Custom Plastic Injection Molding - Trouble
Shooting Your Injection Molding Process - Now that we have established our
basic injection molding process, we can look at a number of the obstacles that
plague all of us in the injection molding industry and that the "battle of
the defects". This is a fact of
life in any injection molding process, and no matter how well we might have our
process established, optimized, and refined, we will always be at the mercy of
certain process variables that are not within our control. By now, if you have been reading the
previous articles I've written, you realize that there hundreds of injection
molding machine settings and variables in an injection molding process, that it
becomes a statistical improbability that your process will not experience
issues with many of these over it's lifetime in the average production
setting. First let's define' what the
"process" is because I know when I am teaching newly graduated
college students and process staff that are relative newcomers to the injection
process arena, that the conception of the injection molding process is the
machine and the mold. While these are
obviously critical to any injection molding process, they are yet only a
portion of it. Let break it down a
little bit.
If you are familiar
with some of the brainstorming techniques used in problem solving, one of them
should be the use of what commonly is referred to as an Ishikawa or "cause
and effect a.k.a. fish bone diagram" because of what it looks like on
paper. I'm not going to explain the use
of this tool in this section but just show it's basic elements to give you an
example of how the injection process interacts.
The "four M's" of the basic diagram are method, machine, man,
& materials. Sometimes
"measurement, mother nature, and or management" are also added to
these diagrams for troubleshooting purposes.
So, why did I just waste a paragraph on that explanation? Because the
injection molding process is a very dynamic and interactive process by nature,
and that is a fact that must always be kept in mind when troubleshooting your
injection molding processes. If you lose
sight of this when you are trying to problem solve an issue you are having, you
could quickly becomes lost in your troubleshooting and end up putting a band
aide on your process instead of really solving the root of your issue to be
able to say that you have implemented a permanent corrective action. You need to understand the entire interaction
component of the injection process, so that you find the true "root
cause" of your problem and correct that and not just correcting the
symptoms of the problem as many will do.
Knob turners (process troubleshooters that stab in the dark until they
get lucky and never know why) are a dime a dozen in the processing world, but
the true process troubleshooters are those who take the time to learn the
injection molding process at a whole different level. This doesn't mean your need to understand
polymer chemistry (although there are times it doesn't hurt) to be a good
process problem solver, but it does mean taking the time to learn and gaining
the experience to do it properly. We
will classify the defects with our injection molded parts into two main
categories for the purposes of this article and they will be "surface,
visual, or cosmetic" and "dimensional or functional". The surface defects are exactly what
they sound like, and that is
defects that can be "seen"
such as knit or weld lines, blush,
splay, and sinks, just to name a few.
The dimensional or functional defects are a little more difficult to
detect, as you most often "can’t see them" and they will require a
mating part or a gauge of some type to take measurements, which can be compared
against an established standard. This
can be as basic as a using calipers, gage pins, or micrometers, all the way to
specially designed gauges and check fixtures that are made for one specific
part. In the next section will begin to
break down the various defects, determining their possible causes and some
possible ways of correcting them.
