The versatility of PVDF as an engineering plastic is well known. Indeed, we have covered the advantages, properties, and applications of PVDF in an earlier article. Properties such as extreme chemical resistance, UV resistance, thermal stability, and piezoelectricity all make PVDF an intriguing polymer that finds application across a host of different industries.

Our own experience with PVDF has thus far stayed within the realm of machining. PVDF rods and sheets are available and can be easily machined to make a final component. The polymer itself throws up no surprises with regards to how it behaves dimensionally, post-machining.

Recently, however, we have encountered a challenging prospect. The part we were asked to develop was far too large to be machined from a rod or a sheet. With an outer diameter of about 200mm and an inner diameter of 130mm, the part would have caused far too much waste were it to be cut from a sheet or a rod. The option, therefore, would be to compression mould it.

Given our expertise in PTFE and PEEK moulding, we assumed that PVDF – whose melting temperature is far less than either PTFE or PEEK – would be simple enough to mould.  We knew from discussions with our suppliers that the equipment we use for PEEK moulding could be easily used for PVDF, provided the processing parameters were adjusted accordingly.

Our first experience with the moulding convinced us that this was a fairly simple affair. We moulded a small rod of 100mm diameter and 50mm thickness and found the part to be uniformly coloured (PVDF should be milky white), with no signs of any blowholes. Skived sections taken from the rod confirmed that the tensile properties were in line with what was expected, while the specific gravity was also in the range of 1.8, as it should have been.

Moving from the test sample to the part we needed to mould showed us that we may have underestimated the material. A few challenges were immediately apparent:

1. The material was extremely sensitive to temperature. While the 100mm sample appeared to have formed easily under a temperature of 210°C, the larger part was getting discoloured and turning brown.

2. The melt flow of the material was challenging to control. If the temperature was held for too long, the viscosity of the material would reduce and cause it to leak from the mould. If the temperature was not held long enough, the part would come out with blowholes, having not been sufficiently melted throughout.

3. Similarly, too much pressure would cause material leakage, while too little pressure would not allow all the air to be expelled, resulting in blowholes.

In effect, moulding PVDF turned into a very precise give and take between temperature, dwell time, and pressure. Furthermore, although the material could be re-melted, doing so would discolour the polymer, rendering it useless. This meant that all parameters needed to be precise and that a cycle could be run only once else the material would be lost. (Incidentally, we are not new to this conundrum. PTFE behaves in much the same way, only we have decades of experience with PTFE and know how to get it right every time!).

Once we had moulded the part, the time came to machining it. Again, although our experience with machining PVDF had always been smooth, here too we observed that compression moulded PVDF behaves slightly differently post machining. Stresses in the material tend to relax overnight, causing slight deviations in dimension. Hence, adjustments needed to be made to the machining process to allow for the same.

There is a reason that engineering polymers are a niche space and that so few have the expertise to consistently manufacture certain high-performance plastics. We pride ourselves in being able to understand our polymers and to investing the time it takes to develop them.

Read More

1. Case Study - PEEK in Coffee Machines

2. Polymers in Low Friction Applications

3. PTFE Extrusion - Ram vs Paste Extruded - A comparison of features

In the ever-evolving world of coffee culture, enthusiasts and professionals alike constantly seek innovations to enhance the brewing experience. One such technological marvel making waves in the coffee industry is the use of PEEK (polyether ether ketone) valves in coffee machines. These valves, though small in size, play a significant role in ensuring a superior and consistent cup of coffee.

Our first introduction to this unusual application of PEEK happened about ten years ago. A manufacturer of high-end coffee equipment came to us with a dilemma. They had been using aluminium valves in their equipment for a while and had never faced any problems. However, as an increasing number of Indians travelled abroad and experienced the flavours of western-brewed coffee, the complaints had started to come in. The issue: the coffee tasted metallic.

The client had done their own research and found out that the Italian coffee machines had replaced aluminium with PEEK.

PEEK is a high-performance thermoplastic known for its exceptional mechanical and chemical properties. Its use in coffee machines brings a range of benefits that contribute to the overall efficiency and quality of the brewing process.

First and foremost, PEEK valves excel in temperature resistance, making them ideal for the hot and demanding environment of coffee machines. Unlike traditional materials that may degrade or lose integrity under high temperatures, PEEK valves maintain their structural integrity, ensuring a reliable and durable component in the coffee brewing system. This resistance to heat is crucial for consistent coffee extraction and flavour preservation. PEEK has a service temperature of 275°C and is therefore more than capable of withstanding the heat within the equipment.

Another notable feature of PEEK valves is their resistance to chemicals and corrosion. Coffee machines often come into contact with various substances, including minerals in water and coffee residues. PEEK's resistance to corrosion ensures that the valves remain unaffected by these elements, leading to a longer lifespan for the coffee machine and reduced maintenance requirements. This not only benefits coffee enthusiasts by providing a more reliable machine but also contributes to sustainability by reducing the need for frequent replacements.

Precision is paramount in the world of specialty coffee, where every parameter matters. PEEK valves offer a high level of precision in controlling the flow of water and steam in coffee machines. This precision allows for fine-tuning of the brewing process, enabling baristas and coffee enthusiasts to achieve the desired extraction profiles. The ability to control water flow with accuracy contributes to the consistency of flavour and aroma in each cup of coffee, a key factor in the pursuit of brewing excellence.

In addition to their mechanical properties, PEEK valves are preferred for their biocompatibility. This characteristic is particularly important in the food and beverage industry, where materials that come into contact with consumables must meet stringent safety standards. PEEK's biocompatibility ensures that it poses no risk of contaminating the coffee with harmful substances, meeting the highest hygiene and safety standards.

The incorporation of PEEK valves revolutionised our client’s coffee machines and allowed them to even build their export business. It should be mentioned that in shifting from Aluminium to PEEK, the client saw the part cost shoot up by a factor of 10 (PEEK is an expensive polymer!). The fact that they still chose to use the PEEK component tells us how vital the material was in ensuring the end product was exactly as needed.

Read More

1. Polymers in Low Friction Applications

2. PTFE Extrusion - Ram vs Paste Extruded - A comparison of features

3. Exploring the Versatile World of PVDF

Polymers in Low Friction Applications: Reducing Wear and Tear and Keeping it Smooth.

The development of faster, more durable equipment usually calls for efficiency in energy utilisation and components that can sustain either rotary or linear motion over a very long product life cycle. This problem always boils down to the management of friction. Moving parts will typically experience wear and tear due to friction, leading to both part failure and an unnecessary build up of heat (and therefore a loss of energy).

Advancements in polymer science have allowed a significant number of metal parts to be replaced with specific, high-performance plastics that combine a low coefficient of friction with a high wear rate (also called the Pressure x Velocity, or PV value). These polymers, often when combined with specific fillers, are able to perform for far longer, minimising replacement costs and boosting energy efficiency.

One of the primary advantages of polymers in low friction applications is their innate lubricating properties. Unlike traditional lubricants that require constant replenishment, polymers can provide a durable and long-lasting solution. Polymeric materials, such as polyethylene and polytetrafluoroethylene (PTFE or Teflon), have self-lubricating properties, reducing the need for external lubricants and minimizing maintenance efforts. In the case of PTFE (Teflon) and UHMWPE, the static and dynamic coefficients of friction are so low that when sliding against certain materials (for example: polished stainless steel) the coefficient could fall to as little as 0.03. In layman’s terms: it would take only 30grams of horizonal push to move a 1Kg block across the surface of the PTFE. This is something we also call ‘near rolling friction’.

In the case of PTFE, the addition of specific fillers – such as bronze, glass, carbon, or MoS2 – can further enhance the wear properties of the material, making it more robust in certain industrial applications. PTFE can itself be used as a filler in other polymers, including PEEK, POM (Delrin), PPS (Ryton) or even Nylons. The addition of PTFE micro powders into these polymers – usually in a concentration of 5-25% - gives an appreciable boost to the low-friction properties of the base polymer, while allowing the polymer to retain its other characteristics.

In addition to their lubricating properties, polymers offer excellent resistance to wear and corrosion. When used in bearings, gears, or sliding components, polymers can withstand harsh conditions and maintain their integrity over time. This resilience contributes to the longevity of the components and reduces the frequency of replacements, ultimately leading to cost savings for industries. PEEK is highly sought after in gears. The hardness of PEEK ensures that the part will not wear out over time, while PEEK’s low density (specific gravity of 1.3) gives the added benefit of weight saving in the system. 

Many polymeric materials excel in low friction applications due to their lightweight nature. In industries where weight is a critical factor, such as aerospace and automotive, using polymers can lead to significant fuel savings. With specific gravities as low as 0.9, the weight saving over a metal component can be as high as 90%. Especially in aerospace applications, this is a benefit that creates immense savings for the end users. The reduced weight contributes to improved fuel efficiency and overall performance, making polymers both an eco-friendly and economically viable choice.

Medical devices also benefit greatly from the incorporation of polymers in low friction applications. Prosthetic joints, for example, often utilize polymer components to mimic the natural lubrication of human joints. The biocompatibility of certain polymers ensures that they can be safely used within the human body, providing low friction solutions for a wide range of medical applications. Similarly, PTFE tubes (usually with radiopaque fillers) are used in medical applications that require the tube to slide in and out of the patient’s body. Amplatz sheaths, for example, are used in urology wherein the tube is pushed in to make a channel through which a guidewire can be passed. The smoothness of the PTFE minimises the discomfort to the patient.

In conclusion, the use of polymers in low friction applications has ushered in a new era of efficiency, durability, and sustainability. Their innate lubricating properties, resistance to wear, and versatility make them indispensable in various industries. As technology advances and the demand for high-performance materials grows, polymers are likely to play an even more significant role in shaping the future of low friction applications.

Read More

1. PTFE Extrusion - Ram vs Paste Extruded - A comparison of features

2. Exploring the Versatile World of PVDF

3. Air Permeability Testing and Water Entry Pressure Testing in Expanded PTFE Membranes