Understanding Extrusion
Chapters 2 and 3
Professor Joe Greene
CSU, CHICO
1
MFGT 144
Chapter 2: Instrumentation and Control
and Extrusion Lines
• Instrumentation
–
–
–
–
One of the most essential elements of an extruder
Measure important process parameters
Window to the process
Essential to troubleshoot problems
2
Instrumentation
• Most Important Parameters
–
–
–
–
–
Melt Pressure
Pressure Transducers
Temperature Measurement
Melt Temperature Measurement
Barrel Temperature Measurement
• Temperature Control
– On-Off Control
– Proportional Control
– Fuzzy Logic Control
3
Most Important Parameters
• Melt Pressure
– Diehead pressure (pressure necessary to overcome
resistance in die) determines the output of the extruder.
– When diehead pressure changes with time, the output
changes as do the dimensions of the product.
•
•
•
•
Pressure Transducers
Temperature Measurement
Melt Temperature Measurement
Barrel Temperature Measurement
4
Most Important Parameters
• Melt Pressure
– Diehead pressure (pressure necessary to overcome
resistance in die) determines the output of the extruder.
– When diehead pressure changes with time, the output
changes as do the dimensions of the product.
– Figure 2.1
5
Most Important Parameters
• Pressure Transducers
– Strain gauge and piezo-electric are most common
– Strain gauge
• Capillary or pushrod transducer
• Two diaphragms, one in contact with the plastic melt and one
some distance away (Fig 1.2). Connection is hydraulic in
capillary and pushrod in the pushrod type.
• Strain gauge is attached to the second diaphragm to measure the
deflection which is related to the pressure at the first point.
6
Most Important Parameters
• Temperature Measurement
– Measured with a thermocouple
• Type-Temp (TC)- when two dissimilar metals are connected
there is a voltage generated between the metals.
• Resistance temperature detector (RTD)- Resistance of metals
changes with temperature.
• Infrared detectors (IR)- objects emit radiation that changes with
temperature. Used for surface temperature measurements.
7
Most Important Parameters
• Melt Temperature Measurement
– Measured with an immersion TC. Insulated probe
protrudes into the melt andreads temperature at the point
of the TC junction.
– Figure 2.4
8
Most Important Parameters
• Barrel Temperature Measurement
– Measured with TC or RTD Sensors pressed into barrel
– Influenced by thickness of metal sheath. Can cause error
as well as air currents around the extruder
– Figs. 2.5, 2.6, and 2.7
9
Most Important Parameters
• Temperature Control
• Maintains temperature based upon as preset value and an actual reading
– On-Off Control
• Power is either on or off. Provides poor control,+/-20C, Fig 2.8
– Proportional Control
• Power is proportional to the temperature within a certain
temperature region called the proportional band. Fig 2.9
• Temperature can be kept steady with the power level adjusting
to keep temperature very close to preset value.
• Proportional control (P-control) works well except when an
upset occurs and the P-control is not able to change it
• Proportional Control with Integrating Action (PI) can control it
• Controller integrates the difference between the actual temp and
10
the setpoint and continues to act until the difference is zero.
Most Important Parameters
• Temperature Control
• Maintains temperature based upon as preset value and an actual reading
– Fuzzy Logic Control
• New method is fuzzy logic control (FLC).
• Artificial intelligence based technology designed to simulate
human decision-making.
• Requires generation of a knowledge base which identify
– Process variables that are important in control
– Membership functions for each variable, (H, M, L)
– Fuzzy rules which define the knowledge of what to do about an
observation based upon previous operating experience
• FLC is being applied a number of times in injection molding.
• FLC can outperform conventional PID control
11
Chap 3: Complete Extrusion Lines
•
•
•
•
Tubing and Pipe Lines
Film and Sheet Lines using the Roll Stack Process
Film Lines Using Chill Roll Casting
Combination of Materials
– Coextrusion
– Extrusion Coating
– Extrusion Lamination
• Blown Film Lines
• Extrusion Compounding Lines
• Profile Extrusion Lines
12
Complete Extrusion Lines
• Tubing and Pipe Lines
– Small diameter tubing (less than 10mm) is usually made
with free extrusion process (No sizing unit)
– Gear Pump may be used depending upon the precision
– Internal air pressure of the tubing is controlled to achieve
proper diameters of the tubing.
– Diameter and wall thickness are functions of
• extruder output, puller speed, and internal air pressure
13
Complete Extrusion Lines
• Film and Sheet Lines using the Roll Stack Process
– No real difference between flat film and sheet extrusion
Can be wither roll-up
or roll down process
14
Complete Extrusion Lines
• Film and Sheet Lines using the Roll Stack Process
15
Complete Extrusion Lines
• Film and Sheet Lines using the Roll Stack Process
16
Complete Extrusion Lines
• Film Lines Using Chill Roll Casting
– Thin films are often cast on a chill roll rather than
extruded into a roll stack.
– Initial contact is established with the use of a air knife
which produces a thin stream of high velocity air across
the roll that pushes the film against the roll.
– Thickness gauge uses a scanning measurement head.
– Surface treatment (flame or corona discharge treatment)
is done to improve adhesion for printing or laminating.
17
Complete Extrusion Lines
• Combination of Materials
– Coextrusion
• Combines two or more plastics through a single die
– Feed block system- different plastics are combined in the feed block
» Simple, inexpensive, and alloys many layers to be combined.
» The materials must have similar flow properties to avoid distortion
– Multi-manifold system- has own entrance and manifold in die and
multiple melt streams combine just before they exit die.
» Plastics with widely different flow properties can be combined.
» Die design is more complicated and expensive.
» Fig 3.5 has 2 inlets, 2 manifolds, and single outlet. Flow of the
upper layer can be adjusted by flexing the choker bar, using
adjustments nuts. 2 plastics combine at the entrance to the land.
» Many multi-manifold dies are possible, ie, flat film and sheet,
pipe, blown film, and profile dies
18
Complete Extrusion Lines
• Combination of Materials
– Extrusion Coating Fig 3.6
• Molten layer of plastic film is combined with a moving solid
web or substrate, e.g., paper, paperboard, foil, plastic film, or
fabric.
19
Complete Extrusion Lines
• Combination of Materials
– Extrusion Lamination (Fig 3.7)
• Involves two or more substrates, such as paper and aluminum
foil, combined by using a plastic film as the adhesive.
• Extruded sheet or film can be laminated with a film
• The laminate is unrolled from a payoff
• After lamination the film is handled as a regular film
20
Complete Extrusion Lines
• Blown Film Lines (Fig 3.8)
– The tubular film is extruded upwards.
• Ratio of bubble diameter to the die diameter = blow up ratio
• Typical ratios for LDPE is 2.0 to 2.5:1
• Can produce tubular products as bags but also film via slit.
21
Complete Extrusion Lines
• Extrusion Compounding Lines
22
Complete Extrusion Lines
• Profile Extrusion Lines
• Consists of an extruder, die, calibrating unit, cooling unit,
measurement unit, haul off, and coil or cutter or saw.
• Gear pump may be used if dimensional tolerance is small.
• Some lines a film or foil is laminated to the extruded profile.
23