Choosing a Mold Compaction Method for Green Sand.

To ensure casting quality foundries must choose the proper mold compaction method to achieve the proper balance of mold hardness and permeability 

AFS Molding Methods & Materials Div. Green Sand Molding committee (4-M)

Principal Author: Greg R. Piontkowski, Georg Fischer Disa, Inc., Holly, Michigan 

The science of green sand mold making is equal parts technology, process and experience. Whether you are in a low-production jobbing shop or a high-production captive operation, successful moldmaking can be broken down into seven critical factors:

* tight flask/matchplate design;

* sand characteristics;

* sand feeding;

* mold compaction force;

* mold compaction head;

* mold draw;

* tooling design.

Each of these factors must be controlled and fitted to the foundry and its patterns to ensure quality molds and castings.

This article focuses on two of those factors, mold compaction force and the choice of the compaction head. Whether a foundry utilizes jolt, squeeze, dynamic compaction or some combination of these compaction methods, it is critical to achieve the proper mold hardness to ensure mold strength for casting yet maintain permeability for gas venting. By analyzing the various forms of mold compaction, this article will provide foundrymen with guidelines in choosing the mold compaction method that is best for their operation.

Compaction Methods

There are four principle methods of applying compaction force--jolting, squeezing, dynamic pre-compaction and dynamic compaction. The appropriate combination of these compaction technologies allows foundries to target a mold hardness on the horizontal plane between 90-95 on the Dietert B scale. Beyond this measurement, the key is to achieve the minimum mold hardness required to produce quality castings. Excessively hard molds may not be permeable enough to properly vent the gases. If a mold is not permeable enough to properly vent the gases, expansion or gas defects may result. Although the determination of the optimum mold hardness for a given casting is a trial-and-error procedure, it is critical to maintain consistency of mold hardness.

Following is a detailed look at the four compaction methods, including their advantages, disadvantages and potential applications.

Jolting 

In this compaction method, a pattern and flask are filled with sand on the anvil jolt table. The molding machine then lifts the table (with pattern, flask and sand) via air pressure and drops it against the anvil base. The result is that the force of the drop is transmitted down the mold to the pattern face. This method creates molds that have the highest mold hardness at the parting surface and marginal hardness on the back face.

Jolting has been shown to cause increased energy and compaction with increased height of the drop and with increased weight on top of the mold. To this end, some foundries will enhance jolting by placing a rack on top of the mold with additional lose-fitting weights, which results in more uniform mold hardness from parting surface to back face surface. Jolting will achieve mold hardness values from the mid 80s to low 90s.

Advantages--The initial cost of equipment that uses this compaction method is low, so a start-up green sand operation can begin with just a single molding machine.

Disadvantages--Although jolting produces good mold hardness at the parting face, the back face and vertical surfaces of the mold are not as strong. As a result, this compaction method typically doesn't provide molds sound enough to proceed in an automated system. In addition, this process, which relies heavily on manual labor to distribute the sand throughout the flask, requires a more experienced moldmaker to ensure quality molds. In addition, the jolting process is self-destructive as it demonstrates a lot of metal-on-metal wear. To combat the wear, the jolting systems typically require frequent rebuilds. Last, the jolting process is noisy.

Applications--Typically, straight jolting moldmaking operations are for low- to medium-volume production of up to 70 molds/hr. As a result, small-to medium-size foundries tend to master this hands-on process for the niches they serve.

Squeezing

This compaction method is defined by its use of brute force. After the sand is introduced to the flask, a squeeze head is placed over the sand opposite the pattern at the back face of the mold. Force is applied to the mold by the squeeze head, compressing the mold from the back face to the pattern. Squeezing imparts the greatest force (compaction) from the squeeze head at the back face of the mold. The compaction decreases toward the parting face due to the internal friction of the sand against the pattern and flask walls.

Squeezing can be applied using various types of squeeze heads, including flat squeeze plates, multiple cylinder heads or contoured squeeze boards. When a flat squeeze plate is used, the areas of highest elevation of the pattern withstand the greatest amount of squeeze force. This results in soft areas where the pattern is recessed and in areas near the flask wall.

The optimum squeeze head provides foundries with the ability to closely mirror the pattern configuration with the head, distribute the squeeze force throughout the mold to the areas that need it most and enhance the uniform mold hardness regardless of pattern contour. While a contoured squeeze board accomplishes this objective, it may be impractical to have a custom squeeze board for every pattern. For this reason, a multiple-cylinder squeeze head may be chosen. A multiple-cylinder squeeze head is designed to distribute the total squeeze force of the molding machine into several zones of the mold area. Typically, the zones include a minimum of an inner and an outer zone (nearer the flask wall). Additional zones may be desired based on flask size and pattern complexity. Foundries can vary the zone pressures to individualize the compaction for each type of casting to be run.

Squeezing can achieve a mold hardness in the low 90s on the Dietert B scale.

Advantages--In addition to being a simple process to understand, this process has a lot of flexibility in terms of the hardness values achieved across the mold due to the ability to control zones of pressure with multiple squeeze heads. The squeeze compaction method also allows foundries to add a tuck strip and pouring cup former during molding. The tuck strip, which attaches around the perimeter of the squeeze head, projects down a few inches around the flask wall and helps to produce higher hardness values at the flask walls. The pouring cup former automatically molds the basin to receive the molten metal, eliminating a process that often adds an extra step.

Disadvantages--Because the squeezing force comes from the back face of the pattern, the mold hardness is stronger at the back than at the parting face. Also, this compaction process usually is a hydraulic operation and has a tendency for oil leaks. It is critical for proper equipment operation and mold quality that these leaks are kept to a minimum. Excessive squeeze force can result in mold spring-back causing a deformation in the mold cavity.

Applications--Squeezing alone can be used in high-production shops making in excess of 300 molds/hr. Due to the squeeze force being applied from the back face, the use of a squeeze-only process typically should be restricted to flasks heights of 14-in, or less in order to achieve the necessary mold hardness near the parting face.

Single-Step Dynamic Compaction

This method uses a single-step air pressure wave to compact the mold. Molding sand is introduced into the flask where regulated compressed air is released, accelerating the molding sand from the back of the mold toward the pattern face. The sand compacts as it decelerates.

During dynamic compaction, the flask and air chamber have to be sealed to ensure effective application of the air pressure. With this method, the air compaction pressure can easily be varied to accommodate specific pattern and mold configurations.

Advantages--Dynamic compaction produces a mold with good degassing properties (without drilling or external vents) as the mold hardness is best at the parting face and decreases toward the back face (increasing permeability) (Fig. 1). The high hardness at the mold surface results in a cleaner casting surface. Also, the easy control of compaction pressure reduces the trial-and-error required to alter mold hardness. The equipment for this compaction method requires few moving parts, reducing the maintenance required. Due to the mold being created from the pattern surface back, there is no spring-back tendencies. This allows for a more accurate mold cavity, hence near-net-shape castings. This process also is less abusive on patterns.

Disadvantages--This compaction method has three main disadvantages. The first is that, due to the air pressure force and resultant mold hardness (stronger at the parting face, weaker at the back face), the top sand surface requires a strike off of the loose molding sand. The second is that this method does not allow a pouring cup to be formed during compaction, forcing foundries to incorporate other means. The third is that foundries must maintain a good seal between the flask and the compressed air chamber. This must be an airtight seal during compaction to ensure the proper force from the compressed air.

Applications--This compaction method can produce molds at a rate of 260 molds/hr. Its best application is for molds with high sand levels with deep cavities (engine blocks and cylinder head, for example), high patterns, and risers and vent pins.

Dynamic Pre-Compaction

Dynamic pre-compaction, as suggested by its name, is a step that precedes compaction. It is a form of compaction, but essentially readies the molding sand for the final compaction, whether it be squeezing or dynamic. Dynamic pre-compaction is used to improve the sand filling into critical pattern areas such as cavities or deep or narrow pockets. Through the use of strategically located vents in the pockets or other difficult areas to mold, air flow is promoted to increase sand flowability, thereby achieving good sand fill. This process allows closer location of the mold cavity to the flask wall.

Advantages--This process allows an improved prefill of sand into difficult pattern areas which promotes a more uniform compaction of the complete mold.

Disadvantages--The disadvantage to the dynamic pre-compaction is that the cycle time of the molding process is extended.

Application--The use of dynamic pre-compaction most commonly is found in foundries that have patterns with deep pockets and other deep vertical surfaces (brake drums, electric housings and camshafts, for example).

Combining Various Compaction Methods

Although each of the compaction methods (except dynamic pre-compaction) can be used alone, it is more common to combine compaction methods to produce more uniform mold hardness on both the horizontal and vertical planes.

Jolt-Squeeze--This method combines the jolting and squeezing process for low- to medium-volume production foundries. This process most commonly is found in jobbing-type foundries with frequent pattern changes.

Dyna-Mechanical Compaction--In this process, the mold is produced via dynamic pre-compaction followed by mechanical squeezing (Fig. 2). The optimum squeeze head would closely reflect the contour of the pattern so that the mold is compacted in a uniform manner. While dynamic mold compaction is characterized by an inverse mold hardness profile with the highest values at the parting face, squeezing is exactly the opposite. The combination of the process provides the advantages of both. This process can be applied to a wide variety of patterns, but is most effective with large, deep cavities; heavy green sand hang downs; and when small distances exist between the pattern and flask walls.

Multi-System Compaction--Multi-System compaction allows the selection of any of the following processes in any combination:

* dynamic pre-compaction;

* dynamic compaction;

* squeezing.

The result is that an optimum method of compaction can be used for the production of good castings.

The wide range of casting requirements has made it essential that the compaction method used in molding is matched to the pattern and castings to be produced.

This article was adapted from an update the AFS Molding Methods & Materials Div. Green Sand Molding Committee (4-M) is writing for the AFS book "Principles of Sand Control".