Process Safety and Energy Efficiency Hot runnings
Process safety and energy efficiency are hot topics in hot runner technology. Precision parts and parts with special surface quality for example are particularly demanding. Modular, low-maintenance systems are required for productive, multi-cavity tools.
The requirements of our customers are similar across all industries," says Dr Stefan Eimeke, director of development at Ewikon Heißkanalsysteme in Frankenberg, Germany: "Faster and more cost-effective production with higher quality and precision." Compact and maintenance-friendly multi-cavity hot runner systems are highly sought after in order to achieve high productivity at the smallest possible injection moulding machine sizes.
Expert register two trends in this respect: Firstly, a great demand for mult-tip nozzle concepts, in particular with nozzles for lateral connection. These allow very compact system layouts that are also easy to maintain. And secondly, an increasing share of valve gate application, regardless of the processed materials and the system’s multi-cavity-properties.
This was mainly due to the increase in process reliability, of course combined with the perfect cut quality this technology provides. Furthermore, modern technologies like electric valve gate technology with stepper motor drives allow users to actively impact processes, for example by traveling to several needle positions during one cycle, or through quick fine adjustment of parameters during production.
Reduced cycle times, elimination of additional energy consumers
Regarding energy efficiency, Eimeke observes, "that a hot runner system is inherently more energy efficient when compared to conventional runner systems (cold runner), where material must be both heated up and then cooled down again." In hot runner systems, once the melting has been heated up, its temperature must only be maintained. Benefits in terms of energy efficiency also come from reduced cycle times and the elimination of additional energy consumers like sprue mills: "When upgrading an 8x-cold runner system to a hot runner system, you can for example achieve energy savings of 30 to 45% through the corresponding reduction of cycle times."
The total energy consumption and thus the efficiency of a hot runner system is largely dependent on the losses occurring during the process. Heat conduction losses at direct contact points between hot runner components and tools are particularly relevant in this respect, As they are responsible for almost 90% of the energy loss of a hot runner system. In order to improve energy efficiency, "one must start here", says Eimeke.
In the processing of thermally sensitive plastics it is of key importance to keep shear at a minimum, to avoid long dwell times in the hot runner system, and at the same time to eliminate dead spaces in the system. For clean room applications, valve gate solutions have proved ideal because unlike with pneumatic or hydraulic actuators, there is no particle emission with this system.
When injection moulding technology is integrated into complex, automated production cells, for example in the production of parts through the insert moulding process, a stable process is one of the key requirements. Valve gate technology is applied in many cases to increase reliability. This allows for production with short cycle times and perfect gate shape and to exclude from the outset potential causes for rejects, like tailing. Ease of maintenance is another important issue, as long downtimes for maintenance must be avoided.
Armin Kölz, sales manager Hot Runner Systems at Hasco Hasenclever in Lüdenscheid, Germany says that heat balancing was "an issue determined by many factors". In addition to constructive measures in the tool to avoid interferences, using alternative materials like titanium or ceramic for supports and sealing surfaces, but also cross sections and diverters of mass channels, and the location of heaters and thermal sensors play a key role in maintaining a constant heat balance in the entire hot runner system.
Controls should be highly accurate, provide monitoring functionality and also have a user friendly and intuitive interface. The processor must be capable of reliably controlling the ever more rapid and sensitive nozzle heaters in virtually all processing windows.
Clean room requirements particularly concern drives used in valve gate technologies. Electric drives are increasingly used beside well-known drives, like pneumatic and hydraulic drives. Servo motors in particular offer 100% clean room compatibility combined with maximum accuracy. But hydraulic systems that do not necessarily need to be opened for maintenance purposes also meet the requirements thanks to their special construction.
More productive, multi-cavity tools require modular, low-maintenance systems that can be integrated quickly and simply into the tool concept already in the construction stage. So-called modular or compact systems, where nozzles can be clustered in small cavity numbers, not only simplify production of the distributor block, but of the whole system, both from a technical and an economic perspective.
The trend within the trend is cooperation with the system supplier, who in this case not only provides the hot runner system, but also assembly right up to the electrical wiring and the complete "hot part".
Dr. Dominic Friederich, Managing Director of Roko Heißkanaltechnik Friederich GmbH, Pforzheim (Germany) and Xintech Hotrunner AG, Dübendorf (Switzerland) sums it up: "Current trends in hot runner technology reflect the latest developments in plastics processing". Material-wise, hot runner technology increasingly faces the challenge of providing reliable hot runner systems for high performance and high temperature polymers. On the other hand, the number of so-called "precision parts" and items with special surface finishes is still increasing.
Both developments ask specific questions of hot runner technology, which manifests itself particularly in regard to heat balancing. Required are hot runner systems that ensure optimum heat management for both flowing and stationary plastic melt. Furthermore, it is increasingly important for hot runner manufacturers to not only provide the product itself, but also application-specific support.
In addition to the thermal insulation of the hot runner system, in particular of the hot runner distributor, heaters that ensure maximum efficiency through optimal heat transfer are also needed in order to increase energy efficiency. Apart from the use of highly thermo-conductive materials, this also requires optimum utilisation of the heat conducting surface.
An appropriately sensitive control technology that can detect and control even minimal thermal changes in the melt is necessary to reliably process thermally sensitive plastics.
The clean room suitability of hot runner nozzles and systems used today in these environments has been proven in recent years. The now fully developed electric version certainly remains the first choice here. A high degree of automation has become a matter of course in today's hot runner technology. However, says Friederich, "maintainability" and "reliability" must be "considered much more important".
Increasing demands on plastic parts' precision and surface quality
"We see a current trend towards ever-increasing demands in terms of precision and surface quality of plastic components, via customizable injection concepts and thus adaptation to the relevant products," explains Ravan Graubner, sales manager at Witos in Frankenberg/Wangershausen Germany. "Moreover, we improve the quality of plastic parts with steadily ever more advanced drive units, valve gate systems, various capabilities for lateral injection and bolted and pre-finished hot runner systems."
To increase energy efficiency, the company offers its customers electric instead of hydraulic or pneumatic drives. Highly precise control of valve gate lifting plates, an external control unit and permanent position sensing through internal path tracking systems ensure precise cut quality.
In processing thermally sensitive plastics, the company would work toward "permanent advancement of hot runner nozzles and temperature profiles as well as optimal coordination of the installation scenario between hot runner manufacturer and tool maker". Graubner says the company has presented an electric drive particularly designed for clean room applications at the Fakuma. The electric servo motor is fully encased, so that there is no risk of a release of lubricants or media in clean rooms. Compared with hydraulic and pneumatic needle drives, the installation effort for servo motor drives is minimal.
For lateral injection, optimally adapted fully hydraulically balanced systems are recommended. All melt guides by default already come with deflectors preventing "dead corners" in the melt flow. According to Graubner, an optimised heater guide in distributor and nozzles ensures a uniform temperature regulation right down to the nozzle tips. All nozzles can be individually regulated and thus optimally configured for a smooth opening and filling of all cavities.
"For us," concludes Witosa's sales manager, "highly complex applications and a high degree of automation do not conflict with ease of maintenance and reliability. All our systems, no matter how complex, are always designed with a focus on ease of maintenance."