Hot Runner System Selection for PET Preform Injection Molds

PET preform molding has strict requirements on temperature control, melt stability, color change efficiency and preform concentricity. The hot runner is the core unit of the mold. Selection should not only focus on procurement cost, but also consider preform specification, output, raw material grade, production schedule and long-term maintenance cost. Unbalanced temperature, dead corners in runners, stringing and crystallization yellowing are typical failures of PET hot runners. These risks should be avoided in the selection stage.

Basic requirements of PET molding characteristics for hot runners

PET is heat-sensitive plastic with a narrow processing window. Long melt residence time leads to hydrolysis, viscosity drop, whitening, black spots and excessive acetaldehyde in preforms. Hot runner interiors must be free of dead zones, with smooth inner walls and fully rounded transitions. PET molding temperature ranges from 270℃ to 290℃. The heating zones of hot runners support independent temperature adjustment from manifold to nozzles for stable temperature distribution. PET cools rapidly, so gate temperature control is critical. Low temperature causes stringing and cold blockage, while high temperature triggers crystallization and yellowing at the preform neck. Food-grade transparent PET preforms require hot runner materials with good corrosion resistance and low precipitation. Acidic substances produced by PET hydrolysis will corrode runner surfaces and introduce impurities into preforms.

Comparison of nozzle types

Open nozzles and valve-gate nozzles are the two major types. Open nozzles have simple structure and low cost, suitable for small-diameter preforms and low-budget projects such as simple daily chemical bottles. Their disadvantages are obvious gate marks, easy stringing and drooling during high-speed continuous production, with heavy cleaning workload after shutdown. They are not suitable for high-transparency food bottles and premium drinking water bottles. Valve-gate nozzles cut off melt by closing valve pins after injection, eliminating stringing and drooling fundamentally. Gate marks are smooth and preform neck consistency is reliable, which is the first choice for mass-produced drinking, beverage and food packaging preforms. Pneumatic valve pins respond fast and are easy to maintain for regular output. Hydraulic valve pins provide stronger driving force and better synchronization for multi-cavity heavy-wall preform molds, but hydraulic leakage may contaminate products. Pneumatic structures are preferred for food-contact applications.

Manifold structure and runner layout selection

Manifolds adopt H-type, X-type or Y-type runner layouts. Balanced runner layout is prioritized for multi-cavity preform molds to ensure consistent melt pressure, temperature and filling time for every cavity, preventing uneven weight, wall thickness and concentricity deviation. Circular runners are preferred, with minimum heat contact area to reduce degradation risk. Runner diameter matches single-cavity shot weight and flow rate. Too small diameter causes excessive shear heat, while oversized diameter prolongs melt residence time. 8-cavity and 16-cavity molds use fewer heating zones. 32-cavity, 48-cavity and 72-cavity high-cavity high-speed molds equip independent temperature zones for manifolds and nozzles for precise control. Heat insulation gaskets and gaps between manifolds and mold plates reduce heat transfer to the cavity and avoid premature crystallization at gates.

Material, coating and food-grade compliance selection

For food-contact PET preforms, melt-contacting hot runner components are recommended to use quenched and tempered H13 and 420 stainless steel for wear and corrosion resistance. Ordinary carbon steel is not recommended for food-grade PET, as acidic hydrolysis products will corrode surfaces and produce metal impurities. For long-term mass production, CrN or DLC coating on runner inner walls reduces material adhesion and carbon accumulation, extending cleaning cycles and improving color change efficiency. Coatings must have supporting compliance documents to meet GB 4806 and FDA migration requirements. Stable coating adhesion is required to avoid peeling under long-term melt scouring. Heater rods and thermocouples adopt mature brand parts. Thermocouple detection points are close to runner walls to reflect real temperature and prevent runaway heating.

Selection based on output and production mode

For intermittent production, fluctuating orders and frequent color or specification switching, simple, easy-to-disassemble hot runners are selected for quick cleaning and short downtime. Valve-gate balanced hot runners from reliable brands are used for year-round continuous high-volume production with sufficient spare parts to reduce unexpected shutdown. Small-batch sampling and frequent specification switching can adopt open hot runners to control initial investment, with clear communication on limitations of gate appearance and stringing control. Hot runner maximum flow rate shall match injection machine shot capacity to avoid excessive pressure loss during high-speed injection.

Auxiliary design and later maintenance consideration

Insulation, sealing and wiring schemes are confirmed during hot runner selection. PET preform molds run at low mold temperature, and the huge temperature difference makes insulation failure a major risk of heat loss and high energy consumption. High-temperature, PET-resistant sealing rings prevent melt leakage. Wiring is protected from short circuit in humid workshops. Wearing parts including valve pins, seals, heaters and thermocouples are stocked in advance. Valve-gate structures contain more parts and require more maintenance work than open types, so long-term operation cost should be evaluated in the early stage.