Demand for quality control never slows down, regardless of the availability of testing programmes and technologies.
Maintaining the proper moisture levels when producing sand moulds guarantees bonding takes place between materials to create the right cohesiveness, permeability, refractoriness, surface finish, flowability and chemical inertness. Without achieving these properties, the quality of castings becomes compromised.
“Moulding sand and core sand depend upon shape, size composition and distribution of sand grains, amount of clay, moisture and additives. The increase in demand for good surface finish and higher accuracy in castings necessitates certainty in the quality of mould and core sands. Sand testing often allows the use of less expensive local sands. It also ensures reliable sand mixing and enables a utilisation of the inherent properties of moulding sand,” explains Jacques Swanepoel, National Sales Manager for DZanetech.
“Sand testing on delivery will immediately detect any variation from the standard quality, and adjustment of the sand mixture to specific requirements so that the casting defects can be minimised. It allows the choice of sand mixtures to give a desired surface finish. Thus sand testing is one of the dominating factors in a foundry and pays for itself by obtaining lower per unit cost and increased production resulting from sound castings,” continued Swanepoel.
DZanetech have imported a machine from India to test the permeability of mould and core sand
“Generally the following tests are performed to judge the moulding and casting characteristics of foundry sands: A moisture content test; a clay content test; a permeability test; the chemical composition of sand; a grain shape and surface texture of sand, in other words a fines test; the grain size distribution of sand and the strength/hardness test.”
“These tests are well-known throughout the foundry industry. Most of these tests require specialised equipment. In some instances there are a number of different measuring instruments available, each using a slightly different test method which may lead to differing results. In all cases the test must be carried out in accordance with the manufacturer’s instructions to ensure safety of the operative and to ensure that accuracy and reproducibility are within acceptable limits.”
Permeability is defined as the physical property of a mould (or core) to allow gas to pass through it. Therefore the permeability of a mould (or core) determines how fast gases created on casting can be released. These gases escape through the voids between sand grains. In general, higher permeability indicates coarser sands whereas lower permeability indicates finer sands. However, there are other factors which affect the permeability of sands. Moisture in the sand will reduce permeability by reducing the void space. A wider graded sand will reduce permeability by packing together more tightly which will reduce the void space. Grain shape can affect permeability by the interlocking pattern of sand grains. For example, rounded grains give a higher permeability than angular grains of the same particle size analysis.
Lower permeability sands may require increased venting to ensure casting defects attributable to trapped gases are avoided (e.g. blowholes, misruns, scabs and buckles).
Higher permeability sands will generally result in a rougher surface finish whereas too high a permeability can lead to burn on and metal penetration defects.
The majority of instruments for measuring permeability work by measuring the resistance to the passage of air through a body of sand with known dimensions after ramming under standard conditions. Measurement can be by volume of air flow and is usually expressed as Permeability Index.
Versatile machine for no-bake sand testing
“We at DZanetech know that sand, chemicals and scrap can cost a foundry owner vast amounts if they do not get the mechanical properties of mould sand correct before casting.”
“Having said this, foundry operators are faced with an insurmountable number of important process variables that they must control. There are so many variables that it’s a wonder anyone ever makes a good casting. The amount of time and attention that every foundry must devote to metallurgical processes is substantial.”
“However, when it comes time to focus on the sand, the attention seems to wane. After all, it’s just sand. Wise foundrymen know that nothing could be further from the truth. A mould is comprised of two things – metal and sand – and each of these is capable of introducing dozens of variables that need to be controlled as closely as possible to ensure a successful process.”
“To ensure the fulfilment of customer expectations and to consistently produce high-quality castings, sand and the sand cores should meet numerous requirements. We have therefore decided to import a state-of-the-art permeability sand testing machine from Versatile in India, a company that has been in the field of manufacturing foundry sand testing and control equipment since 1967.”
“This machine allows us to offer another service for our clients. The machine gives us the ability to give expert recommendations for existing and new projects, so that there are no quality compromises.”
“We have two full time metallurgists working in our laboratory that consists of the standard equipment such as sieves, PH meters, scales, furnace and other equipment necessary for tensile, transverse, bend and bounce testing. We have also invested in a laboratory size cold-box core shooter.”
“Other capabilities offered by the service include structured testing for core-sand analysis, moulding-sand analysis and casting performance evaluation. We are able to assist any foundry with an investment in high-performance sand-preparation quality control and give them the opportunity to do what they do best – cast metal.”
“After all, matching the right product to the right process is fundamental to the task of achieving the best performance.”
For further details contact DZanetech on 079 514 6885 (Jacques Swanepoel), Cape Town 083 454 5465 (Johan Jooste), 083 274 1657 (Koketso Mamogale) or 066 010 0999 (Zaid Syed) or visit www.dzanetech.co.za