Custom PPS Injection Molding Factory

What PPS actually is

PPS ist ein hochtemperaturbeständiger, teilkristalliner technischer Kunststoff, der für elektrische Bauteile, Automobilteile im Motorraum sowie für Anwendungen eingesetzt wird, die eine hervorragende chemische Beständigkeit und Dimensionsstabilität bei anhaltend hohen Temperaturen erfordern.

Chemically, PPS belongs to the Polyphenylene sulfide - engineering family. The polymer chain structure — linear and semi-crystalline — is what sets its shrinkage behaviour, temperature ceiling and melt flow. Like every thermoplastic, its properties are the result of processing history as much as of the resin grade itself: slow cooling increases crystallinity and brittleness, fast cooling locks in an amorphous structure and a different shrink. Resin suppliers offer PPS in commodity, reinforced and specialty grades, and the first selection step is to pick the right family before settling on a specific TDS.

Processing window

Run PPS at a melt temperature of 280–320 °C and a mould temperature of 130–150 °C. The high end of the melt range is for thin walls and long flow paths where the resin would freeze before packing; the low end for thick sections where residence time is already long and thermal degradation is the real risk. Above 320 °C the sulphide linkages begin to break, and the first sign is a faint yellow tint and a measurable drop in impact strength rather than a visible surface defect.

The mould temperature is not optional and not optional at the high end. PPS crystallises as it cools, and the mould temperature controls how much crystallinity develops. A mould at 100 °C or lower produces a fast-cooled, low-crystallinity part that looks fine but creeps and softens above 150 °C in service. A mould held at 130–150 °C produces a high-crystallinity part that is dimensionally stable up to 200 °C. Raising mould temperature adds cycle time — roughly 1–2 seconds per 10 °C — and that is why the specification has to include the mould target, not just a part property.

Drying is required. PPS absorbs moisture readily from ambient air, and at 300 °C that moisture reacts with the melt. The visible symptom is silver streaks and burn marks that look like overheat and cannot be fixed by lowering the barrel temperature. Three hours at 120 °C in a desiccant dryer is the normal starting point, and the dew point should be verified at the hopper not at the dryer controller.

Residence time matters as much as temperature. A large barrel running a small shot leaves the melt hot for too long, and the degradation is cumulative — the second shot is more degraded than the first, the tenth more than the second. Size the shot to occupy 40–70 % of the barrel capacity, and purge thoroughly between grades, especially when moving from a lower-temperature commodity to PPS.

Shrinkage and dimensional tolerance

Design the tool to 0.3–0.7 % shrinkage for unfilled PPS, and up to 0.3 % for glass-filled grades. PPS is semi-crystalline, so flow-direction and cross-flow shrinkage can differ, especially in glass-filled grades where the fibres orientate in the melt flow direction and lock the shrinkage there. That means the correction after first article is not always a uniform offset — it can be directional, and the CMM measurement plan has to cover both directions.

The mould temperature is the single largest influence on crystallinity and therefore on shrinkage. A part cooled in a 140 °C mould develops higher crystallinity and shrinks more uniformly than one cooled in a 100 °C mould, which freezes the amorphous structure and can show warpage. For thin-wall electrical connectors the difference between a 120 °C and a 150 °C mould can decide whether the pin centres fall inside the 0.05 mm tolerance required for a mating fit.

For glass-filled PPS the shrinkage drops to 0.1–0.3 % because the fibres carry the load as the matrix tries to contract. The trade is anisotropy: shrinkage is much lower in flow direction than cross-flow direction on a long thin part. The gate position decides how the fibres align, so the gate has to be placed to minimise the differential, not just to fill the cavity. A side-gated long blade will warp; a centre-gated part keeps the fibre orientation more balanced.

We cut the first sample with a nominal allowance and measure the first shots on a CMM before touching the cavity. Correcting a tool from measured parts is cheaper and more reliable than correcting it from a data-sheet range, and the measurement record is what the next tool for the same grade starts from.

What PPS asks of the mould steel

PPS demands three things from the tool at the same time: heat resistance, abrasion resistance and chemical compatibility. That is why its default steel family is H13 / S136, not the P20 that serves commodity resins. A pre-hardened P20 block runs PPS poorly even for short runs, because the mould temperature (130–150 °C) is already close to the tempering temperature of P20 and the block softens and loses polishability quickly.

Abrasiveness is Hoch and unavoidable. Glass-filled PPS — the grade most commonly specified — carries 30–40 % glass fibre by weight, and the fibres erode the gate, the vent land and any narrow restriction that increases melt velocity. An unfilled PPS runs low abrasion, but the high-temperature requirement still rules out P20 for anything longer than a prototype campaign. H13 hardened to 48–52 HRC is the default production choice; replaceable gate inserts in DC53 or Vanadis take the brunt of the wear and are cheaper to change than the whole tool.

Chemical attack is Moderate but cumulative. PPS itself is chemically inert, which is why it resists solvents and acids so well. The corrosion comes from additives — flame retardants (brominated or phosphorus-based), lubricants and colourants can release acids at 280–320 °C that corrode carbon steel over thousands of shots. The symptom is not pitting on the cavity surface but a brown discoloration that builds up and transfers to the part, and a die build-up that thickens the vent lands. A passivated S136 stainless block avoids this completely and is the correct choice for any PPS tool that may see FR grades.

Cooling layout has to carry more heat than a commodity tool. PPS runs at 130–150 °C mould temperature, but the melt enters at 280–320 °C, so the heat load is higher than a PP or ABS tool. Baffles and bubblers are not optional on thick sections, and the circuit should run through the gate bushing first since the gate absorbs more heat per unit volume than any other spot in the cavity. Insulate the tool from the platens so the heat stays in the cavity where it belongs, not conducted away by the press.

Applications where PPS wins — and where it gets replaced

PPS occupies the band between commodity engineering resins (PA66+GF, PA46) and ultra-high-temperature resins (PEEK, PES). Its characteristic combination — sustained heat resistance up to 200 °C, excellent chemical resistance to fuels, acids and brake fluids, low creep under load, and a moulded surface that needs little or no post-processing — makes it the default choice for three application families: under-hood automotive electrical components, industrial pump and valve components that see aggressive chemicals, and consumer appliance parts that need heat resistance at moderate stiffness.

The biggest application today is automotive under-hood electrical connectors and housings. These parts need to survive 150 °C continuous service, occasional 180 °C peaks from engine heat, exposure to fuel vapour and road salt, and must hold pin-centre tolerances within ±0.05 mm for the connector mating to work. PA66+GF reaches 150 °C HDT but absorbs moisture and swells — the pin centres shift when humidity changes. PPS does not absorb moisture, so the tolerance holds year round. The tool trade is H13 gate inserts for abrasion and S136 cavity blocks if flame-retardant grades are specified.

PPS gets replaced most often by three alternatives. PA66+GF when the heat requirement is below 140 °C and moisture absorption can be designed around — PA66+GF is cheaper, processes at 280–300 °C melt temperature vs 300–320 °C, and needs a P20 or 718H tool rather than H13. PEEK when the service temperature exceeds 220 °C or the part needs to survive repeated steam sterilisation — PPS creeps above 200 °C; PEEK holds its modulus to 250 °C. PA46 when heat resistance is the main driver and cost matters — PA46 reaches 180 °C HDT, processes at 290 °C, and while it absorbs moisture more than PPS, its cost is half or less.

One subtle swap is PPS vs PPSU. PPSU is an amorphous polysulfone that processes at a similar melt temperature (330–350 °C) and mould temperature (120–140 °C), but it is transparent, tough, and survives repeated autoclaving. PPS is semi-crystalline, opaque, and brittle in thick sections without glass fibre. The tool for PPSU is similar to PPS — hardened H13 or S136, hot mould — but the part design is different because PPSU has lower shrinkage (0.5–0.7 % amorphous) and is notch-sensitive like PC.

Design guidelines

• Wall thickness. Keep nominal walls in the 1.5–3.0 mm range for most grades; thicker walls invite sink and shrink, thinner walls risk incomplete fill. Avoid abrupt thick-to-thin steps.
• Ribs. Rib thickness around 0.5–0.6 of the adjoining wall keeps the rib from sinking the face. Space ribs so the gaps between them can be cooled and vented.
• Draft. A minimum of 1° on smooth walls and 1.5–2° on textured walls. Add more where the draw is deep; the part drags most where the texture is deepest.
• Fillets. A fillet at least 0.5 × wall thickness at internal corners avoids a stress concentration that turns an impact into a crack.
• Bosses. Use a wall around 0.6 of the nominal wall, add gussets rather than thickening the boss, and keep the boss height under about 2.5 × diameter.

Cost and cycle-time effect

PPS sits in the middle of engineering thermoplastics on price; the premium over commodity grades buys temperature and stiffness, not a long cycle. The cost triangle is material price, cycle time and yield. A cycle that is a few seconds shorter on a high-volume part is worth more than the material saving, because the press hour is the dominant cost.

Yield is where the cheap resin can become expensive. Wet resin produces silver streaks and scrap; a cold mould produces weak weld lines that fail in the field; a badly vented tool produces burn marks that need rework. None of these are material costs, and all of them are more expensive than the few percent saved by buying a lower grade.

Common selection mistakes

• Using a global shrink number without a measurement loop. The nominal allowance is a starting point, not an answer. Correct from the first measured shots.
• Specifying PPS outside its temperature ceiling. Service temperature above the rated HDT changes the part behaviour over its lifetime, even if it passes first article.
• Ignoring abrasion when the resin is filled. A pre-hardened block in a glass-filled job wears at the gate long before it shows bulk wear.
• Not reading the resin TDS for drying and moisture. Drying is not optional for a hygroscopic grade, and skipping it shows up as a surface defect that looks like a moulding problem.