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Encyclopedia of Physical Science and Technology EN012c-593 July 26, 2001 15:56

Polymer Processing 641

bles somewhat that of blow molding, which is discussed
next. However, the sheets can be forced to take on dif-
ferent shapes because each half of the mold can have a
different shape. Furthermore, different polymers can be
used for each half. The sheets must be held in the mold
long enough for bonding to occur.
Thermoforming can be divided into four sections: (1)
sheet heating without deformation; (2) sheet stretching
without significant heat transfer; (3) part cooling in the
mold; and (4) postmolding operations such as trimming.
The time to make a part is primarily determined by steps 1
and 3 because these are of the order of minutes. However,
the successful functioning of the part is determined by step
2 because the distribution of wall thickness is determined
in this step.

C. Blow Molding
FIGURE 35 Plug-assisted vacuum thermoforming process.
[From Baird, D. G., and Collias, D. I. (1998). “Polymer Processing: Blow molding is a process for generating hollow plastic
Principles and Design,” Wiley, New York.]
articles such as bottles and containers. This process was
initially used by the packaging industry but more recently
has been used by the automotive industry to produce parts
The essential features of the thermoforming process are such as fuel tanks, bumpers, dashboards, and seatbacks.
shown in Fig. 35. Thermoplastic sheet is heated usually In other words, plastic parts are being manufactured for
by means of radiation but sometimes in conjunction with applications where some structural integrity is required.
Although a number of variations are possible in the way
convection cooling to temperatures either just above T g
in the case of amorphous polymers or T m in the case of in which blow molding is carried out, there are a number
semicrystalline polymers. The exact temperature depends of common steps. First, conventional melt processes are
onthedegreeofsagexhibitedbythematerialunderitsown used to make a cylindrical tube (note: the preformed sam-
weight, which is determined by its rheological properties. ple may be of other shapes). When extrusion is used, this
The sample is then removed from the heating system and tube is referred to as a parison, and when injection mold-
brought into position over the mold. The sample is forced ing is used, it is referred to as a preform. The softened
to take the shape of the mold by applying pressure to the preformed tube is transferred to a mold consisting of two
top of the sheet or by generating a vacuum on the underside halves, where it is sealed and inflated to assume the inter-
of the sheet. The forming step occurs in the matter of a nal contours of the mold. The part is cooled in the mold,
second. The sample is maintained in the mold until it is until it reaches a temperature where it will maintain the
rigid enough to be removed from the mold without altering shape of the mold when the mold is opened.
its shape. Extrusion blow molding is used frequently for polymers
There are a number of variations on the basic process. that exhibit high melt strength such as polyolefins. The
For example, in plug-assisted vacuum forming the heated process is shown schematically in Fig. 36. In this figure
sheet is forced by a plug into the mold with the remainder the tubular parison is continuously extruded from a die
of the shape being produced by the application of vac- into position between the two mold halves and then sepa-
uum to the underside of the sheet. This method is used rated from the main stream by means of a knife. The mold
to help maintain a more uniform wall thickness through- closes, sealing the end of the parison, and air pressure is
out the part. Another example is matched-die molding. applied inflating the parison against the walls of the mold.
The heated sheet is forced to take the shape of the fe- The time for inflation is very short, usually in the range
male portion of the mold by the male part. This process of a second depending on the size of the part. The longest
is characterized by the formation of parts with more intri- step in the process is the cooling of the part. The time
cate shapes and uniform wall thickness. Finally, one last for cooling depends on the temperature to which the part
technique is that of twin sheet thermoforming. Here two must be lowered to in order for it to maintain the shape of
sheets are heated and then forced to take the shape of the the mold and the rate of heat transfer between the mold
mold by applying air pressure on the inside of the sheets wall and the part. The mold opens finally and the part is
and possibly vacuum on the outside. This process resem- ejected.