Precious Plastics Brick
The Precious Plastic Brick is an open source product developed by the v4 team in Endhoven, to use as a tool in construction. The brick as designed by the Precious Plastic's team is injection molded, using the PP team's injection molding devices, though it is likely that it can be constructed using other injection molding and compression molding tool sets. It is intended and designed to use recycled, rather than virgin plastic. It's thick walls and hollow internal structure mean that it should be quite robust, even where contamination exists, but also relatively light, and easy to handle.
A collection of example photos to demonstrate how the brick appears, and how it is intended to be used.
Specifications listed by Precious Plastics for the 3x1 brick.
Weight; approximately 1.5kg
Length; approximately 29cm
Height (with teeth); approximately 19.5cm
Height (without teeth); approximately 16.5cm
Width; approximately 10cm
Note that with low shrinkage materials, the molds, which are designed in multiples of 100x100mm produce bricks that are effectively the same size as the mold. Production of bricks with relevant materials was confirmed as possible by experiment with a brick mould.
Potential alternative production methods
Using compression molding may be an alternative where injection molders are not available. Heating the mold, with plastic inside it, in an oven, then compressing it, may produce a good enough result. A similar project was documented on Instructables: www.instructables.com/Plastic-Brick-Compressor/ Alternatively, extrusion and compression could be used as was documented in this video:  Another example of a DIY press for plastics: 
The v4 development of Precious Plastic intended to produce business ready tools and products for recyclers, with the expectation that they would be able to work in the gaps in municipal recycling systems. As such, effort was made to develop products that could be made from plastics that often do not make it into these systems, that can easily be collected. The team of Precious Plastics contributors from Recycle Rebuild worked to develop the brick, initially starting out with a simple, solid brick, then a hollow brick with diagonal walls to reduce material costs and ease of manufacture, arriving finally at a brick with straight external walls, and diagonal internal walls, in what they described as a compromise between utility, easy of manufacturing and construction. Some of the development of this brick is discussed at the davehakkens.nl forum archive as well as the video further down the page on the right. The final development of the product does not appear to be fully documented.
Potential Further development
The potential to utilise these bricks in a variety of materials, with different applications within a building, while maintaining their dimensions and interoperability could be helpful in a wide array of different construction applications. Insulating concrete forms (ICFs) are an example where an existing construction tool; Polystyrene bricks used for insulation and concrete formwork, could utilise either the same form, or a compatible one.
Cinder blocks, made from concrete, or fired geopolymer bricks could also be constructed, and utilised as a form of block formwork, in a similar way to ICFs, while allowing different materials in different parts of the structure. The picture on the left shows plastic bricks and cinder blocks being used interchangeably to create a strip foundation for a shed. In this example, the left over plastic bricks could then be used to form the walls of the structure.
Tooling to produce the bricks in an insulated oven, using compression molding may also be viable, and the barrier to entry to produce the molding could simultaneously be decreased by using a straight walled steel jacket, with internal silicone forms to define the teeth and interior recess. Molds for materials such as concrete could be reproduced from the plastic bricks themselves, using them as negatives for other materials.
Dimensions are another key consideration. The Precious Plastics site states that the bricks are approximately 29cm long. Having standardised blocks across multiple applications might have some advantages, as well as units of measurement in multiples of 10cm.
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