Evaluating Coldbox Binders: A practical Approach.

By identifying which phenolic urethane coldbox resin system is best for your operation, these simple tests can determine opportunities for cost and quality improvements.

To be successful in today's tight market, foundries need a resin system that will make them more competitive as well as more profitable. While resin pricing is an important consideration, resin performance can yield substantial benefits beyond cost. When a resin system shows promise for productivity and quality, the foundry should verify the resin's performance in producing typical cores.

In an effort to improve its productivity, Indianapolis Casting Corp. (ICC), a producer of gray iron castings for the diesel engine and truck industries, began examining its resin system in 1997. The foundry developed procedures for comparing resin system performance, which helped facilitate the evaluation of a new resin system vs. standard practice.

This article details ICC's practical approach to evaluating phenolic urethane coldbox resin systems. By running these tests in your own operation, you can decide which resin is best for your operation and possibly achieve the same level of success.

Testing Methods

The equipment used is available in most foundry sand labs, and, while the test methods used are not necessarily "standard," they do represent a practical approach to comparing resin performance.

These testing methods focus primarily on core productivity improvements (curing speed, core release, sand flowability and bench life) as well as casting and/or core quality (undipped and dipped core strengths, core hot strength, core density, combustible content, humidity resistance and core shelf life). Most of the testing requires making a standard test core (dog bone), but other test cores could be substituted, including a 1 x 1 x 8-in. transverse or disk transverse.

To accurately gauge the benefits of a new system and the impact it will have on your production, begin by using your current resin system as a measuring stick. Prepare laboratory batches of sand from each resin system (including the incumbent) for evaluation. Only one batch of sand should be prepared at a time due to the normal deterioration of sand properties if held beyond benchlife.

Laboratory sand batches of about 3000 g can be made using a lab mixer that will give results comparable to production equipment. Follow proper mixing procedures to ensure homogeneity of the prepared sand. A typical mix cycle might be:

1. Add sand and anti-vein agent (if required);

2. Mix 15 sec (plus 15 sec if an anti-vein agent is used);

3. Add Part I;

4. Mix 45 sec plus 45 sec;

5. Add Part II;

6. Mix 45 sec plus 45 sec.

Once the prepared sand is ready for test core production, be sure that the core lab blower settings are the same for all comparative testing. Some typical settings might be:

* blow time: 0.5 sec;

* blow pressure: 60 psi;

* gas time: 1 sec;

* purge time: 15 sec;

* gas pressure: 10 psi;

* purge pressure: 20 psi.

When comparing one resin against another, use sand from the same source, do the comparative test the same day (and as close together as possible), use common sense in reducing any other variability that might skew results and do several repetitions to verify results. Most core testing is of a destructive nature, and, therefore, the materials cannot be reused. Since there are some inherent variations in the sand, never use one test result for decision-making; make several test repetitions and look at the average result.

Core Strength Development

The strength the resin imparts to the finished core is critical to all phases of production. A core that is too weak may not survive handling and assembly or, if it does, may produce casting defects due to premature core breakdown during casting. A good comparison of resin performance is to evaluate core tensile strength development. Compare the resins at the same resin level, or percent resin, based on a current sand mix in the shop. In addition, use the lowest resin possible, as resin inefficiencies will be accentuated. A core tensile pull test at 60 sec out of the box will be a good indication of the core handling and assembly strength. (An evaluation of core tensile strength 1-24 hr later also will be worthwhile if cores are not dipped prior to placing in the mold.)

If cores are dipped under normal conditions, the test cores should be evaluated for dipped core strength. It is best to simulate the production process as much as possible. The immersion time in the coating should be the same for all test cores because a longer or shorter time will result in more or less coating pick-up, which can affect the dipped core strength. The amount of time the core stays wet also is critical to core strength. Generally, the longer the core stays wet, the lower the dipped core strength will be. All cores should see the same delay entering the oven heat zone. Hot cores are not normally placed directly in molds; therefore, a cooldown of at least 1 hr is recommended before dipped core strength is evaluated.

The core strength development testing will indicate whether, in practice, the resin percentage will have to be raised or lowered to match current resin performance. (A rule of thumb is every 0.1% change in resin will result in approximately 20 psi change in tensile development.)

This information, along with the resin pricing comparison, will give some insight into the economics of a potential switch. Inefficiency at this point might be overcome by some yet-to-be-discovered enhancements. For instance, the new resin may be more costly per pound but might yield productivity or casting quality improvements that may offset the deficiency or result in a cost savings.

Humidity Resistance

Moisture can dramatically change the normal core strength, dipped and undipped. Most foundries do not have a controlled humidity chamber, but if a lab oven is available, a comparison can be made. Dip the test cores and place them in the oven side-by-side for 1 hr at 220-225F (104-107C) with no airflow. This will allow the water to evaporate slowly and the air in the oven to become moisture-laden. A container of water can be added to the oven to accentuate the high humidity. Remove the cores and let them cool to room temperature for at least 1 hr before evaluating core strength.

Another option is to place cores on a shelf above a bucket or drum with water. With a lid in place, this creates a stable high-humidity environment. Cores should be left for a predetermined time (24 hr) before testing core strength. A resin that exhibits more tolerance to high humidity may be beneficial in reducing broken cores and might allow for a resin reduction (if extra resin is used to combat high-humidity conditions).

Prepared Sand Flowability

Flowable sand is important in blowing dense cores to generate the maximum core strength, preventing metal penetration. The resin system plays a large part in prepared sand flowablity. The viscosity of the resin, the amount of resin and the benchlife all influence the way the prepared sand moves. Sand flowability can be determined by weighing test cores made from the two resin systems. With everything else being equal, higher core weights indicate higher core density and sand flowability. One way to accentuate the difference in sand flowability is to lower the blow pressure in 10-psi increments, make cores and then weigh them at each blow pressure. A comparison then can be made to determine if one resin system offers an advantage.

Benchlife Study

Determination of how long prepared sand can be held and still be used to make quality cores is an important feature of any resin system. Core machines do experience downtime, and breaks result in prepared core sand setting up in hoppers. Resin solvents in the prepared sand evaporate as the Part I and II react with each other and result in sand that does not flow well, exhibits lower strength and produces friable cores. Preparing fresh sand mixes and evaluating core strength every 15 min as the sand is allowed to set in an open container can be used as a benchlife comparison of resin systems--both high humidity and temperature conditions will shorten the prepared sand's benchlife. Allowing the sand to set in a humid hot environment will simulate actual production sand deterioration. This test can be run for 1-2 hr, depending on possible downtime.

Another test for benchlife is to take an old test sieve (30 mesh or finer) and fill it with fresh, prepared sand (Fig. 1). Strike off the excess sand so the prepared mix is level with the top of the sieve, and then place some 0.125-in. shims under the sieve rim to elevate it, promoting air exposure to the bottom of the sand mass. Using a "B" scale green hardness gauge, take readings every hour on top of the sand mass. Overtime, the sand will become harder, and higher readings will occur. The higher the readings, the less flowable the sand will be. This simulates what happens inside production sand hoppers and blow magazines.

Bulk Cure

Evaluating cure speed is important to making productivity improvements by reducing core machine cycle times. Prepare each sand mix with the resin to be tested, and then turn the purge time on the lab core blower down to one-third of its original setting. The intent is to make test cores that are not complete (Fig. 2). Some further adjustment of gas time and/or purge time may be required to accomplish this. The bottom portion of the cores will not cure completely (since this is the last part to receive amine), and it will fall away.

Now, a comparison of how much sand is cured with the amount of gas and purge time available can be made. Weigh all core portions that are completely cured and divide this by the total seconds of gas and purge time to calculate cure rate in grams per second. This method determines which resin system is fastest. A resin that exhibits a higher bulk cure means that a core can be cured in less time. This means purge and/or gas timers can be set lower, resulting in more cores produced per hour.

Hot Strength

Both dilatometers (which measure hot compressive strength and deformation testing) and the British Cast Iron Research Assn. hot distortion test require a special core dimension and geometry that takes special tooling. The following methods allow the use of standard test cores (dog bones) to measure hot strength.

Make a test core from each resin system and place them in a 900F (482C) muffle furnace for 10 min. (Place the cores side-by-side so heat exposure is similar.) Take the cores out at 10 min. and immediately break them while they are hot, making sure to record core tensile results. Because cooling time of just a few seconds can make a difference in tensile strength, repeat this test four times, alternating which resin system core is broken first. Average the four test results, and compare the two resin systems. Higher tensile development indicates higher hot strength. The results may not be dramatically different, but, if one shows higher or lower strength, this may be significant to core distortion or core breakdown in the mold.

Another method for testing hot strength is to take a test core from two different resin systems (dip half of each core in coating, if you use coating) and subject them to 900F (482C) in a muffle furnace. When one of the cores visibly fails (usually 20-30 min) (Fig. 3), it becomes obvious which resin provides the greatest hot strength.

Undipped Core Strength

Often, cores that are undipped are held in reserve inventory. When these cores are assembled and dipped later, they may exhibit much lower core strength than cores that are made, assembled and dipped immediately. It can be worthwhile to evaluate the dipped core strength of cores that follow a normal flow-through process compared to those that are held and dipped later. In this case, strength is a function of how the resin system loses solvents and resists moisture degradation. To test for this feature, prepare test cores and simulate the normal process for dipping and drying. After a 1-hr cool down, test the dipped core strength.

Hold other cores undipped for predetermined intervals that might typically occur in production and then dip and evaluate strength after a 1-hr cool down. Some resin systems allow dipped core strength to fall after only 4 hr of storage in an undipped state, while others lose half their strength in 24 hr. On the other hand, some cores can endure 4 weeks or more of storage before dipping and still remain comparable to cores that are dipped immediately.

Core Release

The ease that a core releases from the corebox is important to productivity improvements. If more blows can be achieved before a parting application is required, then more cores can be made per shift. The buildup that occurs on tooling may be caused by resin or silicon from the parting agent. A resin system that releases well or allows more blows between release applications allows the cleaner coreboxes to run longer.

While you can test how cores release from the corebox, a better method is to make a test core and evaluate the stripping force to eject the core. This can be accomplished by making a 2 x 2-in, core in a standard green sand specimen tube (Fig. 4). The specimen tube should be cleaned with acetone to remove any residual material. Since this evaluation is to determine core release, a release or parting agent should not be used.

Ram the prepared core sand using a standard green sand compactibility rammer (similar to the green sand compactability test) into the specimen tube. It makes sense to strive for the same core density as achieved in production cores. This can be determined by comparing scratch hardness between the test core and actual production cores or by weighing the cores to calculate the weight per volume.

The tube should carefully be turned horizontal as the pedestal cup is removed from the specimen tube (remember: the sand is uncured and flowable). Another pedestal cup with holes drilled in it and a screen should be placed over the end of the specimen tube where the first cup was removed. Next, insert a rubber plug and fitting into the other end of the specimen tube to introduce the amine catalyst, and cure the sand. The screened end of the specimen tube should be placed toward the exhaust hood of the core machine to evacuate the amine vapors.

The specimen tube then can be placed in a simple fixture attached to a tensile strength machine. The force necessary to strip the core from the specimen can now be measured. Make several repetitions with each resin system to determine which resin offers a better release.

Another method is to take the resin systems to the shop floor on a job that will allow a core release comparison. Ideally this would be a thin, lacy, deep draw with little draft and high surface area, like a cylinder block or head water jacket core.

Core Shelf Life

A determination of how long dipped or undipped cores can be stored and used can be a function of the resin system. Testing requires making cores, following normal processes as far as dip/no-dip and simulating the earliest time cores would be placed in the mold. Next, test the core strength to establish a baseline. Prepare additional test cores and place them in the production core storage area for different periods of time up to several weeks (if that is possible with production cores). Evaluate core strength to determine core degradation due to storage. High humidity plays a large role in core degradation, so perform the testing during the most humid time of year.

This article was adapted from a presentation (01-003) at the 2001 AFS Casting Congress. Conference proceedings are available through AFS Publications at 800/537-4237 or the AFS E-Store at www.alsinc.org.