History

In 2003 UNESCO and Daimler Chrysler launched the “Mondialogo” partnership. Their aim was to promote develop intercultural learning and to achieve sustainable development. One element of this is the Mondialogo Engineering Award, which invites young engineers to from industrialised and developing countries team up to work together over the course of one year to produce proposals for engineering applications that demonstrate excellence in applied engineering, help develop the technological infrastructures of developing countries and contribute to the Millennium Development Goals of eradication poverty and achieving sustainable development.

In 2004 more than 1,700 young engineers from 79 countries participated, forming 412 international teams. Twenty-one of the best proposals, representing 28 countries received an award. This website describes the concept developed by one of the twenty one winning entries and discusses how the idea has been developed since 2004 and how it might be implemented.


Inspiration

The inspiration for the project was the difficulty in disposing of waste plastic present in many developing countries. The team had connections with St John's Community Centre in Nairobi and so we teamed up with St John's to come up with ideas.

Concept

The concept that we developed was to design a low-tech way of making something useful out of the waste plastic. After discussing what sorts of products could be made, we decided to start by making flower pots.

The choice of flower pots was based on 2 key factors:
  • The waste plastics used could be contaminated by chemicals and so be unsuitable for anything relating to food or drink such as plates, bowls or cups.
  • The shape and size of a flower pot would be a simple starting point for concept development.

Design Development

The first prototype mould was a pair of food tins of different sizes that would fit inside each other. We packed the larger tin with plastic bags before pushing the smaller tin in on top. The tins and bags were then placed in a domestic oven set to 250⁰C for 60 minutes. After this time a downward force was applied by a single person pushing downwards on a wooden board placed on top of the mould whilst it sat on a concrete floor before quenching in water for 5 minutes.

The first problem identified with this method was removing the pot from the mould. The walls of the pot were around 8mm thick, and this produced a surprisingly rigid product, and this property combined with the ribs on the sides of the tins meant that the only method for removing the pot was to cut the mould away from around it, this was obviously not ideal.

It was decided that the walls of the pot need not be as much as 8mm thick as the rigidity associated with this was thought to be impeding the pot’s removal from the mould, and was consuming an unnecessarily large amount of polyethylene. A thickness of 5mm was suggested and agreed upon for the next prototype. It was also agreed that as with conventional plant pots the product should have tapered edges to assist the pots removal from the mould.

Several further prototype moulds were produced using a machined block of aluminium in place of the food tins. Several challenges were overcome through this development such as the difficulty in applying sufficient force to compress the plastic. This was solved by using a conventional car jack, initially by compressing the mould under a landrover, but eventually compressing it within a purpose-built steel frame.

Another challenge was how to prevent the plastic from sticking to the mould. We trialled a number of products and soon found the best to be a silicone-based spray used in the plastics industry for precisely this purpose. We were keen, however, to avoid using a specialist product as it was not in keeping with the concept of appropriate technology. With this in mind we tested a silicone-based car polish and found that, although this smelt mildly unpleasant, it works almost as well as the spray.

Finished Mould

The finished mould looks quite different to the food tins that we started with and can produce pots much more quickly and efficiently than the earlier prototypes.
The male and female halves of the mould are mounted in a simple steel frame with a car jack attached to push the 2 halves together and to pull them apart.


The aluminium mould has a number of simple electric heating elements fitting into it along with an electronic temperature sensor. These are connected up to a standard control circuit which heats the mould to a specified temperature before turning the heating elements off. Once the temperature of the mould drops by a few degrees, the elements turn back on to maintain a steady temperature. The plastic can be heated from room temperature to a temperature at which it can be fully compressed in around 15 to 20 minutes.


The mould also has a pipe running through it which can water pumped through it to cool the mould quickly. This is connected to a standard aquarium pump bought for a few pounds from a pet shop. This arrangement keeps the water isolated from the electrical components and cools the mould enough to open it in less than 5 minutes.
A number of small holes around the top of the mould allow excess plastic to escape as well as a means of removing the pot from the mould once cooled.

 

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