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.

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1. Polymers in Low Friction Applications

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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.

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Extruding a material like PTFE is never straightforward. The nature of the polymer necessitates that any standard methods of processing – such as melt extrusion, injection moulding, or even thermoforming – fail to work with PTFE. The reason for this boils down to the fact that PTFE has no melt flow. Even when taken to its ‘melting point’ of about 380°C, PTFE will merely attain what is known as a ‘gel state’, wherein it goes from opaque to transparent, but stays very much in the same form. This means that the material cannot be injected into a mould to make complex shapes and that it cannot be otherwise shaped or drawn into any kind of final shape. As a result, different techniques have been devised to bring PTFE into a final shape as needed.

PTFE extrusion is one such method used for making tubes and hollow profiles. Rather than melt the material and draw it through a die, the process of PTFE extrusion can be done in one of two ways.

1. Ram extrusion - where subsequent charges of PTFE resin (powder) are layered and compressed one above the other inside a die and the die is heated from the outside to allow for the PTFE resin to fuse into a single piece.

2. Paste extrusion - where a special type of resin (fine powder resin) is blended with an extrusion aid and then squeezed at high pressure through a die at room temperature. The resulting ‘extrudate’ is then heated in ovens to fuse the material.

Between these processes PTFE paste extrusion is considered more complex and it is usually employed when the final profile has a thin cross section. Hence, PTFE thin-walled tubes, or spaghetti tubes are made in this fashion. Ram extrusion is usually used when the cross section is thicker and when the requirements are for a single solid rod.

Comparing properties

1. Material grades - both ram and paste extrusions need to be made with specific grades. For paste extrusion, fine powder resins are used. These resins mix easily with extrusions aids (usually a mineral spirit like naphtha) and once blended, they will form fibrils when pressed between the fingers. In some sense, the extrusion aid forces the PTFE to behave somewhat like a liquid at high pressures, so that the material can be easily passed through a die. Typically, virgin grades are used in paste extrusion, although mixing pigments and even some quantity of additives is done easily. However, when blending fillers (such as glass fibre or carbon fibre) in excess of 5%, there can be complications, as the distribution of the filler is not always uniform. In contrast, for ram extrusion, pre-sintered or free flowing resins are used. Typically, virgin materials would be pre-sintered resins – as they flow easily and can be uniformly distributed in the die without issue. For filled grades, any grade can be used provided it is a free flow grade.

2. Mechanical strength - the difference in the two processes rests largely on the way in which the extrusion is done. For ram extrusion, the final form is built up by adding subsequent charges of PTFE one on top of the other. It is therefore an additive process. As a result, the material will have higher strength in the radial direction and less strength longitudinally (weak points would result in the joint between subsequent charges). For the same reason, since paste extruded resins are formed by passing the resin lengthwise through a die (think of how penne pasta would be made), the strength lies in the longitudinal direction. Un-sintered tube has a tendency to crack or split along its length if it is handled even slightly roughly before being cured in the oven. It is for this reason that paste extruded tubes are preferred in applications where there is likely to be a high tensile load on the tube. On the other hand, ram extruded tubes are used in lining applications, as radial force needs to be applied to the tube when inserting it into a metal pipe to line it from the inside.
   Ram extruded tubes also tend to be much stiffer and lack the flexibility of paste extruded tubes.

3. Sizes - as mentioned earlier, paste extruded tubes would typically be used where the wall thickness was small. Usually, anything within a wall thickness of 2.5mm would be paste extruded, whereas higher diameters and wall thicknesses are ram extruded. Another limitation with ram extrusion is that because the tube is stiff, there is a cap on the length that can be obtained before the finished tube reaches the ground and needs to be cut. With paste extrusion, due to the tube’s flexibility, material can be collected in a coil, allowing for bundles of many of hundreds of meters in a single length. The same flexibility allows the paste extruded tubes to be easily bent and shaped to suit the final requirement.

Fundamentally, both ram and paste extruded tubes are made with the same material. However, the processing techniques dictate that the end properties are different. Care needs to be taken to understand the end application and to choose the tube that best suits this.

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