The advent of precision CNC technology has allowed us to machine components to ever closer levels of tolerance. Metal parts, in particular, will lend themselves to tolerances as close as 1µm (1 micron), or 0.001 millimeters. Not only can such tolerances be attained, but on a piece of stable, well-maintained equipment, their repeatability is guaranteed.

In contrast to metals, polymers do not conform to such close tolerances. Depending on the polymer in question, a variety of factors, including the heat build-up during machining, stress in the underlying material, and the impact of moisture can all play havoc on dimensions. Hence, when we see tolerances of less than 20µm on a customer drawing, we know that the part designer comes from a background in metals and does not appreciate the near impossibility of maintaining the repeatability of such a dimension on a polymer.

 Note that we say, ‘near impossibility’ and not an impossibility. This is because with experience and a lot of trial and error, polymers too can be machined to as little as 10µm tolerances. It is not easy and requires a significant amount of care and caution during machining, but it is possible.

Machining PTFE

PTFE (Teflon) exhibits such a range of properties that it finds use in nearly every industry. As a component – either a valve, seal, seat or a ring – it can be machined either from its raw (virgin) form, or it can be combined with special fillers that add other properties to the material, but which also complicate the machinability of PTFE. Here are some of the key factors to consider when machining PTFE to close tolerances.

1. Filler
The filler changes the composition of the PTFE material and as a result, it affects the behaviour both during and after machining. A filler such as glass, offers higher dimensional stability and the ability to machine down to as little as a 10µm tolerance. Fillers such as carbon and graphite cause the PTFE to become very coarse at a molecular level, resulting in extensive damage to the tool tip. As a result of this damage, the tool itself loses effectiveness and can result in the parts going out of spec.

Fillers are usually employed because they alter the properties of the base PTFE material. Glass is used to improve hardness and reduce creep, while carbon and bronze are used to improve wear resistance. 

2. Sinter cycle
The sinter cycle is a vital part of how PTFE is processed and has a tremendous impact on the PTFE properties. Depending on the size of the part, a cycle of anywhere from 14 hours to 72 hours can be employed. The sintering cycle heats the PTFE up to its melting point, allowing the particles to fuse together. Then the cycle cools the part steadily to ensure that the parts do not crack. A cracked part cannot be re-used, and it is therefore vital from a cost perspective to get the sinter cycle right. Certain blends of PTFE may require a high or lower peak temperature and also a gradual or sharper cool down rate. Once the part comes out of the oven, the easiest thing to check for would be the tensile strength, hardness, specific gravity, and elongation. If these are matching specifications, the material can be considered acceptable.

However, apart from curing the part, the sinter cycle is also vital in how the part behaves both during and post-machining. For one, a part may exhibit good tensile properties, but may still suffer from over-shrinkage after machining. Our own experience has been that parts that exceed 200mm (8”) in diameter need to be observed after machining for shrinkage. In case the part exhibits excessive shrinkage – say over 0.2mm – then attention needs to be given to the sintering as the part may need to be run on a longer cycle to arrest this issue.

In addition to dimensional stability, the part may also exhibit ovality post sintering. This is especially true for larger parts. The issue with ovality is that the part may not clear during machining. Hence, a post sintering re-shaping may be needed to ensure the part is evenly round. However, too much reshaping can also cause problems. Because PTFE has memory, it is likely to try and come close to its original shape over time or when exposed to high heat. In our experience, a temperature of even 150°C for one hour (well below the 250°C service temperature of PTFE) can cause the part to try and regain its post-sintering shape. Hence, care needs to be taken to minimise the ovality at the sintering stage itself or ensure that any re-shaping does not try and alter the part so much that it loses dimensional stability when on the field.

3. Tooling and environment

The type of tooling, machining process (RPM, feed rate, program) and post machining storage all lend themselves to the repeatability of critical dimensions. Very often, we believe that hard, durable tool tips will give us the best and closest finish on the part. However, in many cases we have seen a simple HSS tool tip offer superior results to the more expensive carbide tips.

Similarly, the RPM and feed rates both work towards improving the part dimension and finish. A high RPM may offer a better finish, but may also result in heat build-up, which may show up only post-machining. Hence, each part needs to be treated specially to ensure all the parameters work together for the best possible result.

In addition to tooling, note that PTFE is highly sensitive to temperature. Very often, shrinkage can be attributed to nothing more than a change in the climate. Since PTFE can experience a dimensional change of up to 3% between 0 and 100°C, it is not uncommon to see large parts exhibit shrinkage of a few tenths of a millimetre, when the weather becomes cooler or warmer over a few days. The only way to address this is to ensure that the part is inspected under temperature-controlled conditions.

There are other aspects of machining PTFE that we do not delve into here. Needless to say, as a material, it is as versatile in its processing techniques as it is in its end-properties. However, as a valuable and integral part of so many systems, it makes sense to understand and appreciate these processes so that a consistent and high-quality end-product can be manufactured.

As the world takes a conscious and collective stand against the rising use of fossil fuels, it also disrupts a status quo set in place during the dawn of the industrial revolution. Since fossil fuels began to power our homes, factories and cars, most technological developments have focussed merely on efficiency gains. The improved productivity of coal-fired power plants, combined with better refining techniques and higher efficiency engines have allowed us to do more with less. At the same time, demand has moved steadily upwards, implying that even with these productivity gains, costs have always been rising.

The biggest hurdle to renewable energy was always the fixed costs involved. Consider a solar, or wind-powered plant. Not only is there no option to run these plants 24x7, but the cost per unit of power produced, when added to the cost of the infrastructure itself, always lagged the numbers showcased by their traditional, fossil fuel counterparts. However, efficiency gains have been made in renewables as well. In fact, one might argue that while we have somewhat plateaued with the gains in fossil fuel productivity, the gains in renewables are only just getting started. As a result, the earliest solar plants are now starting to show net positive gains when compared with coal power plants. The same can be said for wind and hydroelectric power plants.

Assisting in this wave of continual improvements in efficiency and productivity are polymer materials that lend their properties to this endeavor. Here we look at a few ways in which high-performance polymers are contributing to the field of renewable energy.

1. Solar Tracker Bearings

The effectiveness of a solar panel in maximizing the energy it can harvest is improved by ensuring it receives the highest intensity of sunlight possible. Solar tracker systems are designed to rotate the solar panels in order to keep them facing the sun at an optimal angle. However, the energy expended in rotating the panels cannot exceed the efficiency gains from the rotation itself. Thus, it is critical to have a low-cost, low-friction process that limits the energy consumption of the tracker.

Solar tracker bearings are made using high-performance, UV resistant polymers that are both light-weight, low in cost and easy to install. In addition to this, we have developed solar tracker bearings that employ fiction reducing additives, leading to higher efficiency gains in the system. Cost-wise, the bearing assembly can be made at roughly 1/10th the cost of a traditional metallic bearing. 

2. Bearings for wind turbines

Like solar tracker systems, wind turbines too have a tracking system which maximizes the angle of the turbine to ensure it receives the most energy. In addition to this, the process of harnessing wind energy is a rotary process, implying higher RPMs and a need for lower friction. Any friction in the system immediately results in a net energy loss. Hence, high-performance polymers such as PTFE find use in these applications. Although PTFE can be an expensive material, the coefficient of friction it offers can be as low as 0.03 when moving against polished stainless steel. This allows a simple PTFE bearing to take on loads and prove effective in applications where otherwise heavy and more expensive metal assemblies would be needed. Furthermore, as PTFE is a self-lubricating material, the need for grease and/or constant maintenance is significantly reduced. This is especially useful for wind turbines, which are usually located in remote areas and benefit from needing less maintenance.

3. EV charging stations

Electric Vehicles are gaining traction over traditional fuel powered vehicles. As their demand and prevalence grows, so too would the infrastructure needed to ensure that they can function smoothly. Investments in EV charging stations have increased significantly and new housing developments are increasingly required to ensure that there are charging stations for all parking slots.

As a superior insulation material, PTFE has been found effective in EV charging stations. PTFE insulation blocks can be used to improve the charging efficiency and ensure that there is minimal leakage of current.

4. Battery separators
One of the key factors with renewable energy is that storage needs to be both ample and efficient. While traditional power plants feed directly into the grid, solar and wind plants will only generate power in bursts. As a result, a lot of effort has gone into battery technologies. Both PTFE and PE (polyethylene) are seen as effective battery separators.

These separators provide internal insulation to the battery, preventing the batteries from discharging when idle. Although PE separators are effective in most applications, high-voltage applications need PTFE films, which possess higher breakdown voltage strengths and can remain effective over a much longer time period. These PTFE films can be made in thicknesses of as low as 40 microns and have breakdown voltages of as high as 100KV per mm.

5. Plastic to diesel conversion - renewable polymer fuels

While polymers are certainly effective in improving the efficiency of renewable energy systems, the irony is that most polymers are manufactured from fossil fuels themselves. However, recent advancements in plastic-to-fuel technologies have allowed waste polymers to be converted back into combustible fuels. While this does not exactly fall in the same category as renewables, it does allow for a lot of waste plastics to be re-used, rather than end up in landfills.

The above are but a few applications where polymers are finding use. In truth, there are myriad different areas when the properties of high-performance plastics can be utilized to improve, enhance and augment existing systems of renewable energy.

The use of PTFE tubes has become quite widespread across industries. As the scale of manufacture of regular thin-walled PTFE tubes has expanded, the cost has become less prohibitive. As a result, applications that were sensitive to the price of PTFE tubes have been able to adopt them.

In truth the properties exhibited by these thin-walled tubes are so diverse, that there is nearly no application where it cannot enhance the efficiency of a system. We have looked before at the overall properties of PTFE tubes and its application across various industries. However, we want to explore further the properties and challenges of thick-walled PTFE tubes.

Challenges in making thick-walled PTFE tubes

PTFE is a material that is challenging to process. Standard moulding processes that can be applied to what we call ‘melt processable plastics’ do not apply. Since PTFE does not melt, it cannot be converted into a liquid form and passed through a die. Instead, other methods are employed to achieve the shapes and profiles needed.

The processes for making PTFE tubes include:

1. Paste extrusion – where the PTFE powder is mixed with an extrusion aid to form a ‘paste’ and this is passed through an extrusion die at high pressure to attain the shape

2. Ram extrusion – where powder is added in successive charges into a die and a ram keeps compacting the powder such that a continuous profile is achieved

3. Compression moulding – where small profiles can be made (usually less that 1000mm in length) by simply compacting the PTFE powder within a die.

Both ram extrusion and compression moulding are employed when the diameters of the tube are in excess of 1” and when the final length needed does not exceed 2-3 meters. In case we want long lengths of tube, paste extrusion is the only option.

The length of a paste extruded tube is limited by the amount of powder that can be loaded into the extruder, since there is no option to add more powder once the extrusion begins. Most extruders would have a maximum capacity of about 10Kgs. Depending on the size of the thick-walled tube itself, the final length will be determined.

For the most part, paste extruded tubes are used to make thin-walled tubing, which is tubing where the wall thickness is within 2mm. The process for making thin-walled tubes follows a continuous sintering process, wherein the tube is extruded directly into the heating ovens and cured at the same rate at which it is extruded. This facilitates the need for a high structure to house the extrusion equipment. Typically, the extruder sits on the topmost floor, a drying oven on the floor below it and a sintering oven under the drying oven. Temperatures are set such that the drying oven is able to remove all traces of the extrusion aid (which is flammable) before the extruded tube reaches the sintering oven.

For higher wall thicknesses, there remain certain hurdles for paste extrusion in this manner. While extrusion itself is not a problem, the thick-walled tube is much tougher to sinter. The quantum of extrusion aid is high, owing to the high cross section of the tube. Hence, ensuring all removal of the extrusion aid in the drying oven is not guaranteed. This can result in fire when the tube reaches the sintering oven, where temperatures would be much higher than the flash point of the extrusion aid.

The other problem with thick-walled tubing is the weight of the tube itself. As thin-walled tubes are light, they place very little load on themselves during sintering, where the tube is soft. In contrast, the load of the thick-walled tube can result in it pulling on itself while in a heated state, causing stretching or deformation in the dimensions. 

For this reason, separate sintering cycles and processes need to be developed for thick-walled tubing. Our own experience has shown that the drying and curing cycles need to be fine tuned to ensure that the tube remains uniform in dimension and does not crack or split during sintering.

Applications for thick-walled PTFE tube

Thick-walled PTFE tubes are used in areas where high burst pressures or high voltages exist. These applications include:

1. Pantographs for the railways

2. Insulation liners for heavy electricals

3. High-pressure pneumatic lines

4. Liners for high-wear applications

5. Shields for chemical equipment

The strength and insulative properties of PTFE ensure that it is often the only material of choice in applications involving strenuous environments.

Properties of PolyTetraFluoroEthylene (PTFE) Thick-walled Tubes