The Advantages of Spin Molded PEEK Tubes for Split Ring Oil and Gas Seals

Because of its processability and its unique combination of chemical, mechanical, and thermal properties, PEEK (polyetheretherketone) is often used for demanding oil and gas applications including split ring seals that are machined from tube stock. It is important to note, however, that not all PEEK tubes are created equal. As with any plastic material, the method used to manufacture a PEEK tube plays a significant role in the performance of a component machined from it. Split rings, in particular, can be tricky. Design engineers must pay special attention to the residual stress that the tube molding process introduces, because it can impact the performance of a PEEK split ring seal. It is also important to recognize that the manufacturing method for a PEEK tube can affect material yield, scrap rates, and machining time for the finished split rings.


Figure 1 – The PEEK split ring on the left was manufactured from injection molded tube stock. When splits were machined into rings, they exhibited springing behavior, which resulted in the parts having to be scrapped. The PEEK split ring on the right was manufactured via spin molding. The extremely low levels of molded-in stress allowed the ring to retain its shaped after it was split.


Due to the favorable mechanical properties of injection molded PEEK tubes, oil and gas equipment OEMs and end-users regularly specify that PEEK parts for sealing applications be machined from tubes made via injection molding. While this ensures consistent and reliable performance for most applications, the residual stress resulting from the injection molding process can lead to machining difficulties for split rings. When the ring is split, stress may cause it to “spring” inward or outward to such a degree that it will not be suitable for use in the intended sealing application. This phenomenon is especially prevalent in filled systems such as 30% glass-filled PEEK and in large-diameter tubes.

Compression molding is an alternative manufacturing method to produce PEEK tube stock. Compression molded tubes, however, tend to have thick walls, which often creates waste. They also tend to have low tensile elongation, and this somewhat brittle behavior limits their use in split ring applications because brittle PEEK split rings may crack during installation.


Figure 2 – The PEEK split ring on the left was manufactured from the compression molded tube that it is resting on. Clearly the large wall thickness of the tube resulted in significant material waste from the OD and ID that was machined off. As shown, the finished ring exhibited brittle behavior and it split during installation. The PEEK split ring on the right was manufactured from a spin molded tube, which resulted in minimal material waste. It also exhibited more ductile behavior and it was easy to install without cracking.


To address these issues, Curbell Plastics offers spin molded PEEK tube stock shapes in unfilled and filled grades that meet major end-user oil and gas specifications. Spin molded PEEK is also known as spun cast PEEK, DCMS PEEK, or melt spun PEEK. Spin molded PEEK tubes offer a number of advantages for split rings when compared with tubes manufactured via other methods. These are listed below and summarized in the table below.

  • Spun cast tubes are manufactured via a proprietary melt spinning process that results in extremely low residual stress and excellent machinability for split rings. Because of this, split rings manufactured from spin molded tubes have less of a tendency to spring open or closed when the split is cut into the seal.
  • Spun cast tubes tend to exhibit relatively high tensile elongation, which allows for easy installation of the split ring without cracking.
  • Because spin molding is a batch process, the minimum order is just one tube. This gives end users greater purchasing flexibility and short lead times.
  • Tubes can be manufactured with customizable inner diameters with no additional tooling charge. This saves material waste and valuable machine time, and it reduces stress in the finished part. Wall thicknesses as thin as 1/8 inch can be achieved.

Curbell offers melt spun tubes up to 20 inches in diameter with a variety of filler packages for PEEK and other high-performance engineering plastics.



Low residual
stress. Low scrap
rates due to
High tensile
Easy to install
without cracking.
Low material
High yield.
Minimum order
is one tube.
Injected Molded
PEEK Tubes
Compression Molded
PEEK Tubes
Spin Molded
PEEK Tubes

This article provides general guidelines and is intended for informational purposes only. Because every situation is unique, many factors must be considered when selecting a material. It is the reader’s responsibility to conduct his or her own research and make his or her own determination regarding the suitability of specific products for any given application.

About the author

Dave Seiler is Business Development Manager at Curbell Plastics who focuses on high-performance engineering plastics. He has literally been involved with plastics his whole life (his first word was “Kynar”) and spends much of his free time reading academic literature on a diverse base of polymer materials. Yeah, he’s that guy. Dave is ready to take your call, set up a webinar, or travel to your facility to discuss your unique application needs and help identify cost-effective engineered plastic candidate materials. Contact Dave.

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