Hot runner technology in injection mold
With the continuous improvement of the performance of plastic raw materials, the proportion of plastic molds in the mold is increasing, and it has been widely used in various sectors of the national economy and daily life, and the hot runner mold has also gained its unique advantages. The more widely used. Hot runner molds have the following advantages over cold runner molds:
(1) Shorten the molding cycle of parts; (2) Improve product quality and reduce scrap rate; (3) Save plastic raw materials; (4) Eliminate subsequent processes, which is conducive to production automation; (5) Expand the application range of injection molding process.
Although hot runner molds have many unparalleled advantages over cold runner molds, we cannot ignore the shortcomings of hot runner molds. To sum up, the general hot runner mold has the following disadvantages during use:
(1) The structure is complicated; (2) The mold cost is high; (3) The maintenance is complicated; (4) The local overheating problem in the hot runner system cannot be fundamentally solved; (5) Because the gate and the hot runner nozzle are away from the mold cavity Recently, after the heat insulation measures are taken, the heat loss cannot be avoided, and the temperature difference between the two ends of the hot runner nozzle is large, and it is difficult to maintain the temperature balance, thereby adversely affecting the injection molding.
Key technologies for hot runner systems
We know that the hot runner is a method of heating to keep the plastic in the entire flow path between the nozzle of the injection machine and the gate of the mold cavity in a molten state, so that only the product is taken out after the injection is completed without generating the sprue. Advanced gating system. The plastic melt needs to flow through the main nozzle, the hot runner plate, and the split nozzle to finally reach the mold cavity. Therefore, how to ensure that the plastic remains in a molten state and temperature balance in the main nozzle, the hot runner plate and the sub-nozzle during use, how to insulate between the hot runner system with a relatively high temperature and the mold cavity with a relatively low temperature. It is the key to hot runner technology. 1.1 Heating of hot runner system 1.1.1 Heating of hot runner plate
(1) Calculation of heating power The heating power calculation formula of the hot runner plate [4] is as follows: where n is the heating power, t is the elevated temperature required for the hot runner plate, and g is the weight of the hot runner plate, t is The heating time is customarily based on 1 hour, but in recent years, the medium and small molds can be changed to 0.5 hours for the improvement of production efficiency, and the thermal efficiency is generally 0.2 to 0.3. (2) Heating mode There are two main heating modes of the hot runner plate, namely, internal heating type and external heating type. The external heating type heats the flow path from the outside of the flow path. Generally, a heating hole is opened in a direction parallel to the flow path, and a heating rod or a heating ring is inserted. This heating method indirectly heats the plastic in the flow channel, and does not cause local overheating. The flow resistance of the melt in the flow channel is small, the flow path is easy to process, the heating device is easy to purchase, and it is easy to repair and replace after the failure, but the thermal efficiency is low. The internal heating type is to place the heating rod in the flow path to heat the plastic from inside the flow path. Compared with the external heating method, this heating method has higher thermal efficiency, but is prone to local overheating, and the flow resistance of the melt is increased, and high pressure molding is required, and plastic parts may generate large internal stress. (3) The heating power calculation of the nozzle of the heating nozzle is much more complicated than the heating power calculation of the hot runner plate. It is still inconclusive and is generally calculated by the empirical formula. The heating method of the nozzle mainly includes no heating type, external heating type and internal heating type.
The non-heating nozzle means that the nozzle is not heated by the heater, but the heat of the hot runner plate keeps the plastic in the nozzle molten. The nozzle is generally made of beryllium copper with good thermal conductivity, and the length cannot be too long, generally 40mm is limited.
The externally heated nozzle is equipped with a heating ring on the outer circumference of the nozzle. This heating method does not cause local overheating. However, the back surface of the mold cavity plate must be grooved to fit the heating coil, and the size of the heating coil is also affected. limit. The internal heating nozzle inserts a heating rod into the nozzle. The thermal efficiency of the nozzle is high, the temperature at the front end of the nozzle is easy to control, and the gate does not solidify. However, since the diameter of the heating rod is small, the density of the tile is increased, resulting in the heating rod. Short life.
In order to keep the plastic in the hot runner system in a molten state during processing, as previously mentioned, various heating methods must be used to heat the hot runner system, so the temperature of the hot runner system is relatively high. However, in order to obtain a qualified plastic product, the temperature of the mold must be lower than the glass transition temperature of the plastic, and the temperature of the mold is relatively low. Therefore, in designing the hot runner mold, it is necessary to fully consider the insulation between the injector template and the mold bottom plate, between the flow plate and the mold bottom plate, between the flow plate and the cavity plate, between the nozzle and the cavity plate. Between the injection machine template and the mold bottom plate, generally 6 to 10 mm asbestos board insulation is used. The air passage plate and the cavity plate and the mold bottom plate are often insulated by an air gap of 3 to 8 mm. All the pressure-bearing parts in contact with the hot runner plate must be controlled in a minimum amount, and it is preferable to use a material having low thermal conductivity. manufacture. Plastic melt or air gap is used between the nozzle and the cavity plate for thermal insulation development.
Hot runner technology is an advanced technology used in the injection molding industry and has been widely used abroad. But in China, the technology is still in its infancy and promotion stage. As mentioned above, there are still many problems in the hot runner system. If it can be improved from the following aspects, the hot runner technology will be more perfect and thus more widely used. Standardize hot runner system components to simplify and reduce maintenance costs for hot runner systems.
Http://news.chinawj.com.cn Editor: (Hardware Business Network Information Center) http://news.chinawj.com.cn
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