Importance of Proper Adhesive Curing

Epoxies can be formulated to cure under a wide range of conditions. However, each epoxy system or resin/hardener combination must be used within specific limits of application temperature, humidity, joint fit, surface preparation and cure time. They rely on a complex chemical reaction to achieve their strength and longevity, and disregarding their limitations can drastically affect the outcome of the chemical reaction and compromise the performance of epoxy bonds.

Mixing an epoxy resin and hardener together starts a chemical reaction which produces heat. This is called an exothermic reaction. The surrounding temperature affects the rate of the reaction and the degree of cure. Warmer temperatures accelerate the reaction, while cooler temperatures mthe reaction and reduce the crosslinking activity of the epoxy molecules, the epoxy may eventually harden, but may not reach a complete cure or achieve its designed physical properties. Although the partially cured epoxy may have enough strength to hold the structure together, it could fail prematurely.

Testing has demonstrated that the heat box method - a common method of curing bows in the archery industry - does not provide an even distribution of heat. Bow limbs must withstand tremendous stresses: tension, compression and shear. They must be cured accurately. Heat boxes heat the bow components from the outside in, there is no ability to control the temperature on the more massive components - the riser area - or on the thinner areas - the tips - during the curing process. Because of the heat boxes' inability to accurately control the temperatures, the interior parts of the boy may never reach the optimum cure temperature.

In comparison, Lone Wolf bows are cured in a specially designed form that's put into a massive, computer controlled press that uses a 3-cycle system to control the cure: 1. Ramping Up is the steady increase in temperature. 2. Soak maintains the set cure temperature for the specified length of time 3. Ramping Down is the controlled decrease of temperature at the end of the cure cycle. The entire process takes 45 minutes and is part of the 'A' phase of the cure cycle. The post cure, or 'B' phase portion of the cure cycle, takes place after the bow is removed from the form. The method we've adapted to our application is also used in the production of today's super figher aircraft. With our computer controlled presses, we're able to precisely cure our bows and make the maximum use of the adhesives properties without degrading or harming any of the components used in the bow.

It is common to read warnings and see statements that traditional bows are subject to breakage in warm tempertures. In many cases this is due to the inaccurate metering of epoxy, inexpensive epoxy and incomplete crosslinking. Most of todays traditional bowmakers choose the same brand of inexpensive epoxy and cure their bows in heat boxes. The use of high grade adhesives and state of the art curing systems can produce bows that can be subjected to a greater range of temperatures with a much higher delaminating threshold.

To advance the point further, each issue of Traditional Bowhunter usually contains an interview with a traditional bowyer. Questions abound regarding limb core choices, etc., but I've never read an interview where the bowyer focuses on what holds the bow together. Issues of durability in hot and/or cold temperatures, spring back speed and resistance to limb twist are all controlled by proper adhesion. Considering the stresses a bow must endure, choice of adhesive and the curing process are two of the most important issues a bowmaker must consider if he is to craft strong, dependable bows.

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3893 Gray Street
Glennie, Michigan, 48737