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Hard Truths About Controlling Resin Balls

April 1, 2019
Identifying the causes and remedies for the soft spots that form in insufficiently blended, PUCB-bonded sand

Q:  We produce molds and cores using phenolic urethane resin-bonded sand, but recently we started to see several small spots in the molds after they are coated. The spots appear randomly and the sand is soft to the touch: no matter how long we wait they do not harden. What are these  spots and how do we control them?

A: The “spots” you describe are known as “resin balls.” Several factors may cause them to form, and often it can be a daunting task to remedy them. Listed here are the most common causes, and some recommendations for preventing them in a molding process.

The most common cause of resin balls is insufficient sand mixing due to mixer inefficiency. There are several causes for poor mixer efficiency, listed here in the order of importance when resin balling is found:

Mixer cleanliness. It is very important for the mixer to be cleaned properly. Most foundries will clean the mixer blades and trough, but cleaning the mixer trough is not recommended unless the blades have been changed. When the trough is cleaned without the blades being changed, it allows sand to bypass the blades without being properly mixed. In phenolic resin systems, the “shell” on the mixer wall will be rebuilt after sand is mixed in the trough and the machine is turned off again, allowing it to cure past the strip time of the mixed sand. Clean the blades after each shift to ensure they perform as designed.

Mixer calibration. Mixers are designed to be operated at a certain rate of mixed sand per minute, and the resin-addition rate also is based on the flow of sand. So, as the sand-flow changes, so will the resin requirements. An increase in sand flow or chemical flow will change the efficiency of the mixers.

Blade condition and configuration. As blades wear, sand-mixing efficiency declines. The blades should be inspected during each cleaning and replaced when they are damaged or worn past 0.25 inch from the new condition.

To promote optimal mixer efficiency, it has been found, the ideal mixer-shaft configuration consists of a ratio of 50% of the positive 45° blades (pushers) and 50% of the 0° blades (mixers). Some foundries have resorted to a ratio of 60/40 to eliminate the possibility of resin balling.

Air assist. Many continuous mixers allow air to come in behind the chemicals to improve mixer efficiency and ensure the chemical ports remain open during and after operating the mixer. Sometimes, the air-assist is not operating as designed, for numerous reasons:

Improperly set pressure. The air-assist function is designed to be set so that the flow of air can be felt on a hand that is held approximately 12 in. from the nozzle, equivalent to about 1 psi of air pressure. This ensures that the chemical ports remain open at all times. Usually, it runs for 5-10 seconds after the mixer is turned off.

Blocked air-assist port. Often these become blocked with sand and chemicals when they are turned upside down during chemical calibration.

Temperature also plays a role in the formation of resin balls. To eliminate this potential cause, control these items:

Resin temperature. We recommend that the resin temperature be maintained at 80°F year-round (+/- 10°F), for consistency. As the temperature of Part One in phenolic urethane decreases, it becomes more viscous (harder to pump.) When using pumps that do not compensate for the change in viscosity, the amount of Part One will not be within the recommended ranges of 50-60% of the total resin being used.

The temperature of the resin system can be maintained in several ways, such as band heaters, submerged heaters, controlled-heat blankets, or by operating the mixer in a climate-controlled environment.

Sand temperature. It is important to control the temperature of the sand for many reasons, beyond preventing resin balls. Colder sand will cool the resin being mixed on it, causing the resin to build viscosity and become harder to mix.

To ensure the sand temperature is maintained, general maintenance of the sand heater is very important and should be done as part of a standardized PM program.

These are the most common of many reasons resin balls form in phenolic urethane resin systems.  With proper care and attention to your equipment and resin handling, resin balls and their resulting headaches can be easily prevented. Join the Conversation. Email Your Questions for ASK Chemicals
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