Tech Talk Blog

Molecular weight and crystallinity are two factors in PTFE that determine many of it’s performance qualities. Both of these properties are highly dependant on the sintering operation of the PTFE production. If the material is cooked (sintered) too long or at too high a temperature it can degrade the polymer. Molecular weight is a measure of the polymer chain length and it affects flex life, tensile strength and elongation. As molecular weight increases so do these properties and paying attention to gel temperatures and dwell times during sintering is extra critical.

Crystallinity of PTFE can vary considerably depending on processing. This value can be seen in small variations in the specific gravity although in practice most PTFE materials fall in the 50-60 percent crystalline percentage. Increasing crystallinity decreases flex life, increases compressive stress and decreases recovery values, permeability and wear life. Other properties of PTFE, such as thermal, chemical, electrical and friction, are not generally affected by crystallinity changes.

So, sintering and other processing steps do a make a difference in deterimining the quality of PTFE. If you need to maximize the performance of your PTFE parts, including Rulon, Fluorosint, Ultraflon or even generic products, be sure you contact Tri Star Plastics first.  We can help you select the right material and the right process.

More and more companies are looking to save weight and costs by changing metal parts to plastic. Processing is always a big question since plastics can be molded, machined, extruded, etc. One method of making large plastic parts is through casting. This process is applied to parts that would be too costly to even machine out of plastic sheet or rod. Some cast nylons can be made up to 6 feet in diameter and weight hundreds of pounds. Several grades of nylon can be used in this process and the cost savings in both tooling and machining is phenomenol. Examples of cast nylon parts include large sheaves used on cranes, railroad locomotive components,  outrigger plates on trucks or trailers and large wear pads for boom cranes.

Next time you consider cost and weight reductions and need a durable, strong plastic, remember Tri Star’s cast nylon process.

Manufacturer’s of plain bearing materials will sometimes reference K factor which is a way of measuring the actual wear of the material over time. What is left out of the equation is how that K factor varies depending on the PV of the application. As an example, Delrin AF is a popular material for plain bearings. If you have an application wher your load is 45 psi and your speed is 100 feet per minute, your approximate K factor is 51. Take that same material and decrease the load to 20 psi and increase the speed to 350 feet per minute and your K factor increases to 70. That’s almost 40% greater wear. Another example is Ertalyte TX, a bearing grade PET from Quadrant. The K factor at 10 psi and 100 feet per minute is approximately 21. At 45 psi and 360 feet per minute the K factor increases to 464. So consider K factor as you do PV and coefficient of friction. These are numbers that are always relative to all of the surrounding factors in the bearing design.

Question: I am using Rulon J on a very small diameter shaft with a light load and medium speed condition. For some reason I am getting more stiction and friction than I expected. Any suggestions?

Answer: One of the issues with Rulon and any PTFE based bearing material is that there is an optimal load range to get the performance you desire. If the bearing is too lightly  loaded you will experience an increase in friction. That is a phenomenom common to PTFE. The thing to remember is the higher the load, the lower the friction. Reducing the surface contact area by either shortening the bearing or machining small reliefs on the ID of the bearing will increase surface loading. That will in turn reduce the friction. It may take some experimenting to get the result you want but it really does work!