Barrel Temperature Profile: Flat Is a Compromise, Not a Default

Three profiles, three different failures

A barrel has four or five zones, and the temperature in each one is a separate decision. Most machines ship with a flat profile because it is neutral and it works on most materials, and most setups keep it because nobody has a reason to change it.

A flat profile runs every zone at the same temperature, usually 230 C for a polypropylene. It is a compromise: the feed zone is hotter than it needs to be, which can cause the screw to slip and the material to melt early, and the nozzle is cooler than it could be, which costs flow at the gate.

A rising profile runs the feed zone cool and the nozzle hot, typically 210 to 245. The feed zone stays below the melting point so the granules convey as solid, the compression zone does the melting, and the nozzle delivers a fully melted, low-viscosity melt. This is the profile that protects the polymer.

A falling profile runs the feed hot and the nozzle cool, typically 245 to 210. It melts the material early, which gives more mixing time and a more homogeneous melt, and it delivers a cooler melt to the cavity, which shortens cooling. It also gives the polymer its longest heat history at the point where it is least mixed, and that is where black spots and colour drift come from.

Table 1 — What each profile does, and how it fails.

Why rising protects the polymer

The feed zone has one job: convey solid granules from the hopper to the compression zone. If the feed zone is hot enough to melt the granules, they form a sticky mass that rotates with the screw instead of being conveyed, and the screw slips. Recovery time rises, the shot becomes inconsistent, and the operator raises the back pressure to compensate, which makes it worse.

A cool feed zone keeps the granules solid and the coefficient of friction between the granules and the barrel high, which is what conveys them. The melting then happens in the compression zone, where the screw geometry is designed for it: the channel depth reduces, the material is sheared against the barrel, and the melt film forms and grows.

A hot nozzle then delivers the melt at the lowest viscosity the material will tolerate, which reduces the pressure needed to fill and lets the cavity be packed at a lower pressure. On a thin-walled part this is worth more than any other single setting.

The limit is starvation. If the feed zone is so cold that the granules do not soften at all, the screw cannot bite and the recovery time climbs until the cycle is limited by plasticising rather than by cooling. The practical minimum is 20 to 30 C below the material's melting point, and the check is whether the recovery time is stable.

Why falling mixes better and degrades more

A hot feed zone melts the material early, which gives it more time in the screw and more turns of shear before it reaches the nozzle. That extra mixing is real, and on a job with a dispersion problem, a falling profile can improve colour uniformity by a measurable amount.

The cost is heat history. The material that melts at zone 1 spends the whole screw length at temperature, and the material in the flight clearance spends it at the highest shear rate. That is where black spots form and where a heat-sensitive pigment drifts.

There is a second cost that is less obvious. A hot feed zone reduces the friction that conveys the granules, so the screw conveys less per revolution and the recovery time can rise even though the melt is hotter. A falling profile is not automatically faster; it is faster only when the material is easy to convey and the mixing benefit outweighs the loss of conveying friction.

The profile is therefore a material decision, not a preference. A glass-filled engineering polymer with a narrow processing window wants rising. A commodity polypropylene with a dispersion problem might want falling, provided the residence time is short and the colour is not heat-sensitive.

A cold feed costs real cycle time

Recovery time is the time the screw needs to plasticise the next shot, and it is set by the rear zone temperature more than by any other setting. At a 200 C rear zone the recovery takes 14 seconds; at 210 it is 12, at 220 10.5, at 230 9.5 and at 240 8.8.

A 40 C drop in the rear zone costs 60 percent more recovery time. On a 25 second cycle that is five seconds, which is 20 percent of output, and it is invisible on a setup sheet because recovery usually happens during cooling and does not show as a separate line.

It becomes visible when the cycle is cooling-limited rather than recovery-limited. If recovery is shorter than cooling, the extra time is free; if it is longer, it sets the cycle. That is why a cold feed zone is sometimes free and sometimes very expensive, and why the check is to compare recovery time against cooling time rather than to look at the profile in isolation.

Measuring the melt temperature properly

The zone setpoints are not the melt temperature. The melt temperature is what the material is when it enters the cavity, and it is measured with an air shot: purge a shot into the air, catch it in an insulated container, and insert a needle probe. The reading is 10 to 30 C above the nozzle setpoint on a fast cycle, because shear heating adds energy that the setpoint does not describe.

That difference is why two machines running the same setpoints produce different parts. A machine with a longer screw, a higher screw speed or a smaller shot generates more shear heat and delivers a hotter melt at the same setpoint. The setpoints are a means; the melt temperature is the specification.

The measurement should be made at the production cycle, not at a slow one, because the shear heating depends on the screw speed and the recovery time. An air shot taken during setup, at half the production screw speed, reads 15 C colder than the same machine in production.

Table 2 — Starting profiles by material, to be tuned against an air shot.

What to put on the setup sheet

A setup sheet that records five zone setpoints records the means, not the result. Four items make it transferable.

1. The profile by zone, and the direction it runs in.
2. The measured melt temperature from an air shot at production screw speed.
3. The recovery time, and whether it is shorter than the cooling time.
4. The screw speed and back pressure, because both change the shear heating.
5. The material lot, because a different lot can shift the window by 10 C.

The recovery time against cooling time is the item that turns the profile from a preference into a decision. A profile that costs recovery time is free on a cooling-limited job and expensive on a recovery-limited one, and the setup sheet is where that distinction is recorded.