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Questions about structural adhesives.

Structural adhesives can offer significant advantages over traditional fastening methods. However questions often arise in the minds of would be users. Following are some of the most frequently asked questions and brief answers: What is a structural adhesive?

It's a chemical compound that increases the quality and performance of joined materials by enhancing the inherent strengths of the materials, while minimizing their weaknesses. Selection requires consideration of a number of interrelated variables: Materials being joined, physical stresses on the joint, resistance to varying environments, application method, cure time, shear and impact strength, resistance to peel, and cost, to name a few.

How do structural adhesives compare to other fastening methods?

Favorably. Chief advantages over mechanical methods are reduced part cost, improved product performance, and improved surface quality. Every bolt, nut, screw, weld, and rivet that can be eliminated from an assembly means less overall cost, weight, refinishing, and cover-up of defects.

Structural adhesives are ideal for joining dissimilar metals, where conventional fastening methods will not work. The adhesives succeed in both joining these materials structurally and in isolating them from each other-important if the materials have a tendency to degrade each other. What are performance characteristics of structural adhesives?

Each class of material has unique performance characteristics. In general, structural adhesives offer durability and resistance to water, humidity, solvents, corrosion, and hostile environments, including temperature extremes and thermal cycling.

Another important property is improved stress distribution-conventional fastening methods concentrate stresses locally, while adhesives do not. How does one choose the right adhesive? This is not an easy task-especially since selection isn't based upon properties of the bonding agents alone. The substrate to be joined is an important factor. Will it be bonded to a like or dissimilar material? Joint type is another factor; i.e., what performance characteristics must the joint have; to what condition will it be exposed?

Adhesive properties should be examined. Cure times and temperatures, surface preparation needs, and substrate type are all factors to be evaluated. Application method and adhesive cost are additional factors. Beyond the basic adhesive cost, the total cost including equipment, parts, labor, and other related expenses, is a more accurate measurement. Because many factors are interrelated, you may be saving on production times, refinishing, energy, or other operations inherent to your manufacturing or assembly process when you choose the right adhesive. Under what conditions will structural adhesives not work?

There are a few situations not conducive to structural adhesive use, primarily due to factors such as product performance, manufacturing process, or cost. For example, temperatures greater than 500 F or lower than - 1 00 F, or very aggressive environments can cause substrate or adhesive failure. For some production processes, cure times or handling properties are not short enough or flexible enough. How are structural adhesives applied?

Adhesives can be applied by spraying, brushing, rolling, silk screening, or as a bead-depending upon the adhesive's physical properties and the design of the bond line. Generally, the bonding/assembly process falls into three categories: Pre-application, application and post-application.

Pre-application considerations include physical joint design, surface preparation, substrate and adhesive characteristics, and cost. Getting the right amount of adhesive in the right place at the right time-with efficient curing-falls into the application phase. Post-application includes bonded assembly performance such as durability in the service environment, load conditions, and final appearance.

If I go the structural adhesive route, will I need special equipment?

For high-volume, automated assembly lines, the answer is yes. For small-volume users, where no equipment or small-volume, easy-to-use cartridge systems will satisfy the need, the answer is no. Are structural adhesives expensive?

This question cannot be properly answered until all parameters involved in a specific application have been examined. Structural adhesives offer a combination of processing and performance benefits that can result in lower production costs.

For more information from Lord Corp on the benefits and limitations of structural adhesives, circle 599.
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Author:Bradshaw, D.R.
Publication:Tooling & Production
Article Type:column
Date:Jan 1, 1989
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