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This article describes the processes used to manufacture the adhesives used in the various projects in BDML. The process of creating adhesives takes four broad steps:
- Preparation of Molds
- Preparation of Backing Material
- Casting Adhesives
- Postprocessing Steps to Integrate Adhesives
Molds for most adhesives are prepared by micromachining features into a wax block for one-off or research geometries.
Details on preparing wax blocks for machining can be found here, and details on the machining process itself is located here. An overview of scripting tools to generate the machining tool paths can be found here.
More durable molds can be fabricated in epoxy using a daughter mold process, using an additional casting step to create a daughter mold by casting epoxy over a silicone master. Ongoing work is exploring two approaches to achieving a solid metal mold: using lithographic techniques to create a metal daughter mold for additional durability and temperature capabilities, and directly micromachining a metal base using the process described for wax molds above.
In most cases, for usability, the adhesive must be bonded to a backing layer capable of supporting adhesive loads.
For grasping curved surfaces, this backing is typically a Kapton film, and the procedure for fixturing and casting on this or similar nonporous films is here. Other applications have used Silnylon fabric; this is slightly porous, and so these backings are simply stretched and fixed around a glass reference surface. Casting on flexible fiberglass sheets is similar to the process for thin films, but uses the aluminum foam backings for rigidity.
For flat surfaces, rigid tiles with tendons are the most used configuration. The tiles can be manufactured via a fiberglass lamination process, as described here. This process is required for later post-treatment of the adhesives. Rigid backings can also be directly 3D-printed; this offers the advantage of much more precise control over tendon locations and routings, simplifying manufacturing in most cases.
For testing samples, it is often beneficial to have the adhesives without backing; to accomplish this while maintaining constant thickness adhesives should be cast with a clean, non-primed acrylic backing of sufficient thickness to be approximately rigid. The adhesive can then be easily peeled away from this backing during the demolding process.
The current process involves mixing and degassing the selected polymer, and then pouring over the mold. The prepared backing is then pressed onto the mold until contacting the reference surface, thus defining the cast adhesive thickness. Excess polymer is allowed to squeeze out of the interface.
In most cases, postprocessing will simply consist of dicing the adhesive into the required sizes, attaching tendons, and installing into the adhesive mechanism.
Some use cases will require post-treatment for improved performance. The inking procedure is detailed in this article, and has the effect of increasing both shear and normal performance. A post-treatment with PEDOT:PSS is in development. Preliminary results suggest a small effect on smooth surfaces, but a significant improvement on rougher surfaces.