---
title: "Barrel temperature zones and temperature profile"
url: "https://www.imspares.com/injection-molding-machine-parts/heaters/barrel-temperature-zones/"
---

# Barrel temperature zones and temperature profile

The barrel temperature zones of an injection molding machine, how to set a rising, flat or reverse temperature profile, and why melt temperature differs.

Barrel temperature zones are the separately controlled heating sections along the barrel of an injection molding machine. A typical barrel has a **rear (feed) zone**, a **middle (compression) zone** and a **front (metering) zone**, plus a separate **nozzle zone**; longer barrels have more. Each zone has its own heater bands, thermocouple and set temperature, and the set temperatures from the feed end to the nozzle make up the **temperature profile**. The drawing highlights the ceramic heater bands that form the zones.

## The zones from the hopper to the nozzle

- **Feed throat.** Not a heating zone. It is water-cooled so the pellets stay solid and free-flowing until the screw takes them in. See [hopper and feed throat](https://www.imspares.com/injection-molding-machine-parts/barrel-screw/hopper-and-feed-throat/).
- **Rear zone.** Warms the pellets as the screw carries them forward. It controls how well the screw feeds.
- **Middle zone.** Where most of the melting happens, as the screw's compression zone squeezes the plastic against the barrel wall.
- **Front zone.** Holds the melt at a stable temperature in front of the screw tip before injection.
- **Nozzle zone.** Keeps the melt from freezing on the way into the mold. See the [nozzle heater](https://www.imspares.com/injection-molding-machine-parts/heaters/nozzle-heater/).

The barrel zones are fixed, but the screw moves back and forth during the cycle, so the screw's feed, compression and metering zones do not line up exactly with the heating zones. The names describe the general position. Controllers also number the zones differently: some count from the nozzle, some from the feed end. Check the temperature screen and the labels on the cables before changing a setting or a part.

## Setting the temperature profile

Start from the processing temperatures in the material supplier's data sheet and adjust from there. Three profile shapes are used:

| Profile | Shape | When it is used |
|---|---|---|
| Rising | Rear zone coolest, rising to the front | The usual starting point, especially when each shot uses a small share of the barrel's capacity |
| Flat | All zones at about the same temperature | Materials and shots where an even heat input works well |
| Reverse | Rear zones as hot as or hotter than the front | Large shots relative to barrel capacity, slow recovery, or materials that need more heat early |

The rising profile lets the plastic heat gradually while the screw adds shear heat toward the front. A reverse profile puts more heat in early, so the plastic is melted sooner when it spends less time in the barrel, and it reduces the torque the screw drive needs.

Change one zone at a time and in small steps. A barrel has a lot of steel and responds slowly, so wait until the zone and the process have settled before judging the effect, and record the settings that work.

## Melt temperature is not the set temperature

The thermocouples measure the barrel wall, not the plastic, and the screw adds **shear heat** as it turns and works against [back pressure](https://www.imspares.com/injection-molding-machine-parts/barrel-screw/back-pressure/). The real melt temperature can therefore differ noticeably from the set points, and it rises when screw speed or back pressure goes up.

To measure it, make an air shot (purge with the nozzle away from the mold) into a suitable container and measure the melt at once with a needle probe that has been preheated. Wear a face shield and gloves: purged melt is hot and can spit.

A front zone that runs above its set point with its heater switched off is a sign that shear heat is doing the work. That is a process or screw matter, not a heater fault. How to tell the difference is in [temperature control](https://www.imspares.com/injection-molding-machine-parts/heaters/temperature-control/).

## Problems caused by the zone settings

- **Rear zone too hot.** Pellets soften and stick at the feed throat or on the screw root, feeding becomes erratic and recovery time varies.
- **Rear or middle zone too cold.** Unmelted pellets reach the front, screw torque rises, recovery slows, and the screw, the check ring and the screw drive are loaded harder.
- **Front zone or nozzle too hot.** Drool, stringing, degraded material, black specks, gas.
- **Too long at temperature.** When the shot is small compared with the barrel's capacity, the plastic stays in the barrel for many cycles. Heat-sensitive materials degrade. See [shot size](https://www.imspares.com/injection-molding-machine-parts/barrel-screw/shot-size/) for how to check the share of barrel capacity you use.
- **A zone that will not hold its setting.** Usually a hardware fault: a failing band, a loose band or a thermocouple problem. See [heater band types](https://www.imspares.com/injection-molding-machine-parts/heaters/heater-band-types/) and the [thermocouple guide](https://www.imspares.com/injection-molding-machine-parts/heaters/thermocouple/).

Cold zones also lead to [short shots](https://www.imspares.com/blog/short-shots-injection-molding-causes-fixes/), and uneven heating along the barrel can cause local wear; the [barrel guide](https://www.imspares.com/injection-molding-machine-parts/barrel-screw/barrel/) covers barrel wear.

## Good practice

1. Heat all zones together from cold, and let the barrel soak before turning the screw.
2. If the controller has a standby temperature, use it during long stops instead of leaving the barrel at full temperature.
3. Keep the feed throat cooling water running whenever the barrel is hot.
4. Keep a record of the profile for each mold and material, and note the actual temperatures as well as the set points.
5. After replacing a band or a thermocouple, check that the zone heats in the right place and reads the same as a handheld probe.

For the whole heating system, see the [heaters guide](https://www.imspares.com/injection-molding-machine-parts/heaters/).

## Questions

### What are the barrel temperature zones on an injection molding machine?

They are the separately controlled heating sections along the barrel, usually called the rear (feed), middle (compression) and front (metering) zones, plus the nozzle. Each zone has its own heater bands, thermocouple and set temperature.

### What is a reverse temperature profile?

A profile where the rear zones are set as hot as or hotter than the front zones. It is used when each shot takes a large share of the barrel's capacity, so the plastic has less time in the barrel to melt, or when a material needs more heat early.

### Why is the melt temperature different from the barrel set temperatures?

The screw adds shear heat as it turns and works against back pressure, and the thermocouples measure the barrel wall, not the melt. Measure the real melt temperature with a probe in an air shot.

### How many heating zones does an injection molding barrel have?

It depends on the barrel length and the machine size. Small machines have a few barrel zones plus the nozzle, and larger barrels have more; the temperature screen of the machine shows them all.

## Replacement parts

- [EC-HT-01 Ceramic Heater Band](https://www.imspares.com/parts/ec-ht-01/): Full-enclosed ceramic heater band for injection molding machine barrel and nozzle heating zones.