Information and expertise on plastic product design

What is Plasticprop?

A properly desinged plastic product is functional, durable and thus environmentally friendly but we fail with this too often. In most cases, this could be avoided by better design, including the right choice of material.

Plasticprop.com is intended for sharing knowledge and experiences on plastic product design among professionals, both green and seasoned. Please join the conversation and share both your questions and insights.

Let’s make better plastic products together!

Plasticprop material selection guide

It is essential for your component to be transparent.

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When you need your product to be transparent, your choices are limited mostly to amorphous plastics. You need to address their common weaknesses:

  • limited chemical resistance
  • the tendency to stress cracking
  • high friction and low wear resistance

In general, amorphous plastics are not well-suited for machine design purposes.

PP Random Copolymer (PP-RC) is the only semi-crystalline exception among the amorphous alternatives. It is always at least slightly milky but has many advantages (chemical resistance, ductility, use of integral hinge) most of the amorphous materials do not offer.

The most common and best known transparent plastics are PC, PMMA and PS. PC is known for its exceptional impact resistance, PMMA for its glass-like clarity. PS is an affordable commodity plastic, familiar from CD-cases.

The Material selection tool presents the following transparent plastic materials:

PP-RC, PC, HT-PC, PS, PMMA, SAN, SBC, MABS, Copolyester, Amorphous PA12, PSU, PPSU, PES.

You are welcome to help expand this list.

 

Examples of transparent materials and their applications:

Lighting industry (optically good): PC, PMMA

Packaging industry (low cost): PS, PP-RC, SBC

Refridgerator boxes: PS, SAN

Sport bottles (FDA approval): Copolyester

Medical industry (chemical resistance): MABS, SAN

Tableware (low cost, FDA approval): SAN, PS, PP-RC

Safety goggles (impact resistance): PC

Process equipment (thermal resistance): High-Performance materials PSU, PPSU, PES

What do you need from your product?

Strength and modulus

Stress-at-break values for common transparent plastics in room-temperature are between 40-70 MPa (PP-random-copo ~20-30 MPa ). From a durability point of view, however, impact strength is usually a more relevant factor than tensile strength.

Transparent materials like PS, PMMA, and SAN have a tensile modulus of above 3 GPa at room temperature. MABS, PC, SBC and copolyester are closer to (below or above) 2 GPa. The difference can be heard if you knock the product with a teaspoon; the higher the sound, the higher the modulus.

Higher modulus usually leads to lower toughness and therefore lower impact resistance.

Please remember that elevated temperature has a significant effect on the strength and modulus of plastic materials. Ýou should not look at the value in room-temperature but at maximum service temperature.

 

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Impact resistance

The most impact-resistant transparent material by far is PC.

Other transparent materials with good or fair impact resistance include MABS, SBC, copolyester, and transparent PA12.

PPSU is the best choice in high-performance materials.

PS and PMMA are fairly brittle. SAN is slightly less so.

Wall thickness has an effect on the impact resistance of a component. The transparent side-plates on the ice-hockey rink are made of thick PMMA plate. Avoid sharp inner corners whenever you can.

When operating in a cold environment, plastic components tend to become harder, stronger and stiffer. However, they also lose some of their toughness and turn more brittle. This will affect their impact resistance.

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Resistance against continuous load

Even when the load is well within the indicated tensile strength, prolonged use may cause creeping and fracture due to stress corrosion. Semi-crystalline materials endure continuous stress better than amorphous materials.

Your product should be designed to endure significantly higher stress than short term experiments indicate.

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Fatigue resistance

Semi-crystalline plastics endure fatigue better than amorphous plastics.

To avoid fatigue fracture, your product should be designed significantly stronger than the cyclic load level indicates.

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Scratch resistance

PMMA is used in boat windows due to its good scratch resistance.

Despite its good ductility, PC is prone to scratching. Motorcycle visors are coated with scratch-resistant lacquer.

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Performance in elevated service temperature

You need to establish the maximum temperature your product is going to be stressed in, choose a material suitable for that range and design the product bearing in mind how the temperature affects its mechanical properties.

In addition to short term service temperatures, you must also take into account that if the product is constantly (long-term) subjected to high temperature, creeping will speed up, and stress corrosion and, in some cases, thermal degradation will appear. Static loading and high temperature are an exceptionally challenging combination for plastics, especially amorphous grades.

High-performance materials offer the best thermal resistance (above 150°): PSU, PES, PPSU.

Thermal resistance is one of the advantages of PC, up to 125°C. PC-HT goes even higher.

Amorphous PA12 can withstand 80-100°C in long term use, which is fairly high for a transparent material.

With SBC, MABS, PMMA, and PS it is safer to stay below 70°C in long term use. Copolyester and SAN have a slightly better thermal resistance.

 

 

 

 

 

 

 

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Performance in low temperature

When operating in a cold environment, plastic components tend to become harder, stronger and stiffer. However, they also lose some of their toughness and turn more brittle. This is especially likely to happen when operating below the material-specific glass transition temperature, Tg. As amorphous plastics are used below their Tg, PP-RC is the only transparent plastic that can be used both under and above its Tg which is app.-20°C. Below that it is likely to turn brittle.

If your product is likely to be used in cold conditions, and especially if it is subjected to impacts, make sure its toughness is sufficient in the expected service temperature. Explore reference products to confirm.

 

 

 

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Chemical resistance

In general, amorphous plastics have significantly lower chemical resistance than semi-crystalline ones.

PP-random-copo is the only semi-crystalline transparent material available and therefore a common choice for applications used in a chemically demanding environment.

Other transparent plastics that are commonly chosen due to their fair (instead of poor) chemical resistance are MABS and amorphous PA12. SAN is favored over PS. PMMA is used in agricultural lighting applications over PC due to its ability to withstand ammonium. Transparent materials with especially poor chemical resistance include PC, PS, and SBC.

Temperature, duration of the exposure and mechanical load are important factors. The lower they are the better.

 

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UV resistance

Fluoroplastics are the only material group than can be considered inherent to UV light. With all other plastics, UV light is an issue.

UV stabilized grades of practically all plastic materials are available. Choose them in order to extend the lifetime of your product.

PMMA is known for its good UV-resistance. Boat windows are typically made of PMMA.

Transparent materials with poor UV-resistance include PS, PC, MABS, and copolyester. Car lighting components are commonly made of PC, a material with relatively poor UV-resistance, which is why they are dipped in lacquer.

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Resistance against water, boiling water, steam

PC is prone to hydrolysis. It might degrade in long term contact with hot water.

If the component is constantly exposed to hot water/steam, high-performance materials like PSU and PPSU should be considered.

 

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You are designing a commodity product.

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If your requirements for both appearance and mechanical performance are low, you are likely to find a suitable material from the lowest level of the plastic pyramid. These materials are typically known as commodity plastics.

Commodity plastics are inexpensive. Often, they have weaknesses in outdoor use, chemical, and thermal resistance or mechanical performance. Please take them into account as the low cost can not be the only defining factor in your choice of material.

Low cost does not necessarily mean that you can not produce a quality product. For example, ABS and PP both have many excellent qualities that enable both mechanically and visually pleasing outcomes when applied properly.

Commodity plastics include both semi-crystalline and amorphous grades. Every product has some requirements in terms of mechanical load and service environment, so as a reminder:

  • Amorphous materials are more likely to provide a straight-edged result with a clean surface finish. However, the product may not be extremely durable in a challenging operating environment or if it experiences long-term loading.
  • Semi-crystalline materials have better chemical resistance and mechanical properties but due to higher, more uneven shrinkage, the product can easily warp slightly.

The semi-crystalline materials come from the polyolefin family: PP and HD-PE. LD-PE is a softer type of PE. In this Material selection guide, it is listed under the Rubbery path.

Amorphous options include PS, HIPS, and SAN. ABS and ASA are also fairly inexpensive and often classified as commodity plastics. PVC is a common material in the construction industry but less used in consumer goods because of its poor recyclability.

Examples of commodity plastic applications:

Storage boxes (ductile): PP, HD-PE

Disposable tableware (common FDA grades): PSPP

Sandbox toys (ductile, no sharp edges): PP

Bottle caps (ductile, enables seal): HD-PE

Disposable razor handles (strong and stiff in short-term use): PS

Small toy characters, dices (easy to print): hard PVC

Sales dispensers (nice surface appearance, dimensional stability): ABS, PS

When designing packages and other disposables, please consider how to minimize the amount of plastic, the possibility to use recycled grades and how to make the recycling of the product as easy as possible. 

 

What do you need from your product?

UV resistance

If your product is used or stored outdoors, you have to take UV light into account. Prolonged outdoor exposure affects the color of your product and may turn it brittle.

All the low-cost commodity plastics, amorphous and semi-crystalline, are somewhat sensitive to UV light. ASA is commonly described as “ABS for outdoors” and often the best choice among amorphous plastics. Ask your material supplier for test data and compare it to your intended use.

PP is a better choice than HD-PE, although all general-purpose grades are somewhat sensitive to UV. Using both UV modified base material and colorant resistance can be significantly improved. As a reference, many roof ventilation products are made of PP and have long warranty times.

Color choice is an important factor also: the darker the color the better it blocks radiation.

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Performance in cold temperature

In some cases, your product may be used in subzero conditions even if it has not been your intention.

ABS and HD-PE retain their ductility well. ASA with a better UV resistance has lower impact resistance than ABS in cold.

PP homopolymer has Tg around -10°C which is not rare in the Nordic winter. Below it, PP-H turns brittle. It may suddenly break under loads it easily withstands in warmer conditions. PP copolymer is a better choice, especially the grades that have been modified to retain their toughness in cold conditions.

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Something else?

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In many cases, it is not possible to split components clearly to visual parts and technical parts. Sometimes we need all, high impression quality, mechanical performance and endurance in challenging service environments. We need to find the best compromise.

As a reminder:

  • Amorphous material is more likely to give you a straight-edged result with a clean surface finish. However, it means the product might not be extremely durable in a challenging operating environment or if it experiences challenging long-term loading.
  • Semi-crystalline materials have better chemical resistance and mechanical properties but due to higher, more uneven shrinkage, the product can easily become slightly warped.

To simplify, it is easier to produce a fairly good-looking part with semi-crystallines than one made of amorphous materials that would reliably work in challenging conditions. Of course, this doesn’t always apply.

The pros and cons of each material below are listed from this best compromise perspective. You can have a closer look at their properties by following the visual or technical paths.

Relevant materials for the best compromise:

PBT (Semi-crystalline)

PBT is listed first for a reason. In many cases, it offers the best compromise.

Why choose:

  • Good mechanical performance and chemical resistance.
  • PBT has a shiny surface without seeming oily. It is printable.
  • Good dimensional accuracy (for a semi-crystalline material).
  • Good UV-resistance.

Why not choose:

  • If the product is in long term contact to boiling water. Polyesters are prone to hydrolysis.

Typical applications:

  • Pure white medical devices that are commonly cleaned with strong detergents.

Note: When comparing the Plasticprop sample between ABS and PBT, it’s hard to tell the difference. The PBT sample is slightly smaller and sways a little more when its set on a level.  If designed well and produced with quality tools, there is no reason PBT wouldn’t have high impression quality.

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PA6 or PA66 (semi-crystalline)

If the use and conditions are harsh, PA might be a safer choice than PBT.

Why choose:

  • PA is a good choice for rough conditions; if clattering and rattling are expected
  • Better chemical and heat resistance than PBT
  • Good UV-resistance (when compared to amorphous options), PA6 slightly better than PA66
  • Low friction in case that is needed.

Why not choose:

  • Compared to PBT, PA has a slightly greasy-looking finish
  • The tendency to absorb moisture

Typical applications:

  • Chainsaw and lawnmower housings

 

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PA12 (semi-crystalline)

If you have doubts about the performance of PA6/PA66, it is an option to upgrade to PA12.

Why choose (over PA6 or PA66)?

  • Absorbs less moisture, better dimensional stability
  • Better impact properties
  • Excellent resistance against stress-cracking
  • Better wear resistance
  • Good weather resistance
  • Better durability against aging

Why not choose?

  • Tensile modulus is lower than PA6
  • Expensive material

Typical applications:

  • Motorcycle parts
  • Bike pedals
  • Housings for high-quality electronic devices
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ABS (amorphous)

ABS is a common choice in visual components. If you consider using it, please pay attention to the limitations.

Why choose?

  • If impact resistance critical, also at low temperature.
  • Strong and tough material properties are needed in short term loading.

Why not choose?

  • If the component is subjected to continuous load.
  • Risk of chemical attack (oil, strong cleaning detergents, etc.)
  • Outdoor product.

Typical applications:

  • Lego mechanisms. Loads are limited, the product is used indoors, the chemical environment is not challenging.

 

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PC (amorphous)

PC is a commonly used material in cell-phone covers. It may be a good option if impact resistance is the driving requirement.

Why choose?

  • Superior impact resistance.
  • Higher service temperature than on most plastics.

Why not choose?

  • Poor chemical resistance
  • Prone to stress-cracking
  • Relatively high friction properties.

Typical applications:

  • Cell phone covers and accessories
  • Electrical boxes
  • Lighting products
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PP (semi-crystalline)

PP is a commodity plastic, but its unique properties might offer you the best compromise.

Why choose?

  • Low cost (it is a benefit, but you should not base your decision on this only)
  • Excellent chemical resistance
  • Low density
  • With proper tooling, the visual quality of the components is nice.

Why not choose?

  • Limited strength and modulus
  • Poor UV resistance (can be improved)
  • The surface is slightly oily

Typical applications:

  • Flower pots
  • Lampshades
  • Storage boxes
  • Kitchen appliance housings

 

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PC/PBT (amorphous/semicrystalline)

PC/PBT is an alloy of an amorphous and semi-crystalline material. This aims to combine the best properties of both.

Why choose?

  • High-quality appearance
  • Better dimensional stability than PBT
  • Better chemical resistance than PC
  • High heat resistance
  • High impact strength

Why not choose?

  • Difficult to find good references. Despite the long list of good characteristics, relatively rare in use.

Typical applications:

  • Gear cases
  • Automotive grills and bumpers

Note: The product sounds so good that I wonder why it is not more widely used. This makes me a little bit doubtful of its reliability but I would definitely still consider it.

 

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What are you looking for?

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You need a technically efficient/reliable material but not necessarily high strength or stiffness.

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When designing parts with demanding technical functions, both mechanical and service environment-related, semi-crystalline materials are typically a good choice. Their ability to withstand continuous load, fatigue, wear, and lubricants are superior compared to amorphous plastics. Levers, gears, cams, wheels, pulleys, etc. are usually made using semi-crystalline plastics.

Amorphous materials (or an alloy of amorphous and semi-crystalline plastics) can be the best choice when the function of the product requires very high impact resistance or dimensional accuracy.

Technical does not automatically equal poor appearance. With proper design and appropriate tooling/process, the result can be visually very pleasing using semi-crystalline materials. Making amorphous materials withstand continuous load or chemicals is a harder task.

The most typical materials used in machine design are POM and different polyamide grades, such as PA6, PA66, and PA12.

Other good options are from the polyester family: PBT and PET.

In some cases, polyolefins PP and HD-PE are good and affordable options for applications that do not require high strength, stiffness or thermal resistance.

If the service environment and the mechanical requirements are exceptionally challenging, you might need semi-crystalline high-performance plastics, such as PEEK, PPS or PPA.

Amorphous high-performance materials, such as polysulfones PSU, PPSU, and PES, are typically used when good dimensional accuracy and high strength are needed under exposure to hot water and steam. They do not, however, are not a good choice for moving components of a mechanism.

In demanding applications, basic polymers may be modified for a certain purpose. They can be combined with oil or PTFE to achieve low friction, aramid fiber for wear resistance or carbon black for electrical conductivity. The possibilities are endless, but tailoring usually requires very large volumes.

Applications:

Chain saw housing: PA6/PA66

The housing of a medical device: PBT

Cable tie: PA66

Keyboard keys: PBT

Electrical connectors: PBT

Side release buckles, plastic zippers: POM

Housings of medical devices (strong cleaning detergents): PBT

Bicycle mudguards: PP

Helmets: ABSHD-PE

Snow shovel scoops: PP, HD-PE (in certain avalanche rescue kits PC)

What do you need from your product?

Low friction

The surface of PP, PA or POM is slippery, even oily, to touch. POM is the most common choice for low friction applications. Different PA grades are typical as well.

Two important factors in your application are surface pressure (P) and sliding velocity (V). If P*V is very high, the surfaces will inevitably heat up. In such cases, semi-crystalline high-performance materials like PEEK or PPA are used.

 

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High quality appearance

When visual high-quality is needed, designers often prefer amorphous plastics. If the mechanical challenges are limited to impacts and short term-stresses, common housing plastics, such as ABS, ABS/PC or PC, fulfill the purpose. However, if resistance against fatigue, stress-cracking or wear are needed, you are likely to find a more suitable plastic within the semi-crystalline offering.

When properly designed, the following materials can deliver pleasing visual quality together with mechanical performance and chemical resistance (in preference order).

  • PBT
  • PA6
  • PA12
  • PA66
  • PC/PBT alloy
  • PP

Compared to amorphous materials, semi-crystalline plastics have a higher and uneven shrinkage, which results in more pronounced warpage and sink-marks. It is therefore important to follow the basic principles of plastic product design: uniform wall thickness, sufficient radius, and rib thickness guidelines. From a tooling perspective, a well-defined parting line, proper cooling, and a properly positioned injection system are vital.

 

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Fatigue resistance

Fatigue is a phenomenon that plastic designers should always take into account. The effects of fatigue do not show in the short term use. A component might withstand 100 cycles with no visible effect but suddenly fail after 15 000 cycles.

POM is known as a “spring plastic”. It endures fatigue well.

All the polyamides are also highly fatigue resistant.

PEEK is a high-performance material but has excellent fatigue resistance at room temperature as well.

To avoid fatigue, a component should be designed so that the repeating stress is significantly smaller than the tensile strength of the material, approximately 10-20%.

 

 

 

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Resistance to continuous stress

Stress-cracking is the most common reason for plastic components to fail their purpose.

Semi-crystalline materials endure continuous stress far better than amorphous materials. POM, PA, PBT, and PP are good options if long term stress is expected. HD-PE is an exception among semi-crystalline plastics, it is prone to stress-cracking.

To avoid stress cracking, the product should be designed to endure significantly higher stress than short term experiments indicate. Semi-crystalline materials endure continuous stress better than amorphous materials.

 

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  • It doesn't need to be a looker, go back

When visual qualities are emphasized amorphous materials usually deliver them. Due to their low and even shrinkage straight and dimensionally accurate parts are easier to process. They also have a visually pleasing surface finish that can, if needed, look shiny.

Please bear in mind the limitations of amorphous plastics, however. They are prone to chemicals, fatigue, and stress-cracking. You will notice that these limitations are mentioned under every listed material.

The most commonly usual materials in this category are  ABS, PC, and their alloy PC/ABS.

Visual parts are often used for covering the inside of a device and its technical functions.

When you have gone through the selection criteria, you may reconsider the use of amorphous plastics. The best compromise between visual and technical qualities is introduced in the Technical path.

Typical products with high visual requirements: 

Mobile phone covers: PC, PC/ABS

WiFi router housings: ABS

TV remote controllers: PC/ABS

Lego blocks: ABS

 

What do you need from your product?

Visual appearance

If your product is well designed and properly molded with a proper tool, its visual quality should not differ much whether it is made of ABS, PC or ABS/PC. They all should come out straight and glossy.

ABS is the easiest to mold. Its tendency to warp is low. Burrs are minimal. Surface gloss is excellent and has a “drier” feel than that of PC.

PC requires a higher molding temperature and is a little more challenging to produce. For a professional manufacturer, this should not be an issue.

PS can be used for making a visually appealing product as well. The rattling echo of the components, however, does not increase the perceived quality.

Perhaps the main difference between the three comes with scratch resistance; in the long term use, ABS is better than PC.

Surfaces can be textured, glossy or satin-like matte. Small and appealing details can be etched or lasered on the mold cavity.

The surface finish of the backside of the component is often forgotten. Remember to specify it when going through the details of the tooling.

 

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Impact resistance

Impact resistance is always a good property, even if your product is mainly visual. A Wi-Fi box might fall off the shelf. From this perspective, PC, ABS, PC/ABS, and PPO are all materials with a good or excellent impact strength. PS and SAN are very brittle. HI-PS is a better choice.

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UV resistance

Amorphous materials are prone to UV light. Long term exposure to direct sunlight affects the strength and color of the product.

Even though ASA is commonly known to resist UV light better than ABS, harsh outdoor conditions unavoidably limit its lifetime.

Interestingly, PC/ABS has a slightly better UV resistance than PC or ABS, but it can not be recommended for products that are placed outdoors for longer periods of time.

The UV resistance of PS is poor.

Practically all amorphous materials can be painted with UV-protective paint or lacquer.

When designing a visually demanding product you have to take into consideration the UV-light effect on the color of the product, also in indoor use. Both the sunlight that penetrates the window and many indoor lights emit UV rays. It is common to see light-switches, printer lids or smoke alarms that have turned from pure white to nicotine yellow. The use of UV stabilized material is not enough, however, but the colorants must be UV-stabilized as well.

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Performance in high or low temperature

When visual qualities are emphasized in the material selection process, the product is not likely to be used in very hot or cold conditions. If your product is always used and stored indoors, service temperature is not a relevant factor.

In some cases, visual housings are used with kitchen appliances or electrical devices that generate heat. If temperatures higher than 70°C are likely, ABS or PS are not the best choices. PC and PPO tolerate high temperatures much better, even above 100°C.

If the product is likely to spend a night in a car in subzero temperature and be used right after that, ABS is a good choice. ABS/PC also remains tough in cold.

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Chemical resistance

Amorphous materials are avoided in medical applications because they are constantly handled with strong cleaning substances. Oily hands might also cause deterioration if the contact is repeating.

Chemicals and continuous load are an especially bad combination for amorphous plastics.

“Excellent chemical resistance” is often listed as an advantage for ABS. This is true when compared to PC, PPO or PS which are chemically very sensitive. However, acetone, for example, is fatal for ABS.

If your product is likely to be subjected to chemicals it is best to research and test it thoroughly. To be sure, see what the Technical path has to offer.

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Which of the following best describes your product?

  • Purely visual
  • Technical
  • Commodity
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Do you consider visual appearance (e.g. high-gloss surface or straight edges) the most important factor for your component? Then choose Purely visual. 

Or is the component only, or also, demanding in terms of mechanical performance or service conditions? If so, choose Technical.

In many cases neither of the above is crucial, and the cost of the material is a more important factor. Then follow the Commodity path.

You need high strength or modulus.

  • Let's not mix things, go back

Adding reinforcement, typically 15-40 % (in some cases even up to 60%) of fiberglass, to plastic can significantly improve the strength, thermal resistance, and stiffness of your product. It also improves chemical resistance but not necessarily impact resistance. Reinforced parts are usually functional components of heavy-duty products and made of semi-crystalline plastics like PA6, PA66, PBT, pPA, PPA or PEEK.

Amorphous plastics with reinforcements are sometimes used in boxes that require, in addition to impact resistance, extremely high strength.

The most common reinforcement by far is glass fiber. Please see below the mechanical properties of PBT with different glass contents. The values are extracts from material datasheets and should be considered as theoretical. In practice, service conditions, fiber orientation, and weld lines may reduce them considerably.

PBT+0%GF

  • Yield stress 60 MPa
  • Tensile modulus 2,7 GPA

PBT+10%GF

  • Stress at break (does not yield) 90MPa
  • Tensile modulus 4,7 GPA

PBT+20%GF

  • Stress at break 120MPa
  • Tensile modulus 7,5 GPa

PBT+30%GF

  • Stress at break 130MPa
  • Tensile modulus 9,5 GPA

PBT+40%GF

  • Stress at break 145 MPa
  • Tensile modulus 12 GPa

The properties of the reinforced material are dependent on the base/matrix plastic:

  • When dry, the modulus of reinforced PA6 and PA66 is on the same level as PBT while their strength is a little higher. When subjected to moisture, their mechanical properties can be reduced by up to 40%.
  • The mechanical properties of PP+GF (with normal short fibers) reach approximately 70% of those of PBT.
  • High-performance plastics like PEEK+GF and PPA+GF have better mechanical qualities than PBT+GF but are still within the same performance range. Their advantage is the ability to retain those qualities in considerably higher temperatures.

Note also:

  • Carbon fiber is an attractive filler, but considerably costlier than glass and therefore usually used in high-end applications only.
  • The effect of temperature is especially important to take into consideration. In most cases, you can note the difference in component stiffness between room temperature and 80°C.

Applications:

Office chair base: PA6+30GF

Ax handle: PA6+30-40GF

Car turn switch: PA66+30GF

 

 

What do you need from your product?

Maximum strength and modulus

Very commonly used glass content with engineering plastics is 30%. Compared to unfilled plastics the product becomes considerably stiffer and stronger yet is still relatively easy to process.

As listed above, an increase of 10% can improve strength and modulus by 20-35%. The same can easily be achieved by modifying the geometry of the component. When the height of a bent element is increased slightly (for example from 10 mm to 11 mm) its stiffness may be improved correspondingly.

It is not rare to use 40% or even 50% glass content in demanding applications. Engineering plastics with up to 60% glass content are available, but not commonly used due to processing difficulties.

If cost is not an issue, carbon fibers lead to higher strength and modulus. In comparison:

PBT+40%CF (CELSTRAN PBT-CF40-09)

  • Stress at break 175MPa
  • Tensile modulus 14,5 GPA

There is also an increasing number of LGF (long glass fiber) materials available. Their main advantage is improved impact resistance.

The highest modulus value found in the Campusplastics.com database is provided by Celstran PPS-CF50-02 AF3002 Natural, PPS with 50% long carbon fiber content. Its tensile modulus is above 40 GPa and Stress at break 155 MPa.

 

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Good impact resistance

Reinforcing plastic increases its strength and modulus. Due to the increase in modulus, however, the material toughness is decreased which might lead to lower impact resistance. In some cases, on the other hand, fibers prevent cracks from propagating. It is useful to perform practical tests with samples.

Polyamide is a common choice when both strength and stiffness are needed together with good impact resistance. For example, the injection-molded shaft of an ax is commonly made of PA6 +30-40% GF. Other reinforced semi-crystalline engineering materials (PBT, POM) are also reasonably impact-resistant. PPS is an exception and very brittle. When high-temperature performance is needed, PPA or PEEK is a much better choice than PPS.

Unfilled Polycarbonate is very impact-resistant. It is usually filled to increase strength and stiffness. Reinforcing may decrease its notch-sensitivity and therefore also makes it more impact resistant in some cases.

Carbon fiber is more expensive than glass fiber and makes the product stronger and stiffer, but more fragile.

Long-fiber (LF) materials increase impact resistance significantly. Long fibers are used mainly with PP and PA.

Note that low temperature affects impact resistance: Your product might be impact resistant in room temperature, but at -20°C it may turn brittle. This is especially the case with PP. Long fibers help in retaining impact resistance in low temperatures.

An impact-modified grade is available for most materials.

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Something else?

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Does the function of your component require exceptionally high strength or stiffness?

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What level of strength and stiffness do you expect from your component? Will it be subjected to forces that might lead to breakage or malfunction through bending? Does it need to be strengthened or made stiffer? Are you considering plastic as metal replacement?

Does the component need to be transparent?

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  • No
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If transparency is a nice option or a potential future modification please consider it carefully. Using transparent materials limits your options considerably and may affect the quality of the product.

You have chosen rubbery and elastic plastics.

  • No soft plastics, thanks, go back

Thermoplastic elastomers (TPEs), is a family of six soft materials: TPE-S, TPE-V, TPE-O, TPE-U, TPE-E, TPE-A. Their advantage is their ability to return to their original shape after being stretched or elongated. TPE-S and TPE-V are the most common choices for consumer products, especially 2K applications.

Some thermoplastics outside the TPE family, like LD-PE, EVA and soft PVC, can also be considered soft or rubbery.

Silicone is a particularly heat-resistant option. It is not a thermoplastic and the manufacturing process of silicone products differs from injection molding.

Examples of soft materials and their applications:

Soft handles on screwdrivers and toothbrush (good grip, low price): TPE-S, TPE-V

Roller wheels, industrial belting (high friction, low wear): TPE-U

Car bumpers, dashboard (good impact strength): TPE-O

Freezer box lid (low price, retains toughness in low temperature): LD-PE

Snowshoes, skiing boots (mechanical strength, performance in low temperature): TPE-ETPE-A

 

What do you need from your product?

Transparency

TPE-A, TPE-S and TPE-U are available as transparent.

The EPDM content of TPE-V and TPE-O prevents this.

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Strength

When selecting a soft material, hardness and strain-at-rupture are probably more relevant than the modulus or the strength of the material.

Tensile strength increases hand in hand with hardness. TPE-A, TPE-E, and TPE-U are stronger than other options, including LD-PE (with the corresponding shore value).

Please note the difference between yield strength and tensile strength. If a component is stretched beyond its yield strength it will not return to its original shape.

 

 

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Specific hardness/flexibility

When extreme flexibility is needed, your choices are limited.

At its softest TPE-S is like a flexible gel. For normal applications, its flexibility is usually adequate.

TPE-V is available relatively soft as well, from app. Shore-A 45.

Soft PVC is available from Shore-A 40.

The hardness of TPE-U and TPE-O is above Shore-A 75-80, of TPE-E and TPE-A roughly Shore-A 80-85.

Some material suppliers may offer products below these values.

Here is a useful comparison between different Shore values.

 

 

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Impact resistance

Improved impact resistance is one of the main reasons why components are made of soft materials.

TPE-U is known for its good impact resistance.

TPE-O is used in car bumpers.

Soft PVC is used in traffic cones and boat fenders.

Note that when operating in a cold environment, plastic components tend to become harder, stronger and stiffer but also lose some of their toughness and turn more brittle. This affects their impact resistance, so the lower end of the service temperature should be taken into consideration. In general, TPE-E and TPE-A serve well in cold conditions.

 

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Resistance against continuous load

All plastic materials, including the rubbery, are prone to creep. If you stretch a plastic component for a prolonged period of time, it will not return to its original length.

The softer the material, the more it creeps.

TPE-A and TPE-E have a relatively good creep resistance although they do not stretch much.

TPE-O can not be recommended for purposes that require prolonged elongation.

EVA has better stress-cracking resistance than LD-PE.

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Fatigue resistance

TPE-V, TPE-U, and TPE-V have good flex fatigue resistance.

 

 

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Low/high friction and abrasion resistance

The harder the material the smaller the coefficient of friction and the less sensitive it is to abrasive wear.

Poor scratch resistance is one of the downsides of both TPE-O and TPE-V.

 

 

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Performance in elevated temperature

The rubbery material with the best thermal resistance is silicone with a service temperature of over 200°C. TPE materials used in injection molding have lower service temperatures.

TPE-S can usually not used in conditions above 100°C. TPE-O is slightly more tolerant.

TPE-V, TPE-U, and TPE-E can be used in up to 120° C.

TPE-A has the best thermal resistance of all TPEs, above 130°C.

The use of LD-PE and EVA can not be recommended to be used at a temperature much higher than 80°C.

The service temperature of each material may vary considerably depending on their hardness.

Note that at the upper end of the service temperature range, the material properties differ significantly from what they are at room temperature.

Please remember also that in plastic product design, continuous and temporary service temperatures are two different things.

 

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Performance in low temperature

None of the TPE materials are especially prone to low temperatures. Their service temperature area starts from roughly -50°C which does not mean that their properties remain unchanged at that level. This is easy to test in practice using reference samples made of each material.

TPE-E and TPE-A are used in skiing boots and snowboard bindings.

LD-PE is used in freezer box lids. Its Tg is -110°C.

 

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UV resistance

Almost all plastics are available with UV stabilization. Prefer those when designing outdoor products.

TPE-S (SEBS), when stabilized, is known for its good UV resistance. It is important not to confuse SEBS and SBS. The UV resistance of SBS is poor.

Aliphatic TPE-U grades offer good color stability.

EVA is a better choice than LD-PE.

Dark colors are better than light ones because they prevent UV rays from penetrating deeper into the material.

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Chemical resistance

Each TPE type has its strengths and weaknesses when in contact with organic solvents, acids, greases, bases, or other substances.

TPE-V is often selected over TPE-S due to its better resistance against oil.

Sometimes, in chemically harsh conditions, silicone is the best option.

Temperature, duration of the exposure and mechanical load are important factors in chemical resistance. The lower they are, the better.

It is always good to test the chemical compatibility in practice by subjecting a sample of the material to the relevant chemicals. The test can be catalyzed by subjecting the sample to different stress conditions and/or by elevating the temperature.

 

 

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Does the function of your component require elasticity, flexibility or rubberiness?

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Welcome to the Plasticprop material selection guide!

Before you start, please go through the questions listed below.

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Choosing the right material is the key decision in plastic product design. A well-considered choice early in the process saves you a lot of unnecessary work and backtracking later on and the outcome has a better chance to be functional and durable.

Do you understand the difference between amorphous and semi-crystalline plastics?

The most important distinction between different plastics is whether they are amorphous or semi-crystalline. Understanding the difference between the two is vital. To simplify, amorphous materials are better for components with high visual requirements, semi-crystalline for those that require technical/functional performance. For more information, read this article.

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How heavy and what type of loading will the component be subjected to?

The strength and modulus of each material (reinforced and unreinforced) are stated in the datasheet provided by the manufacturer. Those values are measured in a short time span, in a limited temperature-range only, and using a method that is unlikely to correspond with your intended use. You should, therefore, use the datasheet information for comparing properties between different materials, not as accurate values to be used in strength calculations.

The strength values (tensile or yield) of unfilled materials are typically 20-60 MPa and those of modulus between 2-3 GPa. Glass fiber reinforcements typically increase those values to 60-170 MPa and 5-20 GPa, depending on the glass content. In comparison, the modulus of aluminum is an app. 70 GPa and steel 210 GPa.

In most cases, you can design a sufficiently strong and stiff component using unfilled or moderately filled materials by simply adjusting their geometry.

Equally important, and too often overlooked, is the type and duration of the load.

Is the product exposed to static loading for prolonged periods of time?

One of the most common reasons why plastic parts fracture is that the effects of continuous loading were overlooked. Even when the load is well within the indicated tensile strength range, prolonged use may cause creeping and fracture due to stress corrosion.

Your product should be designed to endure significantly higher stress than short term experiments indicate. Semi-crystalline materials endure continuous stress better than amorphous materials.

Is the product exposed to repetitive or cyclic loads?

Ignoring the effects of fatigue is another common pitfall in plastic design. Your product might work without a hitch a few times or hundreds of times, but at some point, it breaks.

Semi-crystalline plastics endure fatigue better than amorphous plastics. As with continuous loading, your product should be designed to endure a heavier load than the cyclic load level indicates.

Is the product exposed to impacts?

Different plastics have often completely different impact resistance capabilities. A product made of polystyrene and one made of polycarbonate look similar, but the former easily breaks when dropped on the floor when the latter may be virtually unbreakable.

A product’s impact resistance can be improved also by avoiding sharp corners in the design.

It is always useful to consider what happens when your product is subjected to impact even when it is not its usual service scenario.

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What is the service temperature of your product?

Service temperature has a remarkable effect on the mechanical properties of any plastic product. It is useful to address the following aspects:

Maximum short term operating temperature
Elevated temperature has a significant effect on the strength and modulus of plastic materials. An acrylic PMMA product used at +70°C has totally different characteristics than at room temperature – even if both temperatures are within the given service temperature area according to the material datasheet.

You need to establish the maximum temperature your product is going to be stressed in, choose a material suitable for that range and design the product bearing in mind how the temperature affects its mechanical properties.

Minimum operating temperature
When operating in a cold environment, plastic components tend to become harder, stronger and stiffer. However, they also lose some of their toughness and turn more brittle. This is especially likely to happen when operating below the material-specific glass transition temperature, Tg.

The negative thermal expansion has to be taken into account when defining clearances between components.

Long-term operating temperature
Temporary and continuous operating temperatures require different approaches. If the product is constantly subjected to high temperature, creeping will speed up, and stress corrosion and, in some cases, thermal degradation will appear. Static loading and high temperature are an exceptionally challenging combination for plastics, especially for amorphous grades.

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Will the product be exposed to chemicals?

Chemicals are everywhere: hand lotion, petrol, nail polish remover, olive oil, coffee, even sweat. Plastics consist of chemically bonded polymer chains, so you need to make sure that the bonds of your material are not affected by surrounding chemicals.

You can easily find a table that indicates how sensitive common plastic grades are to hundreds of different chemical substances, but it is harder to establish what exact substances there are in, for example, a marinade sauce or a detergent.

To play safer:

  1. Prefer semicrystalline materials. In general, amorphous plastics have a lower chemical resistance than semi-crystalline ones.
  2. Find a reference case where the material has been successfully used in a similar chemical environment for a longer period of time.
  3. Test in practice: drench a material sample in the substance in question and subject it to a continuous tension. If the combination is poor, results should appear sooner than in a sample not subjected to the substance. You can catalyze the process by elevating the temperature.
  4. Ask your material supplier for recommendations.

While chemical resistance is an important factor to consider, remember that both petrol and turpentine are stored in plastic bottles.

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Will the product be exposed to UV-light?

U -light impacts plastic in two ways:

  1. Fades or changes product colors. White turns yellow. After a few years of use, a red flowerpot might have become baby pink.
  2. Weakens polymer chains which can have a negative impact on the mechanical properties of the material. Ductile material turns brittle.

These issues, especially when used outdoors, apply to all plastics (with the exception of fluoroplastics to an extent).

The negative impacts of UV light can be avoided or reduced. Certain materials like PMMA, PA6, and polyesters endure UV rather well. You should, however, always prefer grades that are specially modified with UV stabilizers and blockers. The same applies to the colorants.  Covering, painting or lacquering the component is an option as well.

With caution, inevitable weaknesses arise so late that they no longer have importance.

The Material selection tool will help you in the process.

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Will the product be exposed to water, moisture or steam?

Water might seem like a neutral agent but its impact can be surprisingly harmful.

Some materials, mostly polyamides, absorb moisture which affects their mechanical properties.

When in longer contact with steam or boiling water, a phenomenon called hydrolysis breaks polymer chains of certain materials, such as POM, PBT, PC and PU.

The Material selection tool will help you to avoid problems that arise from water.

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Are there any standards the product needs to comply with?

Many industries and business areas require certain industry standards to be fulfilled. If you are working in an area with very strict requirements, you probably know them well.

Fulfilling the UL-94 safety of flammability standard, for example, limits the number of suitable materials considerably and has an effect on their mechanical properties. It has, therefore, to be taken into account in the design process as early as possible. Otherwise, you might be forced to make changes to your product after it has been tested and validated.

Some common standards can be found here.

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Plasticprop Essentials sample set

Easy way to explore and compare different plastics hands on: the Plasticprop Essentials sample kit.

The Plasticprop collection of plastic samples was set off by my daily need to assess and compare different plastics in practise. The Plasticprop Essentials contains 20 plastic samples with different characteristics. The set covers a wide range of materials that are commonly used in consumer goods, packaging, kitchen utensils, sporting equipment, vehicles and electronics. A few samples of high-performance and TPE grades are also included.

Markus Paloheimo, Plastic Product Designer

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"A properly designed plastic product is cost-effective but even more importantly, functional and long-lasting. Designing sustainable products requires theoretical understanding and practical experience. Please share your expertise and join the conversation. Let's design better plastic products together!"

Markus Paloheimo

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