Green Sand Sand Mixing Process 1) Shree Vinay Desai had raised a point re water addition- right time & right way. Also hotly debated topic of Dry mixing. 2) About dry mixing & water addition in yester years theory & practice was to have dry mixing generally for 10- 15% if cycle time & then gradually sprinkle water. 3) Idea was to get fry powder additions well disburse & then wet them for bentonite to swell & blend the grains. 4) Some Italian & us mixer manufacturers do not recommend any dry mixing. Their argument perhaps (my interpretation) there is 1.7 to 2 % water in return sand & mostly people recommend so. Therefore dry mixing is only hypothetical. 5) Coming to water addition - with advantage of high speed mixers, fast addition of water that is Flooding recommended - helping short cycle time. Where compatibility controller is installed, 80- 90 % water is added & based on the feedback final water addition is made. 6) Well this is feedback of different Foundries. I am not running a Foundry, & past few years nit as much in touch. 7) However, my final word of advice id that these are views to make a trial in your Foundry, & record the results to customize practice best suited for your Foundry. Sand metal ratios, type of mixer, cooler & geometry of jobs are typical for each Foundry. So treat all advices as guidelines & customize your solution. 8) DRY MIXING:- Purpose of DRY mixing is to first homogenize the return sand and then uniformly distribute the additives like bentonite, new sand and coal dust (or its substitute) which enters the mixer after return sand. The whole process should be completed within 20 Sec max. Under mixing will result in non-homogeneous mix and hence non uniform property of prepared sand within the same batch which may not be detected by lab sand but it will definitely give mould with non-uniform property. Over DRY mixing will remove the layer of bentonite on grains of return sand. For duration of DRY mixing, manufacturer's instructions to be followed. 9) MIXING OF GREEN SAND Continuing discussion on the above subject let follow man, machine, materials, method and environment methodology and also keeping in "process" approach. In put materials in the mixer are:- return sand, new sand, bentonite, coal dust or its substitute. About bentonite, coal dust and water; group members have already shared their opinion now and in near past.
The return sand entering mixer should be uniform with respect to active and dead clay content. It should contain 1.8 to 2.2% moisture and its temperature around 30deg.C For example if a 2 Ton batch in mixer has varying quantity of MB and Total clay will get homogenized during DRY mixing. Bentonite, new sand and coal dust should get uniformly distributed in the batch. Now let us discuss wet mixing. All foundries have now accepted that all batches of the prepared sand should be of compactability within a pre specified range (say 40 plus minus 2) rather than with constant moisture. Before proceeding opinions are solicited from members. 10) I agree to your approach towards unifying the process parameters. All input material with their important parameters have been considered. Well there is no end to considering and discussing their detailed features. The control parameter of the output,i.e. the mixed sand in terms of compatibility is well known and considered all over the world. This is a direct measure of ability of making a good mold in all kinds of molding apparatus. The value of 48-52 is followed by many foundries. The other sand testing parameters will vary quite a bit and control limits vary from foundry to foundry. 12) MIXING OF GREEN SAND (Contd.) After DRY mixing wet mixing cycle starts. Following description is for those mixers which are fitted with c ompactability controller. In this cycle bulk water addition is done immediately after end of DRY mixing time. After few seconds, first sample is collected and its compatibility is checked. Depending on the value obtained, some more water is added, mixed for few more seconds and then the mix is discharged. There is another variant of the above type of equipment in which a moisture probe measures the moisture content of sand after DRY cycle time and bulk water is added, quantity of which depends on moisture content already determined. In this type of equipment also operator sets the desired compactability. Computer of the equipment determines the quantity of water from the ompactability Vs moisture graph in the memory of computer. Above is highly simplified description of the equipment available. For details, operation manual of the equipment should be referred. Let us now focous on the machines involved in the mix preparation which are:Mixer, hoppers of return sand and additives located above the Mixer, weighing systems of all input materials and compatibility controller. Lot of input regarding maintenance of mixer wear parts has already come. Maintenance of hopper and its discharge gate is also very important. Sometimes return sand keeps leaking and
falling on the sand being mixed below which ultimately finds its to mould face with consequent defective casting. Quantity of return sand and additive is also another important aspect. Modern sand plants are having these facilities which is done by mounting hoppers on load cells. 13) one must verify that the additives and have contineous data of prop to be used as Control chart other than given or estimated value of computer 14) the above system described od exactly of RTC fitted mixer of Rihno. if one try to keep input temp of RTn sand in a closr range say 2/3 deg then bery goof in consistency of mixed sand along with hrly tonnage. 15) friends one question which i want to understands is that how does rtc control chart or systems respond when fines or oolitics or dead clay increases in the return sand.. 16) It follows the trend and adjusts to the water demand of system sand. 17) MIXING OF GREEN SAND(Contd.) Quantity of returned sand and additives:- These are displayed on monitor. It is also stored in computer memory. It is advisable to retrieve these data and analysed statistically. For example if set value of return sand weight is 2000kg then whether actual additions, on a statistically significant no. of values, is near to the set value or far away. Such an analysis done over a period will be useful in knowing whether weighing system is working as it was or a drift has taken place. It will also reveal precision and accuracy of the system. A foundry should also calibrate the weighing system on a predetermined frequency say once in six month Till now inputs and action on the inputs have been discussed. Let us now talk about the OUT PUT of the process which is mixed sand or prepared sand. 18) I suggest that the sand mixing process validation approach started by ziauddin should continue till we all contribute and then try to abide by it. 19) In these day's sand availability problems, please find below details of a foundry sand supplier from Bharatpur Rajasthan. I have talked to the party and found him experienced ang knowledgeable. The basic problem of rajasthan sand is that it is crushed from quartz stone,screened to size. The grains are therefore angular and rough as compared to mined sand as Allahabad. Rounded grains are always good for foundry use. This party has both crushed and mined high silica sand. Kindly contact the party directly for your requirement.
20) Why we always use Sodium Benonite in Foundry and not Calcium Bentonite 21) MIXING OF GREEN SAND (Contd.) Out put of the mixing process is mixed sand. In order to know whether the process has been done efficiently a simple test on the prepared sand may be done which is as follows:Take sample from the prepared sand, determine its moisture content and GCS. Remix it in laboratory Muller(with wheels) for a prolong period say 10 minutes, maintaining same moisture. Determine GCS again. Ratio of two GCS values is a measure of mulling efficiency. To clarify it further, assume GCS of sand prepared in shop mixer is 1.8kg/cm2 and after remulling in lab. 22) GCS becomes 2.0kg/ cm2 then the efficiency is 90%.In other words although the ingredients in the sand have the potential to develop 2.0kg/cm2 but it has developed only 1.8kg/cm2. We have to think now how it can be increased to 100%. What is wrong with the shop Mixer? Whether gaps between plough and side wall or between plough and bottom has increased? Or there other reasons related with machine like length of beater blades etc.