Q1 Plans Chapter 3 Page 3-5
- Category: Quickie Plans
- Published: Saturday, 15 November 2008 00:00
- Written by Dan Yager
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areas where heat buildup isn’t a problem and a faster cure is desirable, a fast curing system is used. Both the fast and slow cure epoxies will cure to a firm structure at room temperature within one day. Complete cure takes 14 days.
The RAF epoxy systems are called RAES (for slow curing), RAEF (fast curing), and 5-MIN. Both the RAES and RAEF hardener use the same RAE resin.
Note: If working conditions are hot (85° to 90°F), RAES can be used where RAEF is called out. While this practice is allowable, the best results are obtained working between 70 and 80°F with the specified epoxy.
The working and strength characteristics of an epoxy system are dependent on the resin, the hardener, and on the amount of each in a given mixture. Epoxy systems are engineered for a specific ratio of resin and hardener. It is quite important that the proper mixture be obtained. An accurate balance or ratio pump must be used to accomplish this. A drawing of an inexpensive ratio balance is included in these plans. The mix ratio accuracy is particularly important with RAEF & RAES. The 5-MIN can be adequately ratioed by merely pouring a blob of part A in a cup and adding a blob of part B that looks the same volume before mixing. Never eyeball estimate RAEF or RAES, always carefully use the balance or pump.
Epoxy resin and hardener are mixed in small batches, usually 6 ounces or less, even in the largest layup. The reason for small batches is that, in large batches, as the hardening reaction progresses, heat is generated which speeds the reaction, which causes even more heat, which speeds up in a fast reaction called an exotherm. An exotherm will cause the cup of epoxy to get hot and begin to thicken rapidly. If this occurs, throw it away and mix a new batch. The small volume batch avoids the exotherm. For a large layup, you will mix many small batches rather than a few large ones. With this method you can spend many hours on a large lay-up using epoxy that has a working life of only a few minutes. If the epoxy is spread thin as in a layup its curing heat will quickly dissipate and it will remain only a few degrees above room temperature. However, in a thick buildup or cup, the low surface area to mass ratio will cause the epoxy to retain its heat, increasing its temperature. This results in a faster cure causing more heat. This unstable reaction is called an exotherm. Exotherm tesperatures can easily exceed the maximum allowable for foam (200°F) and damage the foam-to-glass bond. This is why the slow epoxy system is used in joining foam cores as exotherm is impossible as long as epoxy joint thickness is less than 1/8”.
Unwaxed paper cups are used for mixing and ratioing resin and hardener. Convenient 8-oz cups for resin are available from RAF distributors. The hardener cups are the 3-oz unwaxed bathroom paper cups available at any grocery store. Don’t use waxed cups; the wax will contaminate your epoxy.
If you are using the homebuilt balance, follow this procedure. Place the resin (8 oz) cup on the right cradle. The resin cup can be either a new clean cup, one with a little uncured epoxy left in the bottom, or a clean cup from a previous layup with hard epoxy in the bottom (smooth, nor lumpy). Now, take a clean 3-oz hardener cup - pour a splash of hardener into it then scrape the hardener back into the container. This gives the hardener a wet surface, so its remaining hardener will not be counted in the balancing. Now, place the wet hardener cup on the scale, check that it swings freely and balance it perfectly by moving the small weight. Epoxy is then poured into the 8 oz cup (6 oz or less). Hardener is then poured into the 3—oz cup at the other end of the balance until the arm is level. When resdy to mix, pour the hardener into the resin cup and mix completely. If you have the ratio pump, you simply put one cup under the spout, pump out the amount that you want and mix.
Mixing is done by stirring with a stick, being careful not to spill any. If you spill part of an unmixed cup, the ratio of resin and hardener may be inaccurate and it shouldn’t be used. Mix each cup for at least two minutes. You should spend 80% of your mixing time stirring the cup and 20% scraping the sides to assure complete mixing. Do not mix with a brush. The bristles can soak up the hardener, changing the ratio. Use a tongue depressor or wood stick.
The working temperature has a substantial effect on the pot life and cure time. Very hot conditions will cause the cure to speed up. In cold working conditions the cure will be delayed and if it is cold enough, epoxy may not cure at all. Working temperatures must be between 60° and 90°F. A range of 70 to 75°F is best. Be sure to get a wall thermometer (approx. $1.50 at any general store) to check the temperature of your work area. At 75°F, 5-mim must be used within four minutes, RAEF must be used within 20 minutes and RAES must be used within 50 minutes.
Cold epoxy results in increased time required to do a layup, since it takes longer to “wet” and to squeegee the cloth. A layup at 65° may take almost twice the time as st 75°F. On most layups (except for joining foam cores) its best to have 70 to 80°F room temperature and 80 to 90°F epoxy. Resin and hardener can be kept warmer than room temperature by keeping it in a cabinent with a small light bulb on. DO NOT store your resin or hardener on a cold floor if you plan to use it within the next several hours, if you let your shop get cold between
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