How to Control Melt Temperature in Injection Molding? Barrel Temperature, Shear Heat, and Screw Plasticizing Design
In injection molding, melt temperature is a key factor affecting part quality, molding stability, and production efficiency. Although barrel temperature can be set by heaters, the actual melt temperature is also affected by shear heat, plasticizing parameters, screw design, screw speed, and back pressure.
For advanced injection molding machines, temperature control is not only barrel heating management. It is an integrated process of material state control and plasticizing energy efficiency. Melt temperature stability depends not only on process settings, but also on the machine’s plasticizing structure and temperature control accuracy.
Why is Barrel Temperature Different from Actual Melt Temperature?
Melt temperature and barrel temperature are related, but they are not the same. They represent different measurement points and meanings. Relying only on barrel temperature may underestimate the real melt condition, causing quality variation and lower yield.
Figure 1. Core Heating Structure of an Injection Molding Machine
The thermocouple installed on the barrel measures the barrel wall temperature, not the real temperature at the melt center. Heat must transfer from the heater bands through the barrel to the melt, so ambient temperature and machine heat loss may create a gap between the measured value and the set value.
If melt temperature is too high, thermal degradation may occur, causing discoloration, bubbles, silver streaks, burn marks, and harmful gas release. This reduces part strength and appearance quality. If melt temperature is too low, poor flowability may cause short shots, incomplete filling, weld lines, cold slug marks, and unstable dimensions.
Therefore, barrel temperature settings alone cannot fully reflect the actual melt condition. Melt sampling, temperature measurement, calibration, and overall process optimization are required.
How does Shear Heat Affect Melt Temperature in Injection Molding?
Melt temperature is affected not only by external barrel heating, but also by shear heat generated during screw rotation. High screw speed or high back pressure creates strong friction and compression, rapidly increasing local heat. In some cases, shear heat may exceed heater input. This is especially significant with large barrel diameters or high-viscosity materials such as PVC and PEEK, where the temperature difference between the melt center and edge may reach several tens of degrees.
Different industries require different shear control strategies:
- High-speed packaging: Focuses on plasticizing efficiency and stable shear output to prevent filling instability, weight variation, and appearance defects during short-cycle mass production.
- Precision optics: Requires low shear and uniform melt temperature to avoid local overheating, residual stress, and optical defects.
- Recycled materials / microcellular molding: Requires thermal history and shear energy control to prevent secondary degradation and maintain cell uniformity and foaming ratio.
- Heat-sensitive materials such as PVC, POM, and PLA: Require careful control of residence time and low-shear design to avoid decomposition, carbonization, and toxic gas release.
Therefore, better melt flow and filling require more than heater input. Proper screw design, screw speed adjustment, and back pressure control can improve melt flowability and filling performance.
Further Reading: FCS PVC Infrastructure Injection Molding Solutions for Building Pipe Fittings
How does Screw Design Affect Melt Temperature Distribution and Plasticizing Stability?
The injection screw not only conveys and melts plastic. It is also the key control component in the plasticizing energy conversion process.
A standard injection screw consists of three sections:
- Feed section: Conveys solid plastic pellets and preheats them near the melting point.
- Compression section: Melts and mixes the plastic. The shear heat generated here is the main source of melt temperature rise.
- Metering section: Homogenizes the melt and stabilizes output. Precise temperature control is required to prevent degradation or poor flowability.
A proper screw L/D ratio and compression ratio directly affect shear heat generation and melt temperature distribution. In modern injection molding machines, stable melt quality depends not only on screw design, but also on screw speed control, plasticizing load stability, machine rigidity, and drive system precision.
⚡FCS Machine Design Examples:
- SA Series Outward Toggle Injection Molding Machine : Suitable for multi-cavity and high-speed packaging, automotive parts, logistics pallets, and large part mass production. Its high-rigidity outward toggle clamping and hydraulic output help maintain stable plasticizing load during long-term production.
- CT-e Series All-Electric Injection Molding Machine : Suitable for electronics, optics, medical, and microcellular molding applications. High-precision servo control reduces plasticizing energy fluctuation and improves melt quality stability.
How do Screw Speed and Back Pressure Affect Melt Temperature?
Screw rotation generates significant frictional heat, which is one of the heat sources for plastic melting. In production, operators may increase screw speed to shorten recovery time, but this can make the melt center temperature higher than the heater setpoint, causing local overheating, quality variation, and material degradation.
Test results show that when screw speed increases from 10 rpm to 100 rpm, the average melt temperature rises by about 3°C, while local temperature rise may exceed 15°C. This shows that shear heat has a significant effect on melt temperature during high-speed plasticizing.
Figure 2. Effect of Screw Speed on Melt Temperature (Material: PP / Machine Tonnage: 100 tons)
Back pressure can improve melt density, temperature uniformity, and heat transfer efficiency, making temperature distribution more stable.
Test results show that when back pressure increases from 10 bar to 30 bar, average melt temperature rises slowly by about 1–2°C. Color mixing uniformity and viscosity stability also improve, making it especially suitable for products with a higher color powder ratio.
Figure 3. Effect of Back Pressure on Melt Temperature
Therefore, screw speed should be set according to material shear rate, viscosity behavior, and screw diameter. A proper speed range helps avoid quality variation caused by local melt temperature rise.
▶ Thermal Sensitivity and Temperature Control Strategies of Different Plastics.
Thermal sensitivity refers to the tendency of thermoplastic materials to degrade or change during heating in injection molding. Different materials vary greatly in heat sensitivity. Materials such as PVC, PC, and PET require careful control of temperature and residence time.
Table 1. Thermal Sensitivity of Common Thermoplastics in Injection Molding
Material | Thermal Sensitivity | Main Degradation Risks and Notes |
PVC | Very High | May release HCl gas at high temperature; strict temperature control is required. |
PC | High | Moisture may cause degradation at high temperature; thorough drying is required. |
PET | High | Highly moisture-sensitive; thorough drying is required. |
POM | High | May release formaldehyde gas at high temperature; strict temperature control is required. |
PLA | High | Low melting temperature; easily degraded by excessive heat. |
PS | Medium | May degrade at high temperature, but is relatively stable. |
ABS | Medium | May degrade at high temperature; proper temperature control is required. |
PP | Low | May still degrade under excessive temperature or long heating time. |
PE | Low | May degrade and generate black spots at high temperature. |
PEEK | Low | Requires high processing temperature and specialized equipment. |
How Smart Injection Molding Moves from Experience-Based Control to Data-Based Control?
To achieve truly stable melt quality, manufacturers must do more than set barrel heater values. They must understand material behavior under shear, process control, and machine design integration. Modern smart injection molding is adopting nozzle temperature monitoring, in-mold temperature monitoring, and water flow and temperature analysis. Future machine competitiveness will focus on:
- Plasticizing energy control
- Melt quality stability
- Smart data integration
In addition to process optimization, FCS iMF 4.0 Smart Manufacturing System integrates plasticizing energy monitoring, process stability analysis, and energy management to support ESG, energy saving, and sustainable manufacturing goals.
▶ FAQ
Frequently Asked Questions About Melt Temperature Control in Injection Molding
Is barrel temperature equal to actual melt temperature?
The actual melt temperature is usually higher than the barrel temperature. Barrel temperature is measured by sensors installed outside the barrel and mainly reflects the barrel wall temperature. The actual melt center temperature is affected by shear heat, screw speed, back pressure, material viscosity, and plasticizing conditions, so the two values may differ.
Why does excessive melt temperature affect injection molding quality?
Excessive melt temperature may cause thermal degradation, resulting in discoloration, bubbles, silver streaks, burn marks, black spots, or lower strength. For heat-sensitive materials such as PVC, POM, and PLA, high temperature may also cause decomposition and harmful gas release, so precise temperature and residence time control are required.
What molding defects occur when melt temperature is too low?
Low melt temperature reduces plastic flowability and may cause short shots, incomplete filling, weld lines, cold slug marks, and dimensional instability. It may also affect part appearance and structural quality.
What is shear heat, and why does it affect melt temperature?
Shear heat is the heat generated by friction and compression during screw rotation, compression, and mixing. When screw speed is too high or back pressure is too large, shear heat increases. If the barrel thermocouple feedback temperature is higher than the set value, it indicates that shear heat exceeds heater input, which may affect molding stability.
Does faster screw speed always improve plasticizing efficiency?
Not always. Increasing screw speed can shorten recovery time, but it also increases shear heat and may cause local overheating, material degradation, or quality variation. Settings should be adjusted according to material properties, screw diameter, product requirements, and molding cycle.
What is the function of back pressure in injection molding?
Back pressure improves melt density and helps remove gas. It also improves mixing, temperature uniformity, and color powder dispersion, which supports plasticizing stability. However, excessive back pressure may increase shear heat and thermal load, so a balance between mixing effect and temperature rise control is required.
Which plastic materials require special melt temperature control?
PVC, POM, PLA, PC, PET, PA, PEEK, and PMMA are sensitive to temperature, moisture, or hydrolysis. During molding, drying conditions, barrel temperature, screw speed, back pressure, and residence time should be carefully controlled to avoid degradation, discoloration, or gas generation.
How can melt temperature stability in injection molding be improved?
Melt temperature stability can be improved through material drying, barrel temperature settings, screw design, screw speed, back pressure, recovery time, and process monitoring. With smart injection systems and process data analysis, manufacturers can better monitor plasticizing energy, temperature variation, and molding stability.
What solutions does FCS provide for different industries?
FCS provides different machine designs for different industry needs. The SA Series is suitable for high-speed packaging, automotive parts, logistics pallets, and large part mass production. The CT-e all-electric injection molding machine is suitable for electronics, optics, medical, and precision molding applications. With iMF 4.0 Smart Manufacturing System, process monitoring, energy.
