Hot runner system
In the processing of plastics , in particular in the machine injection molding of thermoplastics, a special type of sprue system is referred to as a hot runner or hot runner system , which is thermally insulated from the rest of the injection molding tool and has a higher temperature.
Need / benefit
During the injection molding of thermoplastics, the plastic melt is transported from the plasticizing unit of the injection molding machine through the sprue system into the mold cavity (or cavities). In classic injection molds , the sprue system is not thermally isolated from the rest of the mold. Thus, both the tool and the sprue system in it are tempered to temperatures well below the processing temperatures of the plastic.
As a result, the thermoplastic also solidifies in the sprue system during the production of a component. This has some disadvantages for this type of sprue system. The sprue usually has to be separated from the component. This can be done either through additional functional elements in the tool or manually or by machine after demolding. After the sprue has been separated, a marking can remain on the component.
Some of the raw material does not flow into the product and must either be reused ( recyclates ) or disposed of. The sprue also increases the shot volume; more plastic has to be plasticized than is ultimately present in the product. This can play a role both in terms of energy and in relation to the necessary machine performance.
In the case of a hot runner system, the sprue system is thermally separated from the rest of the mold and heated separately so that the plastic melt in the sprue system remains permanently flowable. This means that the plastic does not solidify in the sprue system, and no sprue remains on the component. Furthermore, longer flow paths can also be realized with the hot runner systems, because the pressure loss in the sprue system is not increased by cooling the melt and the associated increase in viscosity .
The higher investment costs are disadvantageous, since the tool has to be specially designed to accommodate a hot runner system, the hot runner system itself incurs costs and the machines for controlling or regulating the system also have to be adapted. On the other hand, there are not infrequently - especially with expensive technical thermoplastics - significant savings in raw materials as well as savings in post-processing of the product.
The same purpose is also achieved by insulating channel tools in which the sprue is so widened that a plastic core remains. When thermoset and when elastomer injection molding where the plastic is cured under temperature influence, be correspondingly Cold runner systems used.
construction
The hot runner ends in the hot runner nozzle , which represents the transition between the heated and unheated zone in an injection molding tool. Both the hot runner nozzles and the distributor are set to the temperature window in which the plastic can be plastically processed. To regulate the temperature, hot runner control devices are used, which permanently compare and regulate the target and actual temperature.
Hot runner systems are mainly divided into open or closed systems. With closed systems, the injection point can be closed using a special technique. This is usually done by a closure needle , which usually by a separately controllable mechanical, pneumatic , hydraulic or electrically operable.
Hot runner systems are often purchased parts, so they are usually not made by the toolmaker himself.
See also
literature
- Menges, Michaeli , Mohren: Injection molding tools. Design, construction, application , 6th edition, Carl Hanser, Munich 2007, ISBN 978-3-446-40601-8 .