Overview
The ability to make and use NdFeB stir elements to stir very viscous solutions and tightly couple them for high-speed mixing procedures has long been the goal of scientists. Also, the ability to use the flexibility of 3D printing to easily make Stir Elements of any shape is a significant advantage over the molding process typically used to encapsulate magnetic stirrers. The cost of making a mold is significantly high and the temperatures necessary to make the encapsulating material (like PTFE) molten is high enough to destroy the magnetic ability of NdFeB. The ability to 3D print and encapsulate NdFeB means that prototypes of a unique design can be made in a single day for less than $50.00, and if successful, the process can be automated for production runs of hundreds of parts per day.
In addition to the rare earth magnets NdFeB and SmCo, lesser expensive AlNiCo or Magnetic Stainless Steel can also be encapsulated by this method.
The Problems
Most Stir elements on the market today employ PTFE encapsulation via a molding process of AlNiCo or SmCo magnets. No company has been able to PTFE encapsulate NdFeB because the temperature necessary for melting the PTFE destroys the magnetic character of the NdFeB. NdFeB magnets are the strongest of all magnetic materials and therefore the most useful in mixing viscous liquids. Also, the strength of NdFeB stir elements is useful in high-speed stirring applications because they remain strongly coupled to the drive magnet. We were the first to produce PVDF and PEEK encapsulation of NdFeB stir elements. We developed a system of molding simple hollow cores from PVDF and PEEK and then placing a NdFeB magnetic cylinder into the hollow core and sonically welding the two cores together. This method works with simple cylinder designs but not with complex designs because of the limits of sonic welders.
We have also developed a vapor deposition process using Parylene to encapsulate NdFeB magnets that protects them from caustic solutions but the soft parylene coating is quickly worn off by the friction of stirring against a glass or plastic surface. We sought to protect the Parylene coating by taking a sheet of PTFE and using an end mill to make identical pockets on both side of the PTFE sheet leaving a 2 mm thick membrane of PTFE between the two pockets and placing a Parylene coated NdFeB disc into each of the pockets, so they are held in the pocket magnetically. This method protects the Parylene coating and allows for more complex stirrer designs, however cleaning these stir elements requires taking the parylene-coated NdFeB magnets out and cleaning the pockets and the parylene-coated magnets after each use to prevent cross-contamination. A cumbersome and time-consuming process.
The Solution
We start by making a stereolithography 3D printed stir element that has one or more slot openings in the top surface that will accommodate one or more magnets. Once the support structure is removed, the magnet(s) are placed into the slot or slots and a liquid form of the same high-temperature and chemically resistant resin is poured into the slot covering the magnet. The stir element is then placed into a UV light chamber to cure (harden) the resin and encapsulate the magnet. The Stir element is further hardened by placing in an oven at 80° C for 120 minutes. 3D stereolithography printing allows for many custom and complex designs, while still fully encapsulating the magnet. The ability to capture the utility of stereolithography 3D printing makes this method an inexpensive prototyping masterpiece. The resin formulation allows for traditional sterilizing methods, such as autoclaving and gamma radiation, to be used.
Advantages
- Fully hard encapsulated NdFeB Stir Elements
- Unlimited design possibilities due to stereolithography 3D printing
- Solvent/Temperature resistance encapsulation material to 150°C
- Rapid production of workable prototypes
- Easy clean up
- No need for coated magnets
- Can use NdFeB N42 or NdFeB N52, SmCo, AlNiCo or Magnetic Stainless Steel
