Calculation of thermal expansion dimensions of hot injection nozzle
Thermal expansion of hot nozzles is a crucial factor in hot runner system design. Because their operating temperatures (typically 150-350°C) are significantly higher than room temperature, they experience significant axial and radial expansion. Failure to allow for adequate compensation can lead to gaps (material leakage) or excessive extrusion (deformation and damage) between the nozzle and the mold or runner plate. Accurately calculating thermal expansion dimensions and rationally designing clearances and positioning structures are key to ensuring proper operation of hot nozzles at high temperatures, directly impacting the system’s sealing and service life.
The calculation of the axial thermal expansion size of the hot injection nozzle is based on the law of linear expansion. The formula is: ΔL = L0 × α × ΔT , where ΔL is the axial expansion ( mm ), L0 is the axial length at room temperature ( mm ), and α α is the material’s linear expansion coefficient ( 1/ °C), and ΔT is the temperature change (°C, i.e., operating temperature minus room temperature). The α value varies significantly among different materials . Common α values for hot-dip nozzles are approximately 11 × 10⁻⁶/ °C for carbon tool steel (e.g., T10A ), approximately 10 × 10⁻⁶/ °C for alloy tool steel (e.g., H13 ), and approximately 16 × 10⁻⁶/ °C for stainless steel (e.g., 316 ) . For example, for an H13 steel hot-dip nozzle with a room temperature length of 100 mm , an operating temperature of 250 °C, and a room temperature of 25 °C, ΔT = 225 °C, and ΔL = 100 × 10 × 10⁻⁶ × 225 = 0.225 mm , indicating an axial expansion of 0.225 mm . During design, an axial clearance of ≥ 0.3 mm should be reserved between the nozzle and the manifold plate to prevent deformation caused by expansion.
Calculating radial thermal expansion is equally important, especially at the nozzle and sprue bushing. The formula is: ΔD = D0 × α × ΔT , where ΔD is the radial expansion ( mm ) and D0 is the outer diameter at room temperature ( mm ). Excessive radial expansion can lead to a loss of clearance between the nozzle and sprue bushing, causing sticking; insufficient expansion can cause material leakage. For example, for a stainless steel hot-dip nozzle (α = 16 × 10⁻⁶/ °C), with an outer diameter of 20mm at the fitting and an operating temperature of 300 °C, ΔD = 20 × 16 × 10⁻⁶ × (300-25) = 20 × 16 × 10⁻⁶ × 275 = 0.088mm . If the fit clearance at room temperature is designed to be 0.05~0.08mm , the clearance at high temperature will become 0.05-0.088 =-0.038mm (interference) to 0.08-0.088=-0.008mm (slight interference), which may cause sticking. Therefore, the room temperature clearance needs to be increased to 0.1~0.12mm to maintain a reasonable clearance of 0.012~0.032mm at high temperature.
Nonuniform thermal expansion requires special consideration. Temperature differences between different sections of a hot nozzle (e.g., high temperature in the heating section and low temperature in the tail section) can lead to different expansions in each section, potentially causing bending or stress concentration. For example, a hot nozzle with a heating section ( 50 mm length ) at 300 °C and a tail section ( 50 mm length ) at 150 °C, at room temperature of 25 °C, and made of H13 steel (α = 10 × 10⁻⁶/ °C) would have expansions of ΔL1 in the heating section of 50 × 10 × 10⁻⁶ × 275 = 0.1375 mm , and ΔL2 in the tail section of 50 × 10 × 10⁻⁶ × 125 = 0.0625 mm. This 0.075 mm differential in expansion between the two sections could potentially cause slight nozzle bending. The design should optimize the layout of the heating elements to minimize temperature gradients (≤ 50°C/100 mm), or employ an elastic support structure to allow for slight nozzle bending without inducing excessive stress.
The impact of dynamic thermal expansion on the fit clearance must be factored into the design. As a hot nozzle heats from room temperature to operating temperature, the expansion gradually increases with temperature. Temperature fluctuations (± 5 °C) during the pressure holding phase can also cause slight variations in expansion (± 0.005mm ). For high-precision fittings (such as the valve core and seat of a needle-valve nozzle), a compensating mechanism is required, such as a spring at the end of the valve core, to absorb this expansion variation and ensure a reliable seal. For example, for a needle-valve nozzle with an 8mm diameter valve core and an operating temperature of 280 °C and a temperature fluctuation of ± 5 °C, the radial expansion variation ΔD is 8 × 10 × 10⁻⁶ × ± 10 = ± 0.0008mm . Providing a preload of 0.01–0.02mm using the spring fully compensates for this variation, ensuring a tight seal between the valve core and seat.
Coordinating the thermal expansion of hot-dip nozzles with that of other components is crucial. The thermal expansion of components like the manifold and mold plate will interact with nozzle expansion. If the materials are different (for example, a nozzle made of H13 steel and a manifold made of P20 steel), the respective expansions must be calculated to ensure that relative displacement is within the allowable range. For example, if the nozzle is 200mm long ( H13 steel, α = 10 × 10⁻⁶/ °C), the manifold is 150mm thick ( P20 steel, α = 12 × 10⁻⁶/ °C), and the operating temperature is 250 °C, then the nozzle expansion ΔL1 is 200 × 10 × 10⁻⁶ × 225 = 0.45mm , and the manifold expansion ΔL2 is 150 × 12 × 10⁻⁶ × 225 = 0.405mm . This represents a 0.045mm differential between the two . Therefore, a clearance of ≥ 0.05mm should be reserved at the joint to prevent compression. For large hot runner systems, thermal insulation pads (such as mica and ceramics, α≤ 5×10⁻⁶/ °C) can be used to reduce heat conduction between different components and reduce the impact of expansion differences.
Thermal expansion dimensional verification requires integration with actual operating conditions. Test molds are performed to measure the actual expansion of the nozzle at operating temperature, compare it with the calculated value, and then adjust design parameters. This measurement can be performed using a laser length gauge (accuracy ±0.001mm) after the nozzle is heated to operating temperature and held at that temperature for 30 minutes (after thermal stabilization). For example, a hot-dip nozzle with a calculated expansion of 0.25mm may actually measure 0.23mm. This discrepancy stems from the actual temperature being slightly lower than the set value (240°C vs. 250°C). After adjusting the heating parameters to bring the temperature back to the set value, the actual expansion is consistent with the calculated value. For critical applications, long-term thermal cycling testing (≥100 cycles) is also required to verify expansion stability and detect any abnormal expansion due to material fatigue. This ensures dimensional stability over the hot-dip nozzle’s service life (typically exceeding 100,000 mold cycles).
