Vacuum Brick Extruder: How It Works and How to Specify One

A vacuum brick extruder mixes prepared clay with water, removes the air from it in a vacuum chamber, and forces it through a die as a dense, continuous column that is then cut into bricks. De-airing makes the column stronger and more uniform, and hollow and perforated bricks must be vacuum-extruded. Its specification follows from your clay, your brick types and your output.

This guide explains what happens inside the machine, what vacuum level and extrusion moisture mean in practice, how dies and cores shape the brick, which problems trace back to the extruder, and what to write into a specification so that offers from different manufacturers can be compared.

How a Vacuum Brick Extruder Works

Most bricks are formed by the stiff-mud process: ground clay is mixed with water in a pug mill, de-aired in a vacuum chamber and extruded through dies, and the column is cut by wire (US EPA AP-42, section 11.3). On a modern line these functions sit in one two-stage machine. The two stages are stacked: an upper mixing stage on top, and a lower extrusion stage below, joined by the vacuum chamber.

1. Feeding. Prepared clay arrives from the box feeder, crushers and roller mills, and often from a souring house where it rests before forming.
2. Upper stage: mixing and pugging. Twin shafts with paddles mix the clay and add water (or steam) to bring it to extrusion moisture.
3. Sealing. At the end of the upper stage, the clay is pressed into a plug that seals the vacuum chamber from the outside air.
4. Shredding into the vacuum chamber. The plug is cut into thin strands or shreds as it enters the chamber, so that air can escape from a large surface.
5. De-airing. A vacuum pump holds the chamber well below atmospheric pressure and draws the air out of the shredded clay.
6. Lower stage: compaction. An auger (screw) with replaceable wear tips picks up the de-aired clay and compacts it in a lined barrel.
7. Pressure head and die. The auger pushes the clay through a tapering pressure head into the die, which gives the column its outer shape. Cores held in the die form the perforations.
8. Column out. As the column leaves the die it is lubricated (AP-42 mentions oil or another lubricant), surface textures or coatings may be applied (BIA Technical Note 9), and the strip cutter and blank cutter cut it into bricks.

Vacuum Level: What −0.092 MPa Really Means

The Brick Industry Association gives a de-airing vacuum of 15 to 29 inches (375 to 725 mm) of mercury for stiff-mud extrusion (BIA Technical Note 9). That is about 50 to 97 kPa below atmospheric pressure. Two-stage extruders on the lines we supply reach a vacuum of −0.092 MPa, which is 92 kPa below atmospheric, about 690 mm (27 in) of mercury, close to the top of the BIA range.

Air left in the clay does three things: it makes the column less dense, it weakens the bond between the layers of clay that the auger lays down, and it expands when the brick is dried and fired. Good vacuum gives a denser column with higher green strength, which matters when green bricks are stacked high on kiln cars. A brick guideline prepared for Bangladeshi lenders puts it plainly: vacuum extruders should be used to get better green brick quality (Frankfurt School for IDCOL, 2019).

Gauge vacuum and altitude

A vacuum gauge shows the pressure below the local atmosphere, not the absolute pressure in the chamber. At sea level, −0.092 MPa leaves about 9 kPa absolute in the chamber. At altitude the atmosphere itself is lower: NASA’s standard atmosphere model gives about 90 kPa at 1,000 m and about 85 kPa at 1,500 m (NASA Glenn, Earth Atmosphere Model). At 1,500 m a gauge cannot read −0.092 MPa at all, and the same 9 kPa absolute shows as roughly −0.075 MPa. The gauge readings below are our calculation: atmospheric pressure from the NASA model minus about 9 kPa absolute.

For a plant on a plateau in Central Asia or East Africa, write the vacuum requirement as an absolute pressure, or state the site altitude, so that the acceptance test is not argued over a number that cannot be reached.

Extrusion Moisture: Stiff, Soft and Dry-Press Forming

Moisture is set by the clay, not by the machine. Published ranges differ with the source and the basis of measurement:

What this means for the plant:

• Drier columns need more pressure and power. A stiffer column holds its shape better and dries with less shrinkage, but the extruder has to push harder. The extruders on our lines are rated for extrusion pressure up to 4.0 MPa.
• Every point of water is removed in the dryer. Wet bricks contain 7 to 30% moisture and are usually dried for 24 to 48 hours (BIA TN 9). Water added at the extruder is energy spent later.
• The setting method depends on green strength. On a one-setting line, wet bricks go straight onto kiln cars and are limited to about 7–14 layers because wet bricks are weak; on a two-setting line, bricks dried in a separate chamber can be stacked 14–19 layers. The extruder has to deliver the green strength that the chosen setting method needs.
• Moisture must be stable. Variations in water content change the column speed and size, and therefore the cut length. Automatic water dosing on the upper stage, linked to the extruder load, keeps it steady.

The raw material test fixes the moisture window. The Frankfurt School guideline lists the tests a brick factory design needs: plasticity, forming pressure, drying shrinkage, firing temperature, firing shrinkage, density, strength and thermal conductivity.

Dies, Core Bridges and Cores

The die gives the column its outer dimensions. Because clay shrinks in drying and firing, die sizes and cutter spacings are calculated to compensate for normal shrinkage (BIA TN 9). A die that fits one clay will not give the same fired size with another.

Perforations are formed by cores. The cores sit on a bridge (a frame across the flow) just upstream of the die opening. Clay divides around the bridge and must knit together again before it leaves the die. If pressure is too low, or the distance from bridge to die too short, the seams stay weak and the brick cracks along them in drying or firing.

The core pattern decides which standard a brick can meet. In ASTM terms, a solid brick has a void area of not more than 25% (ASTM C62 and C216), and a hollow brick has more than 25% and up to 60% (ASTM C652, classes H40V and H60V), as summarized in BIA Technical Note 9A. European clay masonry units are specified under EN 771-1. Check the current edition of the standard your market uses before the dies are drawn.

Changing the die switches the line to another brick type. Typical products on the lines we supply:

Larger hollow blocks need more flow area and more uniform pressure across the die than solid bricks. If you plan to add hollow blocks later, say so now: the auger, pressure head and drive must be sized for the hardest product, not the easiest.

Raw Materials and Wear

The extruder is the part of the plant that meets the abrasive particles in your body head on. Clay, shale, coal gangue and fly ash are all used, and coal gangue with shale is the most common combination on the lines we supply. Coal gangue typically makes up 30–70% of the body and fly ash can reach 20–40%.

Shale and gangue are harder on wear parts than a fine plastic clay. Three things protect the extruder:

1. Good preparation upstream. Fine, even grinding and a souring house that buffers the extruder against changes in the raw material. The Frankfurt School guideline describes the souring house as a buffer that ensures constant production.
2. Removing tramp metal. A magnetic separator in preparation protects the roller mills (Frankfurt School guideline) and, further down, the auger and die.
3. Wear parts designed for exchange. Hard-faced or replaceable auger tips, replaceable barrel liners and die inserts, with spare sets on site from the first day.

Ask each manufacturer which wear parts their offer treats as consumables, and how they are changed.

Vacuum Extruder Problems and Where They Come From

Many defects first seen after drying or firing start in the extruder:

A falling vacuum gauge reading or a rising motor current are the first signs. Log both every shift.

How to Specify a Vacuum Brick Extruder

A specification written around your raw material makes offers comparable. Include:

A rough throughput check helps compare offers. As our own estimate from brick dimensions: a standard brick of 240 × 115 × 53 mm is about 1.46 liters. A line making 300,000 bricks a day extrudes about 440 m³ of fired brick volume a day, before adding for shrinkage and subtracting for perforations; over 20 operating hours that is about 22 m³ of column an hour. Lines we supply are built for 60–100 million bricks a year in standard brick equivalent, about 250,000–300,000 a day.

Finally, agree how the machine is checked before it leaves the factory. Every shipment comes with a full pre-shipment inspection report — test-run records, functional and dimensional checks, photos and video — before the equipment leaves the factory.

FAQ

What is the difference between a single-stage and a two-stage vacuum extruder?

A two-stage machine mixes and de-airs in an upper stage and extrudes in a lower stage, all in one unit. A single-stage extruder needs a separate mixer or pug mill in front. Two-stage machines are common on new lines because water dosing, sealing and de-airing are controlled together, which gives a more even column.

What vacuum should a brick extruder reach?

The Brick Industry Association gives 15 to 29 inches (375 to 725 mm) of mercury for stiff-mud extrusion. The extruders on our lines reach −0.092 MPa, about 690 mm of mercury, at sea level. Because gauges read relative to local air pressure, state the target as an absolute pressure for sites at altitude.

Do solid bricks need vacuum extrusion?

Hollow and perforated bricks must be vacuum-extruded. Solid bricks can be formed without de-airing, for example by soft-mud molding or dry pressing, but a de-aired column is denser and stronger, dries more evenly and gives fewer laminations. For an extruded solid brick, de-airing is standard practice on new lines.

Can one extruder make several brick types?

Yes. Changing the die switches the line to another brick type, for example from 240 × 115 × 53 mm standard bricks to 240 × 115 × 90 mm perforated bricks. Size the auger, pressure head and drive for the most demanding product you plan to make, and include a spare die set for each type.

What moisture should clay have for extrusion?

Published stiff-mud ranges are about 10–15% water (BIA) and 14–18% moisture entering the vacuum chamber (US EPA AP-42), depending on the clay and how moisture is measured. Your raw material test sets the window. Keep it stable: moisture swings change column speed, cut length and drying behavior.

Planning a Fired Clay Brick Line?

The extruder is specified together with preparation, cutting, setting, drying and the kiln. See our fired clay brick production line for the complete configuration, read the clay brick manufacturing process step by step, and see how the kiln is sized in tunnel kiln for bricks. For the budget side, see clay brick plant cost.

Sources

• US EPA, AP-42 Compilation of Air Emission Factors, Section 11.3 Bricks and Related Clay Products (August 1997): https://www.epa.gov/sites/default/files/2020-10/documents/c11s03.pdf
• Brick Industry Association, Technical Note 9: Manufacturing of Brick (December 2006): https://www.gobrick.com/media/file/9-manufacturing-of-brick.pdf
• Brick Industry Association, Technical Note 9A: Specifications for and Classification of Brick (October 2007): https://www.gobrick.com/media/file/9a-specifications-for-and-classification-of-brick.pdf
• Frankfurt School of Finance & Management for IDCOL, Tunnel Kiln Technology Overview and Project Assessment Guideline (2019), via the Climate and Clean Air Coalition: https://www.ccacoalition.org/sites/default/files/resources/2019_Tunnel%20kiln%20technology%20overview%20and%20project%20assessment%20guideline.pdf
• NASA Glenn Research Center, Earth Atmosphere Model – Metric Units: https://www.grc.nasa.gov/www/k-12/airplane/atmosmet.html
• Digitalfire Reference Library, Laminations: https://digitalfire.com/glossary/laminations