Green sand molding at forefront of productivity gains.

High-density green sand molding offers both diversity and flexibility in achieving casting quality and productivity improvements.

What sets metalcasting apart from other metalworking processes is the flexibility afforded by the molding process. Among the wide variety of molding techniques in use today, green sand is by far the most diversified and widely used practice. Foundries in search of significant improvements in quality and productivity nearly always look toward their molding systems for immediate help.

It is no wonder then that molding machine manufacturers attuned to the needs of their customers continually look to improve their equipment. Much of their efforts in recent years have centered around high-density molding because of its capability to produce net-shape and near-net shape parts. For the purpose of this article, high density will refer to a minimum of 85 mold hardness using the "B" mold hardness tester. This review will look at developments in high-density, tight-flask and flaskless molding systems.

Green Sand Basics

According to the AFS Metalcasters Reference & Guide, the green sand process utilizes a mold made of compressed or compacted moist sand. The term "green" denotes the presence of moisture in the molding sand and indicates that the mold is not baked or dried. The mold material usually consists of silica sand mixed with a suitable bonding agent (clay) and moisture.

To produce the mold, a flask, usually a metal frame, although wood may be used for some processes and types of castings, is placed over the pattern to produce a cavity representing one half of the casting. Compaction is achieved by either jolting or squeezing the mold. The other half of the mold is produced in like manner and the two flasks are positioned together to form the complete mold.

If the casting requires hollow sections, a core consisting of hardened sand (usually baked or chemically hardened) is used. Cores are located in pockets formed by projections on the pattern equipment to produce coreprints. Green sand is the best known of all the sandcasting molding methods, as the molds may be poured without further conditioning. This type of molding is most adaptable to light, bench molding for medium-sized castings or for use with production molding machines.

Advantages: Most metals can be cast by this method. Pattern costs and material costs are relatively low. The method is adaptable to large or small quantities.

Disadvantages: There are practical limits to complexity of design and machining is often required to achieve the finished product. Dimensional accuracy cannot be controlled as well as with other molding processes, although good standards are possible with quality pattern equipment, modern process controls and high-density molding.

High-Density Molding

High-density molding methods often refer to high-squeeze, pressure or impact methods. Introduction of high-pressure molding techniques in the early 1960s greatly improved the standards of accuracy and finish that could be achieved with certain types of castings.

More recently dramatic changes have occurred in green sand molding systems by increasing the capacity and ability of the molding machines to produce molds. This has been accomplished by more effective use of large air cylinders, application of electronics, use of hydraulics, and the innovation of an explosive-type method of compacting sand around the pattern, often referred to as impact molding.

High-pressure or high-density molding has been widely adapted for the casting of most metals because of its capabilities to eliminate many of the problems associated with mold wall movement.

Mold wall movement occurs during the pouring process because of the metallstatic pressure exerted by molten metal as it enters the mold. Generally, the more rigid the mold, the less wall movement that will occur. Factors playing a major role in mold wall movement include the moisture content of the molding sand, density of the molds and mold surface hardness.

High-pressure molding practices have allowed lower moisture contents in the molding sand, so that higher mold densities can be achieved. These castings have demonstrated considerably better dimensional accuracy along with improved surface finish, and they are closer to pattern accuracy.

Advantages: Most metals can be cast using high-density molds. Castings have closer dimensional tolerances, consistently uniform casting weight, better surface finish and increased productivity. Other advantages include reduced feed metal, improved casting soundness, reduced cleaning costs and less setup for machining because of casting dimensional accuracy.

Disadvantages: Tighter process control is required and, as a result, foundries using high-density molding are generally accustomed to more sophistication in equipment, maintenance and operating procedures.

Flaskless Molding

Introduction of flaskless molding--with both vertical as well as horizontal partings--was a significant development for the foundry industry. Contrary to some misconceptions, a flask is used on all green sand molding primarily for containment of sand while it is compacted around the pattern. In flaskless molding, instead of using a "tight" individual flask for each mold produced, the master flask is contained as an integral unit of the totally mechanized mold producing system. Once the mold has been stripped from the integral mold producing unit, it is held against the other half of the mold with enough pressure to allow pouring of the metal.

Through advanced engineering techniques as well as constant modification and improvements, flaskless molding has achieved notable production increases as well as improvements in casting quality through considerably better casting dimensional tolerance and accuracy. Flaskless molding systems are suited to casting gray, malleable and ductile irons, as well as steel, aluminum and brass.

Advantages: The major benefits of flaskless molding systems include the elimination of individual tight flasks along with the elimination of flask cleaning and maintenance, as well as no handling, storing, or shakeout of flasks.

Disadvantages: Restrictions apply to size of casting, use of complicated cores and core assemblies, and number of castings per mold. Mold handling can sometimes be more difficult.

Tight-Flask Molding

The tight-flask molding process uses a flask (made of metal or wood) to contain the sand during the entire mold producing and pouring operation. The flask remains around the sand during coresetting, closing of the mold, pouring and cooling of the casting. After cooling, castings and sand are separated from the flasks, which are then reused to produce more molds.

Advantages: This type of high-density molding offers increased flexibility in type and number of castings poured per mold as well as increased use of the total flask surface when compared to flaskless molds. It also offers the capability of increasing the size and weight of a casting which may be poured. Use of complicated cores or core assemblies is also not a problem.

Disadvantages: Flask handling systems require capital investment and a good maintenance program. The variety and size of castings produced in a "tight" flask molding line may require changing the flask for a larger size casting.

Flaskless Developments

The development of flaskless molding in the early 1960s represented one of the most important innovations ever in green sand molding. Subsequent developments and refinements of both vertically and horizontally parted molding systems brought about even more significant labor savings and economic advantages in many casting applications.

The production efficiencies represented by flaskless molding machines have been impressive. Along with the advantages discussed earlier, some others include:

* uniform mold density and hardness;

* adjustable mold thickness, blow and squeeze pressures;

* maximum output with reduced manpower and floor space;

* elimination of flasks and reduced labor required for flask handling systems and maintenance;

* reduced labor due to continued automating of the molding operation.

The new flaskless matchplate molding machine shown in Fig. 1 is capable of producing 180 molds per hour. It consists of a two-station turntable machine with the molding operations taking place at one station, while coresetting, mold stripping and closing at the other. Both cope and drag mold halves are squeezed simultaneously by a common cylinder to provide uniform mold density. Squeeze pressure is adjustable up to 55,000 lb, delivering a maximum sand surface pressure of 114 lb psi.

A center post between the two stations supports a turntable that has upper and lower double arms. The upper arms support two cope flasks and the lower arm two drag flasks. Each of the drag arms contain a shuttle mechanism that extends the drag mold outside the machine. This facilitates quicker, more efficient coresetting.

The two sets of arms are connected so that, when the turntable rotates 180|degrees~ all four flasks rotate simultaneously. Pattern changes are also performed outside the machine and reportedly can be made in less than a minute. Patterns that are smaller than the machine's flask can be accommodated by using a pattern adaptor frame. Flask dimensions are 20 in. x 24 in. with cope and drag heights ranging from 6.7 in. up to 7.9 in.

Sand is delivered to a single location, acting as a feeder for a common blow tank. The sand tank blow fills into the top of the cope mold at the same time as it blow fills into the bottom of the drag. Introduction of the sand directly onto the face of the pattern by the top and bottom blow fill design provides uniform filling, eliminates shadowing and assures complete filling of the deepest pockets of complex molds. Maximum sand quantity is 450 lb per mold.

Another line of flaskless molding machines recently introduced has been designed expressly for vertically parted green sand molds and does not require mold reinforcements such as flasks or pallets.

Vibration is used to distribute the sand over the pattern prior to the compression cycle. After compression, the machine's front squeeze plate swings up to allow the rear compression plate to push the completed mold onto the pouring conveyor where it is lined up to form the mold cavities that are shown in Fig. 2. A horizontally parted system is also available within this line of molding machines.

The new vertical molding machine pictured in Fig. 3, and its smaller companion, are the latest addition to this established line of flaskless molding systems. Both are vertically parted, shot-squeeze machines that incorporate a number of advances in flaskless molding equipment.

The larger system harnesses the latest in advanced electrical controls and has been specially engineered to reduce wear in critical parts that might affect the dimensional stability of castings. The machine is capable of variable mold thicknesses to improve sand use and to optimize mold dimensions for cored and uncored castings. It also has an adjustable sand shot pressure and profile selection system that is controlled by a PLC process controller. It regulates the optimal sand filling of the mold chamber, a factor particularly important for complex patterns.

Compressibility of the mold sand is continuously monitored by the control system which provides rapid feedback to the sand plant control center. This insures that necessary adjustments in required sand quality and consistency can be made quickly. An enhanced pattern spray system uses nonflammable spray fluid inside the closed molding chamber.

A single operator panel containing all control switches can be located on either the right or left hand side of the machine. Preprogramming of pattern data for the next run adds to more efficient pattern changes and better security against operator error. There are 14 different operation parameters keyed to patterns to control settings for electrical, hydraulic and pneumatic adjustments for any given pattern production selection.

Another automatic flaskless system, designed for smaller foundries, is available for mold dimensions up to 16 in. x 20 in. x 4-12.6 in. thick. The unit can handle 205 medium-sized molds per hour. Mold mismatching is avoided simply by adjusting the vertical and horizontal position of the pattern plates. During squeezing and stripping, vibrators mounted on both squeeze plates can be activated. This facilitates sand filling and smooth pattern stripping, particularly on deep and high patterns.

A new horizontally parted flaskless molding system installed in a Chicago foundry nearly two years ago, demonstrates just how far many of the new flaskless molding systems have come. This system is capable of producing up to 180 molds per hour while consistently producing green sand molds at a 92 hardness. It incorporates a drag rollover station that allows the drag flask, resting on the matchplate, to be filled with sand by gravity.

After vibration, a bottom board is placed over the top of the sand, the drag flask and matchplate are then turned through 180|degrees~ and indexed to a position under the cope flask. The matchplate and drag are lifted up to the cope flask which is then filed with sand. Subsequent vibration and squeeze serve to consolidate the mold halves.

A quick pattern change design allows for a 30-second tooling exchange and pattern temperature is computer-controlled. PLC controls program recipe loading by pattern number to tailor machines to specific jobs. Automatic coresetting is available.

At the heart of the system is an IBM-compatible computer equipped with a high-speed programmable controller. A touch-sensitive 13-in. industrial VGA monitor located on a swivel pendant just above the molding line is in easy reach of the operator and eliminates nearly all of the switches and buttons required by most conventional molding systems. More than 200 information screens are instantly available for control and monitoring purposes of all mold line functions.

Functions displayed include all pump operations and squeeze pressures. Maintenance procedures, self diagnostics and periodic maintenance schedules are displayed along with replacement parts lists and manufacturers' part numbers. The system also provides for continuous monitoring, recording and readouts of sand compactability.

Another system recently introduced is a four-post design that provides rigidity and eliminates mold mismatches. The pour cup and downsprue are air activated and all movements are precisely controlled by a programmable controller. Both cope and drag are filled simultaneously and the drag half features fluidized blow fill. Air enters through the squeeze plate for improved deep pocket filling and flask walls are vented for exhaust and noise control. For flexibility, the mold height can be changed by adjusting the limit switch position.

The self-contained molding machine is able to produce up to 180 molds per hour using a blow/squeeze action. The flasks move to grip the horizontally-mounted matchplate and are enclosed top and bottom by squeeze plates. Sand then is blown simultaneously into the cope and drag flasks from a bifurcated blowhead connected to slots in their walls. The squeeze plate then moves toward the matchplate to consolidate the sand.

Still another automatic matchplate molding machine, illustrated in Fig. 6, uses a horizontal blow-fill system that eliminates the many disadvantages of the older gravity drop sand fill systems. The mold flasks above and below the pattern plate are filled through slots in either side from separate blowheads. This design assures a uniform 45-55 mold hardness prior to the squeeze. Because sand is blown into the mold cavity, less sand movement occurs during the squeeze cycle which means less pattern wear. In addition, the blow-fill system eliminates spill sand and sand-per-mold can be programmed for different patterns.

Tight-Flask Developments

While the use of flaskless molding systems has grown impressively over the last few decades, many casting applications still require the use of cope and drag flasks. Continued refinement of these systems make tight-flask molding machines effective for producing a wide range of castings.

The development and introduction of what is generally referred to as "impact" molding represents a significant step forward for tight-flask molding machines. This equipment uses compressed air or the explosive energy of a gas/air mixture as pressure mediums to drive and compact a measured amount of sand against a pattern plate. With a maximum compaction pressure of 87 psi, the impact process is completed in one step. The degree of compaction can be controlled by changing the air pressure or adjusting the gas/air charge. Depending on mold complexity and size, maximum molds per hour range from 15-180 per machine, producing typical cast iron or steel castings weighing from about 90-450 lb.

The three pressure media used in impact molding are selected by the availability of energy, energy costs and flask dimensions. The three, illustrated in Fig. 7, include:

* gas impact--a gas/air mixture (natural gas, methane, propane) in which a measured amount of gas is injected into a closed combustion chamber and agitated to mix with air. On ignition, the pressure buildup occurs directly above the molding sand. At 60-75 psi, the subsequent compaction is high.

* air impact--the most used system, it relies on the design and reliability of a disc valve that admits compressed air into the mold chamber. The valve opens in milliseconds and the air becomes the impact force that compacts the sand against the pattern face.

* air impact plus--a two-stage compaction process, it is used particularly with bulky or high patterns or patterns with narrow spaces between the pattern and the flask wall. Filling sand into the flask, an initial compressed air impact uniformly compresses the sand in the pattern shadow areas. This impact intensity and duration can be adjusted according to the degree of difficulty of the pattern and the pattern plate layout. The second pressure wave produces the final mold compaction.

A new molding machine has just been introduced to North American foundries that uses compaction technology based on a process successful in many high production foundries. It enables the user to set precise compaction limits for individual patterns and has the capability to store those limits for individual patterns and has the capability to store those limits for instant retrieval. The machines can be integrated into molding systems having varying degrees of automation and they accept all patterns including those of wood. Production rates ranges from 30-50 molds/hour.

Mold Alignment

Matching the cope flask to the drag flask is a key factor in producing good castings. Locator flask pins and bushings that line up the cope and drag molds can become worn or damaged from repeated handling, causing misalignment and flawed castings.

Antishift pattern inserts are a new product that assures a quick, inexpensive means of locating the cope correctly over the drag every time. The two styles available provide close tolerance, reduce mold handling, scrap and cleanup time.

The inserts eliminates lineup cores, are easily installed and create a perfect match on matchplates, matchboards, insert boards in molding boxes and for core alignment. They can be used with green sand, airset, cold molding, vacuum molding and for core alignment.