the heat source until there is sufficient flow of molten material to form a good weld bond. When the
plastic has reached the proper flow temperature and there is indication of a good flow-pattern and
softening of the plastic, it is removed from the heated surface and is quickly joined with another plastic
piece or sheet that has been simultaneously prepared. The two are firmly secured together in the desired
position or shape and held until the plastic has cooled, thus forming a firm welded joint.
A slight pressure should be maintained between the joining surfaces. The pressure range necessary to
make a good bond is between 5 and 15 pounds per square inch of surface. In most applications it can be
applied by hand. For production-type work, jigs are used to apply the pressure.
Heated-tool welding can be accomplished in a number of ways. The use of electrical strip, bar, or hotplate
resistance-coil heaters are the methods most common in the industry. In some applications, gas or
steam is used as the heating medium. Even a soldering iron can be used to make a heated-tool weld
under certain conditions. The material generally used for the surfaces of hot-plate, bar, or strip heaters is
aluminum or nickel-plate. Steel or copper, when hot, has a tendency to decompose plastic and should not
be used, especially with polypropylene. Aluminum is used to a great extent in the manufacture of hotplates,
and with very satisfactory results.
The technique for making a good heated-tool weld depends on the proper temperature of the heater and
the time-lapse between removal of the heated plastic from the hot-plate awl the joining of the two pieces.
The facilities for making heated-tool welds should be so set up that the time-interval between removal
from the hot-plate and the clamping or holding operation is kept to an absolute minimum. The strength of
the weld decreases as the removal-time increases. The optimum time is in the range of a few seconds
from the heated state to joining.
The temperature required to insure good flow or molten plastic for heated-tool welding is high. Indicating
pyrometers for accurate reading and control of temperature are found in the more up-to-date heated-tool
installations. The temperature range in heated-tooI welding is in the vicinity of 400 0F to 650 0F depending
on the weld size. Correct temperatures can be determined from a practice run on the last plastic to be
welded. There should be sufficient flow for proper fusion after about 10 seconds of heating.
Another important application of heated-tool welding is the joining of plastic sheets to form larger areas
than can be obtained in standard sized sheets. This operation is easily accomplished using electric bar