Blow moulding of HDPE is an established process to produce a variety of articles to cater to various needs. Taking care of a few points can help produce good quality blow moulded articles economically.
A blow moulding machine consumes a lot of energy through its various systems. The extruder runs continuously at about 80 to 90 % of the motor Rated Power i.e, a major part of energy flows through the extruder continuously. So it is highly imperative that the extruder system is well designed and maintained properly in order to get more throughput using less energy input.
The following features are worth considering for improvement of existing machines and for specifying new machines.
Replacement of old motors with energy efficient ones will directly yield substantial energy saving.
Even servo motor which has even better efficiency can be considered as replacement.
Over the years a lot of improvements have take place in extrusion screw design. Replacing old style extrusion screw with an improved version will result in more throughput with better quality extrudate and saving of energy.
Even better would be to use a grooved barrel extruder if the gear box torque rating is adequate.
With larger blow moulding machine a complete change of extruder (gear box, thrust bearing assembly, screw, barrel and grooved feed zone) may be worthwhile.
Tungsten carbide coating of extrusion screw is another option available. There are two benefits resulting from tungsten carbide coating. The extruder working life is enhanced. And the throughput of the extruder remains almost constant throughout the life of the screw. Unlike nitrided screw that wears quickly after some period. ie higher performance for a longer period for a given instrument.
Worm gear type speed reducer (gear box) is highly inefficient and should be replaced with helical gear type speed reducer with integral thrust bearing assembly.
Lubricant oil of correct viscosity grade only should be used for speed reducer. A thinner oil leads to wear of gears and a higher than necessary viscosity oil gulps power generating heat and overloading the extruder drive motor.
Lastly alignment of all extruder parts (screw, barrel, feed zone, speed reducer etc.,) should be precisely done and maintained to prevent undue wear and tear.
DIE HEAD / ACCUMULATOR HEAD
Weld line is a common problem with extrusion blow moulding. This can be virtually eliminated by using spiral mandrel distributor. The benefit is better quality articles are produced with minimum reject rate. So replacement of distributor or the entire head is an option for improving the performance.
Hydraulic oil of correct viscosity grade suitable for the pump taking into account the extreme ambient temperatures during summer and winter must be used. A thinner or thicker hydraulic oil will harm the system.
Install a solenoid valve other device for controlling the cooling water flow going into the oil cooler to keep the temperature of hydraulic oil constant in the tank. There are two benefits. One, hydraulic oil temperature and consequently the viscosity remains constant and you can expect steady performance from the machine hydraulic system leading to constant product quality. Two energy saving especially during winter when excessive cooling of hydraulic oil is avoided.
Periodic maintenance is a must. Check everyday level and colour of hydraulic oil (cloudy oil indicates water contamination.) Change all filter elements after first 500hours of running of the machine and thereafter at required frequency.
Never pour leakage oil collected from machine into the hydraulic system. It is penny wise and pound foolish.
It is worthwhile to have a filter cart which can carry out periodic filtration of oil in the tank.
Use of hydraulic pumps with better volumetric and hydro mechanical efficiency is advisable.
A VFD driven motor for hydraulic pump can save energy during idle phase of the cycle time.
Avoid all air leaks.
Use air pressure regulator for feeding part of a circuit that requires lower pressure than the main circuit.
Keep air receiver close to the blow moulding machine in order to reduce pressure drop especially for large machines.
Training of people to keep constant vigil on the performance of the blow moulding machine is a must.