Looking for a soft and flexible plastic?

If you need an injection mouldable rubbery material you have many options to choose from. In addition to the members of the TPE family (TPE-S, TPE-V, TPE-O, TPE-A and TPE-E), some conventional plastics, such as LD-PE or EVA, can be considered rubbery or at least soft. However, if you need the material to be considerably soft and flexible, your options are more limited.

The softness/hardness of the material is usually expressed by a value measured with a device called a durometer. This video provides a good explanation of how the device works and the difference between Shore A and Shore D values.

What hardness values are available for each plastic material type?

The chart on page 21 of this presentation shows the hardness differences between the members of the TPE family.

I wanted to see if it is possible to find the softest grade available of each material using campusplastics, matweb and materialdatacenter databases. It was not as easy as I had hoped. Anyway, the results are quite in line with the presentation above and my earlier understanding.

If you know a material that would exceed these values, please give me a hint and I’ll update the chart.

The members of the TPE-S family offer the best flexibility. TPE-O and TPE-U came to a bit closer to TPE-V than I had expected. TPE-E and TPE-A are clearly on a different level.

Please note that Shore D values are used in the case of LD-PE. Shore D 42 corresponds approximately to Shore A 82. Here you can find a useful comparison chart for the conversion between Shore A and Shore D. Due to the differences in the testing procedure and the material characteristics, however, components with a hardness of Shore A 85 and Shore D 40 might have very different touch and feel.

Components with a hardness of Shore A 35 and less are very flexible. They might not be able to carry their own weight and can be difficult to assemble.

At the other end of the scale, it is questionable if materials with hardness outside the Shore A scale, higher than Shore D 60, can be considered rubbery or even elastic.

Please remember that the elastomers creep just like any other plastics. If you stretch them for a longer period of time, they will not remember their original length and shape. Considerable stretching should be only temporary.

Softness/hardness is only one property to consider in the material selection process. Chemical resistance, UV-resistance, service temperature, etc. must be taken into account as well. Plasticprop.com can hopefully help you in this.

Thank you Jenni Ahola for your great help with the material search.

Hydrolysis and how to prevent it from affecting your plastic product

A phenomenon called hydrolysis degrades certain plastic materials and affects their mechanical properties.
This does not apply to all plastics. The polymer structure of the material must have an -OH group in it. Quite many do. Hydrolysis must be considered with (at least) the following plastics:

  • Polyesters PBT, PET, copolyester, PLA
  • Polycarbonate PC
  • Polyurethane PU, TPE-U
  • Polyamides like PA6, PA66, PA12 (Please notice that the common tendency of polyamides to absorb moisture is another thing)
  • Polyacetal POM

As you can see, the majority of the common engineering materials are included. Should you be worried? Not really, as long as you take hydrolysis into account.
The phenomenon itself is not very aggressive. The key nominators (in addition to the required OH-group) are:

  • Time. In short term use hydrolysis is hardly a problem.
  • Temperature. The water must be hot; close to boiling or steam. Rain or splashes of cold water are not harmful.

Kitchen spatulas are typically made of PA66 and constantly sank in boiling water. At least mine are still in use after several years of use.

The scissor handles I wash on almost daily bases are made of PBT. No problems, although the water temperature in a consumer use dishwasher is typically not higher than 50°C.

How to avoid problems with design?

  1. Understand the service environment of the product. Will it be in prolonged contact with hot water or steam? Are you designing a rice cooker or a coffee machine?
  2. To play safe, prefer materials without the OH group.
  3. Try to find a reference case where the corresponding grade has been used successfully under similar conditions. Your material supplier can probably help with this.

Amorphous vs Semi-Crystalline plastic from a designer perspective

The difference between amorphous and semi-crystalline plastic is something that every product designer should address. Here is what I find essential from a designer perspective

The difference in the thermal behavior

Amorphous and semi-crystalline plastics react to temperature in a different way. If we observe this closer, it will help us to understand the essential difference in their structure.  This can be done in practice by heating some plastic items, for example, Plasticprop samples.

Let’s have polystyrene PS as an example of amorphous plastic. Place the sample into an oven and start heating it slowly. If you take it out every 20°C (gloves recommended), you can notice that the sample gradually turns softer and softer. At 70°C the sample can already be bent quite easily. When the temperature of app. 95°C  is reached, the sample totally (and more rapidly) loses its stiffness and collapses under its own weight. Polystyrene has reached its material-specific Glass Transition temperature (Tg). Amorphous plastic can only be used in temperatures below their Tg.

Semi-crystalline polymers have a partly different structure. A portion, of their polymer chains, 20-80% depending on the material, have arranged to tight and strictly orientated crystals. The remaining chains are in an amorphous state surrounding the crystals. Because of the amorphous part, semi-crystalline plastics do have a Tg as well.

Let’s place a Plasticprop sample made of PA6 in the oven. At 50°C, it feels slightly more flexible than it did at room temperature. But if you take it out of the oven again at 60°C, the difference is bigger. PA6 has exceeded its Tg temperature, which is app. 55°C. The amorphous part of the polymer is now free to move but the crystals still hold the polymer structure together.  The sample has turned from glassy to rubbery (or leathery) state. Semi-crystalline plastic can be used on both sides of its Tg, but the mechanical behavior of the material does not stay the same. Below Tg, in their glassy state, semi-crystalline plastics are stiffer and stronger but at the same time more brittle. Many semi-crystalline polymers are above their Tg in the room temperature (POM, Tg app. -60 °C; HD-PE, Tg  -110°C, both approximately).

Elevating the temperature further will lead to a gradual decrease in modulus and strength. At 200°C the sample feels interestingly similar to the Plasticprops sample of TPE-S (SEBS). When the temperature reaches 220°C, the crystals dissolute and the solid piece turns to low viscosity liquid. The PA6 sample has then reached its Melting Point, Tm.

In both cases, amorphous and semi-crystalline, the process is reversible. In the case of semi-crystalline polymers, the temperature at which the polymer chains have reordered as crystals during the cooling process is known as Crystallization Temperature, Tc. This is far above the service temperature of the material.

 

How to address this in my design?

As said, amorphous plastics can be used only below their Tg. But it is vital to understand that their mechanical properties do not remain the same through the whole service temperature area. The higher the temperature, the lower the strength and modulus of plastic. PMMA, for example, at 60°C has completely different mechanical characteristics as PMMA at 23°C. Many plastic manufacturers are quite optimistic about the service-temperature they provide in their material data-sheets. The given temperatures may be on the very edge of reaching the Tg. You need to be skeptical when reading the data-sheets and bear in mind that temperature has a great influence on the mechanical properties of any polymer.

When using semi-crystalline plastic you need to check if its Tg is within the service temperature of the product. If so, make sure (by testing and exploring reference samples) that your requirements are filled on both sides of the Tg. For example, the Tg of PP homopolymer is -10°C. The ductility of a water bucket in Scandinavian winter might be surprisingly low.

Transparency

Amorphous plastics are generally transparent. Crystals block the light, which makes semi-crystalline plastics opaque. PP-random-copolymer is the only semi-crystalline transparent material, although even at its best it is slightly milky. Some modified grades of PA6 are available as well, but they should be described as translucent rather than transparent.

How to address this in my design?

Amorphous plastics have their weaknesses that are explained later in this article. I might spoil your excitement, but those are related to chemical resistance, continuous stress and friction/wear. Amorphous plastics do not serve well in machine design purposes. Combining mechanical functions to your transparent component must not be done with too much optimism. Problems are likely to occur. PP-RC is often the safest choice.

Straightness and dimensional accuracy

During the crystallization, the polymer chains are packed very firmly into a small volume. A large number of amorphous polymer chains are compressed into one crystal block. Due to crystallization, the shrinkage of semicrystalline plastics is higher. In the case of amorphous plastics the shrinkage is closer to the influence of negative thermal expansion.

Furthermore, amorphous plastics shrink evenly in every direction. In the injection molding process, the crystals of semi-crystalline plastics are oriented in the direction of the flow. As a result, the shrinkage in the direction against the flow is higher than the shrinkage in the flow direction.

Due to higher and uneven shrinkage products that are made of semi-crystalline plastics tend to warp more. It is more challenging to produce them straight.

 

How to address this in my design?

It is easier to produce a straight and dimensionally accurate product using amorphous plastic. Due to the mechanical functions or service environment of the product it is, however, often safer to use semi-crystalline plastic.

Regardless of what plastic you use, the following basic rules should be applied:

  1. Component geometry: Uniform wall thickness, no deep wells (difficult to cool), large radius rather than sharp corners.
  2. Tooling: Efficient cooling, proper gate location. These are important topics you need to discuss with your tooling supplier.

Surface quality

Higher shrinkage of semi-crystalline plastics easily leads to more visible sink marks. The surface of semi-crystalline plastics also tends to be slightly oily. Amorphous plastics typically provide a dry and shiny surface appearance.

How to address this in my design?

Amorphous plastics are the right choice for visually demanding components such as Bluetooth loudspeakers or WiFi routers. If the product is challenged by harsh service environment or mechanical use, semi-crystalline plastics are a safer choice. PBT, PA6, and PA12, for example, do provide a nice shiny surface. If ribs etc. are properly designed thin enough (app 0,6x wall thickness) the sink marks are not evident.

Compounds of semi-crystalline and amorphous plastics, such as PBT/PC and ABS/PA, are also available.  The idea of these is to combine the advantages of both amorphous and semi-crystalline plastics. Sometimes it works.

Paintability, printability, gluability

Amorphous plastics are usually easy to paint, print and glue. With semi-crystalline, it’s the opposite. The crystals for some reason repel chemical substances.

How to address this in my design?

Don’t take it as given that all plastic components would be easy to print, paint or glue.

The higher the level of crystallinity, the more challenging it is to get something to stick on the surface. If you try scratching the printed text on Plasticprop sample made of POM, you’ll notice that the text peels off quite easily. PBT is quite easy to print or even paint, although it is semi-crystalline.

Gluing is hardly a process for modern high volume manufacturing, snap joints should be preferred instead. The same goes for painting. Glossy colored surface can be achieved without secondary operations, although exterior car parts are today commonly painted PP+EPDM+talk. Outdoor components made of PC are lacquered with a UV protective silicate layer.

Chemical resistance

Chemicals challenge every plastic product, especially together with elevated temperature and continuous load. In general, semi-crystalline plastics endure different chemicals considerably better than amorphous plastics.

How to address this in my design?

Amorphous plastics suit well as covers for electronic appliances, toys or picture frames. But in chemically demanding service conditions that include oil, petrol or strong cleaning detergents, etc, the use of semi-crystalline plastic should be required rather than recommended. If you are uncertain, semi-crystalline plastic is always a safer choice.

Resistance against continuous stress

Stress-cracking due to continuous load is the most common reason for plastic products to fail. The long-term load can be also cyclic, fatigue is another common reason for failures. Semi-crystalline plastics endure these both much better than amorphous plastics.

How to address this in my design?

If your component subjected to continuous or cyclic load, semi-crystalline plastic is a better choice. But this is not enough. You should also keep the long term stress level considerably lower (1/5 is a good starting point) that the short term data would suggest. If you must use amorphous plastic, the difference should be even higher. This is not “over-engineering”, it is the most efficient way to prevent failures in the long term use.

Friction and wear

As said earlier in this article, the surface of semi-crystalline plastics is slightly oily. This gives a good indication of their bearing properties. The friction coefficient of semi-crystalline plastics is smaller than that of amorphous. They also resist wear better.

How to address this in my design?

Lower friction and better wear resistance are another two good reasons to favor semi-crystalline plastics in mechanisms and machine design applications.

SLS 3D printing is a common method for prototyping. The material used in SLS sintering is PA12. If you end up using amorphous plastic, the friction between the moving parts of the final product may be totally different than what you experienced with your 3D-printed models.

Plastic products and Impact Resistance

When my wife dropped her old cell phone from the balcony to the concrete-paved yard, the phone took a few bounces and distributed itself then in several pieces around it. That was it, I thought. But as we collected the pieces, snapped them together and turned the power on, the phone worked perfectly again despite the few scratches it had gained. Excellent job on mechanical design, I had to conclude (the phone was, by the way, an ancient Nokia).

Impact resistance (or impact strength) describes a material’s or a product’s ability to absorb shock or impact energy without breaking.

Something that feels strong and stiff might be surprisingly easy to break with a bump to the right spot. Resistance against impacts is one of the key requirements in plastic product design. It is clear that a helmet or a ski boot should have high impact resistance, but almost every plastic product is subjected to impacts at some point of its service life and the consequences are often irreversible. All products should preferably be impact resistant to a degree for more reasons than one. They may fall down from store shelf before they ever meet the customer, to begin with.

Impact resistance is in fact one of the key strengths of plastics. It is why they are used in e.g. helmets, riot shields and sports gear. They replace glass in many applications, and in some cases they can be more durable than metals; if you kick a metal bucket, for example, you easily make a dent in it. A well designed plastic bucket would take the impact with no problem.

Impact resistance is not, however, an inherent material property to all plastics grades. Some grades are rather poor in withstanding impact, as we all may have noticed with cheap plastic toys or other similar products.

Selecting the right material is in main role when designing impact resistant products but the mechanical design/geometry makes a big difference as well. Tips on material selection and mechanics can read below.

Impact Resistance – tips for plastic material selection

Either I am dumb or the Izod and Charpy values expressed on the datasheets are confusing, but I have never been able to utilize them in practise. Apart from the units being sometimes in J/m and sometimes in J/m2, the tests are conducted in circumstances that are not particularly realistic in terms of load case, product geometry, material orientation and temperature. I understand that the general purpose of the data is to help designers to compare different plastic grades, but in case of impact resistance I haven’t found them useful. If you have been able to make use of them in practise, please tell us how in the comments at the bottom of this page. I have instead opted to divide the plastic materials into three groups:

1 Brittle materials that should be avoided when risk of impacts is high

These are for instance amorphous grades, such as PS, PSU, SAN and PMMA (although it is used on ice-skating rink panels). They all share a lack of overall toughness. Lack of toughness is also a factor that reduces the impact resistance of some reinforced plastics. PPS+40GF, for instance, has very high modulus and therefore low toughness. If you knock the edge of the corresponding Plasticprop sample, you can hear a glassy sound. This indicates low impact strength. The same applies for polypropylene. PP-H (homopolymer) is stronger and stiffer than PP-C (copolymer), but its toughness and therefore impact resistance is smaller.

2 Impact resistant materials

Polycarbonate PC is the mother of all impact resistant plastics. However, its superior properties are not fully explained by high toughness. The molecule structure of PC is very firm and it requires a lot of energy to tear the chains apart. The high impact resistance of polycarbonate is utilized in many alloys like PC/ABS, PC/PBT and PC/PA. Here is an amusing video that illustrates PC impact resistance over PMMA:

ABS is also known for its impact resistance which remains good also in low temperatures. This is based on the small butadiene particles that absorb the impact energy. The same principle applies also for HI-PS and impact modified PP (PP+TPE-O). PPO was earlier quite common choice for TV housing. It is often used as an example of good impact resistance, but my personal experience of the material is limited.

3 The materials in between the two

Practically all engineering plastics like PA, PBT and POM withstand impact relatively well. This again depends on toughness. If you can cause a permanent deformation in a material sample by bending it, this indicates that the material has a degree of plastic behaviour. If so, it is probably reasonably impact resistant.

 

Plastic product Impact resistance – what can you do with the geometry

  1. Skilled boxers have good impact resistance. They can absorb the energy of impacts with their whole body, not just one part of it, nose for instance. The same principle applies not only to material toughness, but to mechanics as well:
  2. Try to distribute the energy of impacts to the whole construction. Do not maximise the stiffness of the product without a good reason. Think of the component as a spring.
  3. Impact resistant materials are able to absorb energy in their internal structure. That requires bulkiness to some degree from the construction. A helmet with 0,5 mm wall thickness is not very impact resistant. Robustness is good. Don’t make the shock-absorbing elements too thin or lacy.
  4. Sharp corners and notches operate as stress raisers. This goes with static loads as well, but in the case of impacts, they are merciless.
  5. A rib that is meant to strengthen the product might easily reverse the intended purpose by crack propagation. Ribs should preferably be under compression rather than tension.
  6. If possible, try to avoid weld lines in the areas that are exposed to shocks. The same goes for gating, too.

Other aspects to take into consideration

Reinforcement

Reinforcements are two-edged swords when it comes to impact resistance. Increased strength is a benefit but high stiffness reduces toughness. Being able to decode the Charpy-V or Izod values might help, but I suggest studying each case with a hammer.

The effect of temperature

Service temperature is an important factor in impact resistance. In elevated temperatures it tends to be higher (to a limit), but when it is lowered, plastic products tend to turn stiffer. As the modulus of the material is increased, some toughness is lost. Amorphous plastics do this gradually but semi-crystalline materials might turn from ductile to brittle very suddenly as the temperature goes below their Tg. A water bucket made of PP (Tg app -10°C), for example, might easily break on a cold winter day while a HD-PE bucket (Tg app. -90°C) is still ductile. Based on this, one might expect PA6 (Tg app. 50°C), for instance, to be brittle in room temperature, but it is not. It is not that simple, in other words; each plastic material has its own characteristics.

Finding a stress-strain-temperature curve for each material candidate is therefore helpful and Campusplastics database is worth researching: Type for an example “stress-strain PC” into the search-area to find diagrams available for different PC grades.

Impact fatigue

Resistance against fatigue applies to impacts as well. Some plastic products might withstand single shocks well, but fail with repeated impact, even with smaller energy level. My practical experience is that amorphous materials are more sensitive to impact fatigue than semi-crystalline, as is the case with normal fatigue. Long term testing of impacts must not be overlooked.

Elevated service temperature and plastic products, how to avoid failures.

In practice, all plastic materials come with a datasheet that states their mechanical properties and service temperature area. Usually, the values are measured and given at room temperature only. It is important to bear in mind that elevated temperature has a significant effect on the strength and modulus of plastic materials.

Amorphous plastics gradually soften as temperature increases. For example, an acrylic PMMA product has totally different characteristics when used at +70°C than at room temperature – even if both temperatures are within the given service temperature area according to the material datasheet.

Semi-crystalline plastics turn gradually softer as well, but they may also change their behaviour more rapidly when they cross their material-specific glass transition temperature, Tg. They change from glassy state (below Tg) to rubbery state (above Tg). In the glassy state semi-crystalline plastics have higher strength and modulus, but they are more brittle. PBT, for instance, has Tg of app. 55°C. If you set a Plasticprop sample made of PBT into an oven, you can probably feel the difference between 45°C and 60°C.

Short-term use: how to avoid problems with design

  • Establish the maximum temperature your product is going to be used in.
  • Take this into account when choosing the material. The maximum service temperature should be clearly within the given service temperature area.
  • Explore how the material strength and modulus are changed by the elevated temperature. Stress-strain curves are a good tool for this. It is also informative to set some reference samples into an oven and study how their properties change in different temperatures.
  • Make sure your design is still strong and stiff enough when the mechanical material properties have decreased below the usual service temperature. Take the Tg into account if it is within the service temperature area.
  • Do not exaggerate. The combination of maximum load in maximum temperature is often very unlikely. If your design is based on this, it might be too heavy and too expensive for the great majority of the users.

How does long-term use differ from short-term use when it comes to elevated temperature?

It is important to understand that in plastic product design short-term and long-term service temperatures require different approaches.

In long-term use, we have to keep stress-cracking, fatigue and creeping in mind. The most efficient way to avoid their unwanted effects is to make the product significantly stronger than short-term loading would require.

For example, if a lever must carry a continuous load of 10 units, you should design it in a way that it can easily carry a load of 50 or even 100 units. But in the case of continuous elevated temperature, this ratio must be based on the material properties at the expected temperature, NOT at room temperature.

It is also worth knowing that long-time exposure to high temperatures might degrade a plastic product. This phenomenon is called thermal degradation.