True Melt Temperature in Extrusion

If you ask ten people on the floor what the melt temperature is, you'll usually get two wrong answers before you get the right one. Let's clear this up.

The Two Numbers That Get Mistaken for Melt Temp

The melt display readout. That number on your extruder panel isn't a barrel zone thermocouple — it's a melt bolt, threaded into the steel at the screw tip location, where the melt exits into the adapter. It tells you how hot that steel is at that one spot. That's it. It is not measuring the plastic. The heaters are doing their job holding the barrel at a set point, but there's no fixed relationship between how hot that steel is and how hot the plastic actually is once it picks up shear heat inside the channel. That shear heat never touches that bolt.

The IR gun reading. Point an infrared gun at the extrudate coming out of the die and you're reading the surface. The surface starts cooling the second it hits the air. By the time you pull the trigger, you're already reading something lower than what's happening in the middle of that stream.

The Only Way to Get the Real Number

Stick a melt probe into the extrudate right after it leaves the die, deep enough to get past the cooled skin and into the core of the flow. That's your true average melt temperature. No shortcuts.

Rule of thumb you can count on:

•       True melt temp will usually run higher than your melt bolt reading

•       True melt temp will usually run higher than your IR surface reading

How much higher depends on your process. That's exactly why guessing is dangerous. Take the time to understand what's actually driving that number.

What Actually Changes Your Melt Temperature

1. Shear. This is the heat generated by the plastic getting worked between the screw and the barrel wall. Run the screw faster or run a thicker material and you generate more heat right there in the channel, no matter what the heater bands are set to. This is usually the biggest reason your true melt temp runs hotter than your barrel setting.

2. Barrel conduction. This is the heat coming in through the barrel wall from your heater bands. It's slower than shear heat and it depends on good metal to metal contact. Worn heater bands or bad zone control means less heat is actually getting into the plastic than your controller display would have you believe.

3. Compression and dwell time. How hard the screw compresses the material and how long it sits in the barrel both affect how much heat builds up and how evenly it spreads out. Short dwell time can leave you with hot spots and cold spots even if your average number looks fine.

4. Screw design. Flight depth, compression ratio, mixing sections, overall length. Two different screws running the exact same barrel settings can hand you two very different true melt temperatures.

5. Screw and barrel wear. As the flights and the bore wear down, clearances open up. Material starts slipping past instead of being pushed forward the way it should. That changes your shear and your residence time, and it can push your melt temp up or make it swing around unpredictably. Worn tooling is one of the most overlooked reasons melt temp drifts over time.

6. The resin itself, and its melt index. A lower melt index resin is thicker and generates more shear heat at the same screw speed than a higher melt index resin does. If your incoming lot has a different MI than what you dialed your settings in for, your true melt temp can shift even though you haven't touched a single control.

Can You Predict Melt Temp With a Formula? Sort Of.

There are formulas engineers use to estimate melt temperature before ever running material. Worth knowing, but treat them as a starting point, not gospel.

The basic heat balance. In plain terms: heat coming in from the barrel, plus heat generated by shear, has to equal the heat needed to bring your throughput up to melt temperature.

That's it. It's just an energy balance. Whatever heat goes in has to show up as a temperature rise in however much plastic you're pushing through per hour.

The shear heat piece. Since shear is usually the biggest wildcard, there's a separate estimate for it: shear heat is roughly equal to the material's viscosity, times the shear rate squared, times the volume being sheared.

The catch is viscosity itself changes with temperature and shear rate, so this isn't a plug and play number. It's a moving target that has to be solved alongside everything else.

Zone by zone screw models. More detailed methods break the screw into the feed section, the compression section, and the metering section, and estimate the temperature rise through each one using the actual channel geometry. These can sketch out a melt temperature profile down the length of the screw instead of just spitting out one number at the end.

Why the Formulas Fall Short

Every one of these calculations leans on numbers that are themselves estimates:

•       Viscosity numbers come from a lab test on a sample lot, not necessarily the resin in your hopper today

•       Heat transfer through the barrel wall is assumed to be ideal, not measured on your actual worn equipment

•       The math usually assumes steady state and even mixing, when real melt streams have hot and cold pockets a single average number can't show you

•       None of it accounts for screw wear, barrel wear, or a feed that isn't perfectly consistent

The formulas tell you what melt temperature should be, based on assumptions that are never perfectly true in the real world. They're genuinely useful for designing a new screw or sanity checking a startup condition. But an actual probe reading, taken in your actual extrudate, on your actual screw, with your actual material, at this actual moment, is the only number you can fully trust. Use the formulas to plan. Use the probe to know.

Where Should You Be Targeting Melt Temp?

This depends on whether you're running a semi crystalline resin or an amorphous resin, because they behave completely differently as they heat up.

Semi crystalline resins (PE, PP, nylon, acetal, PET) have a real melting point. Below that point the material is a solid no matter how soft it feels. You need enough heat above the melt point to fully melt every last bit of crystal structure, or you'll get gels and unmelt in your extrudate.

General target: melt point plus roughly 60 to 110 degrees Fahrenheit — and treat the top end as a floor, not a ceiling, for high-shear or high-output processes.

Process matters as much as resin here. Blown film, pipe, and wire coating tend to run toward the low end of that band because you need melt strength and controlled draw-down or fast wall cooling. Cast film, fiber spinning, and high-output profile lines need to run well above it — commonly 130 to 160°F over melt point — because of the added shear and flow length. Treat the range as directional, and let your specific process and grade move you within it or past it.

Some rough real world numbers as examples, not gospel:

•       Polyethylene melts around 230 to 275°F. Real-world extrusion targets typically run 325 to 425°F, which works out to roughly 75 to 150°F above melt point depending on grade and process

•       Polypropylene melts around 330°F. General profile extrusion runs 390 to 450°F — about 60 to 120°F above melt point — while cast film and fiber lines run hotter still, up to 490°F, because of the added shear and output

•       Nylon 6 melts around 425°F. Real-world targets typically run 480 to 500°F, roughly 55 to 75°F above melt point; nylon 66 melts higher, around 500°F, and shifts the whole window up with it

Estimating the target for PET / APET. PET is named above as a semi crystalline resin, and it is — it has a real, well defined melting point around 480 to 490°F. But most PET extrusion is APET sheet, which gets quenched fast enough after the die that the finished part never crystallizes, so it looks and behaves like an amorphous material downstream. That leads some processors to reach for the amorphous rule instead, using PET's glass transition of roughly 165 to 175°F plus 180 to 270°F. Don't. The number that matters is still the actual melt point, not the glass transition. Real APET sheet lines run melt and die temperatures around 464 to 536°F — only 0 to about 55°F above the true melt point. That's a narrow window, and it's driven by hydrolytic degradation: PET reacts with any residual moisture at melt temperature, breaking down the polymer chain, which is why APET has to be bone dry before it ever reaches the barrel. Anchor PET to its melt point, confirm with a probe, and don't assume the general semi crystalline cushion applies — for PET, it doesn't; you're working with far less margin than that.

Amorphous resins (PS, ABS, PC, PVC, acrylic) don't have a true melting point. They have a glass transition temperature, above which they gradually get soft enough to flow. There's no hard line for fully melted, you're chasing a viscosity that's low enough to process well.

General target: glass transition temperature plus 180 to 270 degrees Fahrenheit.

Some rough real world numbers:

•       Polystyrene has a glass transition around 210°F, so you'd typically target somewhere around 390 to 480°F

•       High impact polystyrene (HIPS) shares roughly the same glass transition as GPPS, around 210 to 212°F, but runs cooler in practice — typically 356 to 464°F versus 390 to 480°F for GPPS. That's the dispersed polybutadiene rubber phase talking: it's what gives HIPS its impact strength, and it's more heat sensitive than the styrene matrix around it, so processors keep the window lower to avoid degrading the rubber phase and losing impact properties before the resin itself shows any sign of trouble. The window narrows further by process: sheet and plate run 392 to 446°F, blown film and pipe closer to 356 to 410°F

•       ABS has a glass transition around 220°F, so you'd typically target somewhere around 400 to 490°F

•       Polycarbonate has a glass transition around 300°F, so you'd typically target somewhere around 480 to 570°F — the highest of the group and the least forgiving of grade variation, since flame-retardant packages and molecular weight shift this one around more than most

Running at the high end of the range. It's worth understanding what you're trading away up there. At the high end of the target window the melt is more fluid, which sounds good for filling and output, but it also means less melt strength, so anything that depends on the melt holding its shape (parisons, foam cell structure, sheet sag) can suffer. You'll also pull less horsepower and less torque running hotter and thinner, which can look efficient on paper. The catch is you're now sitting closer to the degradation point for that resin, with less margin for a process upset, a screw hiccup, or a hot spot to push you over the edge into burning, yellowing, or off gassing.

So should you just aim for the middle of the range? Not automatically. The middle is a reasonable default when you don't have a stronger reason to go one way or the other, but the right target depends on what your process actually needs. If you need melt strength for the part or the process (blow molding, foam, thick sheet), you lean toward the low end. If you're fighting to get a hard to melt or high viscosity resin to flow and fill properly, you lean higher, but you do it with your eyes open about the shrinking safety margin. Let the application and the resin drive where in that window you sit, don't just default to the middle because it feels safe.

PVC is the odd one out, and everything about it comes down to how it fails. It does not follow the general amorphous rule above. It has a glass transition around 175 to 195°F, and if you plugged that into the plus 180 to 270°F rule you'd land in a range that would degrade the material before it ever left the die. PVC starts breaking down not far above its actual processing temperature through dehydrohalogenation — a thermal degradation mechanism, not a hydrolytic one, meaning moisture isn't the driver, time and temperature are. It loses HCl as it degrades, and that loss is cumulative: it doesn't just matter how hot you get, it matters how long you stay there.

In practice, rigid PVC is typically processed with a true melt temperature target in the range of roughly 375 to 410°F, and flexible PVC (with plasticizers lowering the effective processing temperature) runs somewhat cooler than that. Both are narrow windows compared to PE, PP, PS, or ABS, and both get run far more conservatively than the general amorphous rule would suggest. This is exactly why PVC needs the tightest control of the bunch: stabilizer package, screw design that limits unnecessary shear, and dwell time all have to be managed together, and the resin supplier's data sheet and stabilizer manufacturer's recommendation should be treated as the real target, not the general rule of thumb.

One more thing. These are rough starting points, nothing more. Your actual target should always come from the resin manufacturer's data sheet for that specific grade, because MI, additives, fillers, and colorants all shift the real number — and for PET specifically, that data sheet isn't optional, it's the only thing standing between you and a degraded part. Use the melting point or glass transition rule to get in the neighborhood and to sanity check a startup. Then confirm it with the probe, every time, and keep checking it, because that's the only number in this whole conversation you can actually stand behind.