Polypropylene (PP) is the workhorse plastic that becomes crates, caps, live hinges, and hot-fill packaging. To form strong parts you must heat PP above its melting range yet avoid thermal damage. This article explains that range, factors that shift it, and simple controls to hit the right window every time you mold, extrude, or weld PP.
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- Why knowing the PP melting window is critical
- Typical melting range and thermal terms
- Compare homopolymer, copolymer, and filled grades
- How crystallinity shifts the melt
- Injection molding settings to watch
- Extrusion and thermoforming heating zones
- Welding and heat-seal guidelines
- Measurement tools for accurate temperature
- Common thermal defects and fast fixes
- Energy cost and cycle time chart
- Need integrated production services?
- Quick checklist before your next run
Knowing The PP Melting Window Is Critical
PP melts over a span, not a single point. Cut heat too low and pellets stay solid, causing poor knit lines. Push heat too high and the polymer degrades, yellowing and losing impact strength. A clear grasp of the correct range helps you run parts that pass dimensional, aesthetic, and mechanical tests in one cycle.
Typical Melting Range And Thermal Terms
DSC (Differential Scanning Calorimetry) measures the melt of PP as a peak, not a step. Most commercial PP grades start melting near 150 °C and fully fuse by 170 °C. Glass-transition temperature (Tg) is much lower—around -10 °C—and tells you when PP starts to soften at cold extremes.
| Thermal term | Symbol | Typical PP value | Meaning in practice |
|---|---|---|---|
| Glass transition | Tg | -10 °C | Below this, PP gets brittle |
| Onset melt | Tm onset | 150 °C | First crystals start to melt |
| Peak melt | Tm peak | 165 °C | Largest heat flow, material fully fluid |
| Degradation | Td | >260 °C | Oxidation, chain scission begin |
Compare Homopolymer, Copolymer, And Filled Grades
Different PP families melt slightly higher or lower because of chain structure and additives.
| Rang | Tm onset °C | Tm peak °C | Why range shifts |
|---|---|---|---|
| PP homopolymer | 152 | 165 | Straight isotactic chains |
| Random copolymer (3 % ethylene) | 148 | 160 | Ethylene disrupts crystals |
| Impact copolymer (ethylene rubber) | 146 | 158 | Rubber lowers fusion point |
| 40 % talc-filled PP | 155 | 168 | Talc acts as nucleant |
| Glass-fiber PP | 154 | 167 | Glass stabilizes heat flow |
How Crystallinity Shifts The Melt
More crystals raise the melt peak. Cooling rate during molding sets final crystallinity.
Cooling Fast (30 °C/s) → Final crystallinity 35 % → Lower heat-deflection temp Cooling Medium (10 °C/s) → Final crystallinity 45 % Cooling Slow (2 °C/s) → Final crystallinity 55 % → Higher heat-deflection temp
Slow cooling increases stiffness but may warp thick parts. Balance cycle time and mechanical needs.
Injection Molding Settings To Watch
Use the table as a baseline, then dial in for your press.
| Zone | Homopolymer °C | Copolymer °C | Opmerking |
|---|---|---|---|
| Feed throat | 40 | 40 | Prevent bridging |
| Rear barrel | 175 | 170 | Start melt |
| Mid barrel | 185 | 180 | Smooth flow |
| Front barrel | 195 | 190 | Fill gate |
| Nozzle | 200 | 195 | Avoid drool |
| Mold surface | 30–50 | 20–40 | Higher temp = better flow lines |
A 10 °C change in front barrel can fix short shots or flash without altering screw rpm.
Extrusion And Thermoforming Heating Zones
Pipes and sheets run through four heaters. Gradual ramps stop burning.
- Feed zone 170 °C
- Compression zone 180 °C
- Meter zone 190 °C
- Die head 200 °C
Sheet for thermoforming needs uniform melt history. After extrusion let the sheet cool, then reheat by IR panels to 155–165 °C just before forming.
Welding And Heat-Seal Guidelines
Hot-plate welding uses two fixtures at 210 °C. Press parts against the plate for 5 s, pull back, then push parts together at 0.6 MPa for 3 s cool. For ultrasonic welding aim for an amplitude of 40 µm and set weld time to 0.2 s/mm joint length.
Measurement Tools For Accurate Temperature
Infrared guns read only surface. A thermocouple in the melt channel reads true.
| Tool | Response time | Nauwkeurigheid | Use tip |
|---|---|---|---|
| K-type screw-in probe | 1 s | ±1 °C | Thread into barrel |
| Infrared sensor | 0.1 s | ±2 °C | Paint surface matte black |
| DSC lab test | 20 min | ±0.2 °C | Grade certification |
Common Thermal Defects And Fast Fixes
If parts fail, link symptom to root cause quickly.
| Defect | Visual symptom | Likely cause | Quick fix |
|---|---|---|---|
| Splay | Silver streaks | Moisture flashes | Dry resin 2 h at 80 °C |
| Char specks | Black dots | Melt over 240 °C | Lower rear barrel 10 °C |
| Sink marks | Dents on ribs | Poor pack pressure | Boost hold 15 % |
| Warp | Bent plate | Uneven cooling | Raise mold to 40 °C both halves |
Energy Cost And Cycle Time Chart
Melt Temp °C | Cycle Time s | kWh/kg 160 | 32 | 0.53 170 | 28 | 0.55 180 | 26 | 0.60 190 | 25 | 0.68 200 | 25 | 0.78
Slightly higher temps drop cycle but raise energy and degrade color. Most plants settle at 180 °C for net savings.
Need Integrated Production Services?
Our facility cuts, molds, and welds PP and other polymers with turnkey metal, sheet, and machining steps. Send CAD today for a unified instant quote online.
Quick Checklist Before Your Next Run
- Verify grade melt peak on the data sheet—do not guess.
- Dry resin if copolymer has moisture >0.05 %.
- Set barrel temps 10–15 °C over Tm peak; avoid >230 °C.
- Balance mold cooling to within 2 °C left to right.
- Log energy and scrap—small heat tweaks show fast ROI.
Follow these steps and your polypropylene parts will run cool, strong, and right the first time.
