Cooling System Upgrade of High-Speed PET Preform Molds
During the molding cycle of PET preforms, the cooling phase occupies 70% to 80% of total duration. Cooling efficiency directly determines maximum mold productivity, preform transparency and residual stress level. Conventional multi-cavity preform molds generally adopt basic drilled water channels, suffering insufficient heat exchange on cores, obvious hot spots at thick-walled gate areas of preform bases and unbalanced cooling among cavities. Such drawbacks restrict continuous high-speed mass production. Cooling system upgrade involves far more than simply enlarging channel sizes. Systematic transformation covering zoned channel layout, core heat exchange structure, fluid state inside channels, matched temperature control hardware and daily maintenance is essential. It shortens molding cycles while steadily reducing defects including preform crystallized whitening, weight fluctuation and deformation after blow molding.
Ⅰ. Restructure Independent Zoned Water Circuits to Eliminate Cavity Temperature Difference and Local Hot Spots
Most outdated molds adopt series cooling circuits. Water temperature gradually rises along circulation paths, creating inconsistent cooling conditions for front and rear cavities and causing discrete preform weight and transparency. The primary upgrade measure is reconstructing the mold into an independent zoned cooling architecture for single cavities. Preforms are divided into three separate circulating circuits: thread neck zone, main preform body zone and thick-wall gate base zone. The gate base accumulates melt with maximum wall thickness, forming hot spots prone to haze and whitening. Water channels here shall be arranged as close to molding surfaces as possible to shorten heat transfer distance. Excessive rapid cooling on thread neck areas should be avoided to prevent dimensional fluctuation and concentrated stress, so inflow temperature and flow rate can be regulated separately. Water channels around preform bodies are evenly arranged circumferentially to guarantee synchronous heat dissipation along the perimeter.
Parallel water supply layout is followed to eliminate temperature gradients caused by long-distance series connection. Thermal simulation is applied during upgrading to locate high-temperature zones and add auxiliary water channels. Surface temperature difference across all mold cavities shall be controlled within 2℃. For high-cavity molds with 48 or 72 cavities, flow distribution manifolds are installed on main water inlet pipes, and adjustable flow limit valves are mounted on each branch pipe to balance cooling water flow for all cavities. This fundamentally improves quality differences between inner and outer ring cavities.
Ⅱ. Upgrade Internal Cooling Structure of Cores to Strengthen Inner Heat Exchange Capacity
Limited space inside preform cores restricts channel layout, making cores the weakest link of cooling systems. Traditional straight-through core cooling easily forms laminar flow with low heat exchange efficiency. During renovation, ordinary straight cores are preferentially replaced with fountain cooling cores. Cooling water directly reaches core tips at preform bases to fully remove heat from thick-wall zones and flows back along core outer walls, maintaining turbulent flow throughout circulation. High-end molds can adopt spiral flow-guiding cores. Internal diversion structures break thermal boundary layers, lifting heat exchange efficiency by over 25% compared with straight structures.
For renovation of long-running old molds without conditions for complete core replacement, inlet and outlet hole diameters of cores can be optimized to improve flow velocity. Meanwhile, scale and rust inside cores should be cleaned. Cooling circuits for cores and cavities are recommended for separate regulation. Inlet water temperature of cores is slightly higher than cavity water temperature to avoid uneven shrinkage and elevated internal stress caused by excessive temperature difference between inner and outer preform surfaces. Core sealing structures shall be upgraded simultaneously to prevent mold rust and product watermarks from water leakage, ensuring stable continuous high-speed production.
Ⅲ. Optimize Fluid Conditions in Water Channels to Sustain Turbulent Heat Exchange
Water flowing inside cooling channels cannot deliver efficient heat dissipation unless turbulent flow (Reynolds number＞4000) breaks thermal insulation boundary layers on wall surfaces. Many on-service molds feature unreasonable pipe diameters, excessive sharp bends and insufficient inlet pressure, leading to long-term laminar flow and weakened heat transfer. During upgrade, channel hole diameters and rounded transitions are optimized to reduce sharp turns. Matching water chillers raise supply pressure to stabilize mold inlet pressure between 3–6bar, with flow velocity maintained above 2.0m/s.
Cooling water quality must be strictly controlled. Closed circulating systems equipped with precision filters and water softeners slow scale formation. Inner channel surfaces can be polished to reduce dirt adhesion, and periodic pipeline pickling maintenance is required. Many factories only monitor chiller temperature while ignoring flow conditions. Even with water temperature reduced to 8℃, heat exchange improvement remains limited under laminar flow. Upgrade schemes require coordinated management of water temperature, flow velocity and water quality to avoid energy waste of "low temperature yet low efficiency".
Ⅳ. Auxiliary Optimization of Matched Temperature Control Hardware and Mold Materials
Cooling channel transformation needs coordinated peripheral temperature control equipment. Outdated single-circuit chillers can be upgraded to multi-channel independent temperature control units to realize separate temperature regulation for cavities, cores and neck circuits with control accuracy of ±0.5℃. In summer with elevated ambient temperature, inlet water temperature is stably maintained at 8–12℃ to prevent unstable cycles induced by temperature fluctuation. For inserts with persistent hot spots, local high-thermal-conductivity beryllium copper inserts can be embedded to accelerate heat export, which must be matched with reasonable cooling circuits to avoid thermal fatigue cracks from uneven heating and cooling.
Cooling for mold plates cannot be neglected. Auxiliary cooling circuits are added to hot runner plates and support plates to stabilize overall mold base temperature and reduce continuous heat accumulation. Heat from hot runners continuously conducts toward cavities. Balanced adjustment of hot runner heating power and mold cooling intensity is needed during upgrades to avoid surging energy consumption from excessive cooling alone, balancing molding efficiency and production costs.
Ⅴ. Establish Standardized Operation and Maintenance System for Long-term Upgrade Effectiveness
Without continuous maintenance after hardware transformation, heat exchange performance will gradually decline within months. Standard periodic maintenance specifications shall be formulated: inspect flow rate and temperature difference of each circuit monthly, and judge pipeline clogging by temperature variation. Perform pickling and scale removal for cores and cavity channels every six months. Drain residual water inside channels for rust prevention during shutdown. Record data of molding cycles and preform appearance under different inlet temperatures and flow rates during daily production to form process benchmarks. High-speed molds operate continuously for 24 hours. Scale and microbial growth gradually narrow effective channel cross-sections, erasing cycle optimization benefits achieved in early upgrades. Routine maintenance guarantees long-term stable production capacity.
Conclusion
Cooling system upgrade for high-speed PET preform molds represents systematic thermal balance transformation, shifting logic from simple water cooling to zoned balanced temperature control and full-range efficient heat exchange. Restructuring independent zoned water circuits eliminates uneven cavity temperatures, upgrading core internal structures reinforces internal heat dissipation, optimizing flow parameters maintains sustained turbulent heat exchange. Combined with temperature control hardware upgrade and regular maintenance specifications, cooling duration can be shortened, maximum mold operating speed lifted, and defects such as preform haze, weight fluctuation and residual stress reduced. Thermal simulation is recommended to locate hot spots during transformation. Distinct renovation schemes shall be adopted for newly manufactured molds and old mold retrofits to balance transformation cost and production profit. Stable and uniform cooling conditions not only boost unit-time output but also improve subsequent blow molding stability, lower full-chain reject rates and fully release production potential of multi-cavity high-speed PET preform molds.
