General Design Principles for DuPont Engineering Polymers - Module

Calculations for Inertia Welding Tools
and Machines
In order to bring a plastic from a solid to a molten
state a certain amount of heat, which depends on the
type of material, is necessary. Engineering plastics
actually differ very little in this respect, and so this
factor will be neglected in the following discussion.
The quantity of heat required for melting is produced
by the energy of the rotating masses. When the joint
faces are pressed together, the friction brings the
flywheel to a stop in less than a second.
With plastics having a narrow melting temperature
range, such as acetal resins, the tool should not perform
more than one or two revolutions once contact
has been made. If the pressure between the two parts is
too low, the flyweight will spin too long, and material
will be sheared off as the plastic solidifies, producing
welds which are weak or which will leak.
This factor is not so important with amorphous
plastics, which solidify more slowly. For all plastics, it
is best to use higher pressures than are absolutely
necessary, since in any case this will not cause the
weld quality to suffer.
To get good results with inertia machines, the following
parameters should be observed:
• Peripheral speed at the joint
As far as possible, this should not be lower than
10 m/sec. But with small diameter parts it is occasionally
necessary to work between 5 and 10 m/sec,
or else the required rpms will be too high. In general,
the higher the peripheral speed, the better the
result. High rpms are also advantageous for the
flywheel, since the higher the speed, the smaller the
mass needed for a given size of part to be joined.
• The flywheel
Since the energy of the flywheel is a function of its
speed of rotation and of its moment of inertia, one of
these parameters must be determined as a function
of the other. The kinetic energy is a function of the
square of the speed (rpms), so that very slight
changes in speed permit adjustment to the required
result. In general, for engineering plastics, the
amount of effort needed to weld 1 cm2 of the
projection of the joint area is about 50 Nm. The
amount of material which has to be melted also
depends on the accuracy with which the two profiles
fit together, and therefore on the injection molding
tolerances. It would be superfluous to carry out too
accurate calculations because adjustments of the
speed are generally required anyway.
• Welding pressure
As mentioned above, the pressure must be sufficient
to bring the mass to rest within one or two revolutions.
As a basis for calculation, we may assume that
a specific pressure of 5 MPa projected joint area is
89
required. It is not enough merely to calculate the
corresponding piston diameter and air pressure; the
inlet pipes and valves must also be so dimensioned
that the piston descends at a high speed, as otherwise
pressure on the cylinder builds up too slowly.
Very many of the unsatisfactory results obtained in
practice stem from this cause.
• Holding pressure
Once the material has melted, it will take some time
to resolidify, so that it is vital to keep up the pressure
for a certain period, which will depend on the
particular plastic, and is best determined experimentally.
For Delrin ® , this is about 0.5–1 seconds, but
for amorphous plastics it is longer.
Graphical Determination of Welding
Parameters
The most important data can be determined quickly
and easily from the nomogram (see Figure 11.26)
which is suitable for all DuPont engineering plastics.
Example: First determine the mean weld diameter d
(see Figure 11.27) and the area of the projection of
the joint surface F.
Figure 11.26 Determination of welding parameters
θ D (mm)
120
110
100
95
90
85
80
75
70
65
60
55
50
20
10
8
6
5
4
3
2
1.5
1
0.8
0.6
0.4
0.3
0.2
F (cm2 )
4
8,000 t,min od 5 mm
7,000 28
6,000 33
5,000 40
4,000 50
3,500 57
3,000 65
2,500 80
2,000 100
1,800 110
1,600 125
1,400 140
1,200 165
1,000 200
3
2
1
30
40
50
60
70
80
90
100
120
10,000
5,000
3,000
2,000
P (N)
1,000
500
400
300
200
100