PP樹脂の受注生産型成形工場

What PP actually is

Polypropylene is the highest-volume commodity resin — low cost, low density, good chemical resistance and a forgiving process window make it the default choice for packaging, caps, containers and automotive interior trim.

Chemically, PP belongs to the Polypropylene - commodity 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 PP 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

The typical process window runs with a melt temperature around 200-240 °C and a mould temperature around 40-60 °C. Inside that band the resin fills, packs and ejects predictably. Outside it, the failure modes are specific rather than vague.

Above the recommended melt temperature, PP can degrade thermally — the symptom is silver streaking, a mottled surface and a measurable loss of impact strength that the part carries into service. Long residence time in a large barrel running small shots is as damaging as a direct temperature overshoot, because the resin sits at temperature longer than intended. Below the lower bound, the melt freezes before it has fully packed the cavity: the part shows visible weld lines, flow marks around the gate and a surface that reads differently across the flow path.

Drying is usually not required. PP absorbs moisture very little, so drying is usually unnecessary unless the resin has been stored in a particularly humid environment or has become contaminated. When in doubt, a quick hopper dry adds schedule but removes a common yield risk. The resin supplier TDS always takes precedence over a generic figure on drying and moisture content.

Shrinkage and dimensional tolerance

Design the tool to 1.5-2.0 % shrinkage and expect the actual figure to sit at a specific point inside that band rather than the middle. The value is set by wall thickness, flow length, gate position, packing pressure, hold time and mould temperature. Thick sections shrink more because there is more melt to contract; long flow paths lose pressure before the cavity is packed, so the far end shrinks more than the gate end.

PP is amorphous, which means shrinkage is comparatively even in flow and cross-flow directions. That is why a single global shrink factor is a starting point, not an answer. On a long, thin part, the difference between the gate end and the far end can exceed the tolerance you are trying to hold. The fix is either to gate where the pressure can reach the whole part, to add a packing profile, or to accept that the tight tolerance belongs on the features near the gate and not on the overall length.

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 PP asks of the mould steel

The default steel family for unfilled PP is P20 / 718H, 30–38 HRC. That choice is driven by three properties in combination: abrasiveness, mould temperature and the risk of chemical attack.

Abrasiveness is Low. The resin carries no abrasive filler, so the gate, runner and vent lands are not eroded by the melt, and a pre-hardened block is normally sufficient. The one common mistake is to skip replaceable gate inserts on a tool that may later switch to glass-filled PP — a P20 gate can lose 0.05 mm in radius after a few thousand shots of PA+GF, and the gate wear shows up first as a weld line shift then as cosmetic scrap.

Chemical attack is Negligible for standard homo- or co-PP. The resin is non-corrosive, so carbon steel or pre-hardened steel is safe without passivation. Where PP becomes corrosive is when flame retardants or certain nucleating agents are blended in — these can release HCl or other acids during moulding, and the symptom is not pitting on the cavity but discoloration of the moulded part and a visible die build-up that thickens over runs. The fix is a passivated stainless insert at the gate, or a switch to S136 for highly loaded tools.

Cooling layout is where PP tools fail, not where the steel grade fails. PP wants a mould at 40–60 °C, and a tool designed for ABS or PE will run unevenly when set to it. Baffles and bubblers belong where the heat actually concentrates — at thick bosses, at the gate, and where the part joins the runner system — not spread evenly around the cavity. A tool that cools the thin walls and the thick boss at the same rate will hold a flat part and still show a sink mark on the boss. The calculation: place each baffle to within 1.5 × its diameter of the centre of the thickest section, and run the circuit through the gate bushing first — the gate absorbs more heat per unit volume than any other spot in the cavity.

Applications where PP wins — and where it gets replaced

PP is chosen where the part needs a rigid cosmetic housing with a good surface at low cost. Automotive interior and exterior trim, appliance housings, electrical connectors and medical device disposable components all sit in its band. Its density of 0.90 g/cm³ means the resin price per cubic centimetre is genuinely low, and its forgiving process window means a first article is usually a valid first article — not an expensive exercise in what not to do.

Three features make PP different from the next commodity up, which is ABS: density (PP is lighter), shrinkage (PP is semi-crystalline so it shrinks more and less evenly), and chemical resistance (PP tolerates acids and bases that ABS does not). For a battery tray, a chemical container or a part that sits in a dishwasher every day, the resin choice is PP because ABS would fail. For an electrical housing that needs a matte textured surface across the whole part, ABS is chosen because PP's semi-crystalline structure makes gloss recovery after moulding inconsistent on a textured wall.

PP gets replaced most often by three alternatives. PC when the part needs a tight tolerance above 100 °C service temperature or impact resistance that won't brittle with UV. ピーピーエス when the part sits in a continuous 180–220 °C environment — PP's HDT drops sharply near 100 °C and long-term creep becomes visible in days. PA66+GF when stiffness at high temperature matters more than finish or chemical resistance. Each replacement changes the tool: PC needs a harder cavity (NAK80 minimum), PPS runs hot and corrosive so S136 or 2316 before P20, PA66+GF is abrasive so gate inserts in H13.

One subtle swap is PP vs HDPE. Both are polyolefins, both low-cost, both low-wear tools. The practical trade is heat: PP's HDT around 100 °C vs HDPE's around 80 °C. The tool trade is shrinkage — HDPE shrinks more (1.5–2.5%) and more unevenly, so the correction from first article will be a larger uniform offset than with PP. For a part that must survive a dishwasher cycle at 90 °C, PP is the answer; for a bottle cap that never sees above 60 °C, HDPE might be the cheaper moulded part despite slightly higher shrinkage tool correction.

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

PP sits at the low end of commodity thermoplastics on price, and its cycle is short because it does not need a hot mould. 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 PP 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.