Manual Clay Ring Forming Machine

Manual clay ring forming machine for making the combustion chamber of a small single-pot Rocket Stove.
New Dawn Engineering
Crispin Pemberton-Pigott June 2 2006

It is operated in the same way as the Terrabric Machine on the New Dawn Engineering website but makes stove components. The rings are not limited to Rocket-type combustors. It could, for example, be used to make chimneys by putting 5-7% cement into soil to stabilise them instead of firing them. Essentially it is a soil-cement brick press with a moving-fulcrum compression system. It can be used to make stabilised soil rings or to mould clay.

An important part of this approach is to try to avoid the drying process essential to an extruded or hand-moulded clay operation. Clay has about 30% water in it when it is made into stoves or stove parts. This has to be reduced to about 10% before firing. It is during the drying process that most of the damage occurs: fractures, micro-cracks, warping and anisotropic shrinkage creating stress. Almost all the literature available on clay stove making is devoted to getting the clay content right in an effort to avoid the problems caused by the 30% to 10% drying process. If the parts can be formed under manual pressure with a lower water content, then the drying is largely avoided. Presto! Far fewer problems.

Further, clays for stove making have been mostly selected on the basis of how they performed when drying rather than for use as a stove. It was from the small 'dryable' range of mixes at each production site that one had to choose for thermal shock. In other words, if the mix was instead selected for the ability to resist cracking during firing and use, it might not be a clay that dried particularly well and was in the past, rejected. If the forming method is a semi-dry manual press, the stove make is free to investigate clay formulations that do not extrude or dry well but which make good stoves.

In the case of the clay rings, the idea is to use 5 rings in a stove with the top three being whole. The bottom two are carved to make a Rocket shelf etc. They nest with the indexing step visible in the pictures. The ring might be carved or cut before or after firing as appropriate.

This machine will be sent to Malawi for use by ProBEC North. They are making domestic Rocket stoves - a portable one that is a metal-clay combination.

At present they are using an insulative fired clay ring that is difficult to form wet, pretty weak and doesn't last well. Like a JIKO, the metal is there to hold the cracked clay parts together. It is a pretty grim combination but that is the state of the art in the rural producer's environment. Scaling up the production is a very difficult challenge for obvious reasons. This machine will allow us to make rings very rapidly with hardly any drying period. An output of 1 per minute is expected. It is also expected that the rings will dry isotropically and that cracking will thus be kept to a minimum. It is not known what further drying will be required though it is recommended that the rings be dried under shade and out of the wind. Under idea conditions it may be possible to take the rings directly from the Ring Maker to the kiln.

The ring is 120mm inside diameter and 220 outside, with a 1 mm taper inside and out to assist demoulding. The indexing lip is 8mm high. The moulding pressure is about 9 MPa. Final compressive strength is linearly related to density. If the water content is correct, no oil is needed on the faces of the mould box.

The machine makes the rings 60mm 55mm or 50mm high. The height is reduced by adding a 5mm shim into the lid section and choosing a different bolt hole on the lid hinge. As the stroke is fixed, the final ring has a constant height. However the density of the brick is therefore dependent on the quantity of mix put into the mould. If a low density brick is desired the quantity is reduced. Using a 'springy' additive like large sawdust will be a problem because it will, after compression, spring back filling the whole ring with tiny cracks.

A 5mm flat bar (or two) can be placed under the bottom of the lower shaft to lift the piston when it is at rest. This shortens the stroke but leaves the final height the same which may suit the producer and the mix.

The bonding of the dry clay does not have to be very good, only enough to hold it together to be fired so the rings pictured here are actually 'wet' by clay brick making standards. When the clay minerals melt during firing, they 'stick' the brick together.

For the technically ambitious, it is possible to make a ring with different mixes on the inner and outer surfaces. This is done by inserting a sheet metal separator (a round sleeve) into the mould and filling the inner and outer portions with different mixes. Suppose the outer portion was a phosphate-containing low-sand clay and the inner was a 60% charcoal powder, high lithium clay. The ring would be compressed and ejected. After firing it would have a low thermal expansion, high shock-resistant insulative region facing the fire surrounded by a mechanically strong clay structural support. And made in your back yard!

Happy pressing!

Crispin Pemberton-Pigott
Regional Technical Advisor
Programme for Biomass and Energy Conservation
GTZ/ProBEC South (an official SADC project)
158 Jan Smuts Ave
Johannesburg
South Africa
(Click on thumbnail to enlarge image.)

1 Ring Brick Maker

2 Fill The Cavity With Dryish Clay