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SELECTING AND USING ADHESIVES 139

Figure 13-9 Two- part epoxy can be
quickly and accurately applied when using a
two- tube plunger.

G So- called 5- or 30- minute epoxies represent the setting time, not the curing time. It takes 6 to
24 hours for most epoxies to cure to 60 percent to 80 percent, then the remainder over a period
of several days. Once cured, the bond achieves its maximum strength.

Using Two- Part Epoxy
To use a two- part epoxy, it’s necessary to first mix the materials together, or else use an appli-
cator that thoroughly premixes them. The latter is not common for consumer use, so we’ll
concentrate on the manual mixing method. With most epoxies for consumer use, the liquids
are mixed in a 1:1 ratio.
1. Apply short (1″ to 2″) but equal- length parallel beads of resin and hardener to a piece
of index card paper. It’s always better to mix too little than too much, and you can
always mix in more as you need it.
2. Use a wooden (not metal or plastic) toothpick to stir the liquids together. Mixing must
be thorough. Try this: mix parallel beads of resin and hardener with a zigzag action (see
Figure 13-10), then scoop the material toward a common center. Stir this center “dol-
lop” for 15 to 20 additional seconds.
3. Apply the mixed epoxy to one or both surfaces to be joined.
4. Most epoxied joints should be taped or clamped to prevent movement of the joint dur-
ing the setting time. If the joint moves while the epoxy is setting up, the bond will be
greatly weakened.
Unused mixed epoxy must be discarded. It cannot be reused. Allow the unused liquid
epoxy to harden on the paper card before throwing it into the trash.

YOU, YOUR ROBOT, AND SUPER GLUE
Super Glue is a trade name, but it’s often used as a generic term for a family of adhesives
known as ethyl cyanoacrylates, or CAs. It’s well known as being able to bond to most anything
within seconds. But with the good also comes some bad. If used incorrectly, CA glues may
provide only a weak and temporary bond. Keep the following in mind when using cyanoacry-
late adhesives:
No gaps, please. The most common cyanoacrylate is water- thin and unable to fill in any
gaps between the materials to be joined (if you need that, get the thicker “gap- filling” kind).

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