FAQs: Plastic Extrusion Dies

Q.  What are some key considerations for getting the
best performance from plastic extrusion dies?

A.  Whether you’re building a new extrusion line or evaluating existing 
extrusion tooling, the first consideration is the flow path. For thermoplastics
and fluoropolymers, you want a constantly converging flow path from the
inlet port all the way to the end of the die.  This will help you prevent dead
spots where material can accumulate, as well as internal area expansion
where material can expand. Since thermoplastics and fluoropolymers
have elastic memory, any increase in internal volume will affect the
dimensional stability at the exit of the extrusion dies.

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Q.  What are the main causes of die drool on plastic extrusion dies?

A.  Die drool and other die flow irregularities are most often caused by internal
die surfaces that are scratched or pitted, or that have poor plating quality.

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Q.  What are your ideas for optimizing sleeving applications?

A.  For sleeving applications, you want to establish the correct drawdown
ratio and draw ratio balance for the material being extruded. Next, you
want to have a land that is appropriate for the die gap.  For pressure
extrusion, you need an opening that is sufficient for the core to pass
through, but not large enough to allow leaking inside the tip.
Generally speaking, sleeving tooling improves wall thickness control,
and pressure tooling is better for faster line speeds.

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Q.  What is the role of the heat profile in getting the
best results from plastic extrusion dies?

A.  A constant temperature profile from the exit of the extruder to the
exit of the die helps keep the material flowing at the same velocity
throughout the die, and across the entire flow path from the outside
surface to the inner surface.  The key is to avoid gaps in heat coverage.
When temperatures are high and extrusion dies are not fully covered by
a heater, deterioration of the flow path can occur.  Variances in temperature
can cause the material to get hung up on the “cooler” spots, leading to
variations in wall thickness.  This causes defects, rejects and lost revenue,
because the cost of the excess material used cannot be recovered.

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Q.  How important is flow distribution to the
performance of my extrusion dies?  

A.  Choosing the correct design for material distribution in the head
assembly is critical.  The material shear and heat sensitivity of the
thermoplastic and fluoropolymer should be considered when choosing
a flow distribution design to avoid material degradation and burning.
For crossheads, this involves a careful choice of deflector. If your material
is shear and heat sensitive, you want a deflector that does not deflect the
material or allow it to stay inside the crosshead too long. The material’s
shear and heat profiles should drive which deflector to use.

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Q.  Are there any special considerations for in-line,
spider-type extrusion dies?

A.  Spider leg designs have 2, 3 or 4 legs, and can vary in thickness.
Again, if the material being extruded is heat and shear sensitive,
it’s important to choose a spider design where the material flows
smoothly, without opportunities to accumulate. A spiderless design
is also offered in the industry. I  once visited a customer who was using
this design on a very shear sensitive material. He reported that they
were having problems controlling the wall thickness. It was apparent they
had the wrong design for the material. We recommended a 3 legged inline
spider die with a balanced tool set and a consistent heat profile and
eliminated these problems.  Again the material’s shear and heat profiles
should drive the design.

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Q.  What are the possible downsides of running extruders faster? 

A.  Just changing speed often produces wider thickness tolerances;
it can also produce product failures and wasted material.  Higher rpms
can also translate into hotter melts – and that introduces multiple risks,
among them breakdown of the polymer itself, and any volatile additives,
difficulties with cooling and sagging, and sticking issues. If faster speeds
are the goal, consider talking with an expert on extrusion dies.  Often,
they can suggest ways to increase speed without negative consequences.

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Q.  I’ve heard that the temperature of the extrusion dies
may or may not match the material.  How do we manage this
potential discrepancy?

A.  You need to take the temperature of the melt using an immersion
thermocouple. You can also use an IR gun, which is pointed at the
melt as it emerges. 

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Q.  How do you determine optimal area draw down ratio?

A. Each polymer has its own optimal area draw down ratio ("ADR"). 
This is the ratio of the cross sectional area between the tip and die,
and the cross sectional area of the product, tube or wire coating. 
Product material properties determine the area draw down ratio that
can be used.  Polymers are elastic, so an area draw down ratio is
needed to design the tooling to control the tube and compensate
for these elastic properties. 

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Q.  Why is the draw ratio balance important when
running material through plastic extrusion dies?

A.  Draw ratio balance ("DRB") is the ratio of the die and tip diameters,
divided by the ratio of the product outer and inner diameters. When
product is pulled from the tip and die, if the material on the inside of the
cone is stretched to the same degree as material on the outside of the
cone, the DRB is 1.0.  When the material on the outside is stretched
more than the material on the inside, the DRB is greater than 1.0. 
The DRB must be 1.0 or higher for a stable extrusion process.

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Q.  Does the use of distressed prime material or scrap affect
the performance of extrusion dies?

A.  Good cost management compels us to use scrap or distressed material
in the feed because it lowers costs.   A few ideas:  if color is not critical,
add it to balance-out the yellowing that reprocessed material can cause. 
Always dilute distressed or scrap material with virgin material.  Keep in
mind that you may need to mix-in additives to meet performance specs
when you use scrap.  Always keep any distressed or scrap material you
have in a clean condition,  Never treat your scrap . . . like scrap.

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