Sinto America Inc.
Palmer Manufacturing & Supply
Cascade Die Casting
Ryzhov Sergey | Dreamstime
Low-pressure sand mold components.
Warut Sintapanon | Dreamstime
Molten iron pouring into a sand mold.
Palmer Mfg. & Supply
A compaction table with a sand mixer, for improving no-bake molding effectiveness and casting quality.

Reduce Casting Defects with Compaction

Jan. 10, 2023
The 20 seconds it takes to properly compact a no-bake mold may be the most profitable sequence in a foundry operation.

When a mold produces a casting with a defect or poor finish, foundry operators often will look at all of its major processes to determine what caused the failure – resulting in analyses of all the process steps from molding to pouring. However, one area that often is overlooked is the compaction sequence. A simple compaction cycle takes about 20 seconds of production time, and yet the effect will be payoff will be considerable, with a variety of benefits to the production process and the products, one of which will be reducing casting defects.

Compaction moves sand grains closer to each other, thereby keeping metal from filling space between the grains. Naturally, this produces a better surface finish for castings. However, compaction also enhances mold strength because less binder is needed, which saves consumable costs for the operation.

In any mold, gases will form defects unless the mold is properly vented. When binder is reduced, the amount of gas that needs to be vented is also reduced. Venting takes time and is a bit cumbersome, as it may require drilling. When vents are filled with metal, they need to be removed and the attachment must be ground down. Additionally, loose parts needed to form the vents must be kept together with the mold box after molding.

The cleaning room has never been an easy area of the foundry to staff and the current labor shortage is only making that more difficult. Proper compaction improves the surface finish and reduces burn-on, which also reduces cleaning time. Any process that reduces cleaning time will be appreciated, especially by the cleaning room staff.

Rotary vibrators are the most common type of vibrator used for compaction. Rotary vibrators have a limited effect because they cannot provide true vertical vibration, even when there are multiple vibrators counter rotating. A vertical vibrator is more versatile as it can be turned down to a low level to properly compact a small mold or core, and turned back up to properly compact larger work. It is critical to closely match the force output to the weight of the tooling and sand.

High-frequency vibration is important to making quality no-bake molds. Vertical vibrators can be turned up or down without sacrificing frequency. The force output of rotary vibrators can only be changed by changing the speed of rotation, or by the very slow process of changing the eccentrics/weights that create the vibration. Rotary vibrators also are very difficult to stop immediately. Unless the rotary vibrator is stopped immediately, ramping down the process may create low speed (“loping”), which can negate the benefits of the compaction process.

While the force changes so does the frequency; with different frequencies for different weights of work coming across the compaction table; compacting is best done with vibratory vibrators. Achieving consistency using rotary vibrators is nearly impossible when compacting varied weights.

In order to select the proper force setting, it is common to run a “drop-and-stop” test. This is a test where the selected box is filled normally and then struck-off before compaction. The compaction cycle is started with the vibration time set at 5–10 seconds.

If the sand is still going down when the vibrators stop, more force or duration of vibration is needed. If the sand drops and stops before the vibrators stop, less force or duration is needed. If this test is performed on every job, the proper effective compaction settings can be determined quickly. It is not as simple as calculating a force/time setting based on weight, as deep work behaves differently than shallow wide work. In order to keep the compaction process simple and repeatable, it is recommended to keep the duration of vibration the same for all jobs (typically at 5 seconds.)

Documenting the force setting is as simple as writing the force output setting on the box. The compaction table “force output” dial is labeled 0–100%. The tech simply performs the drop-and-stop test and writes the number on the side of the box. Then, when the same box is back under the mixer, the operator runs the correct amount of sand into the box, and turns the dial to the correct setting and presses the cycle start button.

At this point, the airbags inflate lifting the box off the rollers or support structure. Once the box is lifted, the vibrators run at the selected force output for the predetermined amount of time. Once this time runs out, the airbags are deflated, lowering the box back onto the conveyor rollers and, completing the cycle. This typically is a 15–20 second process. Then the top of the mold is the struck-off, moved to the next station and the next job is moved into place.

If the operator starts the compaction process when the box is half full and the next operator starts it when the box is completely full, the results will be variable. The simple thing to do is to fill the box and then compact. Although this is simple, sometimes it is desirable to do it differently. A shallow job with a lot of surface area could generate a higher amount of gas and require venting.

However, if the configuration of the mold is such that mechanical venting isn’t practical, then the recommended method is to cover the pattern, compact, and fill out the rest of the box without compaction. This will provide a good mold-metal interface with a permeable zone close to it, and thereby allow gases to escape through the mold itself.

Start time is another variable that needs to be controlled. It is a simple matter to tie the mixer run time into the compaction sequence, where the compaction sequence is initiated after a certain amount of mixer run time. The less strike-off sand that is generated, the lower the mold cost will be.

The 20 seconds it takes to properly compact no-bake molds may be the most profitable 20 seconds in a foundry operation, for reducing costs while producing higher-quality castings.

Jack Palmer is the president of Palmer Manufacturing & Supply, a specialty supplier of no-bake foundry equipment.