A polypropylene data sheet usually carries four temperatures — a melt point near 165°C, an HDT near 85°C, a Vicat near 100°C, and a continuous-use figure near 90°C. To a buyer scanning the COA, they all read like “the heat limit.”
Only one of them tells you how hot the part can run on the line, and reading the wrong one is how a part gets specified for service it can’t survive. The melting point is the easy number to quote; the useful work is knowing which of those four governs which decision.
What Is the Melting Point of Polypropylene by Grade?
Polypropylene melts at roughly 160 to 170°C (320 to 338°F), and where a given grade lands inside that band is set by how it’s polymerized. Isotactic homopolymer sits at the top — about 162 to 170°C — because its regular chain packs into 40 to 60% crystallinity. A homopolymer reference grade like T30S anchors that high end.
Random copolymer (PP-R) runs lower, roughly 130 to 165°C depending on how much ethylene is built into the chain. The comonomer disrupts the crystal lattice, drops crystallinity to 30 to 40%, and pulls the melt point down with it.
PetroChina Dushanzi T4401, a random-copolymer pipe grade, sits at this lower-Tm end — which is exactly why its PP-R processing window shifts down relative to a homopolymer. Impact (block) copolymer lands in between, around 150 to 165°C.
The grade qualifier carries more weight than the headline range. If a spec just says “PP, Tm 165°C,” confirm whether it’s homopolymer, random, or impact before you treat that number as fixed. The grade itself moves the melt point by 20 to 30°C.
Which Number Governs Heat Resistance: Melt Point, HDT, Vicat, or Service Temperature?
The continuous service temperature — not the melt point — sets how hot a PP part can run long-term, and for unfilled PP that ceiling is roughly 80 to 100°C. A part held near its 165°C melt point sags and creeps under its own weight long before anything liquefies.
That melt-vs-service gap is the headline of where PP sits among plastics, and the PP-among-plastics positioning covers it in full; here the job is to separate the four numbers a data sheet actually gives you.
Each governs a different limit, measured under a different load:
| Thermal number | Typical value (unfilled PP) | Test condition | What it governs |
|---|---|---|---|
| Melt point (Tm) | 160-170°C | DSC peak | Processing floor — where the resin starts to flow |
| Heat deflection (HDT) | ~85°C at 1.8 MPa | ASTM D648 / ISO 75, 3-point bend | Load-bearing service limit |
| Vicat softening (VST) | ~5-15°C above HDT | ASTM D1525 / ISO 306, 10N or 50N needle | Surface softening, safe ejection |
| Continuous service | 80-100°C | Long-term exposure | Sustained-use ceiling |
HDT under ASTM D648 loads a bar in 3-point bending at 0.45 or 1.8 MPa and heats it until it deflects 0.25 mm. The 1.8 MPa figure for unfilled PP is about 85°C — and that is the number a design engineer should quote for a part carrying load, not the melt point.
The lighter 0.45 MPa load gives a higher reading because there’s less stress to overcome.
Vicat measures something narrower: a flat needle pushes 1 mm into the surface under a 10N (Method A) or 50N (Method B) point load. It marks where the surface starts to soften — useful for setting a safe ejection temperature, where you want the skin firm enough to demold without warping.
It typically runs 5 to 15°C above HDT for the same grade.
The compound changes all of this. Glass-fiber reinforcement pushes HDT from the ~90-120°C unfilled range up to 130-145°C, so the governing service number depends on the formulation, not just the base polymer.
What Barrel and Mold Temperatures Does a Molder Set for PP?
A molder runs the melt at 200 to 250°C (400 to 470°F) — 30 to 90°C above the melt point, not at it. A typical producer starting point (Formosa Formolene) puts barrel zones at roughly 199-227°C rear, 199-232°C middle, and 199-250°C front, with the melt itself checked at the nozzle by airshot.
The superheat isn’t waste. A polymer sitting right at its melt point is only partially molten and far too viscous to fill a cavity cleanly — you get short shots, weak weld lines, and locked-in stress. Adding 30 to 90°C drops the viscosity enough for consistent flow.
Mold temperature is the opposite end of the window: 15 to 50°C (60 to 120°F). The tool runs cold to set the part, and that cooling rate also fixes final crystallinity. Run the mold warmer and you let more crystal structure develop, which stiffens the part but slows the cycle.
Grade and filler shift the window the same way they shift Tm. Neat grades flow around 190-210°C, talc-filled higher, glass-filled up near 230-240°C. Don’t carry one grade’s setpoints to another without re-checking — a random copolymer’s lower Tm means its window opens at a lower barrel temperature than a homopolymer’s.
Reading the Four Numbers in Practice
Quote the melt point with its grade — homopolymer near 165-170°C, random copolymer lower — but never use it to judge service heat. For a load-bearing part, the number that matters is HDT (~85°C at 1.8 MPa for unfilled PP); for sustained exposure, it’s the 80-100°C continuous-service ceiling; for demolding, it’s Vicat.
The melt point only tells you where processing begins. The COA gives you four numbers; the application tells you which corner of the window you actually need.
The single most expensive misread is treating a 165°C Tm as a service rating — a part run anywhere near that figure deforms while it’s still solid. Match the limit to the load, not to the largest number on the sheet.