Large Structural Brick
Brick types vary from thin brick to face brick to structural through the wall units. Thin brick allow for less weight and less costly installation. Face brick provide some insulation and are practically maintenance free for the lifetime of a building. Structural clay brick provide greater insulation, more protection from the elements, and the appeal and durability that have always made clay brick a desirable building material. The manufacturing of large structural clay brick presents challenges to brick makers in two of the critical processes involved in brick making: extrusion and drying. In this blog post we will focus on how to avoid breakage in the extrusion part of the manufacturing process.
Modern brick plants use machines that are capable of pugging and extruding up to 80 tons of clay or 160,000 pounds of clay per hour under high pressure. There are a few important factors that must be monitored and controlled to maintain quality during pugging and extrusion.
- Pug Level and Pug Knife Condition – Raw materials are normally poured into the rear of the pug tub while being sprayed with atomized water. The atomized water covers more surface area of the raw materials than a dosage of poured water. The level of the materials in the pug tub and the condition of the pug knives can have an effect on how well the raw materials are “worked.” The raw materials should be high enough so that there are two inches of the pug knives showing when the knives are positioned in the vertical position. This allows for the most efficient use of the knives surfaces to move the raw material toward the sealing augers and to “work” the raw material. The pug knives should also be in good condition so as to have sufficient surface area to work the clay mix and so as to not break off and cause a catastrophic mechanical breakdown.
- Extrusion Vacuum – The pugged raw materials are moved from the pug tub to the extruder by a set of augers that seal the extruder’s vacuum chamber and send the raw material through a set of shredder knives that work the material further. Depending on atmospheric conditions and the brick plant’s relative position to sea level, the material should be subject to vacuum levels of no less than 25 inches of mercury. Materials that are extruded at less than 25 inches of mercury may not be properly de-aired and may be produce weak green brick. Vacuum levels are normally kept above 25 inches of mercury by having a preventative maintenance program for the vacuum pump and lines.
- Extruder Wear Parts – The extruder contains wear parts including swipes, augers, and liners that all work to move the raw materials along the axis of extrusion toward the extrusion die. The extruder wear parts begin to wear out due to friction very soon after they are installed. The parts wear at different rates depending on the abrasiveness of the raw materials being extruded and the quality of the wear parts. Also, different wear parts will need to be replaced at different intervals as they will not wear at the same rate. It is important to change wear parts as needed in order to maintain good extrusion and prevent laminations and/or stresses that can occur with worn parts.
- Dies, Bridges, and Core Tips – The final part of extrusion involves the compression of the raw materials as they are moved through the forming section and finally through the die. The raw materials are compressed at high pressure over the bridge and around the cores tips where the void sections of the extruded brick are formed. This part of the process is critical as the large structural brick are extruded with large void areas and thick outer shells. In order to insure that the raw materials knit and form properly without stress defects or cracking, the column should be balanced regularly and the die, bridge, and cores should be inspected and changed regularly. Balancing can be performed by cutting the extruded column off at the end of the die and jogging out a foot of the column and looking for uneven extrusion. Any uneven extrusions can be corrected through adjusting die lubrication or adjusting the core tips or bridge. The particular extrusion equipment supplier can make further recommendations.
- Raw Materials – The characteristics of the raw materials being used can be a factor as to whether the formed brick crack or have stress defects. It may not be possible to make adequate adjustments to the clay formulation to correct the extrusion issues, but clay conditioners can be used to make clay extrude more easily. Clay conditioners may have the dual benefits of dispersing the clay particles so as to make them form together more evenly and to lubricate the clay mix so as to make it extrude with less stress due to radial and axial pressure. The lessened pressure can allow for extrusion with diminished stress and less tendency to crack.