Technical data on use of melt pressure and temperature sensors, rheometers and other instrumentation for maximizing polymer processing
- Guardian Tech Note
- An Investigation into the Problems Associated with Extrusion of Reground LDPE Tech Notes
- Burst Plug Temperature Effects Tech Notes
- Closed Loop Inlet Pressure Control for Melt Pump Extrusion Tech Notes
- Closed Loop Pressure Control for the Extrusion Process Tech Notes
- Coping with Transducer-Crippling Pressure Spikes Tech Notes
- Dynisco Pressure Transducers Relay Vital Information for Closed Loop Control Tech Notes
- Evaluating Extruder Screw Performance Tech Notes
- Gage vs. Absolute vs. Sealed Pressure Tech Notes
- Guide to Pressure Transducer Selection Tech Notes
- Improved Part Quality Using Cavity Pressure Switchover Tech Notes
- LMI Weights and Combinations Tech Notes
- Melt Pressure Measurement: Environmental Effects Tech Notes
- Melt-Pressure Transducers Part I Tech Notes
- Melt-Pressure Transducers Part II Tech Notes
- Mercury Fill Sensors Tech Notes
- Oil Fill Offset Tech Notes
- Operating Standards and Records Tech Notes
- The Doís and Doníts of Pressure Transducers Tech Notes
- The Importance of Monitoring Extrusion Stability Tech Notes
- The Strain Gage Pressure Transducer Tech Notes
- Utilization of Pressure Transducers for Improved Control of the Extrusion Process Tech Notes
The Machinery Directive standard EN ISO 13849-1 is a European safety regulation. This is a generic standard defining the requirements for the safety-related parts of control systems.. The primary goal of the Directive is to ensure that equipment will function in a safe manner. There are many specific standards that can be used to show compliance to the Machinery Directive. To that end, machines and their components are expected to function safely. A pressure sensor—while not the only component—is a machinery component that is expected to comply under the Directive, when they are used in a safety function on a machine.
A burst plug (aka rupture disk) is used to mitigate overpressure conditions. The burst plug is used as an intentional weak link in a process system. It is designed to rupture at a predetermined process pressure called the “burst pressure,” thereby protecting people, products, and processes.
In order to obtain an extruded profile of consistent quality, it is necessary to keep both the extruder output and melt viscosity constant. In most cases, the extruder output varies to sosme degree with time, as does the melt quality.
Extrusion is a continuous process and successful economic production depends on maintaining stable output and melt quality at an accurately controlled rate.
Here are some techniques to help reduce or eliminate their detrimental effect
One of the fastest growing areas for the use of pressure measurement instrumentation is in the co-extrusion of multi layer barrier structures. Many leading co-extrusion design and manufacturing firms, such as the Cloeren Company, engineer melt pressure transducers into their systems for closedloop process control.
As technology advances in screw and extruder design, the plastics producers can easily lose track of what is happening in the fast moving world of plastic processing. People offer extruder screws to improve performance but it has become increasingly difficult for the average extrusion engineer or manager to evaluate one screw's performance vs. another or to compare to his production performance. Buying a screw without performance situation can lead to screw reworks and perhaps, unnecessary disappointments.
The conventional units for defining pressure are PSI, pounds per square inch. However, in all pressure measuring devices, a reference pressure has to be established and the PSI qualified to indicate the reference datum. The suffixes “G”, “A” and “S” identify the measured pressure as follows:
PSIG -- Gage Pressure
PSIA --Absolute Pressure
PSIS -- Sealed Pressure
Most applications for pressure transducers are relatively straightforward. Install the transducer according to manufacturer’s recommendations, and obtain an accurate reading from an exceptionally reliable device. But life does not always follow the straight and narrow path. The purpose of this article is to provide some guidelines for helping a transducer user detect atypical conditions and compensate for them through the proper selection and application of the transducer
This study examined the improvements in injection molded part quality using cavity pressure to initiate switchover from injection pressure to holding pressure. Cavity pressure produced more controllable and uniform part dimensions than the time and position switchover typically employed in older point controlled injection molding machines. the controller was relatively easily incorporated into an older machine. When the cavity pressure set point was determined from the position switchover conditions, the pressure may not have been optimized, but provided better parts than the other transfer techniques, Direct determination of the cavity pressure set point is still being evaluated.
With the introduction of Dynisco’s new stackable weights for the LMI, no longer are large, heavy weights needed to achieve all of the ASTM D1238 testing conditions. Operators will now be able to stack combinations of smaller, easier to handle weights to attain the desired overall weight. With the introduction of these new weights, we are also introducing a new, easier to understand part number system. Please use this document as a guide to the new part numbers as well as for suggestions on how to achieve the ASTM D1238 weight values.
The Need For Pressure Measurement in Extrusion
In order to maintain the dimensional stability necessary to produce extruded products that meet todays precise quality and tolerance specifications, it is necessary to keep both the output rate and the melt condition constant (1,2). Although it is not possible to measure these quantities continuously, closely related variables such as melt temperature, and constant melt pressure at the die, the output rate can be considered constant (2).
Extrusion processors are employing melt pressure transducers more and more frequently to help them improve output and melt quality, enhance production safety, and safeguard machinery. To select the right transducer to meet their needs, processors must familiarize themselves with the performance characteristics of various transducers. Once a specific transducer is selected, proper application and maintenance are key factors in ensuring that these instruments provide optimum performance.
When selecting a melt pressure transducer, the first factor you should consider is the pressure range that will be measured. Available in ranges between 0 and 25 psi and 0 and 30,000 psi, transducers must be operated within their stated pressure range to ensure proper functioning. A transducer can be damaged if the extruder is operated at pressure levels beyond the transducer’s rated capacity, resulting in inaccurate pressure readings.
Why Is Mercury Used in Melt Pressure Sensors?
A transducer is an electronic device that converts energy from one form to another. In the case of a Dynisco pressure transducer, the process fluid exerts a pressure force on a thin flexible metal diaphragm at the tip of the sensor. This, in turn, stresses a small sensing device called a strain gage. The strain gage then converts the pressure stresses to a low level millivolt signal, which can be further conditioned to yet another signal type.
An oil‐filled sensor has the advantage of being non‐toxic and therefore compliant for many food, medical or other critical applications.
The overall performance of an extrusion line is strongly dependent upon selection of processing conditions and proper maintenance of extrusion hardware. Management should not expect to achieve a high-quality product at good production rates if extrusion equipment is not well maintained or if operating conditions are not well defined.
When specifying a pressure transducer for a process measurement, a number of items have to be considered. Some of the more important ones are discussed in terms of the transducer itself as well as the overall measuring system. This information is user-oriented and serves as a practical guide in the selection and application of strain gage pressure transducers.
It has long been known that pressure instability for polymer melt entering the die usually results directly in ouput variation. From 1960 to 1964 various investigators at DuPont and Union Carbide published articles citing examples of situations in which output fluctuations were found to be up to three times the measured pressure fluctuation. However, instrumentation used in these early studies was too delicate and expensive to be practical for routine commercial use. An excellent article written by B. H. Maddock, covering some of his early studies is title “Measurement and Analysis of Extruder Stability.” It appeared in the December 1964 issue of the SPE Journal.
Pressure transducers use a variety of sensing devices to provide an electrical output proportional to applied pressure. The sensing device employed in the transducers under discussion is bonded, metal foil strain gages. The strain gage because of its unique set of operational characteristics has easily dominated the transducer field for the past twenty (20) or so years.
Due to the precise quality and tolerance requirements of today's extruded products, it is very important to keep both the output rate and the melt condition constant (1). Melt pressure, which is one of the variables in this process, can be both measured and controlled with the use of a process, can be both measured and controlled with the use of a pressure transducer and some type of instrumentation. The transducer is a highly sophisticated sensor which is capable of measuring very small changes in head pressure can result in extrudate variation. Studies have shown that a head pressure change of 1% can result in a change f up to 3% in the extruder output (2).