long as load is continuously applied. Therefore, for plastic parts subjected to long-term loads, creep data must be calculated and analyzed to ensure that they will not experience excessive deformation, cracking, or yielding due to creep over product's service life.
Creep Testing and Influencing Factors
Creep testing involves applying a tensile or bending load to a specimen and measuring strain generated at different sections. Main factors affecting creep are:
Temperature: The higher temperature, the greater creep deformation.
Load Magnitude: The greater load, the greater creep deformation.
Load Duration: Long-term load application can lead to stress cracking, even at low stresses.
Residual Stress: Internal stress (residual stress) should also be considered along with external stress.

 

Creep Strain at Different Temperatures (HDPE)
Stress Relaxation: If amount of deformation is constant, stress resisting deformation decreases over time. This is called stress relaxation, a corollary of creep. Stress relaxation is a crucial consideration in product design for plastic parts subject to long-term, fixed deformation. For example:
In snap-fit assemblies, stress relaxation can reduce clamping force.
Loss of elastic stress in seals or gaskets can lead to leakage.
Decreased spring tension can cause component failure.

 

Design Implications:
Use materials with low creep sensitivity (such as POM + 30% glass fiber) for load-bearing components.
When selecting materials, consider temperature profile and service life to avoid accumulation of creep deformation over time.
Reduce creep-related failures by balancing design features (such as increased wall thickness) with material properties.
2.2.2.2.5 Fatigue
Fatigue is process by which mechanical properties of plastic parts weaken or fail under cyclic loading. Under cyclic loading, small fatigue cracks in specimen gradually grow and eventually lead to complete failure. Plastic parts subjected to vibration or repeated deformation require excellent fatigue performance (for example, snowplow headlight housings, integrated salad tongs, and high-frequency snap-fit switches).
Main Influencing Factors
Fatigue performance of plastics is affected by many factors:
Shape Characteristics: Notches, stress concentrations, and sharp corners accelerate crack initiation.
Material Defects: Internal defects, surface scratches, and rough surface treatments all generate stress.
Product Usage Environment: Chemical exposure, temperature fluctuations, and UV radiation can reduce fatigue life.
Load Magnitude: Frequency, amplitude, and stress type (tension, bending, torsion) affect crack growth rate.
Orientation: Aligned glass fibers enhance fatigue resistance in their orientation.
Fatigue Testing Methods: Fatigue testing involves applying cyclic stress (tension, bending, torsion, impact, or a combination of stresses) to a specimen under specific conditions (fixed frequency, temperature, and load). Number of load cycles and corresponding stress magnitude are recorded until fatigue failure occurs. This yields a fatigue S-N curve.

 

N curves of PP at different frequencies

 

S-N curves of ABS at different frequencies
Types of fatigue failure
Complete fracture: Specimen breaks into two or more pieces, typically occurring in brittle materials.
Reduction in stiffness: Stiffness of plastics decreases to a specified value because crack growth in these materials is slow, and complete fracture requires multiple cycles.
Insights from S-N curve
Stress-life relationship: As number of load cycles increases, stress required to cause failure decreases.
Fatigue limit: Many materials have a specific fatigue strength limit. Beyond this limit, plastic will fail due to fatigue caused by cyclic loading.
Design applications
S-N curves can help us:
Compare fatigue properties of different plastics;
Predict service life of plastic parts under cyclic loading;
Optimize designs by avoiding stress concentrations or selecting materials with higher fatigue limits (such as reinforced nylon used in automotive suspension components).
Important Note: Unlike metals, fatigue limit of plastics is not readily apparent, requiring testing under simulated operating conditions. For high-cycle applications, prioritize materials with documented fatigue resistance and employ designs that minimize stress concentrations (e.g., rounded corners and uniform wall thickness).
2.2.2.3 Thermal Properties
2.2.2.3.1 Heat Deflection Temperature (HDT)
At high temperatures, mechanical properties of plastic parts can degrade significantly, leading to deformation and cracking. Therefore, operating temperature often limits choice of plastic.
Heat deflection temperature (HDT), or deflection temperature under load (DTUL), indicates short-term heat resistance of a plastic under load. For safety reasons, maximum short-term operating temperature of a plastic part should be approximately 10℃ below HDT to avoid deformation.

▲ Heat Deflection Temperature of Common Plastics
Practical Example: For components subject to moderate loads at 120℃, it is recommended to use ABS with 30% glass fiber (HDT at 1.8 MPa is 145℃) rather than pure ABS (DTUL at 1.8 MPa is 88℃) to provide a safety margin. Ensure that HDT is verified based on actual thermal loads to ensure component reliability.
2.2.2.3.2 Coefficient of Thermal Expansion (CLTE)
Dimensional change of a plastic part due to temperature increases. Coefficient of linear thermal expansion (CLTE) is rate at which a plastic expands when heated, expressed in cm/cm/℃.
CTE of plastics is much higher than that of metals (5-10 times higher), temperature changes significantly affect dimensional and mechanical properties of plastic parts. Therefore, plastic part design must consider operating temperature range. When plastics are mated to metal, large temperature or dimensional differences can lead to deformation or failure due to differential expansion and contraction.

 

▲ Design Considerations for CLTE
2.2.2.4 Environmental Properties
2.2.2.4.1 Water Absorption
Plastics can change their size, shape, and physical properties when they absorb water. PA expands after absorbing water; a 1% increase in humidity results in a 0.9% volumetric expansion and a 0.3% linear expansion, which can affect performance of metal-PA joints. While mechanical and electrical properties also change with moisture content, these changes can be reversed by drying.
Some plastics can hydrolyze due to water absorption, where polymer chains chemically react with water, reducing their molecular weight. Hydrolysis is affected by exposure time, type, temperature, load, and chemical properties of water. Plastics such as PC, PA, PET, and PBT are particularly susceptible, high temperatures and humidity accelerate this process. Environmental testing is essential to verify performance of plastic parts.
Always dry plastic before injection molding to prevent degradation during processing. Follow supplier's recommendations for best results.
2.2.2.4.2 Chemical Resistance
Chemical resistance refers to a plastic's ability to withstand attack by acids, bases, salts, solvents, and other chemicals. This property is complex and is influenced by many variables, such as type and concentration of chemical, duration of exposure, temperature, and applied load.
Mechanism of Chemical Attack
Molecular Chain Degradation: Strong electrolytes can break down molecular chains, reducing molecular weight and degrading mechanical properties (such as tensile strength and impact resistance).
Stress Cracking: Some chemicals can penetrate stress concentrations (such as notches and weld lines), causing cracking or brittle fracture.
Swelling/Dissolution: Weaker solvents can cause plastic parts to swell, soften, and change mechanical properties.
Design Considerations for Chemical Exposure
When designing plastic parts, consider all potential chemicals throughout product lifecycle:
Manufacturing Phase: Cutting oils, mold release agents, cleaning agents, and adhesives used during assembly.
Use Phase: End-use chemicals (such as detergents, fuels, pharmaceuticals) and environmental factors (such as humidity and UV light).
Key Applications: Medical devices, automotive parts, or industrial equipment may need to withstand highly corrosive chemicals (e.g., bases, aromatic solvents).
Limitations of Chemical Resistance Data
Formulation Variability: Chemical resistance charts for plastics are publicly available, but should be used with caution. Chemical attack may vary depending on manufacturer's formulation, additives, or impurities.
Environmental Synergy: Higher temperatures and higher chemical concentrations can significantly reduce chemical resistance. For example, a plastic that withstands a 10% concentration of a chemical at room temperature may not withstand a 5% concentration of same chemical at 66℃.

 

▲ Portion of a Plastic Chemical Resistance Chart
Mitigation Strategies
Material Testing: Conduct real-world environmental exposure tests under conditions consistent with the application (e.g., temperature, chemical concentration, load).
Material Selection: Plastics with strong chemical resistance are preferred for harsh environments:
Fluoropolymers (e.g., PTFE): Offer excellent chemical resistance to most chemicals.
Polypropylene (PP): Resistant to acids, bases, and many organic solvents. Polyethylene (PE)
Suitable for aqueous solutions and non-polar solvents.
Optimize Design: Avoid stress concentrations (e.g., sharp corners) and apply chemical-resistant coatings or barriers where necessary.
Important Warning: For critical applications, do not rely solely on published data. Verify chemical resistance through testing, as even minor formulation differences or environmental factors can cause unexpected failures.
2.2.2.4.3 Weatherability
Plastics used outdoors are subject to harsh elements of outdoor environment, such as sunlight, heat, cold, wind, and rain, which can cause degradation. UV radiation is a major cause of plastic degradation, causing embrittlement, discoloration, loss of strength, surface cracking, and chalking.
Material Differences:
Weatherability of different plastics (e.g., ABS vs. polycarbonate) varies, and even different grades of same plastic can have varying weatherability.
Formulations containing UV stabilizers (e.g., hindered amine light stabilizers (HALS)) can significantly extend service life of material, while untreated plastics may degrade within months.
Dark pigments absorb more UV energy than light pigments, accelerating material degradation. Certification Testing:
UL 746
Specifies testing requirements for outdoor suitability, including:
UV Testing: Simulates long-term sunlight exposure using a xenon arc lamp test (ASTM G155).
Water Resistance Testing: Tested in accordance with IEC 60068-2-30.
Test results are documented on a UL Yellow Card, including:
Relative Temperature Index (RTI) for outdoor use.
UV stability rating (e.g., "F1" indicates full compliance with outdoor use requirements).
Pro Tip: For critical outdoor applications, be sure to select materials with UL 746 "F1" certification—this certification ensures performance after more than 720 hours of accelerated UV/water exposure testing.
2.2.2.5 Flame Retardancy
UL94 is a global standard for evaluating flame retardancy of plastics. It measures their ability to extinguish after ignition. Material color and thickness affect flame retardancy rating, and rating must include appropriate thickness (e.g., "V-0 @ 1.5mm").

 

Horizontal Burning (HB)—Basic Flame Test
UL 94 HB rating indicates the lowest flame retardancy and is applied to plastics that exhibit slow horizontal burning. Materials less than 3 mm thick must either burn at a rate of less than 76 mm/min or self-extinguish before burning to 100 mm to pass the test.
Vertical Flame Test (V-2, V-1, V-0) - More Stringent Standards
When testing plastics vertically, they are graded based on duration of burning and presence of flaming particles:
UL 94 V-2: Burns cease within 30 seconds, and flaming particles are permitted.
UL 94 V-1: Burns cease within 30 seconds, and non-flaming particles are permitted.
UL 94 V-0: Burns cease within 10 seconds, and flaming particles are not permitted. This is one of the most commonly used flame retardant grades.
5V Grade (5VA, 5VB) - Highest Flame Resistance
UL 94 5V is designed for materials that withstand more intense flames than those tested in standard vertical test, is commonly used in high-risk applications such as electrical enclosures and structural components.
UL 94 5VA: Burns cease within 60 seconds, with no dripping or holes (highest flame resistance). UL 94 5VB: Burns must cease within 60 seconds. No dripping is permitted, but holes may form.
A material marked "V-0, 5VA" on UL Yellow Card indicates that it has passed both vertical burning test and 5V test.
To meet UL 94 standard, plastics often require addition of flame retardants, but this can increase costs, complicate molding process, and reduce mechanical properties. Avoid excessively high flame retardancy ratings unless required for market access certification.
Pro Tip: It is recommended to cross-verify UL Yellow Cards with the UL iQ™ database, which provides real-time compliance checks for over 50,000 material grades.
2.2.2.6 UL746
UL746 is a series of material performance test standards developed by UL in United States. It includes:
UL746A: Short-term material performance tests, such as CTI (Comparative Tracking Index), HWI (Hot Wire Ignition), and HAI (High Arc Ignition Index).
UL746B: Long-term material performance testing, such as Relative Temperature Index (RTI).
UL746C: Evaluation of polymer materials for electrical equipment.
2.2.2.6.1 Relative Temperature Index (RTI)
When plastics are exposed to high temperatures for extended periods, their mechanical and electrical properties gradually degrade over time. Relative Temperature Index (RTI), also known as Long-Term Use Temperature, refers to maximum temperature at which a plastic can withstand long-term use without additional loads. Above this temperature, key plastic properties (such as tensile strength, impact strength, and dielectric strength) significantly degrade.
RTI requires UL certification testing. Plastics that pass this test receive a UL Yellow Card, which displays following three parameters:
RTI Elec: Indicates that after 100,000 hours of use at this temperature, plastic's electrical properties will degrade by 50%.
RTI Imp: Indicates that after 100,000 hours of use at this temperature, plastic's mechanical properties under impact will degrade by 50%.
RTI Str: Indicates that after 100,000 hours of use at this temperature, non-impact mechanical properties of plastic will degrade by 50%.
2.2.2.6.2 Comparative Tracking Index (CTI)
Comparative Tracking Index (CTI) is an indicator of a material's resistance to electrical tracking. When a voltage is applied to surface of an insulator, an electrolyte is dripped onto the surface of a plastic sample between electrodes. Maximum voltage material can withstand without electrical damage is evaluated. Material is graded on a scale of 0 to 5 based on withstand voltage value. The lower number, the higher electrical tracking resistance.
2.2.2.6.3 High-Voltage Arc Tracking Rate (HVTR)
HVTR evaluates speed at which carbonized tracks (conductive paths) form on a plastic sample when exposed to a 5200V arc generated by moving electrodes. Materials are graded numerically: the lower number, the slower arc carbonized tracks, indicating better arc resistance.
2.2.2.6.4 Hot Wire Ignition (HWI)
Hot wire ignition test involves wrapping a nickel wire around a plastic specimen and passing a specified current through it. Material's flame retardancy is graded based on the time it takes to start burning:
Grade 1: Time to start burning is 1-2 minutes (high flame retardancy).
Grade 4: Time to start burning is 7-15 seconds (low flame retardancy).
The higher number, the more flammable material.
2.2.2.6.5 High Arc Ignition Index (HAI)
HAI is a grading of material's flame retardancy based on number of arc flashes required to ignite plastic specimen after a high current arc flashes repeatedly past it. The lower number, the more flammable material.
2.2.2.6.6 Outdoor Weatherability
Outdoor weatherability is divided into (f1) and (f2):
(f1): Meets both "Ultraviolet UV Test" and "Water Shower Test."
UV Test: After 720 hours in a dual-lamp sealed carbon arc or 1000 hours in a xenon arc, flammability rating remains unchanged and ≥70% of original tensile, flexural, and impact strengths are retained.
Water Shower Test: After 7 days in 70℃ hot water, flammability rating remains unchanged and ≥50% of the key mechanical properties are retained.
(f2): Only one of above tests must be met.
2.2.2.7 Electrical Properties
2.2.2.7.1 Dielectric Strength
Dielectric strength, also known as breakdown strength, refers to maximum voltage a plastic can withstand before electrical breakdown occurs. It is usually expressed as voltage per unit thickness at breakdown point (e.g., V/mil or kV/mm). This property directly reflects plastic's insulating ability; generally, higher dielectric strength indicates better insulation quality.
2.2.2.7.2 Volume Resistivity
Volume resistivity measures electrical resistance of a plastic material through its bulk. It is determined by applying a voltage between parallel electrodes sandwiching a plastic sample and measuring resulting current flowing through sample. Volume resistance is calculated using Ohm's law (resistance = voltage/current).
Note: Volume resistance is a key indicator of a plastic's insulating properties. Most plastics exhibit excellent insulating properties, with a volume resistivity of at least 10⁸ ohm-cm. Sensitive to temperature and humidity: Volume resistance decreases with increasing temperature or humidity.
2.2.2.7.3 Surface Resistance
Surface resistance measures resistance to current flow along surface of a plastic specimen. During test, a voltage is applied between two electrodes on same surface of specimen and surface current is measured. Higher surface resistance indicates better surface insulation.
Importance: Surface resistance is crucial in applications where surface contaminants (such as moisture and dust) could compromise insulation. Volume and surface resistance are crucial in evaluating plastics for use in electrical and electronic components.

Select the most appropriate plastic category based on application requirements:
Is tensile strength critical? Is flexural strength critical? Is impact strength critical? Is hardness critical? Is dielectric strength critical? Is heat deflection temperature critical? What is maximum operating temperature? What kind of chemical resistance is required? Is UV resistance required? What are transparency requirements?

 

Choose plastic type that meets all requirements and offers the best price-performance ratio.

Same plastic material can be supplied by multiple suppliers. Even from same supplier, same type of plastic material may have multiple brands, and each brand may have multiple specific plastic materials. For example, ABS alone has 3,305 grades listed on UL Prospector (see figure below), each with varying properties such as mechanical strength, flame retardancy, and cost to suit specific applications.

 

After selecting a plastic material, further comparisons of performance and cost are necessary to determine the most suitable plastic material.
Sometimes, property tables may lack information such as chemical resistance. In these cases, it is necessary to communicate with material supplier for detailed information or conduct relevant testing.