Successful
injection molding process design and simulation
Today, everything is different. the mixture of excellent
mathematical models of the rheology of resin melts, a far better understanding
of metallurgy and warmth transfer, and therefore the formalization of years of
“rules of thumb” has allowed specialist engineers to coach specifically in
plastic mold engineering.
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Iterative mold development, however, remains considerably
a tooling strategy. The difference today is that this is often done virtually
with simulation software. But why? Modelling of fluid flow in closed channels
has been finished decades. the rationale is within the resins, says Caitlin
Tschappat, Moldflow technical specialist with Autodesk and a specialist polymer
engineer.
“People don’t realize how complex plastics are and the
way they don’t behave like metals. They flow by non-Newtonian principles, with
very different flow properties. for instance , they’re highly shear sensitive.
It’s complex. many of us check out an easy part and think ‘oh, that’ll be easy
to fill.’ that straight forward shape might be one among the toughest parts to
manufacture due to many considerations, like warpage,” she explains.
At its simplest, injection molding is about orienting
cavities in three-dimensional space with a parting line that permits free
ejection of the cooled, solid resin parts. Simply determining the situation of
the parting line are often nontrivial. Parts with zero or negative draft angles
could also be impossible to eject with conventional ejector pins, forcing a
designer to use fewer cavities, or non optimal cavity orientation during a mold
to urge clean ejection. In some severe cases, there's no thanks to orient the
part to facilitate ejection, and core pulls must be used, adding complexity and
price .
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The complexity doesn’t end there. Co-injection, over
molding—especially of TPU’s over commodity thermoplastics—and in-mold
decorating all complicate the mold design process.
If very high-volume production is important, like within
the packaging industry, stack molds may require complex designs with multiple
parting lines and a requirement for fast, clean ejection. Symmetry helps, and a
64 or 128-cavity small part mold could also be an easy matter of design
recursion. A family mold, however, or parts that need special features like the
favored “living hinge,” are often very difficult to style. There also are
multiple other issues involving gates, runners and other essential mold
features.
The advantages of simulation are obvious. Tschappat is an
industry veteran with experience within the packaging and automotive industries
and has seen this complexity up close.
“Think a few larger part,” she says. “The automotive
industry, for instance, has many long, thin parts. For these, you want to consider
the ratio with reference to part length to wall thickness, because it would be
difficult to fill uniformly without the inclusion of complex runner systems,
hot drops and valve gates. On the opposite hand, small parts, like those for
medical or electronic applications, can also require an equivalent complexity
with reference to the runner systems, as they can also be difficult to fill
thanks to small features and limited filling pressures.”
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Modern mold design partially addresses the complexity
issue by the utilization of off-the-shelf mold bases, inserts, gates and other
standardized components wherever possible. Advanced simulation software like
Autodesk Moldflow works with these components to permit accurate approximations
of mold performance in gating and cavity filling.
Cavity balance is usually a high priority when filling
multi-cavity molds, which may sometimes be remedied through the utilization of
mold cavity symmetry. These instances also are ready to simplify mold
simulations, like how other Finite Element Analyses use symmetry for model
simplification.
For family molds, or large single cavity molds with
complex shapes, simulation makes the difference between a productive and
cost-effective tool, and a design that's revised such a lot it “goes through
the alphabet.”
Before simulation became more widespread, it had been not
uncommon to change important mold components on the fly, like quick-fixes to
gates for improving mold balance. Unfortunately, these are even as it
says—quick-fixes—leading to effects on other aspects like part quality like
jetting, knit lines and even dimensional instability. that sort of
experimentation may solve a drag , but it frequently requires a complete
rethink of the general molding strategy, with new machine parameters which will
require many shots to perfect.
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The ability of simulation to attenuate rework not only
reduces time spent on the mold, but also the training curve on machine found
out with a replacement job. for outlets with a captive press operation, the savings
for reduced downtime and improved machine scheduling ability are obvious,
except for mold shops there are additional benefits. Rework costs and delivery
delays are reduced, customer satisfaction is improved, and therefore the too
frequent finger-pointing (who pays for that modification?) are often greatly
minimized.
Complexity may be a given with modern injection molding
and lots of jobs simply can’t be attempted without advanced simulation.
Tschappat describes how she uses Moldflow to deal with higher level problems,
saying, “We have many modules within the software counting on what questions
you would like answered, whether it's co-injection, two-shot, over-molding or
insert molding. for instance , with gas assist, we will help identify where the
gas void will settle within the cavity, or with coinjection how two plastics
are getting to bond together within the analysis.”
Simulation helps address the previously mentioned topic
of cavity balance for multi-cavity tools.
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"We see numerous tool cavity layouts when talking
with our simulation customers. But the one thing that always seems to throw
people off is that if they will escape with molding quality parts from family
tools, where each cavity could also be a special part, and technically would
wish different processing conditions,” says Tschappat.
“Using Moldflow simulation, we model up the mold layout
and predict the part quality of the various cavities. Then we experiment with
artificially balancing the filling through the utilization of adjusting runner
diameters between cavities or changing the runner design, to aim a more uniform
filling from cavity to cavity. albeit the filling is balanced, other factors
like shear-induced imbalances can occur as a results of the various geometry
features. this is often where simulation is basically cool—seeing something
that we will not even see with our eyes when actually molding the parts at the
press," she adds.
The types of runners are another source of complexity.
Hot runners are standard for volume production thanks to zero or minimal wasted
material, but cold runners leave re-purpose of their runner material toward
regrind to feature to material savings through reusing small percentages. But
as Tschappat observes, “based on your part, what's the simplest gating scheme?
counting on how big your part is, maybe you're employing a fan gate, or if it
is a smaller part it'd be alittle pin gate. Then the pressures and therefore
the pressure drops are an element throughout the runner system. These are all
things that make it complex, yet you've got to weigh them out at the top of the
day to work out what's best for your business.”
Even a seemingly simple change during a commodity resin
can introduce issues. “They teach you in class that if you're getting to be
employing a different resin, particularly very dissimilar materials to people
who you’re wont to , you ought to build your mold thereto material,” Tschappat
says. “We all know that that's not necessarily the practice. you'll start out
with a non-filled material then plan to switch to a glass-filled material after
manufacturing a couple of shots, for instance . Now you've got to stress about
more abrasive decline the tool over time. What does one neutralize those
situations? More frequent tool inspections and reworks, maybe even welding up
the gate and re-cutting it so it allows for fewer shear of the fibrous
material. These kinds of things just take overtime out of the method and slow
you down, but it's normal practice.”
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The ability to rerun a simulation with a replacement
material virtually can flag a molder about potential problems before they
translate to expensive tool rework and downtime. Warpage is another common
reason for modifications to the tool design. an honest designer who has
familiarity with simulation packages like Moldflow can gain insight into root
causes of those problems.
“Once you actually get comfortable, you'll actually
isolate causes of the warpage to raised understand why something is warping,”
states Tschappat. “For example, if you're getting tons of warpage thanks to a
thicker cross-section because your material's not freezing off and you've got
tons of shrinkage therein area, Moldflow allows you to ascertain that then make
a design change and rerun an analysis to see how that style change reduces
overall warpage. this is often why we are working more and more with part
designers. they will use simulation as they design the part to flag these
problem areas before escalating to the tool designer, resulting in fewer
iterating between the 2 , making them appear as if they're superstars!”
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The cooling issue is critical and is usually harder to
perfect than the cavity and runner design. Although 3D printing promises truly
conformal cooling, most production molds are cooled by drilled and manifolded
channels carrying coolant, usually water or oil, which carries the warmth away
by the thermalator.
There’s an old rule of thumb that for efficient cooling,
set the machine to eject the part at 80 percent of the part’s heat distortion
temperature. However, for complex parts, multiple cavities or complexity added
by factors like thermoplastic elastomers, coinjection or gas assist, rules of
thumb are quickly replaced with empirically derived settings during the mold
runoff. Cooling is typically the determining think about overall cycle time, so
here time considerably is money.
Simulation of cooling channel layout and flow capability
are often equally or more important than efficient cavity filling for a high
capacity mold, and within the world of injection molding, only a few molds
aren't thought of as high capacity, meaning cooling is nearly always a critical
factor.
Mold designers are frequently faced with customer
requirements which will be difficult or maybe impossible to realize . Most
production shops understand their press plate size and tonnage, chiller capacity
and target cycle time, but know little about the mold.
Customer expectations can sometimes be unrealistic. “Part
designers and mold engineers need to work with each other ,” says Tschappat.
“Time is money, and everything must be finished yesterday. It goes through
several phases. How are you getting to lay your model out? you've got to think
about where you are going to inject resin and locate your gates. then what sort
of gate you are going to use. What sort of runner systems? Is it getting to be
a hot manifold or are cold runners getting to be sufficient? Or is it a hot to
cold runner? what is the ejection unit look like? Where are you able to squeeze
cooling in?”
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“Often in my job, you get parts from customers and you
ask them, ‘what's your cooling layout look like?’ you'll get a solution thereto
question, or they'll not know. Often a designer will just squeeze cooling in
there wherever they need extra room left over,” Tschappat adds.
Even the potential of the machines that run the mold are
often assessed critically with mold simulation. Simulation allows a mold
designer to run “what if” scenarios which will show a customer where
expectations are unrealistic and nudge them to a far better mold design without
an argument.
Math is definitive. “If you're limited within the types
or how large your injection molding machines are, then you're limited to the
dimensions of the mold that you simply can put therein unit,” Tschappat says.
“And then in fact , what proportion pressure it's getting to fancy fill these
parts? is that the machine large enough to not just fill out the cavities, but
hold the pressures needed to pack those parts? If it isn't , you are going to
possess some problems and you're either getting to need to redesign the tool or
reconsider it. Or buy a replacement machine.”
This isn’t a theoretical consideration. Tschappat has
seen customers who were spared the value of a replacement machine when shown
the advantages of a far better optimized mold design.
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Does simulation replace the tooling designer?
“No,” declares Tschappat. “I would say there's still an
art thereto. When I'm in conversation, I regularly tell people they ought to be
using simulation as an additional datum or another datum , and use their
experience. This actually ties into that aging workforce, too, as we're
beginning to lose that skillset. People are retiring, so how can we build that
have base up? Simulation may be a specialized thanks to help. children got to
be sponges; if you are a toolmaker or a tool designer, work closely with the
older generation to select up a number of their skills.”
Simulation software for injection mold design has
progressed from “nice to have” to a “must have” for cost effective tooling
“I think it did wonders for the industry because you are
able to create an ROI case and see where you'll improve cost savings at the top
of the day. Everybody wants a more complex part cheaper, faster, quicker, and
by incorporating this type of technology into your workflows and work
processes, we will help achieve that goal for our customers.”
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