Sand fill and compaction: getting them right.

Sand Fill and Compaction: Getting Them Right

One of the important tricks in making consistently reliable "lost foam," or evaporative pattern castings, is engineering the filling of the flask with sand. Sounds fairly straightforward. You place the coated foam cluster into a flask, fill the flask with sand, compact, and make your casting. No wonder the lost foam process is becoming more peculiar; it's so simple.

But like so many "simple" things, there are some difficulties. Last month in modern casting, mold coating technology, one of those difficulties, was discussed. I would like now to make some comments on another--flask fill and compaction of sand around the mold.

The foam cluster must be totally surrounded and structurally supported by sand without being distorted or causing dimensional changes, no easy engineering task given the weight of sand and the relative fragility of the foam replica itself.

Sand fill around the foam can be accomplished in several ways, two of which, fill tubes and rain fill, are described below. Since no overt compaction devices are used other than flask vibration, the success of a properly-filled flask depends upon understanding and using the fluid dynamics of sand to fill all voids and support evenly the stresses caused by the heat and weight of the cast metal.

Fill Tubes--Filling the flask with silica sand must be carefully controlled by the spacing of the fill tubes. Fill tubes, developed to overcome damage to the foam from free-falling sand filling techniques, were designed based on mold cluster configuration.

They were placed to fill evenly around the foam and arranged so that they could be withdrawn as the filling and compacting process proceeded. This was to allow for greater fill uniformity of sand around the foam, shorter sand free-fall and less chance for "shadowing" of a particular area of the foam. Shadowing results from incomplete sand fill and compaction under portions of foams, resulting in molding defects.

Fill tubes require devices that will withdraw the tubes at a specific rate during the fill process to facilitate adequate sand distribution. They can be attached to a batch hopper or to any controllable mechanical extractor, the extraction rate of which allows the sand feed rate to be varied by the withdrawal rate. Today, fill tubes have been almost totally replaced by better application and control of rain-fill systems.

Rain Fill--Rain fill is perhaps the most widely used method for filling lost foam casting flasks today. The pattern of raining sand can be adjusted by use of masks and mask screens to void particular areas of sand that would subject the foam to impact damage, or to control stresses.

One design consideration for the rain fill method is to minimize the free-fall distance of the sand into the flask. This lessens the impact of the sand against the foam. The fill rate can be varied in a number of ways depending on the physical characteristics of the rain gate feeding system.

A rain fill method weakness is fugitive dust. Due to the free falling effect of the sand, more airborne contamination tends to occur compared to the tube fill method which "places" the sand in the flask. This is easily overcome with proper design of the gate/hopper combination. Masking screens also allow for a final screening of the sand just prior to flask filling to remove only tramp materials.

Controlling Sand Volume

There are several alternatives to controlling the amount of sand to be introduced into a flask. One can weigh the sand, allowing for a programmable controller and load cells to coordinate a particular amount of sand to be introduced for a particular job. The established weight of this sand then could be stored as a value for later use for the same or similar sand fill and compaction cycles after testing confirmed the value in actual production.

Sand also can be controlled volumetrically with a batch hopper, computing the volume of sand introduced and correlated with sand fall height as an adjustable value. The adjustment can be one that is manual or automated to produce a consistent amount of sand each time within a flask. However, if the foam-to-sand ratio in a particular application is to vary widely, this method may not be practical strictly by manual control, except for prototype setups.

Generally speaking, the controlling and conveying of sand in the lost foam process is like controlling water. The sand is an extremely free-flowing material, and an operator should be provided with the ability for quick and positive sand shutoff when it is required. The segregation of sand is another concern and anti-segregation devices must be used at storage points in the sand system.

Compaction

Proper compaction of the dry sand is an essential element of a successful lost foam system. The sand must be totally compacted in all areas of the flask to correctly position and support the foam cluster for dimensionally stable castings to be produced consistently. Universally, vibration is now utilized to both fluidize and compact the sand. Compaction tables for foam pattern castings have evolved, and many design variations are commercially available including horizontal, vertical, multi- and omni-axis tables for both laboratory and production work.

Vertical Axis Compaction Table--Vertical axis compaction tables, widely used in the foundry industry, are usually positioned below the flask-handling system, elevating the flask from the bottom. Pneumatic actuators, such as air bags, are the most common method used to elevate these tables, but the type of elevator used depends on a foundry's flask-handling equipment as well as the space the overall envelope allows. (Flasks for use with vertical axis tables are generally cylindrical.)

Some foundry procedures require hydraulic clamps to secure flask and table to form a rigid unit; others do not. Using the former, some contend, assures uniform compaction, and test data gathered without clamping is inconclusive. Many production facilities are successful in producing castings without the use of clamps.

As with the problems associated with sand fill and compaction, the question of clamping is subject to individual foundry testing to determine its own requirements. It should be pointed out, though, that clamping at the frequencies and accelera tion levels being used is difficult to do successfully and is expensive in equipment and maintenance costs.

Vibration of the vertical axis tables is provided by electric motors mounted horizontally and equipped with adjustable force wheel weights at the end of each motor. The number of motors required is a function of the flask size and the total mass to be vibrated.

Mounted in pairs, two or four motors ideally are located at the end of each compaction table where they are readily accessible for servicing vulnerable bearings. Their general operating frequencies range to as high as 100 Hz or 6000 rpm motor speed, but normal operations appear to be 1700-4200 rpm (28-70 Hz). "G" ranges for vertical axis tables in successful applications have been observed at fractional "G" to about 4, but this can only be determined by the complexity of each casting.

Horizontal Axis Compaction Table--A horizontal axis compaction table is firmly coupled to a flask by side-mounted hydraulic clamps that allow the flask to be removed from its conveyor by means of pneumatic actuators. Without benefit of gravity to help secure the flask to the table, the clamps are used to form the inflexible flask/table linkage, letting them act as a single unit basic to the transmission of forces generated by the table.

As with the case of the vertical axis table, three points of connection are used, but with the table placed beside the flask a pneumatic support mechanism on the flask side opposite the compaction table must be used to keep the flask and table level. Again, vibration is provided by motors, vertically-mounted and equipped with adjustable force wheel weights located within the table's power head.

A means to maintain motor weight phasing must be provided, and the wheel weights also are adjusted manually to obtain a given acceleration at frequencies determined by compaction table testing. Operating frequencies appear to be in the 34-60 Hz, or 2000-3600 rpm ranges, with acceleration between 1.0-3.0 "G" limits. A possible advantage over vertical tables is quieter operation.

Multi-axis Compaction Table--Multi-axis, or x-y-z, tables used many of the same components found in vertical and horizontal axis tables. By using complex control systems and multiple pairs of drive motors, virtually any axis variation theoretically can be programmed. Clamping is still integral, but the complexity of the multi-axis design in a production situation adds to the difficulty of meeting foundry production requirements and few are in use.

Omni-axi Compaction Table--Currently available is an omni-axis compaction table which is claimed to produce a gyrator motion over a full adjustable range on both vertical and horizontal axes vibration intensities. Its manufacturer states that by using a three-point flask support system, the need for clamping is eliminated. The vibration of the omni-table is provided by a single vertical squirrel cage motor equipped with adjustable force, unbalanced wheel weights at each end.

The builder claims the frequencies of horizontal force components mirror motor operating speeds while gyratory motion develops vertical force component frequencies at multiples up to ten times motor rpms. This produces action intensities of up to 30,000 impacts per minute for flasks sizes as large as 60 inches.

Table Controls

Perhaps the single most important component of any compaction system is the control array for the compaction table. The operating controls must be capable of assuring repeatability, reliability and consistency from one flask to another. Visual readouts for acceleration, or "G" force, rpm and displacement in mils should be provided to allow monitoring of compaction table functions.

The system also must control the sand fill, since filling and compaction must interact.

State-of-the-art control systems with user-friendly programmable controls are a prerequisite for successful lost foam casting production. Such systems have been developed only after intensive experience with foundries producing a variety of castings under actual production conditions. Therefore, many weaknesses and limitations in earlier systems were not discovered until recently.

If a foundry is beginning the lost foam process by doing prototype work, it is important to remember that controls should be designed so that data from prototype setups can be collected to allow this information to be used to establish production recipes.

Conclusions

Foam pattern casting is a relatively new technology, and production equipment specifically designed for its filling and compaction requirements is pivotal to success. Only prototype casting systems using production-type fill, compaction and control parameters will allow engineers to gather functional data transferable to manufacturing environments.

A word of caution: Lost foam has been oversimplified and oversold in some cases to foundries whose greatest need now is for more information about the practical problems inherent in this new casting process. The process is being used successfully today under full production conditions, by captive and jobbing foundries, in ferrous and nonferrous metals. Sand fill and compaction are keys to this success, and must not be taken lightly.