For previous article, please refer to PART 03: How to Calculate Clamping Force? A Comprehensive Guide to Projected Area, Cavity Pressure,.
Barrel temperature is a crucial setting parameter among five essential elements of injection molding: temperature. It directly affects melting state and process of plastic granules, as well as final plasticizing quality, filling flowability, and product performance.

Setting barrel temperature should be guided by type of plastic used. Different plastics have vastly different melting points, thermal stability, and viscosity characteristics. Setting must be based on processing temperature range recommended by material supplier.
Set temperature must be compatible with product and mold. For example, thin-walled, complex structures, long runner products require higher temperatures to ensure flowability; high-gloss or precision products require more precise temperature control to reduce weld lines and internal stress.
At the same time, it is essential to be familiar with material properties to prevent excessive temperature from causing thermal decomposition of material. It is strictly forbidden to exceed material's maximum heat resistance temperature.

 

General Screw Structure Diagram
Diagram above shows a common three-section general screw structure. Frontmost components—rocket head, check ring, and sealing ring—are what we often refer to as "three small parts," and are crucial to stability of injection molding production. We will discuss this in more detail later when we talk about static sealing test of injection molding machine.
Besides "three small parts," the entire effective length (L) of screw is divided into: metering section, compression section, and feeding section.
Feeding Section: Often called feeding section, it is located at the rear of barrel, near discharge port. Its main function is to preheat material, perform preliminary compression, expel air and moisture.
Compression Section: Middle section is often called plasticizing section. Its main function is to completely melt and homogenize solid material, and establish melt pressure required for injection. Quality of plasticizing largely depends on temperature setting of this section and coordination of screw's shearing action. Metering Section: Located at the front of screw, this section is called homogenization section, where melt temperature is almost uniform. Its main function is to store a fixed quantity of melt at a uniform temperature, preparing it for injection. Stroke length of this section is maximum injection stroke of injection molding machine.

Common injection molding machines typically have five to seven temperature zones. More temperature zones mean higher precision in heating and temperature control, directly impacting process flexibility, plasticizing quality, and product quality.

 

Multi-Zone Precise Control

 

Common Five-Zone Temperature Control
Image above shows a common five-zone temperature control setting interface. It controls temperature of heating devices in areas such as nozzle flange position, metering section, compression section, front and rear ends of feeding section through real-time monitoring by temperature sensors.

 

Zone 1: Rear end of feeding section: Lower temperature setting. For semi-crystalline plastics, general setting principle is to subtract 30℃~50℃ from Tm to ensure that plastic remains in a loose, solid particle state throughout this section, preventing premature melting and "bridging." For amorphous plastics, setting principle is to take a temperature above material's glass transition temperature (Tg), but significantly lower than its heat distortion temperature or median of its processing temperature. Appropriately increasing discharge port temperature for amorphous plastics can initiate plasticizing process earlier, distributing heating load of compression section. This makes the entire plasticizing process smoother, with more uniform shear heat, contributing to a more uniform melt temperature and viscosity.
Zone 2: Feed Section Front End: This is "throat" controlling the transition of plastic from solid conveying to molten compression. Its function is to ensure material enters compression section in correct physical state, preparing for upcoming melting through "preheating" or "holding." It is a crucial link in the entire stepped heating curve. Therefore, temperature setting is generally close to or slightly lower than plastic molding and processing temperatures.
Zone 3: Compression Zone: Temperature rises significantly, set above material's melting range or melting point. This is critical zone for plastic melting, mixing, and compression. It ensures complete melting and homogenization of solid material, establishes melt pressure required for injection. Temperature in this zone has the greatest impact on plasticization quality and melt viscosity.
Zone 4: Metering Zone: Temperature reaches its peak, which may be slightly lower or higher than nozzle, depending on situation. It ensures complete melt homogenization and consistent temperature. Fully molten plastic in this zone has a uniform and consistent temperature, preparing for filling.
Zone 5: Nozzle and Flange Position: Individually controlled. Usually same as or slightly lower than front end temperature (to prevent drooling). For heat-sensitive materials or to prevent cold material, it may also be slightly higher. It ensures melt enters mold runner at optimal temperature. Too low a temperature can lead to cold material blockage or product defects; too high a temperature can easily cause drooling or decomposition.

As shown in diagram above, temperature settings generally follow a high-to-low approach. At nozzle, temperature can be appropriately increased or decreased depending on actual situation. However, in actual production, to cope with certain special circumstances, low-high-low and low-to-high setting methods are also necessary. Regardless of method chosen, temperature at discharge port should be kept as low as possible, following setting principles mentioned earlier.
Below, we will discuss application scenarios of these three temperature setting methods:

Curve Shape: From nozzle to discharge port, temperature is set from high to low, decreasing sequentially.
Physical Meaning: Creates a smooth and controllable heating process, allowing plastic to gradually soften, melt, and homogenize.
Application Scenarios and Materials: Standard Setting for Amorphous Plastics:
Typical Materials: ABS, PS, PC, PMMA, SAN.
Principle: These plastics do not have a melting point, only a glass transition temperature, and gradually soften over a wide temperature range. A stepped heating curve allows for:
Preheating at discharge port and initiating venting (removing moisture and volatiles).
Main plasticization is completed in compression section.
Optimal flowability is achieved in metering section and nozzle, ensuring smooth filling.
The entire process is smooth, with stable shear and uniform melt temperature.
Example (ABS): Discharge port 180℃ → Front section of feeding 200℃ → Compression section 220℃ → Metering section 230℃ → Nozzle 230℃.
For shear-sensitive or heat history-sensitive materials:
Such as some transparent materials (PC, PMMA), steady heating can prevent black spots or yellowing caused by localized overheating.
General strategy, stability first: When material properties are not fully understood, or when process stability is desired, starting with "low and gradually increasing" is the safest and most universal starting point.

Curve Shape: Low at discharge port, highest temperature in compression section (plasticizing section), then slightly lower at metering section and nozzle.
Physical Significance: Concentrates energy in compression section to complete phase change of plastic from solid to molten (especially for crystalline plastics), followed by "heat preservation" or "fine-tuning" at the front end.
Application Scenarios and Materials: Standard settings for crystalline and semi-crystalline plastics:
Typical Materials: PP, PE, PA (Nylon), PBT, PET.
Principle: These plastics have a definite melting point and require a large amount of heat (latent heat of fusion) to change from solid to molten. Setting the highest temperature in compression section can:
Provide maximum heat energy, combined with screw shearing, to efficiently complete melting.
Prevent premature melting at discharge port (leading to unstable discharge).
Prevent overheating of metering section and nozzle (crystalline plastic melts are also susceptible to decomposition from prolonged overheating).
Example (PA66): Feed section rear 230℃ → Feed section front 260℃ → Compression section 280℃ (peak) → Metering section 270℃ → Nozzle 265℃.
For applications requiring high shear and high plasticization uniformity:
For materials containing fillers such as glass fiber and flame retardants, maintaining high temperature and high shear in compression section is beneficial for uniform additive dispersion.
When using a high proportion of recycled materials, high peak temperatures help to thoroughly melt and plasticize materials with different thermal histories.

Temperature profile: Set from low to high at the nozzle, like climbing a staircase step by step.
This setting method is uncommon and is generally used in specific situations. Its main purpose is to compensate for material and screw defects. It uses a strategy of advancing melting point, primarily to address phenomenon of unstable melting.
1. Handling Difficult-to-Convey Material Forms
Scenario: Processing powdery materials, recycled fine powders, flaky recycled materials, or materials with extremely low bulk density. Problem: These materials tend to "bridge" or form unstable solid beds at feed inlet, resulting in insufficient friction with barrel wall and screw, leading to screw slippage, unstable feed, and significant fluctuations in plasticization.
Solution: Increase temperature at the front end of feeding section.
Principle: Heat slightly softens or melts material surface, increasing friction between material and barrel wall. Stronger friction allows for more effective material gripping and pushing, forming a stable solid plug for conveying. This is equivalent to artificially "enhancing" screw's conveying capacity.
2. Solving "Screw Slippage" Problem for Specific Materials
Scenario: Certain highly lubricating materials (such as modified plastics containing a high proportion of silicone oil, release agents, or internal lubricants) or ultra-high molecular weight polyethylene (UHMW-PE).
Problem: Material itself has extremely low friction. Even in a standard low-temperature feeding section, screw cannot effectively grip material, resulting in severe slippage and idling, making it impossible to establish a stable plasticization rate.
Solution: Increase temperature at the front end of feeding section.
Principle: By melting material surface, viscosity of melt is used to replace and enhance solid friction, thereby improving conveying. This is a last resort method because it touches boundaries of solid conveying principles.
3. Compensating for Screw Design or Wear Defects
Scenario: Using a screw with a low compression ratio to process high-viscosity materials, or screw's feeding section is severely worn.
Problem: Standard solid conveying capacity is insufficient; material cannot be effectively compressed and pushed to compression section.
Solution: Increase temperature at the front end of feeding section to initiate plasticizing earlier.
Principle: Allowing some plasticizing to occur "earlier" at the end of feeding section reduces load on compression section and utilizes viscosity of melt to aid conveying. This is a process compensation for equipment defects.
4. A Risky "Ventilation" Strategy (Requires Extreme Caution)
Scenario: For engineering plastics that are extremely hygroscopic and incompletely dried (such as PA, PET), but production is urgent.
Problem: Moisture will vaporize violently in high-temperature compression section, causing bubbles, silver streaks, and material degradation in product.
Risky approach: Significantly increase temperature at the front end of feeding section (but still keep it below material's melting point).
Principle: This attempt aims to heat and evaporate moisture into steam in lower-pressure feeding section before material enters high-pressure compression section, hoping it will be discharged in reverse from loose material in hopper. This is a high-risk approach with a low success rate and is highly likely to cause material agglomeration in feeding section. The best practice is always thorough pre-drying.