Shape memory alloy (SMA) wires represent a transduction technology, which allows to design highly compact and transmission-free actuator applications. The possibility to use SMA wires as actuators is due to the reversible phase transformation of the material crystal lattice, which can be properly triggered depending on the thermo-mechanical loading conditions. In particular, when the wire is heated via an electric current, it transforms into an austenitic phase and, in turn, undergoes a macroscopic contraction in length (on the order of 4 %). The initial length of the SMA is restored once the wire is cooled down below a specific transformation temperature and transforms to a martensitic phase, provided that the SMA is loaded with a mechanical pre-tensioning system (e.g. a linear spring). One of the biggest advantages of SMA wires, compared to conventional actuators, is their high energy density. This feature results in small construction space requirements, and opens up application areas which are hardly feasible, or even not possible, with conventional actuator technologies.

One of the application fields which can potentially benefit from the features of SMA technology is the active venting of cavities in injection molding. Depending on the shape of the injection mold, trapped air can prevent the mold from complete filling, thus degrading the overall system performance. To avoid this detrimental phenomenon, electromagnetic valves are commonly used to let the air flow outside of the mold chamber. However, these electromagnetic components are hard to miniaturize to a desired level, as required by many applications. Especially, dense packaging is often not possible due to the outer diameter of electromagnetic actuators.

To address this issue, this paper presents a novel SMA valve actuator configuration for injection molding applications. By means of the novel design, the same dynamic performance of conventional electromagnetic actuators can be obtained via an overall more compact and lightweight device. The new developed valve prototype has a total outer diameter of 7 mm. The proposed actuator concept is based on a pair of SMA-spring configurations working against each other. One of the SMA-spring configurations ensures opening and closing of the valve, depending on whether the wire is activated or deactivated, respectively. In order to avoid back injection, and thus ensure a correct functioning, the timing of valve opening and closing must be accurately triggered. The slow cooling times required by SMA wires makes it difficult to ensure these time specifications. For this reason, the second SMA-spring configuration is used for rapidly closing of the valve in an active way, independently of the cooling behavior of the first SMA. This function is realized by decoupling of the two actuator systems. Additionally, the two SMA-spring configurations are designed to effectively work at high ambient temperatures, compatibly with the requirements of injection molding applications. Functionality at these higher temperatures is ensured by higher pre-stress levels through the mechanical springs.


Decoupled Antagonistic SMA Actuator for Valve Applications

Division / Sponsors: 
Aerospace Division

Paper Proceedings: https://asmedigitalcollection.asme.org/SMASIS/SMASIS2020/volume/84027

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