Examining Key Variables of a Coldbox Coremaking Operation.

By monitoring the key variables that affect coldbox coremaking processes, you can apply the 'best' techniques for optimizing your unique operation.

Coldbox coremaking operations come in a variety of sizes, from the independent core operation that handles varied high-production work to a modest foundry coreroom that sees smaller quantities and more complex jobs. While there is a spectrum of "acceptable" coremaking facilities, there is little variation in the factors that truly great operations monitor regularly.

The importance of each factor to the overall coremaking process differs from core shop to core shop. For example, a large, busy foundry may not want to take the time to minimize core scrap rate. Having the "best" core shop may rank behind making enough cores to satisfy the mold line.

Approached with the task of enumerating the factors that distinguish exceptional core shops, the AFS Molding Methods & Materials (Div. 4) Cured Sand Committee identified the following 19 characteristics. By examining these variables and determining which are most important to your operation, you may begin to reassess your coremaking plan and tailor your system to best suit your needs.

Core Scrap Rate

Core scrap rate is calculated by recording the amount of scrapped cores divided by the number of machine blow cycles. This rate is an excellent measure of efficiency. The type of machinery, the condition of tooling, the type of resin and sand mix, and the experience of the machine operator all have a bearing on core scrap. Tooling in excellent condition that is sized correctly for the machine usually results in a lowerscrap rate than over- or undersized tooling for a particular machine. In addition, properly vented tooling with good blow tube arrangements will yield lowerscrap rates.

Chunkier, less complex cores will show lower scrap rates than complex, rangier cores, so a good scrap rate for one core may not be good for another. The sand mix quality, in terms of flowability and consistency, also affects the core scrap. Decreases in flowability caused by sand additives, high resin percentage, and fines in base sand aggregate increase scrap.

High-production corerooms that consistently make the same core can approach core scrap rates of less than 5%, while jobbing shops with many different cores, coreboxes and core machines may see scrap rates in the 10-15% range.

Gas or Co-Reactant Usage

There are different methods to measure the amount of catalyst or co-reactant being used in relation to the amount of resin. One method is to total the pounds of resin used over a particular period (1 yr or several months), and then total the amount of amine, [SO.sub.2] or methyl formate used in that same time. Amine or [SO.sub.2] usage should be less than 10% of the total resin. Methyl formate usage should be 35-50%. These figures measure the spectrum of the total coreroom capabilities, including gas or vapor generation equipment parameters, the quality of tooling and seals, scrubbing capabilities and foundry commitment to low odor.

Another method used to measure gas or vapor usage is determining the exact amount of gas used for each core. This can be accomplished if the gas generation equipment measures amount of gas (ccs) per cycle. This figure can be compared to the core weight or the amount of resin in each core. Typical values include 0.5 cc/lb sand for the phenolic urethane process and 35-50% methyl formate based on the amount of resin for the estercured phenolic resin process.

Binder Levels

The types of cores produced, including size and shape, influence the binder level, since not all cores can be made with the same level of binder.

Lower binder levels do not always indicate that the coreroom is better run than a facility with higher levels. Binder level depends on core geometry and the type of casting being made.

In general, lower binder levels lead to increased flowability, lessresin wipe-off and denser cores, all of which should lead to less core scrap. However, this benefit is realized only if the low binder level does not result in unacceptable core strength.

Cycle Times

Coremaking cycle times include total time from blow to blow. Good corerooms can achieve cycle times of 40-60 sec, depending on core geometry.

Other cycle time measurements include the length of blow time, gas time and purge time. All can be measures of certain capabilities of the equipment and tooling as well as coreroom expertise.

Machine Uptime

This variable is a measurement of the total machine run time vs. the amount of scheduled running time. Machine uptime is measured in percentage of scheduled time and can depend on the amount of downtime for clean-up, breaks, and sand delivery as well as machine idle time while waiting for core racks.

Many well-run corerooms average more than 90% uptime for the core machines. This depends, however, on the binder system and all relevant equipment. It also may depend on the incentive pay and/or quotas of the machine operators.

Residual Odor

Well-run corerooms have lower residual gas (whether amine, [SO.sub.2] or methyl formate) left in freshly made cores. Low residual gas means that the coreroom is not using higher-than-needed amounts of gas for curing. It also means that purge times are followed and that the tooling is well-vented and properly maintained.

Low residual gas throughout the core-making area indicates that the scrubber system is efficient, the equipment is well-maintained, the cores do not have a large amount of residual gas and there is proper ventilation throughout the system.

Cost per Ton of Mixed Sand

The binder type and sand type have a large effect on cost per ton of mixed sand. This variable is an important metric for a coreroom. It will assess the raw material costs of the sand and resin and addresses the binder level needed to make cores, which is dependent on type of sand, sand additive (if needed) and robustness of the machinery. Cost of castings out the door, however, is the final criterion when it comes to costs. Higher costs per ton of mixed sand may be worthwhile if they lower cleaning room costs.

Maintenance/Tooling Program

Every well-run coreroom must have a preventive maintenance program to anticipate machine breakdowns before they occur. Corerooms with machines that are not functioning will certainly not achieve some of the metrics, such as machine uptime, that signal well-run operations.

Preventive tooling programs also assist in achieving high marks in productivity by ensuring the availability of core boxes ready for high-production runs.

Tooling Changeover Time

Some of the best corerooms design tooling change processes and purchase core equipment to change coreboxes in 15 min or less. This benchmark depends on the type and size of corebox as well as the type of machine, length of production run, design of changeover space near machine, preventive tooling program and engineering tricks designed to speed changeover times.

Quality Tests

Quality control tests for raw sand and mixed sand and destructive core tests (such as tensile or transverse strength tests) are important tools to monitor the coremaking process. Raw sand tests, performed on a sample taken at the mixer, might include grain fineness, percent moisture, acid demand value and pH. These periodic checks will establish consistency ranges or target levels to be achieved when the sand is in question due to poor coremaking performance.

Many corerooms will use tensile or transverse tests to determine operating ranges of the mixed sand. These tests should be run consistently with sand samples taken at consistent periods. If 2-hr tensiles have been used in the past, always look at 2-hr tensiles. If sand is taken immediately after mixing, it should always be taken immediately after mixing and from the same spot.

Some corerooms run destructive tests on production cores periodically. When completed scientifically, this practice is preferable to sand testing (preparation of dogbones or tensile or transverse specimens), as the destructive test also will evaluate the process used to make the core, not just sand mixing.

Raw Material Handling

There are many different methods of handling raw materials efficiently. Large facilities need bulk resin and gas delivery to feed many core machines. Based on resin usage, some smaller corerooms can completely exhaust their resin supply before restocking. The key metric in regard to material handling of resin and gas is the trade-off between manhours needed to change packaging (drums, totes, etc.) vs. cost of operating on a bulk material system.

Sand handling and distribution also is an issue in the coreroom. Systems designed to deliver consistent sand quality at an even temperature are optimal. In addition, they must be designed to deliver the same quality sand at each machine. Flexibility of sand delivery and mixing may be an issue if specialty sands are required for specific jobs.

Core Wash Methods

Core operations that apply a refractory coating (core wash) to improve casting finish or prevent defects must design the workflow for proper application and drying. The most prevalent sequence of applying a water-based refractory coating is dipping the core into the coating as soon after coremaking as possible. Dipping the core immediately lessens the effects that water has on the chemical binder systems. Cores then should be processed through a drying oven with enough airflow so that the cores dry at moderate temperatures of 250-350F (121-177C). Ovens that do not have adequate airflow actually create a humidity chamber in which the moisture driven from the coating will not be evacuated.

When using alcohol-based coatings with phenolic urethane coldbox cores, cores should cure for 15-30 min.

Assembly/Breakage Level

The core scrap associated with assembly through placing the core in the mold should be monitored. This variable shows how well the operation is designed for core movement through the molding area. The figure also shows the ability of the foundry to adapt to the specific complexity of cores it is making. More intricate cores will have a high scrap figure unless the foundry finds methods to reduce core handling or to engineer better methods of setting fragile cores.

Core Inventory

Just-in-time inventory programs were the norm a few years ago. However, given the harsh environments that some corerooms endure, these programs are not very practical. Corerooms that must keep large inventories of certain jobs due to high core scrap, high casting scrap and long tooling changeover problems usually will not measure up to the metrics of a well-run core shop. A lower inventory level indicates an accurate materials management and finished product system.

Gas Generator Efficiency

The most important criteria for a gas generator is to be sized correctly for the amount of sand that will be cured per blow. Under-sized generators lead to excessive catalyst or co-reactant usage and extended cycle times. The efficiency of the generator also must be judged against the cost of the equipment being installed. "Expensive" generators probably are not needed for simple, job shop operations with non-repetitive tooling runs.

The type and size of generator, relative to the size of cores produced, also affects the measurements of gas used per pound of sand and total gas used for the process.

Scrubbing Equipment

Properly maintained scrubbing equipment is essential for a well-run coreroom. This maintenance reduces the gas cycle times, residual gas in the cores and, most importantly, residual gas odors in the core area. Scrubbing equipment also helps keep the foundry in compliance with local and state regulations. Preventive maintenance checklists for scrubbing equipment are essential to achieve low residual gas goals.

Performance Tracking

Corerooms that keep track of performance metrics through charts, graphs and visible results often will be the shops that lead the industry in these metrics. Well-defined goals and programs to reach these goals enable core personnel to aim at established targets.

Casting Scrap vs. Cores

The ultimate measure of a successful coreroom is the casting scrap caused by the cores. Defects associated with cores include gas blows, subsurface gas defects, sand erosion from cores, dimensional stability of cores, veining, metal penetration and core scabs. Each defect can occur when the system deviates from standard processes.

In general, the percentage of casting scrap depends on the type of casting made, including geometry and metal. Casting scrap also is measured as in-house and customer scrap. Medium-sized iron jobbing foundries may run at 3-4% in-house casting scrap and less than 1-2% customer machining scrap. Steel foundries will do more welding repairs to salvage the castings. These foundries will measure scrap through actual scrapped castings, amount of welding rod used and cleaning room hours.

Sand Mixing Equipment

Sand mixers that are properly sized for the equipment and process are critical for the delivery of fresh mixed sand. This fact is critical to the phenolic urethane coldbox and estercured phenolic coldbox processes. For the acrylic-epoxy system, mixed sand benchlife is not an issue, so core operations can be designed differently in terms of sand mix equipment. The important issue is to have a mixing system designed specifically for the equipment and resin used.

What's Right for You?

There may be no real-life foundry that is able to achieve optimal numbers in every coremaking variable, but there are standouts that excel in particular categories. While every shop may not run the "best" possible processes, they certainly may operate in an optimized way for specific concerns and needs. With some attention to the factors that are most important to your own operation, you may re-engineer processes to optimize efficiency and cut bottom-line costs associated with the casting process--meaning both happy customers and better profits.