In-depth analysis of hot runner injection mold hot runner plate design

In the continuous development of the mold industry, hot runner molds have gradually become an important part of the injection molding field due to their advantages of time and material saving, high efficiency and stable quality.

However, the design and application of hot runner mold is not an easy task, in which the design of hot runner plate is even more critical. Today, we will discuss in depth about the design points of hot runner plate in hot runner injection mold.

Introduction

The core of a hot runner mold lies in the hot runner plate, which is a steel plate heated by a heating element, with a main flow channel and a diverter channel in between, used to keep the plastic in the molten state.

The design of the hot runner plate needs to consider two main issues: one is the eccentricity of the nozzle centerline due to thermal expansion, and the other is the accurate calculation and control of the heating power.

Compensatory measures to avoid nozzle centerline eccentricity

1.Use of nozzle deflection compensation

Figure 1 shows the multi-cavity main flow channel gate hot runner mold, hot runner plate 6 rely on air adiabatic and increase the diameter of the manifold method to make the manifold in the plastic to maintain a molten state, the nozzle 8 by the heating coil 9 for heating.

Manufacturing nozzle 8 material for the soft steel or high-strength nickel bronze, the use of its good deflection to compensate for the thermal expansion generated by the work of the hot runner plate, to ensure that the center line of the nozzle is in the correct position.

Figure 1 Multi-cavity hot runner mold with main flow channel gate

1. fixed mold seat plate 2. pad 3. cylindrical pin 4. plug 5. screw 6. hot runner plate 7. bracket 8. nozzle 9. heating coil 10. fixed mold cavity plate 11. moving mold cavity plate

2.Compensation by adiabatic layer

Figure 2 shows a multi-cavity point-gate hot runner mold with electrically heated runners and beryllium copper alloy nozzles with strong thermal conductivity.

The front of the nozzle is insulated with plastic, and the middle is 0.4-0.8mm thinnest. the nozzle is slidingly connected with the runner plate, and sealed by expansion ring.

Temperature difference causes thermal expansion, nozzle and sprue bushing have adiabatic gap to prevent interference, design needs to be reserved eccentric distance.

Figure 2 Multi-cavity hot runner mold with spot gate

1. core 2. fixed mold cavity plate 3. gate plate 4. gate bushing 5. nozzle 6. expansion ring 7. hot runner plate 8. Heating holes 9. Fixed mold seat plate 10. Set screw 11. Sealing ball 12. Set screw 13.Set screws 14. Brackets

3.Compensation by mutual sliding of parts

The laser heats the material to the ignition point, pure oxygen reacts exothermically to produce molten oxides, and an auxiliary gas blows off and initiates continuous combustion, which is suitable for material cutting.

This technique is mainly applicable to the cutting of materials such as titanium and steel. In Fig. 3, the thermal expansion of the hot runner plate is compensated by the sliding of the nozzle and the cavity plate along the A surface;

In Fig. 4, it is realized by the sliding pressure ring, and the beryllium copper nozzle and the fixed template are equipped with a sliding pressure ring and a plastic adiabatic layer, which constitutes a fully adiabatic nozzle.

Figure 3 Structure for compensating eccentricity by relative sliding of nozzle and cavity plate along the A-plane

1. Cavity plate 2. Heating coil 3. Nozzle 4. Support 5. Heated runner plate 6. Plug 7. Heat insulation pads 8. Positioning support nails 9. Fixed mold seat plate

Figure 4 Structure to compensate eccentricity by relative sliding of sliding pressure ring

1.stencil plate 2. nozzle 3. sliding pressure ring 4. heated runner plate 5. manifold

4.Compensation by floating gap

As the center distance between hot runner plate nozzles increases, so does the thermal expansion. When the temperature difference between the fixed mold and the moving mold is large, it will lead to inconsistent thermal expansion.

In high-precision molds, half of the mold (e.g., concave mold or convex mold core) can be designed as a floating structure to compensate for the thermal expansion of the hot runner plate through the floating gap.

As in Figure 5, the core and cavity plate are tapered to fit together, and the nozzle drives the cavity

Figure 5 Structure of eccentricity compensation by floating gap

1. moving plate 2. core fixing plate 3. core 4. cavity plate 5. cavity fixing plate 6. nozzle 7. heating probe 8. manifold 9. heated runner plate

Hot runner plate heating power calculation

Hot runner plate heating power required by two parts,

one is the molten plastic to reach the set injection temperature of the electric power required; the second is to supplement the hot runner plate conduction, convection and radiation heat loss of power. At present, the commonly used calculation methods are the following three:

1.Rough calculation method

Laser cutting technology shows significant advantages in the processing of metal materials. It can not only significantly improve the processing efficiency, reduce material loss, but also improve the processing accuracy.

Especially for metal sheet materials, laser cutting technology has become one of the important indispensable processes.

2.Chart calculation method

According to the volume or mass of the mold to check the chart, and then use the empirical formula for calculation.

The commonly used empirical formula for calculating the electric power required for heating the hot runner plate is P=ηG, where P is the electric power, G is the mass of the mold, and η is the electric power required for each kilogram of the hot runner plate when it is heated up to the set temperature.

η can be obtained by checking the chart directly according to the mass of the runner plate.

Figure 6 Curve of electric power required to heat up the hot runner plate with mass from 1 to 10,000kg per kg

1. 1kg~ 10kg 2. 10 ~ 100kg 3. 100 ~ 1000kg 4. 1000 ~ 10000kg

3.Formula calculation method

The formula calculation method takes into account factors such as the mass of the hot runner plate, the specific heat, the warming time, the set temperature and the room temperature, as well as the electric heating efficiency of the heater and the supplemental efficiency of the heat loss of the runner plate.

The heater power can be calculated by the formula P=WC(t1-t2)/860ηT. Where W is the mass of the hot runner plate, C is the specific heat of steel, t1 is the temperature of the hot runner plate, t2 is the room temperature, T is the heating time, and η is the efficiency coefficient.

Conclusion

The design and application of hot runner mold is a complex and detailed process, in which the design of hot runner plate is even more critical.

Through the in-depth discussion of the thermal expansion compensation measures and heating power calculation method of the hot runner plate, we can better understand the working principle and design points of the hot runner mold.

This not only helps to improve the mold design level, but also provides strong support for the successful application of hot runner molding technology.