Breathable Aluminum for Thermoforming and Vacuum Clamping

METAPOR is a new composite aluminum material that offers one unique characteristic – it breathes. The micro porous structure is capable of conducting air and gases for applications that use vacuum or air pressure. Such applications are separated into two industrial segments; thermoforming and material handling where it is used to build vacuum clamping devices and air-cushion systems.

This porous metal composite is a combination of aluminum powder and epoxy resin. The secret recipe and a proprietary manufacturing process enables production of large cast blocks having dimensions of 20" x 20" x 16". The blocks are subsequently cut into plates of varying thickness for distribution as illustrated below.

Several plates can be fashioned together using a special adhesive to accommodate deep draw or large molds and large vacuum holding applications. Since the material is created by casting, alternative technology is available to produce custom porous shapes using existing patterns. This technology is called ESPOR and Portec – North America licenses it and materials to qualified users within the U.S. and Canada. With ESPOR the user can cast a reverse impression with incredible precision reproducing complex surface textures and elements.

For thermoformers, the materials offer new technical capabilities that would have never been considered using traditional aluminum tooling. The micro-porous nature of this product eliminates the limitation of introducing intricate details, and infinitely increases the final resolution, making it possible to form extremely complex surface patterns and details. In the past, if a tool maker wanted to add lettering to a mold, tiny vent holes would have to be drilled in every letter and the size of the letters would have to be large enough to accommodate the size of the drill. Some users of METAPOR material use extensive engraving and even carving to create artistic designs that would be unthinkable with traditional tooling materials.

The uniform porosity of the material means that the vacuum is present on every surface of the mold, so as the plastic is formed it not only stretches down, but it is sucked against all surfaces of the mold resulting in extremely sharp definition and crispness. The same effect takes place during the "blow-off" cycle when the air-pressure through all surfaces of the mold helps to release the formed part without deformations.

A common problem in thermoforming occurs on flat surfaces of the mold when air is trapped between the sheet and the surface of the mold. This causes waviness of the plastic as well as shiny spots. Now, thermoformers can use METAPOR to eliminate such problems on flat surfaces. It is not necessary to build a complete mold using METAPOR, and many toolmakers are effectively using METAPOR inserts in combination with aluminum and other materials.

The surface finish of METAPOR molds is generally quite good, however, for forming transparent plastics and parts requiring optical clarity, the mold surface has to be polished. This requirement was addressed last year with introduction of high density METAPOR HD 100 AL material. This formulation has a much finer grain structure and can be highly polished.

The complete surface porosity results in extremely fast air evacuation. This accelerated cycle helps to maintain the plastic within a tighter temperature formability window. Also, some users have been able to avoid using a plug assist for drawing plastic into deep cavities.

Mechanical and thermal properties of the material are different than standard aluminum, so it is important to understand the benefits and limitations of each material in order to use them effectively. It is softer than aluminum, so for very high production tooling, the overall frame could be made using aluminum and inserts within each cavity can be made using METAPOR. The material is less thermally conductive than aluminum, however, so it cools faster by convection when air is cycled through the porous structure. In some cases, water-cooling lines are inserted into METAPOR molds or they are mounted on aluminum "chill plates".

The other main area of application is material handling. Vacuum clamping systems, vacuum chucks, and air bearings. The pores within the material do not form direct passageways or channels as compared to drilled holes. This characteristic causes the build-up of air pressure within the material thus avoiding significant air leaks. So when a METAPOR vacuum chuck is partially covered, the substrate is still held in place. This feature is important in cases when certain manufacturing operations create openings in the substrate, or when the substrate is not perfectly solid.

Three METAPOR products are currently being used in material handling applications. One is a good solution for partially covered surfaces, or for applications requiring high flatness and good surface quality. Vacuum clamping of thin films, circuit boards, and other delicate devices is common for this product. Another is a highly porous and is used in applications where the full surface area is covered. Due to increased air-permeability, this product does not maintain as much air pressure within the material. However, the evacuation of air is very fast and the holding forces are stronger. Lastly, is a new product with very high air-permeability and a good surface finish.

Ease of machining and stability makes it possible for METAPOR to be used in highly precise systems. It has been used to build large vacuum clamping tables (6’ x 6’) in order to hold very thin sheets. The tables had to have a flatness of ± 0.002" from corner to corner, and they were constructed from 16 individual plates of the material.

ESPOR casting technology is sometimes used to create vacuum clamping systems by casting them from existing parts or patterns. With this material, the air permeability can be varied depending on the application. METAPOR and ESPOR micro-porous material technologies offer unique technical capabilities and solutions in areas of thermoforming and material handling.

For more information, contact Michael Tsenter, portec - North America, a division of NEST Technologies, 3849 Ridgemoor Drive, Studio City, CA 91604, 818-761-6500, Fax: 818-761-6116, E-mail: nest@primenet.com, Web: www.nesttechnologies.com.