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When it comes to fluoroplastic convoluted tubing it can be very confusing just what you are getting. Do you want standard and if so, what is standard? Is it PTFE, FEP or PFA? Is it annular or helical? For each user, “standard” depends on the user’s application and preferences.
In the early 1960s, fluoroplastic convoluted tubing was originally developed to provide a light-weight, non-flammable, corrosion resistant alternative to the heavier metal interconnects in aircraft. Later on, when the fluid handling industry became interested in using fluoroplastic convoluted tubing, the application requirements began to change in regard to flexibility, color, wall thickness, and how to attach the tubing to a compression fitting. As these issues were addressed, many new variations of fluoroplastic convoluted tubing were developed.
Around 1980, one of the new variations was fluoroplastic corrugated tubing. Unlike convoluted tubing, where the convolutions are molded into the tube, corrugations were actually made by creating an annular ring with a unique angle that allows for extension and compression and the tightest possible bend radius. The result was a flexible tube that could handle an extremely tight bend radius and goose neck around obstacles without collapsing the tube. This was not possible with convoluted constructions.
In those early years, when fluoroplastic convoluted tubing and corrugated were both supplied in annular form, corrugated tubing was sometimes incorrectly referred to as “convoluted tubing.” Annular tubing or corrugated tubing is referred to as “rings” of tubing formed into a continuous tube. If you cut this tube apart you could cut it on the valley or peak and you would have a straight perpendicular cut. If you cut through a valley on one side you will be in a valley on the other side. However, over the years, most of the fluoroplastic convoluted tubing evolved into a helical convolution. Helical convoluted tubing is a spiraled convoluted tubing. If you cut the tubing perpendicular you will probably cut a peak on one side and a valley on the other.
Convoluted tubing uses twice the material of a standard smooth bore tube, creating a tube reinforced within itself to handle higher pressures and offering increased flexibility. Convoluting the tube increases the flexibility of a tube by adding hoop strength to the circumferential strength to resist collapse, which results in a tube capable of turning corners and winding between machinery. It can be supplied in coils (sometimes as long as 1,000 feet continuous) or cut to length with formed cuffs on each end. These cuffs allow the convoluted tubing to be attached to machinery or fittings without comprising the integrity of the tube and ensuring a sure fit.
Today, almost all convoluted tubing is created by machines. In PTFE, the smooth bore tubing is run through an automatic convoluter that heats the tubing back up into the gel state and then pushes the soft material through a die to create the convolutions. It takes approximately 100 feet of smooth bore tubing to create 50 feet of convoluted tubing.
This article was written by Janine Kruit, Texloc/Atlantic Tubing, a Division of Parker Hannifin Corporation.
|In This Issue:
The ins and outs of standard fluoroplastic convoluted tubing
Convoluted tubing, often made from PTFE, can handle higher pressures while offering more flexibility.
PTFE — fluoropolymer
Diversity in PTFE
Test your knowledge
Plastics Education in Philadelphia
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Members include plastics distributors, processors, manufacturers, resin manufacturers, manufacturers’ representatives and associated products and services, all of whom are dedicated to the distribution channel.
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The fluorocarbon resins are an outgrowth of the fluorine, carbon and chlorine compounds developed during the 1930s, as part of the same research which developed refrigerants. Dr. Roy Plunkett’s research led to one of today’s most widely used fluoropolymers — polytetrafluoroethylene, better known as PTFE.
Properties and characteristics
PTFE has an impressive array of physical properties which make it the optimum material of choice for applications ranging from wire and cable insulation to medical catheter linings to active wear fabric. Those properties include:
There are also some characteristics which can limit the use of PTFE, including relatively low mechanical properties and the inability of materials to adhere to the surface. Mechanical properties can be dramatically improved with the addition of fillers (glass, bronze, stainless steel, etc.) while etching the surface with sodium naphthalene or sodium ammonia will prepare the surface for adhesives. Note that the relatively low compressive strength of unfilled PTFE makes it an excellent gasket material, while the filled and newer chemically modified resins greatly expand the usefulness in these applications.
Processes and grades
Many people think of PTFE in two forms — mechanical and virgin. This is a slippery slope, since today what is typically sold as “mechanical” is probably lower grade virgin while some virgin resins may not be suitable for all applications, especially those of an electrical nature. It is far more prudent to classify PTFE according to ASTM D-3308-01, which contains four types and two grades, that can help to differentiate between end use suitabilities (e.g., semiconductor, electrical, food use, gaskets, etc.). Your PTFE supplier can assist you or your customer in making the correct choice.
Physical, especially mechanical, properties can be greatly affected by fabrication techniques, including how the stock shape was processed as well as machining techniques employed. Poor techniques will result in finished parts with less than optimum properties.
It is especially important to choose correct tools and working speeds when machining PTFE components. Post annealing is sometimes required to ensure very close tolerances can be maintained.
This article was written by Russ Consentino, CPMR, Plastic Solutions, Inc..
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Since its discovery in 1938, polytetrafluoroethylene (PTFE) has revolutionized the progress of modern man, as did the discovery of fossil fuels. PTFE has opened new doors for significant advancements in a variety of different technologies. From the aerospace industry to the food processing industry, PTFE has aided in many benefits to mankind. Most commonly known throughout the industry as Dupont’s brand name, Teflon®, PTFE has been commonly used in its virgin state. The material’s chemical inertness, low coefficient of friction and high temperature resistance are but a few advantages that it has to offer. As use of this material increased, so did the applications for it. Chemists soon began to add additional materials within PTFE to increase a variety of properties for their respective applications.
The virgin grade exhibits the lowest coefficient of friction of all solid materials making it extremely slippery. This tendency to repel also adds to the chemical inertness of the material. It has a chemical resistance to most chemicals except fluorochemicals and molten alkali metals. It has an extremely high electrical resistance and excellent dielectric strength. The virgin grade is also FDA approved for the food processing industry. Some virgin grades are also approved for insertion within the human body as possible prosthetics or even valves within an artificial heart.
The modified grade offers an excellent weldability characteristic that makes the fabrication of complex parts economically feasible. This improved characteristic also lowers the porosity and improves permeation resistance. Also, environmental safety concerns are improved because of lower fugitive emissions.
A variety of other fillers also exist, such as calcium fluoride, mica and stainless steel, just to name a few. The blending capabilities are endless as long as the required application deems it necessary. The diversity found in PTFE, from the virgin grades through the filled grades, offer unique characteristics not found in other materials, making it one of the most valued discoveries of our time.
This article was written by Carlos A. Baez, Plastomer Technologies: Texolon.
PTFE was the first fluorocarbon. It is the most chemically resistant plastic known. Only a few chemicals react with it. Its mechanical properties are low compared to other engineering plastics, but its properties remain at a useful level over a great temperature range — from -400 to 500°F (-240 to 260°C). Mechanical properties can be improved by the addition of fillers such as glass fiber, carbon, graphite, molybdenum disulfide and bronze. PTFE has excellent thermal and electrical insulation properties. And, it has a low coefficient of friction. It is difficult to make anything adhere to PTFE. A material may stick to it, but the material can be peeled off or rubbed off.
PTFE is not melt processible; instead it is processed by paste and ram extrusion or compression molding.
For more information on PTFE and other plastic materials, IAPD’s Introduction to Plastics is an invaluable training manual. Details about it and other IAPD educational resources are available online at www.iapd.org.
What do you know about PTFE? Answers are at www.iapd.org/popquiz.html.
1. What filler would you add to PTFE to best improve its load bearing properties?
2. What is the dynamic co-efficient of friction for PTFE?
Your IAPD Distributor is your choice in finding the right material for your application. Go to www.iapd.org to find a distributor in your area. You can search by company name, location or product category.
The IAPD Magazine web site at www.theiapdmagazine.com allows you to search by material, trade name and fabrication process. You can also search for fabrication capabilities.
Join IAPD in Philadelphia September 20 for a special one-day educational package specifically for engineers and specifiers to learn about innovations in both plastic materials and applications.
IAPD experts will first present the popular IAPD Plastics Applications Seminar, which offers a look at a wide variety of plastics applications. The seminar introduces and reintroduces materials in a variety of eye-opening applications. After the seminar, be sure to attend the IAPD Plastics Exhibition, a great opportunity to experience first-hand the best of the plastics stock shapes and pipe, valves and fittings distribution industry. Last year’s exhibition featured 70 exhibitors, and the show is the only one of its kind in North America.
Information is available online at www.iapd.org/new/pdf/events/ac/one_day.pdf.
Designing with Plastics is published by the International Association of Plastics Distribution. While every effort has been made to ensure accuracy, IAPD encourages you to verify information with a plastics distributor to ensure you select the correct plastic products to meet your needs.