Wet Sock

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Wet Sock

Release status: concept


A paste or slurry extruder with a knitted wrap
CAD Models
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Imagine 3D printing a house.

Well the concept has been demonstrated but not having heard big news it obviously means that there are still some kinks to iron out.

I have an idea that may make it practical a lot sooner, perhaps not the way it will be done always but something that might be put into practice with just a bit of putting together of parts.


The robot

The robot would have to be reasonably robust to handle the wight of the head and the longer spans required to reach across at least half of a typical build structure. A number of building robot structures have been proposed and any could be tried. Having one that can move reasonably fast would be a benefit as the extrusion process would potentially be higher than a thixotropic paste extruding system that cannot rush the next layer. The head will also require the sock dispensing or knitting system which may start out quite heavy at the prototype stage. Perhaps the first prototype could be manually directed just by hanging from a single suspension point with a tool weight balancing spring. Then one could point and build a bit like the 3Doodler but an a massive scale.

Printing Steps

Think of combining three concepts. First a continuous tube knitting (or woven if it can be made easily) machine that can make an endless stocking. Then think of a regular sausage making machine that extrudes the mince into a tubular casing. Lastly consider that some things built with sandbags have a long life. These days houses can be built with them, they have been used in fortifications, flood and erosion embankments, and consolidation of all sorts of other earth works. When filled with a cementitious mixture (wet or dry) on wet curing stacked bags are almost as strong as a single casting.

Continuous perforated tube

A tube knitting machine is positioned to surround a nozzle say 5cm (2 inches)or more in diameter that supplies a continuous tube to be filled with the build mix.

While most tube kitting machines or open at the top allowing the thread to reach the working edge it is possible to knit around a core if the thread is supplied on a bobbin or the knitting is done in a reciprocating arrangement with 360 degrees plus 1 stitch at a time before direction is reversed, this will result in a technically irrelevant seam where the extra stitch links to the other side of the tube. The industry trend seems to be to have the needle carousel rotate with the production and the thread supply (often more than one for increased speed) to be fixed. All this has been reasonably well engineered in the continuous hose reinforcing knitting machines. They are however intended for stationary use and are not optimised for size.

The tube does not have to be perforated for it to be used in a building application but this will add a lot of layer to layer strength if a cementitions filling is used. With a sealed sand sock some form of framing will almost certainly be required to maintain stability on higher walls.

The tube also does not need to be knitted as a tube on the spot. It could be supplied in a flat section and folded around the nozzle and sealed into a tube by stitching as needed. If it is a plastic type of material it could be heat or ultrasonically welded. It could even be formed with a solvent or RF cure adhesive or even a pressure sensitive sealing tape. The ultimate strength of the structure will not rest exclusively of the tube seam if a cementitious mix is used and if the seam is manipulated to position it under the current extruded sock it would likely provide almost the same benefits. In the same way the sock does not have to be a knitted or woven material, it could be some other perforated material that would support the mix while it settles and sets without allowing it to burst open during the build.

Finally it is quite possible that a batch process for the sock could be implemented in production (preferred for prototyping) as it may be reasonably simple to preload a nozzle with the correct length of tubing for the specific number of repeats on a specific section of wall. It would start with a closed end so take-up is guaranteed. The system could alternate between a pair of nozzles like on a microscope turret with one extruding and the other being loaded with flat coiled tubular sock. With 4 alternating nozzles on the turret it should be possible to have them in two different sizes. This would give a number of wall thickness combinations with thick and thin socks with various cavity spacing.

Extrusion of mix

The build head would either have a slurry or paste extruded like a sausage or a dry mix blown into the sock like stuffing is inserted into a teddy bear or pillow. The slurry could be plain sand or earth or a weak mix with cement. If the mix is dry then it would make sense to wet the casing and the receiving surface as it is extruded.

Pumping of a slurry with a concrete pumps is common though they have a significant pulsing flow profile. Peristaltic and other pump types are also an option. The pulsing flow profile can be modelled and the head movement can be synchronised with it or the pump drive speed can be adjusted for a synchronised flow rate. Dry mix blowing is an option but might be harder to work with materials found on site as they would need to be dried first. Also any fines (clay) would cause a lot of dust during the build process. An alternative is to use a dryish mix (just enough water to activate all the cement) that is extruded with the help of gravity and an auger or even a ram.

The mix could even be a expanding construction foam in some parts of the building like the eves where load bearing is not required but still constrained by the sock to form even layers placed and adhered as required.

Sand Bag building

Most sand bag projects use discreet bags that are of a size that is convenient to carry. Some programs have adopted longer bags that are filled in position.

When the sock has been laid down (with water spray to activate the cement) it can be tamped down slightly if that seems to be of assistance. However if building progresses continuously the layers above will help to compact the layers below as more mass rests above them. The top layers could be tamped down manually at the end of the build.

It would make sense to make window and door openings higher than required in the early days until the settling characteristics have been established. The top gap can be closed with foam or plaster after the walls have set.


23 December 2013

The reinforcing that will be available from the embedded polymer fibres could be significant. The surface of the structure can be allowed to erode and decompose with time if it is not plastered over but this will not reduce the internal strength from the embedded sock that is between the layers and protected from the elements. With a suitable material the amount of mix that oozes out could almost encapsulate the exposed sides and would certainly make a very good key for a plaster coat.

Additional fibres that are linear (as opposed to knitted) could be introduced with (outside) or inside the sock that will provide primarily longitudinal reinforcing and could be supplied at the heights of the top and bottom of openings (windows and doors) an at the top and bottom of the wall to tie it in. They would provide the same function as 'brick-force' ladder wite that is inserted between every 5th (4th, 6th) course of brick work. They are not as important because the extruded bag will be continuous already but can be put in to consolidate the edges of the walls.

Further if the fibres are made of metal they would be very strong. Also if the fibres are placed in such a way that the ends are exposed at the corners and openings they could be made of memory wire and electrically heated after the structure has set to tension them and make the wall super strong.


  • Easy to test with discreet lengths of sock knitted on the side
  • Can build faster than any other system that needs to gain setting strength before next layer
  • Can be designed to add tensile strength with choice of fibres
  • Wet or dry application possible
  • Can be used with low cement mixes
  • Can be used with most pasty materials that will ooze through the sock a bit
  • Sock can be made from cheap organic fibre or long life geo-textile fibre for exposed surfaces
  • Can jump over gaps by pausing extrusion with empty sock
  • Can bridge over gaps by maintaining sock tension (catenary span)
  • A double skin wall can have staples easily inserted between skins
  • Sock diameter could be potentially adjusted on the fly


  • A second consumable needs to be supplied
  • Full strength only after setting period
  • Extrusion head has added complexity
  • Bridging would be best done with a prop placed across the gap on the layer below


  • Investigate suitable knits
  • Investigate suitable mixes
  • Compare pumped slurry and blown dust transport
  • Test out the new LRC licensing model, this idea is free for private use, available for commercial licensing
  • Try and find out if prior art exists

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