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Hot Runner Solutions for Micro Parts

Return list Reffer:NITONI Click number:- Release time:2018-10-15

Injection molding of micro parts requires the use of innovative hot runner solutions to meet the needs of injection mold workers and mold manufacturers.

Injection molding of precision micro parts places special requirements on mold manufacturers and hot runner suppliers. Since the weight of such parts is only 0.05~2g, the weight of the runner system also has a great influence on the price of the parts. In fact, most miniature parts are produced with very small tolerances. In order to ensure that all cavities can be uniformly injected into the melt, a naturally balanced runner system is used.

With a solidified runner system, the weight of the runner is many times greater than the weight of the part. This will inevitably increase the cost of materials for each part, and it will also require the addition of separation equipment for the flow path and parts, which will increase the overall price of the parts.

On the other hand, if an ordinary commercial hot runner system is used, the size of the mold and the manufacturing cost of the mold will inevitably increase due to the size and the number of nozzles required to directly inject the parts. In order to reduce the cost of molds and materials, a comprehensive and balanced hot runner system is required for a large number of mold cavities with a small control area and a small pitch.

Minimum pitch size balance

In injection molding of small parts, the pitch dimension of the hot runner tip plays a decisive role in the size of the mold. In order to reduce the size of the mold, the tip of the hot runner is tightened directly onto the manifold. Then it is naturally balanced by a special dispensing device. Figure 1 shows the hot runner system used in 4, 8 and 16 downstream runner applications. The internal distribution of the melt is shown in Figure 2. A radial-milled cylindrical manifold diverts the melt to the tip of each flow path with the same flow length. This design allows a fully balanced hot runner system to be contained in a limited space.

The use of technical resins (especially those containing flame-retardant additives) is very important and it allows the material in the manifold to maintain a short residence time. In fact, the lower pressure loss and the short residence time are contradictory. For a lower pressure loss, a large flow rate is required, while for a shorter inflow time, a smaller flow rate is required. With an externally milled cylindrical manifold, both requirements can be met simultaneously.

If strict materials are chosen, the residence time and pressure drop in this system must also be calculated. There are many calculation programs available on the market and the best solution can be calculated.

The cylindrical element is mounted on the manifold body and is heated at the center using a heating cartridge. The small tolerance size and precise manufacturing process, together with the unique sealing plug, ensure that the system is free of any leakage.

Influence of temperature distribution on quality

The qualitative change of the material depends not only on the residence time but also on the temperature distribution of the hot runner elements. The processed plastic must not exceed its allowable temperature range to avoid deterioration of the material in the manifold. For this reason, it is necessary to have a balanced distribution of temperature throughout the area. In a hot runner system, a certain amount of heat is often delivered to the mold via a heating element. There are three ways to transfer heat to the mold: radiation; convection and heat transfer.

It is also necessary to minimize the number of these three heat transfer methods. If radiant heat transfer is used, it means that the surface of the neon tube is similar to a white light emitter; if convective heat transfer is used, it should be kept as small as possible. This is very important; if heat conduction is used, it should be Ensure the smallest contact surface.

Figure 3 shows a 16-channel downstream manifold with special titanium alloy components to reduce heat transfer, and an optimized heater to evenly distribute radiant heat. At both ends of the manifold, the heat transmitted by the radiation is higher than the heat in the middle of the manifold, which means that the two ends of the heater need to introduce more heat than the intermediate stage.

Micro hot runner mold design

As mentioned earlier, the best effort to reduce the contact surface between the mold and the hot runner reduces the amount of heat transferred to the mold (see Figure 4). The melt chamber adopts a front seal around the titanium alloy ring. This can compensate for the coefficient of thermal expansion of the helium tube and avoid the tension caused by thermal expansion. The pressure on the load surface is small, so the lifting force can be reduced. A small amount of material in the area of the injection chamber facilitates rapid discoloration and maintains a short residence time.

The manifold must be centered to ensure uniform thermal expansion in all directions. The small pitch between the tips facilitates the accurate calculation of the coefficient of thermal expansion and allows the diameter of the small injection port to reach 0.4 mm.

Cutting-edge materials must have good overall performance, maintaining a level of compromise between wear resistance and thermal conductivity. To achieve this goal, molybdenum alloys can be used as the tip material. If all the guidelines in the mold design can be taken into account, then a micro hot runner system can be used on a small injection molding machine to create a high-quality part with minimal imprint.

In the mold, there are many applications using micro hot runners. They can be used with very small injection molding machines or four branch manifolds under a distributed manifold (see Figure 5).

The combination of micro hot runners and standard manifold parts provides many types of mold concepts. Although the number of control zones has been reduced to a minimum, the use of a two-in-one hot die ensures the correct installation of the system, avoiding some of the problems that occur when the die starts to work.

The progress made in the development of hot runners has provided mold manufacturers and injection mold manufacturers with new solutions that will enable them to remain undefeated in the ever-changing competitive market.

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【Paper label】:Hot Runner Hot Runner system Parts