Plastic materials suitable for low hot runner temperatures are mainly low heat-resistant resins, with heat distortion temperatures typically below 100℃ and processing temperature windows mostly between 160℃ and 230℃. This includes polyethylene (PE).
As your technical partner, I know that when maintaining hot runner systems, your primary concern is how to match low-temperature solutions for heat-sensitive materials such as ABS and PC to reduce the risk of material degradation and extend mold life. Considering your frequent handling of heat-sensitive materials, the following outlines the characteristics of these low-temperature materials and their configuration strategies in hot runner systems:
1. List and Characteristics of Core Low-Temperature Materials These materials are heat-sensitive and easily decompose or produce odors at high temperatures. Therefore, the hot runner temperature must be strictly controlled within a low range.
Polyethylene (PE):
Heat Resistance Grade: Low heat resistance (heat distortion temperature <100℃).
Reference Processing Temperature: 180℃ ~ 230℃.
Hot runner strategy: PE has good fluidity and relatively good thermal stability, but excessively high temperatures can lead to fiberization or degradation. Low-density polyethylene (LDPE) should have lower values (approximately 180℃-200℃), and high-density polyethylene (HDPE) should have higher values (approximately 210℃-230℃). In a hot runner system, it is necessary to prevent the melt from "drooling" at the nozzle due to excessively high temperatures.
Polypropylene (PP):
Heat resistance rating: Although it belongs to the medium heat resistance category (100℃-200℃), the actual processing temperature is relatively low.
Processing temperature reference: 200℃ ~ 240℃.
Hot runner strategy: PP is a typical "low-temperature, high-efficiency" material. If the product wall thickness is large, it is recommended to set it at around 210℃ to shorten the cooling cycle; for thin-walled parts, the temperature can be increased to 230℃. Note that PP is prone to oxidation at high temperatures, and the hot runner temperature should not exceed 240℃ for extended periods.
Polystyrene (PS):
Heat Resistance Grade: Low (Heat Deflection Temperature <100℃).
Reference Processing Temperature: 180℃ ~ 230℃.
Hot Runner Strategy: PS has low melt viscosity and excellent fluidity, requiring a relatively low hot runner temperature. Excessive temperature can cause yellowing or blistering; a temperature of around 200℃ is generally sufficient for most needs.
Polyvinyl Chloride (PVC):
Heat Resistance Grade: Low (Heat Deflection Temperature <100℃).
Reference Processing Temperature: 160℃ ~ 190℃ (Slightly higher for rigid PVC, slightly lower for flexible PVC).
Hot Runner Strategy: Extremely heat-sensitive. PVC readily decomposes and releases corrosive gases (hydrogen chloride) above 200℃, severely damaging hot runner components. A low-temperature, high-pressure strategy is necessary, and the hot runner system must possess excellent corrosion resistance (e.g., using special coatings or materials). The runner system must be thoroughly emptied before shutdown.
ABS Resin:
Heat Resistance Grade: Low heat resistance (heat distortion temperature <100℃).
Reference Processing Temperature: 230℃ ~ 260℃ (Compared to high-temperature materials like PC, ABS falls into the medium-low temperature range, but caution is needed regarding its decomposition threshold of 260℃).
Hot Runner Strategy: A key material you should be monitoring daily. ABS is very sensitive to temperature; the recommended hot runner temperature is between 230℃ and 245℃. Exceeding 260℃ will cause rapid decomposition, producing silver streaks and black spots. For flame-retardant ABS, the set temperature should be further reduced by 5℃ to 10℃.
Polymethyl methacrylate (PMMA/Acrylic):
Heat Resistance Grade: Low heat resistance (heat distortion temperature <100℃).
Reference Processing Temperature: 230℃ ~ 250℃.
Hot runner strategy: PMMA is extremely sensitive to temperature fluctuations. Excessive temperature can cause bubbles or silver streaks, while insufficient temperature results in poor flowability. A high-precision temperature control chamber (±0.5℃) is required, and the set value should typically not exceed 250℃.
2. Key techniques for setting low-temperature hot runner temperatures: For the aforementioned low-temperature materials, the following principles should be followed when operating a hot runner system to match your core objective of reducing degradation risk:
"Sufficient is good enough" principle: Do not blindly pursue high temperatures to improve flowability. For materials such as PE, PP, and PS, as long as the mold filling requirements are met, the lower limit of the temperature window should be used as much as possible. For example, if 210℃ can fill the cavity, do not set it to 230℃. This can significantly reduce the thermal history of the material in the runner and lower the probability of degradation.
Gradient temperature field control: Even for low-temperature materials, a reasonable temperature gradient needs to be constructed:
Runner plate: Set to the reference temperature or slightly lower by 5℃. In large-volume runner systems, the melt residence time is long; a slightly lower temperature can prevent prolonged thermal decomposition.
Nozzle tip: Set to the reference temperature or slightly higher by 5°C. This prevents excessive heat dissipation at the gate, leading to cold material, but excessively high temperatures are strictly prohibited to avoid carbonization at the gate.
Strictly control residence time: Low-temperature materials are often heat-sensitive (e.g., PVC, ABS). If production is interrupted, the temperature must be reduced to a safe holding temperature (e.g., 150°C) or the runner must be emptied. It is strictly forbidden for the melt to remain stationary in the high-temperature runner; otherwise, even if the set temperature is not exceeded, prolonged heat accumulation will lead to degradation.
Shear heat management: Actual melt temperature = Set temperature + Shear heat. For shear-sensitive materials such as PE and PP, if the screw speed is too high, the generated shear heat will cause the actual temperature to far exceed the set value. It is recommended to reduce the screw speed and back pressure, relying on external heating for plasticization to avoid localized overheating caused by internal friction.
3. Special Precautions
PVC Corrosivity: When processing PVC, in addition to setting a low temperature, it is essential to ensure that the hot runner components (nozzles, runner plates) have undergone anti-corrosion treatment. Ordinary steel will be rapidly damaged by the corrosive effects of PVC decomposition gases.
ABS Drying and Temperature: ABS is highly hygroscopic. If drying is insufficient, the temperature needs to be appropriately increased to reduce viscosity, but it is crucial to constantly monitor whether it approaches the 260°C red line. A better approach is to enhance the drying of the raw material, thereby allowing processing at even lower temperatures.
Sensor Calibration: In the low-temperature range, the impact of temperature deviation is more significant. It is imperative to calibrate thermocouples regularly to ensure that the displayed temperature matches the actual melt temperature, preventing the actual temperature from being too high due to sensor errors.
