Q:  I’m designing laboratory equipment that will be utilized at 325-400F and I need screws that have immunity to organics and low concentrations of acids, as well be FDA approved. Would that be a ceramic?

A:  You probably don’t want a ceramic screw. Though they are corrosion resistant and offer high purity, they are typically reserved for extreme temperatures. In addition, ceramics typically aren’t FDA approved. For your situation, you might want to consider PEEK screws. They offer an economic solution to temperatures up to 500F. In addition PEEK screws are an FDA approved. PEEK screws also offer excellent corrosion resistance to organics, salts, oils, and moderate concentration acids and bases. Depending on the strength you need, PEEK may be an excellent choice and are also available in reinforced glass and carbon if extra strength is needed.

Q: I have an application where I need pan head, insulating, high vacuum compatible, screw. It will also need to be usable at a service temperature up to 150C. Do I need a ceramic?

A: Though ceramics are excellent insulators, they are usually only used for high temperature applications, where polymers are not compatible. For example Alumina ceramic has a usable limit of 1650°C. Since this is a relatively low temperature, your best bet is a polymer like PEEK or PVDF screws. Depending on the strength, inertness and chemical resistance needed will determine which polymer PEEK or PVDF is best for your fasteners.

Q: How does creep occur and what polymer is best to prevent this?

A: Creep is the permanent deformation of the material caused by constant load or stress, usually accompanied by constant high temperature. This can result in permanent changes of the fastener shape as well weakening and fracturing of the material. Depending on your conditions, it may not be possible to completely prevent this but it certainly could be minimized with creep resistant polymer fasteners like PEEK. PEEK screws are extremely creep resistant and could be further strengthened with glass or carbon fiber reinforcement if needed.

Q: Need to replace a standard steel washer with a non-conductive material.

A:   After speaking with this customer, I learned that he needed to replace a standard steel washer with a non-conductive material for a critical bolted joint. These washers needed to withstand 70°C temperatures while maintaining preload. He had been experiencing creep with non-metal washers  at elevated temperatures and needed to minimize this issue.

While Vespel (polyimide) fasteners may have superb creep properties, it may be overkill for your application.  Vespel is also very expensive. I think you may be better off considering PEEK fasteners.  It is much more affordable that Vespel fasteners and could hold up to the 70C temperatures easily. It is also very well known for excellent creep resistance (as well as chemical resistance and temperature resistance). To further improve the creep resistance, glass and carbon fibers can be added.  Glass fiber and carbon fiber will improve stiffness and creep resistance.

Q: Can you provide alternative for a nylon pan head screw?

A:   Here the customer needed an alternative to nylon. Resistance to electrical fields was a primary concern. In addition, he also needed a material that had a better dissipation factor than nylon. In this situation we would recommend PEEK. It has similar properties with regards to its electromagnetic properties, better dissipation.