|Monthly Tech-Tip |
The practice of removing air from clay as it is pugged. Deaired clay has better forming properties and produces a smoother fired surface.
Key phrases linking here: de-airing pugmill, pugmill - Learn more
Pugmills are often equipped with a vacuum pump and feature a chamber near the end of the barrel where the vacuum is applied to remove air (just before the auger that moves the material to the nozzle). The knives on the rotating shaft cut the clay within the chamber to expose as much surface as possible to the vacuum.
Prevailing knowledge and opinion is that deaired clay normally has better forming properties and produces a smoother fired surface than that prepared by other methods. This is especially true for bodies of lower plasticity or of certain formulations (e.g. high silt, high talc). However for most plastic terra cottas, stonewares and porcelains; slurrying, dewatering and wedging produce similar workability.
Traditions in many places are to age clay after pugging to improve plasticity. However on closer examination, it becomes evident that the body has low plasticity, any increase is considered worth the effort. However, for plastic bodies (e.g. those used on a potter's wheel to make large ware), the clay is fine right out of the nozzle of the machine. Today, low plasticity is normally managed by a simple bentonite addition or substitution of kaolin for ball clay.
Pugmills can be a part of a larger body-making process or it can be the only one. Theoretically, the ideal is a slurry mixing process to thoroughly blend the materials and wet all particle surfaces (called blunging). The blunger then feeds a screening device that removes coarser particles. That in turn feeds a filter press that dewaters the slip. Flat filter cakes produced by the press are then fed into a premixer that re-blends the separated layers in the cakes. The premixer then feeds the pugmill and it finalizes mixing and de-airs and extrudes the material. However, the more plastic the clay, the less practical some of the steps in the above process are. This is because highly plastic bodies do not screen well, they do not filter press well and they stick inside the premixer. If a de-airing pugmill has enough blades and they are angled for the best mixing possible (rather than speed), the dry material and water can be fed directly into the head of the pugmill chamber and it can do all of the steps by itself. For super plastic bodies, there is not really another practical method.
Studio potters often use de-airing pugmills to re-pug incoming material and to reprocess production scrap. Very capable small pugmills are commonly available and potters highly value them.
The machine has been reassembled after cleaning and is ready for startup. This pugmill is powerful and capable of injecting alot of energy into the material. Premixed powder and water are fed into the main mixing chamber by a screw conveyor at the far end. Dozens of blades on the rotating shaft inside cut and mix the material so that by the time it has reached half way in the main chamber all traces of powder are gone. At the end of the main chamber an auger delivers the materials to a venturi terminated by a shredder. This slices the material with dozens of tiny blades as it enters the vacuum chamber (yellow cover). This exposes as much surface as possible to the vacuum. Additional blades on the main shaft further mix the material and finally an auger compresses it and delivers it to the nose where a column is extruded for cutting to length and packaging.
A quality extrusion depends a lot on the design of the nose of a pugmill. On this industrial pugmill at Plainsman Clays this is the custom design it took to prevent laminations and peeling at the edge of extrusions, especially on lower plasticity clay bodies. This nose assembly weighs about 70lb. It is fed by a long auger from a vacuum chamber. This design achieves compression by easing the extrusion outwards and then abruptly narrowing it.
The machine is being cleaned in preparation for a porcelain run (after producing stoneware clays). The pugmill has been stripped down completely and all the casings and augers and other parts have been washed and dried separately. These must be installed (in the main chamber, the vacuum chamber and in the nose). Clean-downs like this are costly but necessary to insure a quality porcelain.
Clay is soft, but when under pressure in the nose and shredder feeders the abrasive particles within it (grog and quartz) take their toll.
Augers like these force the clay into the nose of the pugmill for extrusion. The one on the left was originally the same size as the one on the right. But the wear of grog particles in the clay have worn it down. Particles of quartz also take their toll on pugmills.
The same pugmill (back and front). One is stainless steel. Potters claim that they can dump almost anything into these machines (even dry scrap) and as long as they add the right amount of water the devices will mix and vacuum extrude a finished slug. Considering how portable these are they are an amazing device. However, these are no match for a large industrial pugmill. In the quantity of material they can produce, but also in the quality. They have few or even no blades on the main shaft, only augers. They contain no or only a rudimentary shredder feeding the vacuum chamber and little dwell time in the both chambers.
Example request-for-bid for an alumina extrusion pugmill, much more demanding, technical and specialized than a pottery pugmill (like that shown here). Alumina and cordierite extruders much work at very high pressures and shear much more.
-Purpose: De-air and extrude pure alumina (having 15-20% water and 2-4% methyl cellulose or HPMC binder).
-Type: Twin-screw feed (counter-rotating) horizontal.
-Capacity: 35-50Kg per hour.
-Barrel dia: 80mm.
-Screw RPM: 1-36 (adjustable).
-Extrusion pressure: 150-200 bar.
-Vacuum capability: 50 mbar.
-Cooling System for screw and barrel: Water cooling to maintain 10°C – 30°C during extrusion.
-Sensor system: Display of operational parameters like pressure, speed, torque, temperature etc.
-Cleaning of screw and barrel: Self-cleaning function/design.
-Coating thickness on auger and barrel: Alumina coated with 1500–2000 um thickness having excellent adhesion and wear resistance.
-Design: Specifically designed/configured, with conveyor, to extrude profiles like rod, tube, column etc. up to dia. of 70 mm.
-Spares: Two extra complete compatible augers.
At first it appeared to be the lack of a tapered die. Two were designed. The first did not solve the problem (right). Although it was tapered, it was only 5/8" thick (left overlay on the bottom right). Before going to a longer tapered die (bottom right overlay) we did a rethink. The clay being fed into this pugmill has already been processed in an industrial pugmill having 40 blades in the main chamber, a compression auger to a very efficient shredder, a vacuum chamber with about 20 more blades and another auger feeding a very long nose with dual compression dies. And a very powerful motor to drive the clay out the nose. The small pugmill here only has one shaft with few auger-like blades in the two chambers. A "screen" shreds the clay on the way into the vacuum. Each time the clay goes through the screen could its cohesiveness actually be compromised more by not being knitted back together properly in the nose? That could have been solved by the longer gently tapered die. But, it would not likely have worked, the clay would simply back-feed into the vacuum chamber because the design and power would not be sufficient to push it out the die. In the end the problem was fixed, but no how you might think: They turned off the vacuum!
These are cross-cuts from slugs of a production run of clay that was improperly pugged (inadequate vacuum). The problem is not often evident at time of extrusion but exhibits after weeks or months of storage. This clay body does not contain significant fine-grained material, but 2% talc was added to aid vitrification. The production crew claims that this talc makes it doubly important to monitor vacuum pressure at all times (or laminations will result). These are not actually a problem if you wedge the clay well. But if you do not (e.g. inserting them into a hand-extruder, pressing them in a mold) then they "build in" failure points that will initiate drying and firing cracks later. Even if pieces survive the drying and firing processes, weaknesses will persist making them more prone to failure-on-impact or stress. This being said, does that mean you do not need to wedge plastic clay bodies if they are not laminated? No. All clays laminate to some extent when pugged.
This company was plagued with drying cracks in their solid porcelain pieces. After some time they discovered that the deaired plastic material received from their suppliers had laminations (revealed in a cross section cut of the slug). Since they were not wedging, but simply inserting the clay into their hand extruders and presses, these laminations produced built-in weaknesses, the stresses of drying later exploited these. The obvious fix seemed to be to buy a vacuum pugmill to remix the clay. But that did not work. Why? Commercial pugmills commonly have multiple shafts, hundreds of blades, large powerful motors, separate mixing and vacuum chambers, shredders, high-compression heads, etc. Small studio pugmills have none of these features. They are still great for recycling and mixing clay that will later be wedged. But for the machine-forming purposes of this company, this pugmill actually made the laminations worse!
The brick-halves on the left cracked in two during drying, the crack opened at the center. I dried six of them and all cracked in the same way. The one-inch-slices were cut laterally from an extruded slug of clay and sun-dried. The radial pattern of the laminations are clearly visible on the break. These laminations are "a weakness" formed-into this extruded and unwedged clay, they would, of course, extend to fired integrity, weakening the piece. The halves on the right are from a brick that I made by first wedging (kneading) the clay, then forming and cutting it to size. It was likewise sun-dried. But did not crack. I broke it (with difficulty), notice the break followed the stresses of the breaking process, not internal lines of weakness.
This clay was slurried in a mixer and then poured onto a plaster table for dewatering. During throwing it is splitting when stretched and peeling when cutting the base. Yet when this same clay is water-mixed and pugged in a vacuum de-airing pugmill it performs well. One might think that the slurry mixer would wet all the particle surfaces better than a pugmill, but it appears the energy that the latter is putting into the mix is needed to develop the plasticity when there is a high talc percentage in the recipe.
These two close-ups of a fired cone 6 porcelain showing a big difference in surface smoothness. The deaired material on the right has a much smoother fired surface even though the non-deaired material on the left has been wedged much more. The transparent glaze does not hide the roughness.
Here is how the pugmill operators at Plainsman Clays gauge the stiffness of the clay coming out of the pugmill. The machined nylon roller is on a slant and weighted. The softer the clay the more lines show. When they are like this (5th line steady) the operator knows the water content is around 22% for this clay, Polar Ice. For each type of clays it is different. Stiffness at pugging must also compensate when the clay tends to stiffen or soften over time. Over the years they have tried many devices to measure stiffness, but this has proven the most reliable.
This is an example of an angle iron utility table being made into a plaster table. The cardboard sides extend upward to make the slab thicker and create a buffer gap to prevent the expanding and setting plaster from pressing outward on the frame. 150 lbs plaster (92 lbs water) was poured into the plastic-lined space (the bottom cardboard sections were supported from below). In a dry enough climate, this table could make enough clay to support slurry-to-plastic production for a potter (a thicker slab and a fan would enable even more capacity). The slurry-up process is better than a pugmill for small operations. It's much cheaper and is an easier way to utilize scrap material and weigh out custom recipes. The clay quality and de-airing is better (without hard chunks and contamination common to pugmilling). The procedure generates much less dust and the tank is easily cleaned. Slurries are easily sieved, especially if you have a sieve shaker.
Starkey Machinery Inc.
Xiangtan Weida Electrical and Machinery Co., Ltd
Clay mixers and pugmills
|By Tony Hansen|
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