Hot runner is a method of heating, so that plastic in entire runner from nozzle of injection machine to gate of mold cavity is always in a molten state, so that after injection is completed and mold is opened, only product is taken out instead of solidified waste in runner. Through functions of hot flow plate, hot nozzle, and its control system, mold can improve quality of injection molded parts, accelerate production speed, reduce production costs, and make difficult products when mold is formed.

Hot runner usually contains following components: hot drop, manifold, hot runner controller and valve controller, see figure below.

 

Hot runners can be divided into two types according to heating methods: internally heated hot runners and externally heated hot runners; advantages of internally heated hot runners are: no flash, no thermal expansion, no heat loss, will not affect mold temperature, short heating time and high mold stability; advantages of external heating hot runner are: completely designed cross-sectional shape, low pressure loss, easy color replacement, no temperature difference and easy balance.

 

Heat balance is one of core issues of hot runner system. Main task of hot runner plate is to send melt from main runner to each individual nozzle at a constant temperature. Hot runner plate must be in a thermal equilibrium state, and heat loss must be compensated by heating. Therefore, function of hot runner is heat preservation. Temperature balance of hot runner: In thermal system of hot runner, heat is transferred in following three ways:
1) Heat conduction: According to Fourier's law, if there is a temperature gradient ΔT=T1-T2, a certain amount of heat QC flows through a single-layer, flat mold wall, formula is QC = λ/δ x A x ΔT
Qc ----- heat flow, W; λ ----- thermal conductivity, W/(m•K); A ----- cross-sectional area, ㎡; δ ----- wall thickness, m; ΔT ----- temperature gradient, K; heat transfer caused by conduction in hot runner can be divided into following two aspects:
Heat source: heating power of manifold, heating power of nozzle, if necessary, heating power of main glue feed.
Heat consumption: positioning ring, positioning pin, pressure plate, air gap. Heat loss will cause uneven temperature distribution, so-called thermal imbalance.
2) Thermal convection: amount of heat transferred by convection Qconv = αA (Tw-TA)
Where Qconv ---- heat flow,α ---- heat transfer coefficient, A ---- wall area, Tw ---- fluid temperature, TA ---- Wall temperature. Since heat generated by heat source in manifold is transferred to surface, temperature gradient between it and mold plate causes heat convection under action of air.

3) Thermal radiation: According to Stepin-Boltzmann law, QR = A c1│c2[(T1/100)Λ4-(T2/100)Λ4]

 

A ---- wall area, c1│c2 ---- radiation exchange coefficient T ---- thermodynamic temperature of the part, K radiation energy exchange occurs between splitter plate and surrounding template. Part of radiation is reflected back by surface of object, part of it is absorbed and then converted into heat.

How does Moldflow predict effect of temperature changes on molding? Software can be set as follows:

 

 

 

Through above settings, it is possible to accurately simulate heat transfer and heat loss of hot runner, as well as influence on mold temperature; at the same time, following three points are recommended to reduce influence of hot runner on mold temperature
To avoid chimney effect, hot runner is completely contained in mold except for wiring hole;
Reduce contact area between hot runner and mold, and use materials with low thermal conductivity;
Keep a sufficient distance between mold and manifold and hot nozzle;

Total pressure loss in hot runner is sum of forces from compression of material itself and flow resistance in runner that must be overcome when pushing material, which is flow resistance suffered by melt during flow process. It is difference between input pressure and output pressure: PL(ΔP) = PIn-POut. In injection molding process, pressure loss is firstly based on viscosity of plastic material (fluidity index); composition of filler; and compression ratio of plastic itself. If divided according to different equipment, pressure loss mainly comes from following 3 equipment:
1. Pressure loss in barrel of injection molding machine;
2. Pressure loss of runner system;
3. Pressure loss inside mold cavity; this requires us to adjust pressure balance and distribution of entire hot runner system when trying hot runner system. Unbalanced pressure will cause insufficient partial pressure to cause product glue; excessive partial pressure will cause product burrs and increase tonnage of injection molding machine, which will affect normal production of products. Simplified diagram of pressure loss at each gate of hot runner is as follows:

 

Moldflow has a mature and complete heat pump Beam unit, which can accurately simulate pressure drop. Valve gate time controller can be used to open and close each hot runner, which is suitable for solving welding line problem and controlling tonnage of injection molding machine; you can also use Dynamic Feed system to adjust pressure at each gate to control flow speed of individual gate to prevent flow marks (tiger skin patterns).

 

Needle valve hot runner and open hot runner

 

 

Moldflow's latest 3D technology can accurately simulate flow state and temperature distribution of hot runner; especially influence of shear heat on temperature, which can avoid problems of local excessive temperature and flow dead ends, improve forming performance of hot runner.

 

 

 

Summary: In summary, using Moldflow to simulate hot runner mold can accurately simulate impact of temperature change of hot runner on mold, predict product performance and defect causes in advance; it can accurately simulate opening and closing time, pressure distribution of hot runner gate to improve forming performance of hot runner; it can also simulate temperature and shear heat in hot runner to prevent flow dead angle and black spots from being excessively high, and optimize hot runner design.