Getting Ideas for Energy Management Opportunities
Mould making may consume as much as 20% of all energy in the foundry, depending on the technology used. The prevalent green sand mould making consists of sand and clay receiving, conveying, preparation, delivery to the mould area, mould handling, shakeout, reclamation, reconditioning and more conveying. This uses all forms of energy in a foundry, and the whole system must be designed in such a way that there are no bottlenecks.
Depending on application, virtually all processes that are used in core making also produce moulds: oil sand, hotbox, shell, nobakes, coldbox and green sand. One process also uses just loose dry sand around the lost foam moulds (see below). Similar principles apply in energy efficiency improvement efforts also in the mould department.
Drying of mould coating
To protect sand moulds and cores from high molten metal temperatures, foundries are using refractory coatings. This reduces scrap, cuts labour costs and improves productivity. The coating is in the form of liquid wash. The carrier liquid can be based on alcohol-chlorinated compound mixtures, which dry quickly without heat, but their toxicity and volatility require emissions controls. Water-based washes are less expensive and are non-toxic, but require a long drying period in gas or electric-resistance heated ovens. The ovens are effective. Nevertheless, application of infrared heating for mould coating, developed in joint collaboration by the Electric Power Research Institute (U.S.), can reduce the drying time by as much as 85%. The energy saving stems from the controllability of the infrared unit, which brings the mould surface to the desired temperature and then cycles off in a predetermined time sequence. Less heat is dissipated into the surroundings. The infrared element directs the heat more effectively at the mould and can dry deep cavities and mould pockets – thus contributing to better castings quality. The subsurface of the mould is not affected. An additional advantage was that only 25% of the floor space occupied by the resistance ovens was required. The payback was less than one year.
Lost foam casting
This relatively new technology (currently, just two lost-foam casting plants operate in Canada) offers substantial energy savings (up to 30%). The foam pattern can be of very intricate design. For example, a foundry replaced the need for assembly of 125 pieces with a single foam-cast part. The lost-foam casting can achieve the dimensional precision of high-pressure die-casting. It uses low expansion sand as a moulding medium.
In lost-foam casting, a machined foam pattern is dip-coated or sprayed with a thin ceramic film. After passing of the coated pattern through a dryer, the film now offers protection against molten metal penetration. The pattern is then placed in dry sand (no binders necessary!), which is compacted around it by vibration. Molten metal is poured onto the foam through a sprue. It displaces the foam by melting and subliming it away.
In the monitored installations (by the University of Alabama in 2000), machining and assembly were mostly eliminated for lost-foam parts. Production rates increased several times. Energy consumption decreased also by load reduction from auxiliary equipment. Metal waste was significantly reduced. Moderate pressure application during the solidification phase reduced porosity by an order of magnitude, increased tensile strength by 10%, and showed 100% increases in elongation. The useful life of foam-making machine tools is virtually unlimited, while conventional tools must be replaced after about 200 000 cast parts.
Unique to the lost-foam process is the burning off of the Styrofoam® patterns as the metal is being poured. It produces additional large quantities of recoverable heat in the vented gases; a heat reclaimer can be put to profitable use there.
While the costs of a lost-foam casting machine are about double what a die-casting machine would cost, the several-times-higher output and the advantages described above make the overall economics very attractive.
Other EMOs
Housekeeping
- Do not let equipment run when it is not needed.
- Apply preventive maintenance.
- Ensure regular calibration of the controls.
- Monitor sand composition.
- Control spillage of shell sand (costs).
- Control spillage of sand (eliminate losses and double handling).
Low cost
- Consider installing an automatic sand analyser.
- Install automatic controls for the maintenance of sand temperature.
Retrofit; high cost
- Analyse and optimize the system: look at conveying and handling patterns.
- Automate and synchronize the key elements of the mould-making line.