Laboratory tests

Tests for green sands

A number of tests exist which are specifically designed for use with Greensand systems of the synthetic, semi-synthetic or naturally bonded varieties; these are well known throughout this sector of 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. Therefore it is not considered practical to provide the testing method for each type of instrument available. 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.

The following section describes the common tests performed,but other tests, such as particle size analysis (Section 3 ) may also be pertinent to greensand testing and are described elsewhere in this publication.

Moisture content

Moisture content is important to greensand users as water is an integral part of the binding system. Many properties of the sand are related to and are affected by the level of moisture and many moulding problems can be attributed to too little or too much water in the sand. Strict control of the moisture content is required at the mixing (milling) stage.

The normal test method employed is direct reading apparatus using calcium carbide in a pressure vessel to give a quick indication of moisture content. There are also more accurate techniques such as loss of weight on drying at 105°C±2°C or a moisture balance. Care must be taken when heating greensand that materials other than water are not lost.

Permability

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.

Green compressive strength

The green strength of a sand has a direct bearing on its moulding properties; particularly its cohesiveness* and compaction (known in the industry as mouldability and compactability) when subjected to forces generated by the metal casting process.

Low green compressive strength may cause: mould failure, severe penetration, erosion and di- mensionally inaccurate castings.

High green compressive strength may cause: expansion defects such as buckles and scabs as well as contributing to residual stresses in the cast- ings upon cooling.

Green compressive strength is normally meas- ured on direct reading equipment in either psi or KN/m².

*Note "cohesiveness" is the preferred term. Cohesion: act of sticking together. ing to unite a mass.

Shatter index

Shatter Index is an indication of mouldability derived from the traditional moulder's practice of throwing a ball of sand at a wall and seeing if it stuck! Generally, the apparatus drops a core of sand from a set height in a controlled manner and the amount of disintegration upon impact with a target anvil is determined by measuring the weight of frag- mented sand that is retained by the anvil and a sieve of specific mesh size. Many using this test actually weigh the material collected in the pan and apply the subtraction from the tolerance weight of the ini- tial sample to obtain the amount retained.

Compactability

This test seeks to indicate, by measuring compaction, the ability of a sand to flow into a mould (known in the industry as flowability). This test uses a direct reading technique which measures the decrease in volume after ramming under specific conditions.

Compactability is related to both green strength and, in particular, the moisture content. Modern automatic green sand control systems, typically attached to high pressure moulding plants, measure compactability as the basis of their method of control. This is maintained at a constant level by adjustments to the moisture content. Moulding properties are therefore consistent and moisture content, while varying, is always at a level appropriate to all other conditions within the sand system.

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