Page 33 - Plastics News June 2018

FeAtures

Process Documentation Is Crucial to Making ‘Identical’ Parts

- JOHN BOZZELLI

Use a machine/mold setup sheet to document plastic parameters and use them on any acceptable machine
into which the mold is placed.

njection molding is one of the most complex processes between shifts.
Iin manufacturing. Our industry has a common problem 1. Fill time, within a range of ±0.04 sec, for most jobs.
in setting up a molding job to make “identical” parts on This does not mean that the processor inputs the same
every run, especially on different machines.
injection rate in mm/sec or in./sec on the controller.
Defining what “identical” means depends on the Even if you are using cubic centimeters (volume) the
application; but whatever the requirements, the goal is to input number may vary as the machine, hydraulic or
make identical parts run to run, year
to year, even on different machines.
How we, as an 80-year-old industry,
attempt to accomplish this varies and
is something of a Pandora’s Box.
There is no general agreement on best
practices. It is desperately needed,
as it would enable us to compete
more competitively within the world
markets. Your future in the plastics
industry may depend on it. Where do
we begin?
There are five components to making
a successful plastics application: part
design; resin selection/handling; mold
design/construction; processing; and
testing. Each has hundreds of details and putting them electric, may not give the same result from one run
together requires a trained and talented group of people. to another. A machine is composed of mechanical and
One person cannot do it all. Most likely your shop has electrical components, and these do wear (take the
talented people in two to four of these components. non-return valve as just one example). The input for
The point is that if we want to make “identical” parts we injection rate can vary to provide the documented
need to ensure these processing parameters are controlled first-stage fill rate of a given weight, size and fill time.
and measured to be the same within a reasonable 2. Plastic pressure at transfer; it may or may not be
tolerance. That mandates that when a processor sets up a peak pressure during injection. It should be the same
process, he or she inputs the process setpoints that provide between runs and machines. This requires use of the
the same processing conditions (parameters) documented. same type and size of nozzle tip. The nozzle becomes
The numbers input on the machine may be different— part of the mold from a pressure-loss point of view.
for example, hydraulic pressures vary from machine to 3. Cushion—this does not have to be the same from press
machine—but the specified actual plastic pressures are the to press but must be consistent shot to shot. I target
same. What process variables do we duplicate? That list less than about 1 mm variance. I define cushion here
is too long for this column, but here are six that must be as the screw position at the end of second (hold)
duplicated. I suggest that once a shift, have someone visit stage. This is not minimum screw position. There are
each machine running, document each of these process two definitions of cushion, depending on which brand
conditions to ensure part reproducibility/accountability of machine you are using.

33 June 2018 Plastics News