Understanding Extrusion Chapters 2 and 3

1. Understanding ExtrusionChapters 2 and 3 Professor Joe Greene CSU, CHICO MFGT 144

2. 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

3. 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

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. • Pressure Transducers • Temperature Measurement • Melt Temperature Measurement • Barrel Temperature Measurement

5. 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

6. 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.

7. 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.

8. 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

9. 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

10. 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 the setpoint and continues to act until the difference is zero.

11. 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

12. 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

13. 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

14. 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

15. Complete Extrusion Lines • Film and Sheet Lines using the Roll Stack Process

16. Complete Extrusion Lines • Film and Sheet Lines using the Roll Stack Process

17. 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.

18. 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

19. 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.

20. 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

21. 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.

22. Complete Extrusion Lines • Extrusion Compounding Lines

23. 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.