IBAG North America

Holding Parts with a Porous Vacuum Chuck



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In past blogs, we've discussed many ways to use vacuum workholding systems to hold parts. Up till now, these vacuum chucks were made of a solid material and were machined to supply vacuum to the part surface. In addition, they rely upon a flexible seal (O-ring or pad) to make sure the vacuum does not leak out under the part. As long as we have vacuum under a sealed part, atmospheric pressure (14.7 PSI) will apply force down on the part. However, not all vacuum chucks rely upon a flexible seal. Some vacuum chucks are entirely constructed of porous material. The porous surface of the chuck exposes 100% of the part to the vacuum and generates holding force. So, how are these porous chucks made, and why are they needed?

Porous Chuck Basics
To build a porous vacuum chuck, we first need some porous material. What exactly is a porous material and how is it made? A porous material is a semi-solid material that allows the flow of gas, like a filter. The most common example of a porous material is sintered bronze. These materials are commonly used for bushings and pneumatic filters. To make sintered bronze, a mix of bronze powder is heated with a binder. When the process is done, the material has physical strength but voids remain in the material that makes it porous. Although sintered bronze has been around for many years, a much greater variety of porous materials exist today. Let's review how these different materials are used for vacuum chuck applications.

Sintered Bronze Vacuum Chucks
Sintered Bronze Vacuum ChuckSintered bronze is a great material to use for many simple vacuum chuck applications. It has high porosity (good vacuum flow), is very hard (good wear characteristics), and is inexpensive. High porosity allows high flow rates. High flow rates will compensate for leakage and result in the best holding force on your part. Sintered bronze material is used for many general purpose vacuum chuck applications including CD’s, credit card production and laser cutting & marking. In fact, a temperature-controlled vacuum chuck was produced for laser processing that holds the part and removes the heat.

Porous Aluminum Vacuum Chucks
A special material now exists called Metapor®. It is a porous material that can be made from a variety of materials, including aluminum and ceramics. Metapor® is made by mixing a fine powder with a binder. When the binder cures, small voids are created in the material that results in porosity.  Metapor® can be machined like aluminum to different shapes. For some applications, a flat chuck surface is required. The chuck can also be machined to exactly match the contour of the part, like a vacuum mold.

Porous Aluminum Vacuum ChuckThe best applications for Aluminum Metapor® vacuum chucks are thin-walled materials such as paper, foils or films. They are also often used to hold optical glass. Parts are also held with porous vacuum chucks for many inspection operations.

Some very special applications also require porous chucks. One automotive supplier actually machines leather dashboard covers using a Metapor® fixture. The leather sheet has small grooves cut to precisely control how the air bags are deployed.
 
Porous Ceramic Vacuum Chucks
Porous Ceramic Vacuum ChuckMetapor® CE100 is a porous material made from ceramic powder and a binder. This material has many applications in semi-conductor and wafer processing. One extremely critical requirement in this industry is ultra-high precision. When a wafer is held for processing or inspection, it is critical that the wafer surface does not deflect due to clamping forces. This can occur when using a perforated vacuum chuck that relies upon grooves cut into the surface of the vacuum chuck. A Metapor® CE100 chuck can be produced as a solid surface that completely supports the wafer as well as applies vacuum to the entire part surface without deflection. And, it is possible (and necessary) to produce a vacuum wafer chuck that has a flatness tolerance of under 10 microns! That is very difficult to do with a metal chuck.

OK, What Type of Vacuum Pump Do I Need for This Type of Chuck?
A porous vacuum chuck does not use any sealing gaskets or O-rings. It operates with an expected amount of vacuum leakage when it is used. So, you must use a vacuum pump that has the capacity to provide sufficient vacuum with flow due to leakage. Not all vacuum pumps can do this. Most venturi vacuum pumps will not be able to compensate for the losses. Make sure your pump is big enough!

Conclusion
Traditional vacuum chucks rely upon a seal to create holding force on a part. If the seal is broken, holding force is lost and part comes off the chuck. Porous vacuum chucks rely upon using a porous material made from a metal or ceramic. They provide 100% support under the part and are an excellent solution for many special applications including holding thin materials and semi-conductor wafers. Porous vacuum chucks are also used for many part holding applications including inspection and low force operations like engraving or laser marking.