Polyphenylene Sulfide (PPS)

Solvay’s 40% glass-reinforced Ryton R-4 has been a dominant workhorse among PPS engineering materials, serving innumerable applications across a broad range of industries for five decades. However, for most of its history, it’s primary use had been for injection-molded parts. Its potential was far more limited in machining applications where part configurations were either too complex or whose quantities were too small for injection molding tooling investments.  The limiting factor had been a lack of high-quality machinable stock shapes that carried the resin certifications for machined parts and prototypes required by many industries.

To fill this need, Drake invested in the technology to produce 40% glass-reinforced Ryton R-4 PPS in a wide range of machinable rod and plate sizes. Because Drake’s Ryton R-4 PPS shapes are extruded from 100% virgin Ryton R-4 240 resin, properties of the material are at an optimal level, and material certifications to ASTM D-4067 PPS 000G40 and ASTM D-6358 PPS 011G40 are available.

Glass fiber-filled PPS blends are commonly offered in the form of compression molded shapes or ram extruded rod. PPS powder is blended with short glass fibers in-house. Short fibers are used to prevent fiber bundling and glass agglomerations in the blend. These short fibers act more as fillers compared to the reinforcement that long fibers impart. The PPS – glass fiber blend is then heated close to the melting point of PPS (545⁰F/ 285⁰C). Under high pressure, the glass fiber – powder blend is consolidated and fused into the desired shape.

Since the raw material form of PPS used in compression molding and ram extrusion is a variable powder blend, it cannot be certified as such to ASTM standards for PPS resin. Moreover, powder and short glass fiber blends would not likely meet the ASTM property requirements for 40% glass-reinforced PPS.

A characteristic weakness of ram extruded rod is low elongation. The process tends to form ‘poker chip’ stratifications between every press cycle as more powder blend is added and pressed against the solidified material ahead of it.  The effect is evident when flexing a ram extruded rod into a circular shape. Typically it will break along one stratification and expose others.

In addition to the drawback of short lengths, the compression molding process is prone to variations that lead to inconsistent quality and physical properties.  They include blend ratio discrepancies, non-uniform mixing, drying and moisture control problems, air entrapment and inaccurate control of pressures and temperatures. Also, given the inherently low thermal conductivity of polymers and the low bulk density of the powder-glass fiber blends, heating them to their molding temperatures often requires long thermal cycles that can degrade the PPS polymer. This is especially true when producing larger sizes. In addition, variations in forming pressures due to frictional losses along the sidewall of the mold often yield compression molded shapes with different densities and physical properties in the top and bottom vs. the middle section.

Melt extrusion by contrast is a continuous process using the resin supplier’s uniformly pre-compounded 40% glass fiber reinforced Ryton R-4 resin which is  tested and certified in pellet form. The pellets are melted uniformly in the extruder barrel.  Under Drake’s rigid conditions for controlling internal pressure and output rate, the extruder screw moves the molten Ryton R-4 PPS through a die in the desired stock shape. The extruded shape is then uniformly solidified under tightly controlled cool-down conditions that minimize internal stresses.

The resultant Ryton R-4 PPS stock shapes produced by Drake’s extrusion process exhibit consistent properties and uniform machinability throughout the cross-section and from size to size.