FAQs: Getting the Most from Your
Extrusion Tooling Operation

Q. What is the largest tube I can extrude on my head assembly?

A. To make this calculation, you’ll need to consider the physical
constraints of the extrusion tooling and the rheological constraints
of the polymer being extruded. Regarding the physical constraints
of the head assembly, first determine the die’s largest possible ID
opening. For our BH275C-1, for example, the die ID can be opened
to a maximum of 1.75". So, in calculating the tooling sizes for a tube
with an optimal Area Drawdown Ratio (ADR) and Draw Ratio Balance,
there is a ceiling of 1.75”. When you approach that limit, it’s time to
consider variables, such as a lower ADR, a larger head assembly,
slower line speeds, etc.

Regarding the rheological constraints of the polymer, the first
step is to determine the optimal ADR for the specific material
the you plan to extrude. For example, Flex PVC has an
optimal ADR between 2:1 and 3:1. You need to stay within this
range, generally speaking, in calculating the tooling dimensions
(e.g. Tip OD and Die ID). If you know the material you plan to
extrude and the largest tube OD, one of our online calculators,
located here: www.bhtool.com/calculator.htm can be helpful.

To summarize, the variables involved in determining the largest
tube a particular head assembly can extrude include the polymer,
the optimal ADR for that material, the maximum open die ID for
that head assembly, and extrusion line speed. With this information, it
is relatively easy to calculate the largest tube that can be extruded. 

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Q.  What does B&H recommend for set-up and cleaning of
plastic extrusion tooling?

A.  Several Operations Manuals are available with detailed
recommendations. Click here for more information.

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Q.  Can you help me with information about designing tooling
to extrude tubing?

A.  Yes, we recently added a white paper on tooling design for tubing
by extrusion expert Chris Rauwendaal.

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Q.  What are the major considerations in looking at extrusion tooling?

A.  To optimize extrusion tooling for a new or existing extrusion line,
first consider flow path.  For thermoplastics and fluoropolymers, you want
the flow path within the head assembly to have a constantly converging
flow path from the inlet port all the way out to the end of the die.  

The reason is, you want to avoid dead spots where material can get hung up
and you also want to avoid internal area expansion where the 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 die.

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Q.  We’re looking at extrusion tooling for sleeving applications. 
What are some things we should watch for?

A.  For sleeving applications, you need to make sure you have the correct
drawdown ratio and draw ratio balance for the material used with your extrusion
tooling. And, you want to have a land that is appropriate for the die gap.

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Q.  We’re doing pressure extrusions.  What are the factors we
need to be aware of in buying extrusion tooling?

A.  For pressure extrusions, it’s important to have an opening sufficient for
the core to pass through, but not too big 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.  How does heat profile figure in to decisions about extrusion tooling?  

A.  A constant temperature profile from the exit of the extrusion tooling
to the exit of the die is critical, because it keeps the material flowing at
uniform velocity through out the die and across the flow path, from the
outside surface to the inner surface. 

Avoiding gaps in heat coverage is very important. For high temperature
extrusions, any area not covered by a heater can adversely affect the
flow path. Temperature variances can cause the extrusion material to get
hung up on the “cooler” spots, leading to variations in wall thickness.

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Q.  What are some things to consider in choosing extrusion
tooling for best material distribution?  

A.  Choosing extrusion tooling for best material distribution in the head
assembly is critical.  Thermoplastic and fluoropolymer material shear,
and heat sensitivity should be considered when choosing the correct flow
distribution design to avoid degrading and burning the material.

For crosshead extrusion tooling, this means choosing the correct deflector.
If your material is shear and heat-sensitive, a deflector that does not
deflect the material, or allow it to stay inside the crosshead too long,
is preferable. The material’s shear and heat profiles should drive what
deflector to use with your extrusion tooling.

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Q.  We’re adding in-line spider dies to our extrusion tooling.
What are my options?

A.  For in-line spider dies, you can choose 2,3, or 4 spider leg designs.
Also, the spider leg designs can vary in thickness.  If the material being
extruded is heat and shear sensitive, avoid a spider design where the
material can get hung up.

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Q.  What about “spiderless” designs in extrusion tooling?

A.  This design is widely regarded as controversial, and very experimental. 
I visited a customer who was using spiderless extrusion tooling on a very
shear sensitive material. And they had problems controlling wall thickness.
It was apparent that the customer had the wrong design for the material.
We recommended a 3-legged inline spider die with a balanced extrusion
tooling set and a consistent heat profile, and eliminated these problems.  
The material’s shear and heat profiles should drive the design of any
extrusion tooling.

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Q.  Are there any smart "tools" available that can simplify the
process of ordering crossheads and dies?

A.  Yes, there are 7 of them here.

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