If you’ve ever microwaved last night’s leftovers in the typical plastic to go container, you’ve witnessed the effect that high heat can have on thermoplastic. The plastic begins to soften and lose its stiffness as the material temperature increases and if you heat it long enough or exceed the limit of its operational temperature range, it will begin to distort. Worst case scenario, when you open the microwave door to enjoy your meal, you are presented with something that can be quite unrecognizable from what you put in.

While this example may not be relevant in all cases, it does demonstrate the importance of selecting a plastic thermoforming material with the appropriate temperature properties for your application’s operating environment. Imagine a similar scenario on an essential safety, structural, or functional component for a medical, transportation, or industrial application. Loss of stiffness (flexural modulus) and material distortion (heat deflection) are just a few of the factors to account for when addressing the temperature requirements of a project.

Material considerations for prolonged exposure to excessive temperatures

Most of the effects of temperature to thermoplastic occur at high heat levels, although excessively low temperatures can have an impact as well. Mechanical properties, chemical resistance, electrical conductivity, material fatigue, and many other attributes can be affected by increased temperatures. Below is a list of the most common considerations.

Note: The exact temperature thresholds and performance will vary for each different plastic material. In addition, factors like part geometry and material thickness will also affect material properties under extreme temperatures both high and low. The considerations below are just a general behavior characteristic of plastic in relation to temperature. (reference our article on material testing and data sheets for more information on standard testing of a material’s temperature performance)

Distortion

1. Exceeding a material’s approximate heat deflection temperature can cause the material to distort.
2. Prolonged exposure to heat while subjected to a load or force can also cause plastic to deform or “creep” over time.
3. Most thermoplastic materials have a heat distortion temperature (HDT) of less than 500 degrees F
4. HDT is a good comparative specification of how different materials respond to the HDT test conditions but provides little information regarding the long term effects of continuous high temperature exposure on their physical, mechanical, thermal, and electrical properties.

Softening

1. As temperature increases, material stiffness (flexural modulus) will decrease.

Expansion

1. As with most materials, plastic expands as temperature increases (coefficient of thermal expansion – CTE). This can be a consideration when the plastic is mated with another material, such as metal, that may have conflicting thermal expansion rates.
2. If the dimensional change is obstructed, stresses can be induced in the plastic part due to excessive tensile, shear, or compressive stress loads that could result in unexpected failure.

Service Life

1. Thermal Degradation – Plastic materials subjected to prolonged exposure to high temperatures will lose strength and toughness, becoming more prone to cracking, chipping, and breaking, at a rate in proportion to the temperature and time of exposure. Materials exposed to higher heat for longer duration will wear substantially faster than those exposed to more moderate temperatures and exposure times.
2. The Continuous Use Temperature Rating is based on a thermal aging test that predicts the temperature at which a 50% loss of the original mechanical properties will occur after 100,000 hours of continuous exposure at that temperature. (see table below)

Thermal Conductivity

1. The quantity of heat that passes through a cube of the material in a certain period of time when the difference in temperature between the two surfaces becomes one degree.
2. Plastic materials generally have a much lower Thermal Conductivity than metals. This makes them excellent replacement materials when thermal insulation is important.

Some questions to consider to determine your application’s temperature profile and ideal material candidates

During the product development process, Productive Plastics uses the following questions to zero in on the plastic material options that will be temperature compatible for a customer’s application:

1. What environmental temperature range (high and low) will the part be exposed to operationally?
2. What dimensional and stiffness (flexural modulus) tolerances are required of the part at the high, mid, and low points of its expected temperature range?
3. What loads or forces are expected on the part at the high end of its temperature range?
4. What is the time/temperature relationship? A low temperature for a long time can result in comparable properties damage as a high temperature for a short time.
5. What is the projected service life of the application?
6. Will the plastic part be mated to any other material types, such as metal, as part of the application design?
7. What are the specified (FST) flame, smoke & toxicity requirements?

Available thermoplastic options and temperature performance

As discussed in previous posts on material selection, when it comes to plastic material options, there are many choices and each has different thermal and mechanical performance properties. The information below will give you a general understanding of the operating thermal ranges of the common plastic material options available.

Note: The options listed are generic plastic material formulations. Many plastic material companies have specific plastic material products formulated and designed to meet the demands of a wide range of industry requirements. For information on the thermal performance of these products, visit our material supplier datasheet page or our thermofoming materials page.

Click here for a full list of plastic abbreviations and acronyms.

Productive Plastics is top contract manufacturer for heavy gauge thermoforming, including vacuum forming and pressure forming. Contact us or request our complimentary thermoforming design guide for more information.

When it comes to plastic thermoforming materials, one of the greatest advantages is that they can be manipulated and alloyed at the polymer level as well as being co-extruded to produce a multitude of variations. The result is a large list of available and even customizable plastic material products, each with its own unique properties and often formulated to meet the requirements of a particular industry.

The task of comparing and selecting the appropriate thermoforming material can therefore be daunting. To make the selection process easier, plastic material manufacturers have their material products independently tested to provide potential users with general characteristic and performance data, which is then presented via material product datasheets. These data sheets are excellent for material property comparisons but not always a exact indicator of field performance due to test sample preparation.

See some examples of thermoforming material datasheets here.

This data can give the end user an indication on how that material will behave once thermoformed into a finished component and if it is compatible with their application.

Measuring Plastic Thermoforming Material Properties:

Below is a list of the common tests that are performed on each plastic thermoforming material product, a description of what is measured, and how it should be used to assist in material selection.

Note – The data from raw material testing may not be exactly representative of how a material will perform on your finished product and in the field. Testing performed on material samples is done in a very controlled environment, at a uniform thickness, and as a flat extruded sheet of plastic or often injection molded. Your component, once thermoformed and assembled into a finished product, will likely have complex geometry, varying part thickness, and environmental factors such as temperature that are unique to your application and unaccounted for in standard material testing. For example, a raw material test may indicate a heat deflection (material distortion) of 200 degrees. However, once that same material is thermoformed and assembled on your finished product, it may have a heat deflection of only 190 degrees. So, ultimately, while testing will give you a ballpark indication on how your product will perform, keep in mind that results may vary. For more accurate data, conduct product testing on a finished and fully assembled prototype.

Physical Property Testing

Notched Izod Impact Strength (Ref ASTM D256)

Test definition: Pendulum style impact test of a notched sample subjected to a shock force. Typically used on more notch sensitive materials such as HIPS and ABS. The force absorbed by the notchedsample is measured and the type of failure is described

Material selection application:

• Good comparison test between similar materials
• Not a direct indicator of field performance

Specific Gravity (Ref ASTM D792)

Test definition: The ratio of the density of any substance to the density of an equal volume of water. Because Specific Gravity is a ratio it is a unitless quantity.

Material selection application:

• The Specific Gravity of plastic materials are an indication of their density
• Higher Specific Gravity will result in heavier material so caution must be taken when estimating and comparing part weights with varied materials

Chemical Resistance (Ref ASTM D543)

Test definition: Evaluation of plastic materials for resistance to chemical reagents (ex. lubricants, cleaning agents, inks, foods) The test includes provisions for reporting changes in weight, dimensions, appearance and strength properties.

Material selection application:

• The published chemical resistance properties are a good guideline for material selection. However since variable factors can affect chemical resistance one should always test under their own conditions
• Chemicals can affect strength, flexibility, color, surface appearance, and dimensions of plastics.
• Plastics often fail even under very low stress when in contact with some chemical agents. This is called environmental stress cracking and is of great importance in material selection

Stiffness (Flexural Modulus) (Ref ASTM D790)

Test definition: Rigidity of material / a measure of stiffness

Material selection application:

• Provides design criteria to determine the necessary thickness required for a given load
• Good for comparison of different materials

Hardness (Ref ASTM D2240)

Test definition: A measure of how resistant a material is to various kinds of permanent shape change when a compressive force from a harder body is applied

Material selection application:

• This is a good measure of resistance to wear by friction or erosion
• Material resistance to abrasion, chipping, and cracking

Tensile strength (Ref ASTM D638)

Test definition: Resistance to being pulled apart

Material selection application:

• Tells how material stretches before breaking
• Provides an indication of overall toughness
• The most important indication of strength of the material

Dielectric Strength

Test definition: Electrical insulation- the maximum voltage that can be applied to a material without it breaking down

Material selection application:

• Plastics are generally considered insulators but they can transmit some electrical energy at high frequency
• Many variables such as material fillers and additives, part thickness, and environmental conditions will affect the plastic’s dielectric constant

Thermal Property Testing

Thermal Conductivity (Ref ASTM E1530)

Test definition: A measure of the ability of a material to transfer heat.

Material selection application:

• Most plastics are insulators and not good conductors of heat

Coefficient of Thermal Expansion (CTE) (Ref ASTM E831, ASTM D696, and ISO11359)

Test definition: Amount of expansion and contraction at a given temperature

Material selection application:

• Impact relating to very narrow dimensional tolerances
• Potential interference and fitment issues when plastic components are combined in assembly with dissimilar material components
• Tooling and process design considerations are affected by CTE
• Strict control of temperature during forming, post forming, trimming, and QC processes must be understood and maintained

Heat Deflection (Ref ASTM D648)

Test definition: The temperature at which the material will distort

Material selection application:

• Usually listed at 2 loading values (264 psi and 66 psi)
• Lower heat distortion materials will require a greater processing time
• The temperature up to which rigidity for mechanical loads is retained

Flammability

Test definition: Extent to which a material will support combustion

Material selection application:

• Plastics made up of organic chemical materials can have violent oxidation reactions in the presence of air at elevated temperatures like any other organic materials such as wood, paper and textiles.
• Many plastics are now available compounded with flame, smoke and toxicity suppressant ingredients
• In many applications, government mandated standards will dictate the required testing (See FST testing)

Fire, Smoke, and Toxicity (FST) Property Testing

In industries such as aviation and mass/rail transit, there are very strict regulations on the fire, smoke, and toxicity properties of utilized materials. Many thermoplastic suppliers produce material variations that are specifically designed to meet U.S. and international regulatory requirements. These very industry specific thermoforming material products will typically list the particular industry regulation directly on the material’s data sheet.

Some common industry regulations:

FAR25.853a

FAR25.853d iv & v

ADB-0031

D6-51377

DIN 5510

ASTM E662 & E162

Look & Appearance Property Testing

Accelerated Weathering (Ref Q-Lab)

Test definition: Provides a simulated exposure sequence to ultra violet radiation that allows weatherability to be categorized

Material selection application:

• The advantage of thermoforming and the use of coextruded material stands out here with the ability to form parts subject to ultraviolet radiation on the outside with a coextruded material that has high weatherability coextruded with a lower cost rigid substrate material
• Evaluation of color fade relating to time and UV exposure

Scratch & Mar (Ref Taber Method)

Test definition: Provides a measurement of the scratch or mar resistance of plastic sheet

Material selection application:

• The visual appearance of a scratch or mar normally involves changes in surface topography, color or brightness
• Some plastic materials have elastic recovery properties that occur after removal of the applied stress

References:

Click here for more detailed information on the testing of thermoplastic properties. (Society of Plastics Engineers – Thermoforming Quarterly 2015 Q1)

For additional information on ASTM standards of material testing, visit the official ASTM website.

You can also visit our Plastic Thermoforming Materials page for more in depth information.

Productive Plastics is top contract manufacturer for heavy gauge thermoforming, including vacuum forming and pressure forming. Contact us or request our complimentary thermoforming design guide for more information.