PDC (pressure die casting) has remained rather static in India for many years now, with very few users opting for high pressure die casting technique, or employing the pulsed technique of molten metal feed that results in more superior castings. This story takes a look at several such innovations which have been proven in industrial usage abroad, and describes them in detail. It lists out the various inherent strengths of these innovative ideas that can help the users battle the everyday problems that plague the productivity routinely e.g. early onset of heat checks, metal adhesion, non-uniformity in castings, low life of dies and machine parts that come in contact with molten metal. It also takes a look at the vastly advanced science of duplex treatments that combine plasma nitriding and multilayer coatings via the physical vapour deposition route. Also the plasma assisted chemical vapour deposition applications. The article draws latest information from reliable sources. Casting as a technology for producing metal artifacts and products has been known by mankind for well over 5000 years as the discovery of cast metal statuettes and implements in early civilisation areas such as the Mayan civilisation around the Andes mountain range in Latin America has shown. Die casting or gravity die casting as the original technique is now referred to as, came to be established only in the last couple of centuries soon after the metallurgical processing took several leaps in new directions. The period after the industrial revolution saw remarkable progress in casting technology development.
One cannot overlook the fact that initially casting methodologies used to employ only sand moulds but with the arrival of more sophisticated toolings technologies, it was just a matter of producing better surfaces in the steel moulds (or dies) and then pour molten metal into it. Of course, there were rather persistent internal defects, and a lot of these like porosity, blowholes, entrapped gas bubbles, etc were directly caused by failure of gas to leave the solidifying melt. This usually caused gas entrapment leading to more disastrous results. Obviously one logical solution that presented itself was pressurising the melt - this is easier said than done, and it must have taken a whole lot of coincidences on part of the technology developers then: pressurising and feeding molten metal into an enclosed cavity inside a die is not easy at all. But they did and today one finds an abundance of die cast parts that show us tremendously sharp features, need hardly any cleaning or post-casting finishing, and utilize the metal efficiently.
What are the die cast parts one sees all around us today? A perfect example is those tiny sports cars, in their true replica form, sold globally and treasured rather jealously by collectors: cast, creed, colour or age no bar. These tiny cars have perfectly recognisable formats due to pressure die casting. What else? All kinds of two or four wheelers will have carburettors, the unit that mixes air and fuel in perfect proportions before feeding it into the cylinder. These carburettors as a rule are die cast, and they have pretty complex geometries and complexities. Ordinary casting or even gravity die casting cannot produce all those tiny projections and compartments inside the body of the carburettor which is a marvel of modern engineering and after due machining and assembly, it has to work with hair-breadth levels of precision. A tiny spec of dirt totally invisible to the eye can block the fine jet of fuel that injects the air-fuel mixture into the cylinder. Pressure die casting then, has changed little from the days when Henry Ford used to sell his hugely popular Model T car universally and to repeat an oft-heard cliché, offer any colour scheme as far as it is black… To the casual industry observer there may be hardly any conspicuous changes in the design of the giant machines used especially in the automotive spare parts sector, when one compares a century old antique machine or the latest one duly computerised and gleaming with modern colour schemes. The difference lies all inside. The difference is in adoption of several innovative ideas that we shall soon examine, and also in adoption of microprocessors, numerical control or digital control and use of PCs as the heart of the system.
Automation has made all the difference. Microprocessor controls gave way to the PLCs (programmable logic controls) which became necessary as the number of processing parameters steadily climbed. As it is, larger industries have always been reluctant to rely too heavily on human skill. One of the major benefits of computerisation in production processes clearly has been this decreasing necessity of reliance on human skill. Now the skill part is taken over by the computer, sliced into smaller digital entities, and then put together in a particular menu suited to that particular use. Change the alloy, just select another menu and all the related processing parameters will be automatically reset to handle the new material with all attendant vagaries and uncertainties. All guesswork has gone out of production technologies now, with the use of PLCs, CNC or computer controls.
Some important innovations in all aspects of gravity/ pressure / high pressure die casting:
High pressure die casting
The pressure die casting technique has shown a lot of promise with the variant of high pressure die casting becoming more and more accepted for higher quality parts with lower rejection rates. Specialised machines are needed and sometimes it may not be worthwhile to use add-ons, but rather invest in a single piece HPDC system so that more reliability can be assured. Heres then a quick look at what CSIRO, Australia, has been working upon, in this particular area:
Reduction in metal pressure in the HPDC process
Aim: To investigate the role of metal pressure on the production of quality parts in high pressure die casting.
Outcome: In the final six months of this project, effort was focused on innovative technologies. Following findings were noted:
[a] A special new technology was designed to absorb impact pressure spikes that cause detrimental flashing,
[b] Another technology involved total revamping of the hydraulics of the ageing die casting machines to improve overall product quality.
[c] A novel shock absorbing technology was developed that utilised existing facilities of casting overflows. Die casting trials at CSIRO confirmed the effectiveness of this technology in absorbing impact pressure shocks upon cavity filling. Through several in-plant trials at Nissan Casting Plant the limits of hydraulic valve timing and circuit functioning were confirmed.
[d] A proposal for a complete revamp to improve intensification pressure response was suggested. The project concluded in December 2001. Ford Australia adopted the reduced pressure operating parameters for the production of their new Barra model engine sumps later in 2002.
Improved quality aluminium automotive castings [by low pressure die casting]
Aim: To improve the overall performance of low pressure die casting operations by implementing improved tools in design and process control to reduce casting defects. [Note, this happens to be an area of great relevance for the Indian pressure die casting community, as it does not look at equipment replacement and yet promises to deliver much improved performance. Most industries are very reluctant to completely replace their bread-earning old equipment with new machinery.]
Outcome: Successful development of appropriate tooling design and process control was achieved for the low pressure die casting (LPDC) process to cast small automotive components.
A multi-cavity die design was selected and optimised by solidification simulation. At present, several dies of this design are being used to produce high-volume, high-integrity parts. Casting parameters were also minutely investigated to improve the casting quality and reduce the cycle time.
Die trials were conducted on an LPDC research die to investigate the effect of casting geometry and process parameters on shrinkage defects in castings having several fundamental features of cylinder heads. The die trial successfully produced castings with shrinkage defects in one particular area sandwiched in the sand core, as predicted. Analysis of castings made on the LPDC pseudo-cylinder head research die will later be completed to establish relationships between porosity defects and process parameters.
Integrated gravity die design methodology
Aim: To develop an integrated die design methodology for gravity die casting that can achieve optimal die filling, optimal feeding and optimal yield, and dimensional stability.