March 2005

QUESTION: We are pumping a high-viscosity sealant through a robotic application system. Can you suggest a sensor that will monitor flow through the tubing and nozzle?

ANSWER: There are several types of flow sensors that you might investigate for your system. However, I would recommend that you look at two types, namely acoustic sensors and ultrasonic Doppler sensors. An acoustic sensor is essentially a very sensitive microphone that can detect movement of the liquid inside a tube and will be the simplest type of sensor if it works in your system and factory environment. A Doppler sensor is very sensitive and relies on the reflection of ultrasonic sound waves from solid particles or air bubbles in the sealant. Several companies manufacture these detectors, and both can be used on the outside of your pumping system.

QUESTION: I am confused by the terms "form-in-place gasket" and "cure-in-place gasket." Could you please clarify these terms?

ANSWER: Both terms are used to describe liquid gasketing compositions designed to replace cut or molded gaskets, with the major benefits being that they avoid the necessity of stocking a wide inventory of different-sized solid gaskets and they can be very cost effective in high-volume production situations, like the automotive industry. Unfortunately, the inventors of these terms probably got the definitions backward. Form-in-place is the term used to describe technologies like liquid RTV silicones or anaerobic sealants that are applied between two flanges immediately before assembly and then cure in place to form an elastomeric gasket (see where the confusion comes in?). Form-in-place gaskets rely on good adhesion to surfaces to maintain seal integrity. Cure-in-place gaskets are usually liquid silicones, acrylics or ethylene acrylic rubbers that are "formed in place" by applying to one flange only on a part and then curing by heat or UV radiation. Form-in-place gaskets are often applied in a production line of an OEM auto manufacturer, whereas cure-in-place gaskets are particularly suitable for a component manufacturer who can cast, stamp, or mold a part, apply a gasket and then supply a gasketed part to his customer. Form-in-place gaskets can be readily replaced in repair situations by applying the liquid gaskets from a cartridge or tube. Form-in-place gaskets can sometimes replace cure-in-place gaskets, but this may not be possible because of differences in flange designs for these two types of systems. In the latter case, one has to use a cut or molded gasket or buy a new part with the gasket already attached.

By the way, you may also come across the term "flow-in-place gaskets," which is another way to describe cure-in-place systems...but let's not confuse the situation further.

Q&A Exchange is written by Dr. Dave Dunn of F.L.D.Enterprises, a technical consultancy and full-service industrial market research firm specializing in the adhesives, sealants, specialty rubbers, and plastics fields. Dr. Dave is a former vice president and director of Loctite Corp., and has spent many years troubleshooting adhesive and sealant problems. Questions for publication should be directed to him at 242 Trails End, Aurora OH 44202; (330) 562-2930; FAX (865) 251-9687; e-mail DrDave242@att.net ; or visit http://www.fldenterprises.com .