Originally published in the May 2017 issue of Foundry Management & Technology.
by Alexander Brown
Controlling raw materials is critical to the success of iron casting production from green sand systems. The base silica sand is often overlooked, with the main focus on bentonite additions. Carbonaceous additives can be considered a “necessary evil” to ensure a good surface finish and reduction in sand-related surface defects. Other additives are used when systems get out of balance, and these will further complicate the complexity of green sand systems.
For castings requiring cores, this becomes a bigger issue, as many different resin systems are employed for core production, and these must be taken into consideration when controlling both the carbonaceous levels and the overall grading of the sand system. The twin effects on additional carbon and loss-on-ignition and overall sand grading need careful understanding and control.
Control of raw materials
Much has been documented about additives to green sand systems. What follows are some common features that give cause for concern. This is not because the operators do not have the means to control the system, but because their decisions are based on a lack of validated data. Some concerns are:
• Poor sampling
• Frequency and timing of samples
• Lack of calibrated testing
• Focus on the wrong control areas
• Lack of understanding of primary and secondary sand tests
• Little focus on silica sand or sand grading
• Bentonite emphasizes detriment to carbonaceous additive
• Poor incoming test procedures
• Over-reliance on suppliers
All of these are important, but equally good castings can be produced from systems with little or no control. By control, I mean a basic understanding of all the raw materials used together with in-house testing and/or approved certification, coupled with good, consistent casting practices. This is measured by general scrap rates and the costs associated with knockout and shot blasting, costs so often not taken into consideration when selecting the raw materials to be used in the system.
Mass balance checklist
- Return-sand storage silo/bunker capacity
- Return-sand temperature/moisture (at various points)
- Weight of sand on line/in boxes etc.
- Core weight input (if applicable)
- Fines extraction/Sand losses
- Additive control/weigh calibration/stock against usage
- Sand carryover at knockout
- Shot blasting times/consumption
- Scrap levels/sand related defects
- Sand to metal ratio/casting weight data
- Volatiles data at mill and in return sand
- Loss-on-ignition data at mill and in return sand
A great deal of information can be gathered from doing a complete mass balance on a green sand system. This exercise picks up so much useful data that it should be a regular exercise in all foundries, especially if the casting weights or size of castings alters over time. Consider the simple question “What is the total sand weight in the green sand system?” Normally, this is met either with a wild estimate or a complete blank response.
From the data gathered a much clearer picture may emerge. Most important, we know that control systems can actually monitor the burn-out rates of bentonite and carbonaceous additives. Most observers agree that additions at the muller require time to be effective, and by understanding how the sand reacts to varying sand-to-metal ratios, proactive steps can be taken. Various foundries have used predictive software or even a simple traffic light system to monitor heat demand, with good results. It is not the actual system that matters, but, importantly, understanding what is happening and being one step ahead of it.
Mixer controls, technical support
The need for control in this area is obvious as changes here are reflected throughout the system. Regular mixer maintenance coupled with calibration of additives and usage checks against actual stock purchases often may spot problems before they give concern. This should be part of the mass balance procedure that should be a routine feature. Getting the primary and secondary testing balance right is difficult for foundries. Too much, and the testers see no positive advantages other than routine test control. Too little, and there is no starting point for investigation and correction once things go wrong.
Total carbon testing
As a control measure, many foundries rely on loss-on-ignition and volatiles tests to measure carbonaceous addition. Coupled with AFS clay grade and active clay measurement, this was considered adequate testing, along with the usual series of permeability, strength, and moisture tests. Experience has shown that active clay levels in most systems range from 1% to 2% higher than required, and these levels are regarded as a safety feature. Whereas we know foundry workers make excellent moldable sand, this does not always translate into top quality castings.
Evaluating volatiles, loss-on-ignition properties
The whole matter of the use of carbonaceous additives and their purpose has been well documented. This work simply adds to the conclusion that there is an interaction between the total carbon in a sand system, allied to a level of active effective volatiles, and a loss-on-ignition maximum for successful casting production. Often overlooked in problem solving is the amount of volatiles, which must come from the bentonite, and typically this is classified as 10% of the AFS clay grade. To investigate this further, washed and unwashed tests reveal the true active volatiles content from the carbonaceous additive. Many foundries are simply unwilling to move their loss-on-ignition numbers above 5%, which limits the active volatiles, which in turn limits casting performance.
Other control methods
The Mass Balance predictive method is used to ensure heat losses after casting (burn out) are reacted to before pouring, and to ensure that additional additives are added or decreased ahead of heavier or lighter castings. This method finds favor in well organized foundries that control additive levels effectively. It allows for lower levels to be added as normal and larger additions ahead of the heavier casting load. This depends on the sand-to-metal ratio, but with all foundries aiming for maximum efficiency it makes good sense to understand how the system will react to pattern changes and varying heat loads.
This system can be incorporated into the sand plant computer controls, and can equally be used for manual adjustments, using what some foundries call the “traffic light” system. They use red to indicate the heaviest castings on the program, amber for mid-range products, and green for lighter heat loads on the system. Irrespective of the system, the important aspect is to be aware of the system, the heat loads proposed, and to be able to take action before corrective measures are required. Rushed changes based on “feel” may work, but informed change is the safer, more positive approach.
Return sand control is one of the most neglected operating practices in green sand foundries. For most foundry workers, what happens to sand after casting is a mystery. What manages to stay in the system simply re-appears for mulling, and so it goes on. Here is a massive area of potential savings, coupled with the knowledge that a well-prepared return sand will be lower in temperature, higher in moisture (ideally at 2%+), and better developed in terms of bentonite and general condition.
A well-developed return sand is much easier to re-mull. In fact, lower addition levels will be required, which in turn leads to lower moisture in the system sand. With lower moisture levels the conditions for a reducing atmosphere are better and, coupled with adequate carbon levels and active volatile content, this will ensure better performance and result in improved casting quality
Casting knockout is another standard operating practice that often is overlooked as an area of potential quality control. The results of all calculations and control are available simply by looking critically at the resultant castings. The sand peel from the castings, the amount of sand carried over, and the color of the castings is often a good early indicator of casting quality.
Simple, complex, effective
Green sand systems can be simple and complex — simple in terms of additives and control; complex in the amount of variables possible and the need for constant vigilance — but they are simply the most cost-effective method for high volume production of iron castings.
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