Unravelling Polymers

The Definitive Blog on Polymers by Poly Fluoro Ltd.

Solar Tracker Bearings - Cost Benefits from Injection Moulding

Our foray into Solar Tracker Bearings over the past few years has not come without a fair share of learnings.

While, the initial assumptions we had regarding the bearing design and polymer grade selection have not altered much, we have had to revise our understanding of the manufacturing process significantly.

Primarily, a solar tracker bearing needs to conform to a few parameters in order to be suitable for the application involved:

  1. Lightweight – this is essential because the more load the bearing places on the solar tracker mechanism, the higher the inefficiency of the system. Solar energy is sensitive to net energy losses within the system and the bearing should not be a drag on energy

  2. Low coefficient of friction – the smoother the motion of the bearing, the easier the system can move. Again, if it requires extra load to rotate the bearing within its casing, it leads to a new energy loss

  3. UV resistant – since the bearing is being used in a solar application, it is essential that the polymer does not degrade over the 15-20 years that the system is expected to be operational

  4. Load resistant – while different projects have different parameters on how much load the bearing needs to take, it is safe to assume that a single solar tracker bearing would need to withstand somewhere between 700-1200 Kgs of load without experiencing any fatigue or deformation over time

  5. Low cost – with most projects needing about 1000-2000 sets of bearings per megawatt, there is a high volume of bearings needed for a single project. One reason that standard pillow block bearings are not used in these projects is that they are prohibitively expensive in comparison to the polymer bearing assemblies
     

Below, we look at some of the key learnings along the above metrics and evaluate how our understanding has changed and allowed us to offer a more tailored and effective solution for this product.

  Existing designs Poly Fluoro designs Benefits
Weight Machined from rods, so parts tend to be bulky Injection moulded using special lattice structure Injection moulding offers significant weight reduction of up to 40%
Grade of polymer UHMWPE or PA6 Special blend of thermoplastic with PTFE PA6 has high moisture absorption, while UHMWPE is not injection mouldable in its standard form. Poly Fluoro uses special blends to give a low moisture absorption polymer that has the benefits of PTFE's self-lubricating properties
UV resistance Usually black pigment Uses HALs HALs offer a far superior resistance to UV radiation over the long term
Coefficient of friction 0.15-0.2 0.1 Using a thermoplastic-PTFE blend allows us to reduce coefficients of friction to within 0.1 and possibly below
Load capacity 700-1100 Kgs 700-1100 Kgs Poly Fluoro designs offer the same load capacity, but at much lower part weights
Cost 1X 0.6X The use of injection moulding allows us to lower material consumption, machining cost and gives a much higher productivity overall. This brings the cost down significantly

 

To summarise – we too had initially opted for the machining route in making these bearings. However, we find that using injection moulding allows us the freedom to lower the part weight, improve the blend of the polymer grade being used and increase productivity, thereby reducing cost while enhancing the effectiveness of the part.

The downside to injection moulding is that it requires a significant up-front cost in making the mould. However, our understanding is that the mould cost is effectively recovered within the first 1500-2000 sets of bearings made. In other words, a project that requires many thousands of bearings would recover the cost quite quickly. Furthermore, since the life of the mould is anywhere between 150,000 and 200,000 sets of bearings, solar companies would save significantly by standardising a bearing design and using this across multiple projects.
 

Poly Fluoro Ltd. is currently making solar tracker bearings under the brand name: HelioGlide. We offer standard sizes of bearings which can be incorporated even in smaller projects, where the total requirement may not be high.

ePTFE Gland Packing - An Effective Sealing Element

Expanded PTFE (ePTFE) is a material that is constantly evolving. From simple mono-directional (uni-directional) gasket tapes, the material has been further worked into ePTFE membranes, multi-directional tapes and sheets and ePTFE Ropes.

Expanded PTFE (ePTFE) Gland Packing is becoming increasingly popular because of its incredible properties as a sealing element and the ease with which it can be used. Here we look at some of the variants and benefits of ePTFE Gland Packing.

Variants of ePTFE Gland Packing

Typically, we see two variants – braided ePTFE gland packings and pure ePTFE gland packings.

Braided gland packings, as the name suggests, uses thin filaments of PTFE/ePTFE yarn to make a braided square profile. This product possesses the properties of PTFE while having the added strength lent by the braiding process. The trade-off is that the material is not as compressible as pure ePTFE packing and is hence used only in applications where the pressure and torque being applied on the system are high enough to induce compression in the braided material.

Pure ePTFE gland packings are also referred to as PTFE Rope, ePTFE Rope and ePTFE Round Rope. It is a round cross section of PTFE that has been mono-directionally stretched to give compressibility to the material. The process is nearly identical to making ePTFE gasket tapes, although care needs to be taken to ensure that the outer surface of the rope remains smooth and that the material is handled carefully during processing to ensure no ovality develops in the shape. The benefit of using this material is that it has a far higher seal-ability and can offer sealing even at very low pressures and torques.

Both variants described above can be infused with fillers of glass and graphite to offer added properties such a creep resistance and improved tensile strength.

Applications of ePTFE Gland Packing

Like ePTFE Gasket Tape, gland packing finds uses in applications involving one of the following:

  1. High temperature environments

    The capability to withstand between -250°C and +250°C immediately makes ePTFE Gland Packing an essential sealing element in applications involving high temperatures. It remains true that very few materials combine high seal-ability with temperature resistance. While rubbers may offer sealing, they melt beyond 150-180°C. Similarly, some silicones have shown high resistance to temperature, but cannot form as effective a seal as ePTFE unless pressure are high.

    Apart from high temperatures, ePTFE is also the material of choice in cryogenic applications, since there is no risk that the material will become brittle and suffer a loss in properties at low temperatures.

    Further enhancements – by adding fillers of glass, carbon and graphite – can be introduced into the material to improve properties such as thermal expansion and creep. These allow for the properties of the ePTFE to be fine tuned to ensure the application parameters are being fully addressed.

  2. Chemically corrosive environments

    As an inert material, PTFE resists numerous chemicals and remains one the few materials that designers turn to in the event that a system may churn our corrosive elements that are unpredictable.

    Furthermore, while ePTFE is structurally a porous material, the pore sizes are so small that water is not absorbed into the material. ePTFE is hydroscopic and hence offers no risk that the fluids within the system will stagnate within the material.

  3. High pressure environments

    We have spoken above about pressure. A key feature of ePTFE is that even with low torques, it can offer sealing at high pressures. Most standard sealing materials will perform well at lower pressures, but once the load within the system increases, they may experience fatigue and failure.

    Certain systems – such as those using brittle materials such as glass, may not have the option of being tightened beyond a point, as this would crack the materials. In such cases, ePTFE is ideal in taking the load.

  4. Food grade and sterile environments

    PTFE is FDA approved in its resin form and certified for food grade applications. As a result, ePTFE gland packings can also be employed in applications where the fluids would be used in or come in contact with food.

    In addition to food, many labs that manufacture pharmaceuticals and specialised chemicals need an inert material that will not contaminate the final product in the even that there is contact. In such cases, making the sealing elements using PTFE and/or ePTFE is the only viable option.

Expanded PTFE (ePTFE) Gland Packing is usually supplied in rolls and in diameters ranging from 3mm up to 20mm. Rolls have a practically infinite shelf life, which means they can be stocked and kept for both installation and maintenance requirements.

For more information, please do visit us at www.polyfluoroltd.com

PTFE (Teflon) Tubing - Cost and Quality Concerns of Weaker Substitutes

It is not uncommon in an industry catering to high-performance polymers to frequently come up against the issue of pricing. Polymers such as PTFE, PEEK, PVDF and PPS command prices that are sometimes up to 20X the cost of cheaper plastics such as polypropylene or polyethylene. This results in suppliers sometimes passing off a low-grade polymer as the high-performance variant, with the client being none-the-wiser. There is also the very real and commercial issue that in many cases, it is the purchase department who procure the material, and their KRAs are tied closely with the cost of the product, rather than the authenticity. In addition to this, supplying a test certificate that claims the polymer is genuine is simple enough and so the material is passed without much ado.

It is only when the material fails, or when the client’s engineering teams takes the initiative to have the material tested in a lab that the true nature of the polymer becomes apparent. We have had clients come to us with material procured from local vendors that has failed miserably. The client – now concerned with quality – seeks clarification on the root cause of the failure. When we inform them that the material is not what they think it is, we are usually met with shock.

A similar pattern tends to follow specifically around PTFE tubing. For the most part, PTFE (Teflon) tubes look and feel like any other polymer. The colour of the tube ranges from milky white to translucent to nearly transparent (more on this later). This is not much different that the shades seen in tubes of nylon, polypropylene or polyurethane, to name just a few. In all these cases, the price of PTFE (Teflon) tubing is likely to be anywhere from 5X to 10X more expensive. Hence, many times the client will inform us that we need to check our costing, since their current procurement rates are far below what we have quoted.

In this article, we focus on both the cost benchmarks and quality metrics of PTFE (Teflon) tubes. We hope that with this information, a client would be able to better distinguish genuine PTFE (Teflon) tubes, from the cheaper variants that the market sometimes tries to push.

PTFE (Teflon) Tubes – Pricing Benchmarks

When we set out to install our PTFE paste extruder for making PTFE (Teflon) tubes, our first concern was competing with China. We had already witnessed the market for PTFE stock shapes (rods and sheets) become flooded with cheap – albeit low quality – Chinese material and did not want our product to suffer the same fate. Since nearly all PTFE tube is imported into India (we remain one of the very few with the technology to manufacture PTFE tubes in India), we procured import data from the preceding 2 years to analyse the landed cost of the imported material.

The table below shows the average price per Kg for PTFE (Teflon) Tubes from China, the US and Europe. Although as an engineering company, we rarely ever quote a client by weight, it is useful to compare the same to understand the costing.

 

China

Europe/US

Ex-works Price per Kg (US$)

31.0

45.1

Add: Freight @ US$1 Per Kg

1

1

Add: Customs Duties @ 15%

4.8

6.9

Landed Cost per Kg (US$)

36.8

53.0

Unsurprisingly, Chinese tubes are cheaper by over 30% when compare with Europe/US. This clearly does not consider any of the quality variations between the two sources or indeed between Chinese tubes and our own tubes. But having the China price – and the hard data to support this – has allowed us to refute the claims of clients who say they are buying PTFE (Teflon) tubes are US$10-15 per Kg.

We also found that our own tubes could be made for as low as 10% below the China price. Obviously, this depends highly on the grade od raw materials used. Since some grades – such as Daikin, DuPont and 3M need to be imported, our pricing is also impacted accordingly.

Therefore, our standard response when a client claims to be getting a lower price is to first quote the China price – which everyone agrees is about as low as the price can get – and then request the client for a sample of tube so we can verify whether it is PTFE or some other polymer. Our experience here has been fruitful. Genuine OEMs are not keen to use material that is inferior. They will usually respond positively to such an exercise and appreciate our efforts to supply authentic PTFE material. Traders and price players would rather ignore our data – which is an immediate warning signal for us to stay as far away from them as possible!

PTFE (Teflon) Tube – Variants and Substitutes

We mentioned above that tubes made from nylon, polypropylene and polyurethane are sometimes passed off as PTFE (Teflon) tubes. Notably, there is a vast difference in the basic properties of these materials (which is the reason PTFE is opted for in the first place). Most notably is the temperature capability. Quite simply, PTFE can withstand a temperature of 260°C without suffering any deformation or loss in properties. Since most of the cheaper polymers would melt at anywhere above 150°C, popping a small length of tube into an oven would be the easiest way to check the authenticity.

It should also be noted that PTFE (Teflon) tubes are rarely fully transparent. We will elaborate on this below but as a client, if someone is offering you low-cost PTFE (Teflon) tubes that have a glass-like transparency, you are most likely being sold a variant of polycarbonate or acrylic tubes.

In addition to substitute polymers, there exist variants within PTFE (Teflon) tubes that should also be investigated. The range of PTFE (Teflon) tubes described above gives us some clue as to the composition and quality of the tubes.

  1. Milky white tubes – good quality PTFE (Teflon) tubes should never appear milky white and opaque. This is a property of low-quality tubes – usually from China – and is a result of adding Titanium di Oxide to the PTFE. This both reduces the cost of the material and allows any dark spots or blemishes within the material to be masked. Property-wise, the additive diminishes the tensile strength of the material, leading to a lower burst pressure.

    Many traders will sometimes insist that milky-white tubes are genuine. They are sadly mistaken and have most likely been convinced of the same by a manufacturer wishing to squeeze out a lightly higher margin.

  2. Translucent tubes – most PTFE tubes made with good quality resins will appear translucent. With higher quality resins, there results in a bluish tinge in the tube. Suffice to say that if you observe these properties in your PTFE tubes, the quality is probably good.

  3. Transparent tubes – as mentioned above, this is a rare variant within PTFE (Teflon) Tubes. Transparency can only be attained if the resin is a ‘modified grade’. Companies such as Daikin, DuPont (Chemours) and even GFL have variants of extrusion grade resins which have been modified. This modified resin allows for the final tube to exhibit transparency. However, even with an extremely high grade of resin (we are told that the modified grade of Asahi Glass offers the highest clarity), the transparency would never be glass-like.

We hope that the above information gives a clear picture of the pitfalls when ordering PTFE (Teflon) tubes. It should be emphasized that PTFE (Teflon) tubes offer a set of properties unlike any other polymer. In addition to this, the techniques involved in processing the tube are both expensive and proprietary. All these factors contribute to the end-result that PTFE (Teflon) tubing is a high-end product and that getting it at a price comparable with a regular polymer should be viewed with caution.