Ken Mintz Composite Information pg.8 ==
Page 1 Introduction=================================
Page 2 A Short Description of Fiber Types and Properties=====
Page 3 Advantages/Disadvantages of Composites===========
Page 4 The Matrix===================.==============
Page 5 Setting Up The Shop===========================
Page 6 Preparing to Layup and More Tools=.==============
Page 7 The Core and Its Preparation======.==============
163 Home Page====================================
Composites Corner No. 8 - Hot Box Pictures and Dimensions
I gave a brief description of the hot box used to keep your resins and hardeners warm in Composites
Corner No. 6. As noted there the box can be easily built from a sheet of 1/2 inch 4' x 8' sheet of
aluminized urethane foam house insulation. The dimensions are chosen by the builder to cover the
resin and hardener containers used. Usually these will be combined in the form of an automatic
dispenser that doles out the proper proportions of each material into the mixing cups. In any case the
resin and hardener need to be kept warm.
The dimensions of my hot box are 23.25" wide X 13" thick X 22" tall which covers my automatic
dispenser and allows additional room for the light bulb heater and various drip catching cups, etc. It is
composed of six panels bonded together with a hot glue gun. The floor panel is not bonded to the rest
of the box but is left to act as a drip pad for the dispenser from which the rest of the box can be lifted to
allow free access to the dispenser for replenishment and clean-up. I cut an access door into the front
panel in order to permit operation of the dispenser without having to remove the whole box. I even
bonded handles made from scrap foam and wood pieces to the top of the box and to the access door
to make moving them easier. A smaller inspection door is cut into the front access panel to allow
viewing of the thermometer glued to the resin tank on the dispenser. All this cutting can be done easily
with a sharp knife or a hacksaw blade.
Vent holes are cut into the top. These allow vapors to escape as well as hot air. The latter is a crude
method of temperature control; however, I added an automatic thermostat to control the light bulb.
Vent holes are still advisable in case the thermostat fails and for the aforementioned vapor removal.
If you use heat leakage from the vent holes to control temperature you can cut enough holes to allow
too much heat to escape and then plug them back up until the correct temperature range is maintained.
As you can imagine this is done by trial and error and takes a fare amount of time to achieve. The first
box I made used this method but the one depicted here has an automatic thermostat, a King Model PT-2
in my case, obtainable from any home supply store for about $10.00. Any of them are rated high enough
to handle a light bulb but do check this out. It is also set by trial and error but this can be done much
faster than using the vent hole method.
The bulb used should be between 40 and 60 watts and no more. The bulb should be no closer than
about 6" to any of the resin/hardener containers or anything else inside the box that could be heat
damaged or cause a fire. It should not touch the walls of the box but can be within and inch or so of them.
Radiant heat from the bulb is the culprit here. The thermometer should not be directly exposed to light
from the bulb nor in contact with the metal parts of the dispenser. I glued mine to the plastic resin
container out of line of sight with the bulb. The temperature range is usually between 85F and 95F but
can vary somewhat depending upon the type of resin used. The manufacturer can usually supply this
information if it is not listed in your aircraft construction manual.
During initial setup of the hot box check your temperature often until it stabilizes in the proper range.
Too hot and you will cook your resin especially the hardener. It will often become thick and will not
pump easily from the dispenser. Also its chemical characteristics will change affecting the quality of
the cure. Resins and hardeners that get cold thicken up and also won't pump easily or at all but they
usually do not change chemically. A gelled hardener can often be restored to useful fluidity simply by
heating it up slowly. This can be done in your hot box or you can heat it up faster by placing its
container in another container of hot (not boiling) water - a double boiler without the boil. Don't use
direct heat from a stove burner or torch or anything similar. You will burn the hardener or resin.
If it boils it is ruined for all practical purposes again especially the hardener. Don't breath this stuff
either. Once all this is done your hot box will be relatively trouble free. Replace the bulb and clean it
up on occasion is about all that is required.
Chapter 163 Tech Counselor
Phone: (702) 567-1938