Tips for Melt Pressure Transducers
Extruders use melt pressure transducers to improve output and melt quality, enhance production safety and safeguard machinery. It is important to maintain the optimum processing pressure during produc...
Extruders use melt pressure transducers to improve output and melt quality, enhance production safety and safeguard machinery. It is important to maintain the optimum processing pressure during production to ensure that product quality specifications such as part dimension and surface finish are met with minimal material waste.
Transducers placed at the entrance to the die, in conjunction with a pressure control device, help maintain stable output, thereby reducing scrap and material waste. Pressure measurements at the screen pack and melt pump are also important. A pressure gauge mounted downstream of the screen pack will alert operators if flow from the screw to the die is restricted, while a transducer upstream of the screen pack will warn of a high-pressure situation that could result in excessive wear to the screw’s thrust bearing. For processors using melt pumps, measurement of both inlet and outlet pressure ensures a constant melt flow at the die and helps prevent damage to the pump that a restriction in melt flow could cause.
Melt pressure instrumentation can be as simple as one transducer measuring a single pressure point, or as complex as a series of transducers measuring the entire process, along with instrumentation that records data, sounds alarms, gives warnings to take corrective actions and relays information to process control systems.
Knowing both proper installation and maintenance form the key to optimum transducer performance and longevity. To obtain the greatest number of accurate, reliable melt pressure measurements, follow these installation and maintenance guidelines and recommendations.
Proper transducer mounting
One of the most common causes of damage to pressure transducers is the installation of these units into improperly machined holes. By trying to force a transducer into a too-small or eccentric hole, the transducer diaphragm will be crushed and the instrument will not function. Mounting hole machining tool kits can help make sure holes are properly sized.
Although proper mounting torque–100 to 200 in.-lb. –is essential to form an adequate seal, excessive mounting torque will cause seizing. A high temperature anti-seize compound should be applied to threads prior to transducer installation. Transducers installed at a mounting torque above 500 in.-lb. will be difficult to remove.
Use clean mounting holes
Once the transducer mounting holes are correctly machined, it is important that they are kept free of any plastic buildup. Before cleaning an extruder, remove all transducers from the barrel to avoid
damage to the units. Once they are removed, however, plastic is likely to flow into the mounting holes and harden. If this plastic is not removed before inserting the transducers, extensive tip damage will result. Use a cleaning tool kit.
Ensure correct location
If a transducer is positioned too far upstream along the barrel, unmelted plastic pellets can be forced against the transducer tip, resulting in damage. If a transducer is positioned too far back in the mounting hole, room is left for unmelted plastic to build up between the transducer tip and the screw flights. This unmelted plastic will degrade to carbon, prohibiting the transmission of an accurate pressure signal. Conversely, if a transducer is pushed too far into the barrel, then the screw flights could shear off the unit’s tip.
Care in cleaning
Always remove transducers before cleaning an extruder barrel with either a wire brush or special cleaning compounds. These can ruin the transducer diaphragm. Remove the transducer with the barrel hot and wipe the tip clean. Remember to go back and clean the hole with a cleaning drill/guide sleeve.
Keep transducers dry
Most standard transducers are not watertight. Although a transducer’s electronic circuitry is designed to withstand the rigors of the extrusion process, it will not operate when wet. To avoid damage, make sure there are no leaks in the water-cooling jackets in the extruder barrel. If wet operation is unavoidable, specify watertight transducers.
Ensure proper thread size
The friction caused by trying to screw a transducer into a mounting hole with incorrect thread size will damage the transducer’s threads and the instrument will not function properly. The proper mounting hole dimensions must be used to avoid thread galling. A mounting well gauge plug can be used to verify that your mounting hole is correctly machined and cleaned.
Avoid cold starts
Both the transducer and extruder can be damaged if the heater bands are not left on for a while before the extruder begins operating. Sufficient ‘soak time’ must be allowed for the plastic to return to a molten state. Also, if a transducer is removed from a cold extruder, melt and material could adhere to the transducer tip and rip the diaphragm off the unit. Make sure the barrel is warm before removing the transducer.
To avoid the risk of applying too much pressure to
a transducer, make sure you are using a model designed for your operating pressure range. It is also wise to use transducers that are built to withstand twice the rated pressure. This way, the extruder would have to be operating at an extremely high and unsafe pressure level in order for the transducer to fail.
This article was supplied by Cyronix Inc., Don Mills, Ontario, the exclusive Canadian representative of Dynisco Instruments, Sharon, MA.