How to choose a suitable heat-resistant plastic?

How to choose a suitable heat-resistant plastic?

Before selecting a suitable material for a heat-resistant plastic product, we first need to understand several temperature concepts. These theoretical values for temperature resistance form the basic framework and boundaries for selecting heat-resistant plastics. Then, we need to analyze the gap between the actual application of your product and the theoretical values. This is because we cannot ignore the material’s real-world performance under long-term loads, chemical corrosion, and temperature fluctuations. How are these requirements defined? The decision rests with the end customer, and different industries have different testing standards.

Considerations and practical experience when selecting heat-resistant plastic materials

Generally, the differences in testing types across industries stem primarily from the environmental stresses of the application scenarios. For example, electronics focuses on high humidity and long-term thermal aging, automotive on weather resistance and media contact, and medical on sterilization resistance. Certification, on the other hand, requires compliance with industry-specific standards (such as UL, IEC, and ISO). The same material may need to undergo multiple specialized tests in different industries before it can enter the market.

In addition, there are many sub-types of heat resistance testing and certification for plastic parts, such as “Double 85” and “UL RTI”, which are environmental adaptability, long-term reliability or specific scenario simulation tests. They are usually strongly tied to industry application scenarios (such as high temperature, high humidity, long-term load, contact with specific media, etc.).

I’ve drawn on my practical project experience at Elimold to provide an in-depth analysis of temperature-resistant engineering plastics from the end-customer’s perspective, highlighting a different selection logic. A common, fatal misconception among engineers is that Tg ≠ operating temperature! hdt ≠ operating temperature! Tb ≠ low-temperature resistance. This is because too many engineers simplify temperature resistance to a single heat distortion temperature (HDT) or glass transition temperature (Tg) and low-temperature resistance (Tb), neglecting the material’s true performance under long-term loads, chemical corrosion, climatic conditions, and temperature fluctuations.

Indicators such as heat distortion and Vicat softening often fail to accurately reflect the actual requirements of a product in a project. Therefore, relying solely on theoretical concepts in practical applications without considering real-world conditions and the specific operating conditions of the product can lead to the selection of the wrong materials. This can result in product recalls or even fatalities. Blindly believing everything in books is worse than having no books at all.

In commercial applications, the requirements for heat resistance in plastics are not simply based on temperature resistance parameters. Different industries and brands will consider the specific application scenarios and require different materials.

Heat resistance + thermal aging
Heat resistant + UV resistant
Heat resistant + chemical resistant
Heat resistance + low temperature resistance switching
Heat resistance + radiation
Heat resistant + hydrolysis resistant
Heat resistance + other

…

Products that emerge from market competition always meet the real needs of the end-user market. Therefore, choosing the most suitable heat-resistant plastic material from the vast ocean of plastic materials is an extremely difficult task.

Many engineers are accustomed to using materials from previous generations of products, believing that “what worked in the past will work now.” However, when the product structure changes, using the previous generation of materials is no longer suitable.

Performance parameters of plastic materials

plastic materials are numerous and complex, not only difficult to remember, but even more challenging to understand the practical meaning behind each parameter and its actual impact on the quality, reliability, and performance of plastic parts. For low-cost, smaller-brand plastic materials, some performance parameters may not even be available, especially chemical resistance. Even when data is available, different testing standards (ISO vs. ASTM) and units (metric vs. imperial) can hinder material comparisons.

category	Key parameters	Impact Instance
Physical properties	Density, Melt Flow Index (MFR), Shrinkage	The density difference between PP (0.9 g/cm³) and PPS (1.8 g/cm³) significantly affects product weight.
Mechanical properties	Tensile strength, flexural modulus, impact resistance (notch sensitivity), creep, fatigue	The notch sensitivity of PA66 caused unexpected breakage of automotive gears under cyclic loads.
Thermal performance	Heat distortion temperature (HDT), coefficient of linear thermal expansion (CLTE), Vicat softening	PPO’s 120°C HDT cannot withstand a high-temperature environment of 200°C and needs to be replaced with PBT.
Environmental aspects	Hygroscopicity, chemical resistance, UV resistance	ABS tensile strength decreased by 30% after 1000 hours of UV exposure, while ASA only decreased by 5%.
Compliance	UL94 flame retardant rating, FDA/USDA certification, RoHS/REACH restrictions	A medical device was recalled because its chemical resistance data had not been verified.

Factors influencing the selection of heat-resistant injection molding materials

Never wait until the product mold is opened to discover material problems during testing. Material selection should be completed during the product design phase, ensuring it meets the requirements of part structure, manufacturing process, and performance specifications. This proactive approach can significantly reduce subsequent risks and guarantee the economic benefits of product development. Therefore, the selection of suitable heat-resistant injection molding materials can be based on analysis and selection of the following factors.

Define the final product’s usage environment

The product’s operating environment includes ambient temperature and the medium it comes into contact with. Plastic materials can only maintain their performance within a certain temperature range. Below or above this temperature range, plastic parts are prone to becoming brittle and failing under mechanical stress or chemical attack; moreover, different plastic materials have different operating temperature ranges.

The selection of plastic materials should also take into account the effects of various chemical media (such as mold release agents, oils, degreasers, lubricants, cleaners, dyes, adhesives) that the product may come into contact with in the production and use environment, as well as the effects of rainwater, ultraviolet rays, sand, etc. on the plastic materials, to ensure environmental compatibility.

Load conditions of plastic parts

When selecting plastic materials, it is essential to carefully analyze the load conditions that the parts will bear in actual applications, including the magnitude, type, and duration of the load. Then, compare the physical properties of the plastic materials to select the appropriate material.

Appearance

Consider aesthetic requirements such as transparency, texturing, electroplating, and surface gloss. These factors all influence the choice of plastic materials.

Assembly requirements

The selection of plastic materials must take into account how the parts will be assembled into the final product, because different materials are suitable for different assembly methods. Some plastic materials are suitable for adhesive bonding, some are suitable for ultrasonic welding, while hook and latch connections require plastic materials to have sufficient strength, elasticity, and dimensional stability.

Dimensional stability

Amorphous plastics possess excellent dimensional stability, while semi-crystalline plastics exhibit poor stability due to their higher shrinkage rate and tendency to warp. Therefore, it is essential to select appropriate plastic materials based on the dimensional stability requirements of the parts and products.

Compliance

When selecting plastic materials, you should check the 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 depending on the type and rating of the plastic.

cost

Cost is paramount in product design. When selecting plastics, we should consider both price and density, choosing the more affordable material while still meeting the product’s application requirements.

density

Density has a significant impact on the cost of plastic parts. Higher density means a heavier part for the same volume. Therefore, when evaluating material costs, both price and density must be considered.

melt flow rate

Melt flow rate (MFR), also known as melt index, is the weight of molten plastic passing through a melt indexer in 10 minutes under certain temperature and pressure conditions, measured in grams per 10 minutes.

Shrinkage

Plastic parts shrink after molding due to cooling, and their dimensions are always smaller than the cavity dimensions. Shrinkage rate refers to the percentage of the difference between the mold cavity dimensions and the product dimensions relative to the cavity dimensions. The shrinkage rate differs in the flow direction and the vertical direction, and is also affected by the plastic part structure, part wall thickness, injection molding process, type of additives, and gate location.

Mechanical properties

Mechanical properties of plastics, such as tensile strength, flexural strength, impact resistance, creep, and fatigue, are crucial for material selection, performance evaluation, and stress behavior prediction. It is important to emphasize that plastic mechanical property data are derived from laboratory testing and cannot be directly used to evaluate actual plastic parts. These data are for comparative reference only; the true mechanical properties of plastic parts need to be tested and verified in real-world application environments.

Tensile strength

Tensile testing involves applying a tensile load along the longitudinal axis of a specimen until it breaks under specific temperature, humidity, and tensile speed conditions. The magnitude of the load and the magnitude of the deformation are recorded and converted into stress and strain curves.

Bending strength

The bending performance test involves applying a bending load to a standard specimen under specified test conditions until the specimen breaks or the strain reaches 5%.

Impact resistance

Impact strength is a key factor in selecting plastic materials. It is defined as the energy absorbed per unit cross-sectional area of a specimen when subjected to an impact load, and is an indicator of a material’s toughness. Because factors such as specimen shape, thickness, stress concentration points, injection pressure, ambient temperature, and impact velocity all affect impact performance, test data often deviates from the actual performance of the parts. Therefore, different impact testing methods are used for different application environments.

creep

Creep is the process by which the deformation of plastics increases over time under a constant load. Even under low loads, plastics will continue to deform as long as the load is applied continuously. Therefore, for plastic parts subjected to long-term loads, creep data must be calculated and analyzed to ensure that the plastic parts do not undergo excessive deformation, cracking, or yielding due to creep during the product’s service life.

fatigue

Fatigue refers to the process by which the mechanical properties of plastic parts weaken or fail under cyclic loading. Under cyclic loading, tiny fatigue cracks generated on the sample gradually increase in size, eventually leading to complete failure. Plastic parts need to possess good fatigue performance to withstand vibration or repeated deformation.

Heat distortion temperature (HDT)

At high temperatures, the mechanical properties of plastic parts decrease significantly, leading to deformation and cracking. Therefore, operating temperature typically limits the choice of plastics. The heat deflection temperature (HDT) or load deflection temperature (DTUL) indicates the short-term heat resistance of a plastic under load. For safety, the maximum short-term operating temperature of plastic parts should be approximately 10°C below the HDT to prevent deformation.

Coefficient of thermal expansion (CLTE)

The dimensional changes of plastic parts due to increased temperature are described by the coefficient of thermal expansion (CLTE), which is the ratio of dimensional expansion of plastic when heated, measured in cm/cm/°C. The CLTE of plastics is much higher than that of metals (5-10 times), and temperature changes significantly affect the dimensions and mechanical properties of plastic parts. Therefore, the design of plastic parts must consider the operating temperature range. When plastics are used in conjunction with metals, large temperature differences or dimensional variations can lead to deformation or failure due to differences in expansion and contraction.

water absorption

Plastics change their size, shape, and physical properties when they absorb water. The mechanical and electrical properties of the material change with the moisture content, but these changes can be reversed by drying.

Chemical resistance

Chemical resistance refers to the ability of plastics to withstand the erosion of acids, alkalis, salts, solvents, and other chemicals. This property is very complex and is affected by a variety of variables, such as the type and concentration of the chemical, the duration of exposure, temperature, and applied load.

Weather resistance

Plastics used outdoors are subjected to the elements, including sunlight, temperature fluctuations, and wind and rain, which can cause them to age. Ultraviolet radiation is a major cause of plastic aging, leading to embrittlement, discoloration, reduced strength, surface cracking, and powdering.

Flame retardancy

UL94 is a global standard for assessing the flame retardancy of plastics. It evaluates a plastic’s ability to extinguish itself after being ignited. Material color and thickness affect the flame retardancy rating, and the rating must account for the corresponding thickness.

Relative Temperature Index (RTI)

When plastics are exposed to high temperatures for extended periods, their mechanical and electrical properties gradually decline. The Relative Temperature Index (RTI), also known as the long-term service temperature, refers to the highest temperature at which a plastic can be used continuously without additional load. Above this temperature, key properties of the plastic (such as tensile strength, impact strength, and dielectric strength) will significantly decrease.

Relative Tracking Index (CTI)

The tracking index (CTI) is an indicator of a material’s resistance to electrical leakage. It is determined by applying a voltage to the surface of an insulator and then dripping an electrolyte solution onto the surface of a plastic sample between electrodes. The highest voltage the material can withstand without leakage damage is evaluated. The CTI is graded from 0 to 5, with lower numbers indicating higher resistance to leakage.

High voltage arc tracking rate (HVTR)

HVTR refers to the speed at which a plastic sample forms a carbonized trail (conductive path) under a 5200V electric arc generated by a moving electrode. Materials are graded numerically: the smaller the number, the slower the arc carbonization trace travels, meaning better arc resistance.

Hot Wire Ignition (HWI)

The hot wire ignition test involves wrapping a nickel wire around a plastic sample and passing a specified current through it.

High Arc Burning Index (HAI)

The high arc flammability index (HAFI) is a classification of the flame retardancy of a plastic sample based on the number of arc flashes required for combustion after multiple high-current arc flashes near the sample. The lower the number, the more difficult the material is to burn.

Outdoor weather resistance

Outdoor weather resistance is divided into two categories: simultaneously meeting the “UV test” and the “water spray test”. The UV test involves 720 hours under a dual-lamp sealed carbon arc or 1000 hours under a xenon arc, with no change in flammability rating and maintaining ≥70% of the original tensile, flexural, and impact strength. The water spray test involves treatment in 70°C hot water for 7 days, with the flammability rating remaining unchanged and key mechanical properties retaining ≥50%.

Dielectric strength

Dielectric strength, also known as breakdown strength, refers to the maximum voltage that a plastic can withstand before electrical breakdown occurs. It is usually expressed as the voltage per unit thickness at the breakdown point (e.g., V/mil or kV/mm). This characteristic directly reflects the insulating ability of the plastic; generally, the higher the dielectric strength, the better the insulation quality of the plastic.

Volume resistivity

Volume resistivity measures the resistance of a plastic material through its bulk. It is determined by applying a voltage across parallel electrodes clamping a plastic sample and measuring the resulting current flowing through the sample. Volume resistivity is calculated using Ohm’s law (resistance = voltage/current).

Surface resistance

Surface resistivity measures the resistance encountered when current flows across the surface of a plastic sample. During testing, a voltage is applied between two electrodes on the same surface of the sample, and the surface current is measured. Higher surface resistivity indicates better surface insulation performance.

Why choose Elimold’s injection molding services?

Elimold specializes in injection molding services for a wide range of industries. We offer a variety of plastic materials, color options, and finishes to meet your specific needs. Our experienced team will work closely with you to determine the best solution for your unique requirements, ensuring your parts are produced quickly, efficiently, and to the highest quality standards. Whether you need assistance with design, prototyping, or mass production, we have the expertise and resources to help you get things done. Contact us today to learn more about how we can help you achieve your production goals.

Summarize

There are many types of materials available for plastic injection molding, each with its unique physicochemical properties and applications. Choosing the right material is crucial for ensuring product quality, performance, and cost-effectiveness. With advancements in science and technology and increased environmental awareness, plastic injection molding materials will evolve towards a more environmentally friendly and sustainable future, contributing to the green development of the manufacturing industry.

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