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Kingston Hot Press: Process Improvements
Check out the project on wasteforlife.org Hot Press Discussion Page
Currently, I am investigating the potential to design a plastic/paper composite extruder which would facilitate the production of feedstock for the Kingston Hot Press.
- 1 Introduction
- 2 Problem Definition and Scope
- 3 Constraints
- 4 Prior Art
- 5 Theory and Methodology
- 6 Final Design and Analysis
- 7 Cost Analysis
- 8 Conclusions and Recommendations
- 9 Future Work
- 10 References
The organization Waste for Life (WFL) defines itself as "a loosely joined network of scientists, engineers, educators, architects, artists, designers, and cooperatives who work together to develop poverty-reducing solutions to specific ecological problems." Through a collaboration with researchers and community members at Queen's University, the Centro Experimental de la Produccion (CEP) in Argentina, the Rhode Island School of Design (RISD), Smith College, and the University of Western Australia, the Kingston Hot Press has been designed and developed to provide the means of production to smaller cooperatives in communities in Argentina and Lesotho. The Hot Press allows the user to produce a value-added composite tile out of waste plastic and fiber (most commonly cardboard and paper). Currently three prototypes have been built, one at Queen's, one at RISD, and one at CEP. Detailed design drawings are available at the WFL website.
Problem Definition and Scope
The WFL team has identified several key areas of design development that they would like to pursue :
- Dimensions - The current Kingston Hot Press can produce a 24"x24"x1/4" sheet. A wider gap in the press could allow more width and perhaps allow for 3D molds. The size constraints could be circumvented if pieces could be produced in modules and then connected post-production.
- Heating Sources - Currently the plate heaters require electricity. CEP has expressed interest in a gas powered system.
- Temperature Consistency - Initial tests had shown inconsistencies between the temperature distribution on the top and bottom plates. To remedy this, researchers at Queen's replaced the 1/4" steel heating plates with 1/2" aluminum plates.
- Opening between top and bottom plates - Currently molds are slid between the heating plates. A "clam shell" lid design could allow for a more diverse range of geometries and facilitate the use of 3D molds.
- Safety and Environmental Issues - Fire safety, emissions, pollutants.
- Production Speed - The current system requires a time intensive production process to arrive at a single 1/4" composite tile. CEP has expressed interest in finding ways to improve throughput.
Currently the temperature consistency problem has been resolved with very expensive aluminum plates. A heat transfer model could assist in the evaluation of different solutions and hopefully provide an optimal solution that could use less costly and more widely available materials.
There are several key stakeholders that I have identified for this project. Here at Queen's I am working to assist Dr. Matovic with design improvements based on the Hot Press prototype in Kingston. More broadly, I hope my work can contribute to the Waste For Life team. Finally the end-user of the hot press is the cartoneros, the workers who partake in the informal economy of waste in Argentina.
I would like to develop a useful heat transfer model which could assist the WFL team in reducing the cost of the Hot Press. I would also like to take this opportunity to expand the reach of the WFL team and share their innovative design. Finally, I would like to provide a clear pictographic instruction manual for users of the Hot Press.
The hot press should provide a low-cost tool to access the means of production and add value to "waste" products. As such, materials should be as economical and accessible to the carteneros communities as possible.
Design work must be limited to theoretical analysis as significant empirical testing would require equipment not currently available for the budget and scope of the project. Future work could include an empirical evaluation of several plate designs using an array of thermal transducers (thermistors, or thermocouples) to determine realized temperature gradients in the hot press.
At Queen's University Dr. Matovic has produced CAD drawings fully detailing the design and dimensions of the Kingston Hot Press.
The original design used 1/4" steel plates to press and heat the tiles. To overcome temperature inconsistencies which were producing burnt profiles the prototype plates were replaced with 1/2" aluminum plates.
Theory and Methodology
Although the design of a hot press plate must ideally involve both a heat transfer model and a finite element static stress analysis it will be assumed for the following optimization that based on previous work by the WFL team, the 1/4" steel plate provides a minimum benchmark thickness for the plate and a lower limit for allowable bending and deformation of the plate. Therefore the following analysis will focus on better understanding the heat transfer mechanisms in the heat press, and how these effect the performance of the press.
(see also the Wikipedia article)
The Kingston Hot Press presents a particularly challenging heat transfer system to model. Six 750W Omega OT-2107 strip heaters are clamped to the two "press" plates which transfer heat and pressure to the mold and tile composite material. A cork rubber gasket provides insulation between the press plates and the steel weldments which provide the necessary structural support for the device. A simple on/off controller regulates the temperature of the center of the plate with a thermocouple transducer. Standard setpoints range between 150°C and 250°C. Since a tile (or film) can be pressed within 5-35 minutes depending on the thickness of the mold, the problem almost certainly falls within the transient time period. However, the plates are preheated to the setpoint temperature, and so a steady state model can assist in determining the ideal performance of the heating plates upon the initiation of the press cycle.
A preliminary 2D model of the plate was developed to allow for a general understanding of the behaviour of the plate which could be made accessible for the community through open office. Using a 2D finite element control volume approach, a rough estimate of the temperature distribution can be arrived at in a simple spreadsheet. Using each cell as a finite control volume the energy balance equation can be used to derive an equation for the control volume (spreadsheet cell) temperature.
The energy balance equation for a 2D control volume is given by:
Where q is the energy leaving (or entering) each boundary of the control volume, denoted by n=north, e=east, s=south, and w=west. Note this assumes no generation or storage within the control volume.
This simplifies in a medium surrounded by a material with identical conductivity to the average of the neighbouring cell temperatures:
Assuming heat loss through the edges due to convective heat transfer we can derive the temperatures for edge cells and corner cells:
Edge Cells (example N edge)-
Corner Cells (example NW corner)-
These equations when put into Excel or Calc can be solved iteratively (See this help for Excel). The screenshot below shows the three strip heaters (approximated as constant temperature surfaces) surrounded by the aluminum plate and the resulting temperature distribution. Although this is a rudimentary model, it provides an immediate sense of the geometry and gradient in a way that can be easily distributed and modified through OpenOffice.
The file can be downloaded in Open Office format Media:2d_Plate_model_3_strips.ods or Excel Media:2d Plate_model_3_strips.xls. Another model was developed for four square heaters which could be used to investigate the design of a modified hot press that used stove element heaters instead of the more specialized Omega heaters and can be found here Media:2d Plate_model_3_strips_square.xls.
If the top of a plate is unevenly heated, with finite thickness and conductivity the bottom of the plate will not have a perfectly even temperature distribution. In practice, an effectively trivial temperature gradient can be arrived at with a plate of high enough conductivity and thick enough profile to provide adequate opportunity for the heat to take the path of least resistance.
By modeling the temperature distribution on the press plate through SolidWorks COSMOSWorks heat transfer tool an optimization of the plate material and thickness can be undertaken. SolidWorks was convenient as previous CAD work had already been done for the Kingston Hot Press in this software and was readily available to undergraduates at Queen's. Unfortunately, this software is not readily available to all interested readers and therefore I must leave it to future work to transcribe the CAD to an open source program.
After a brief review of the fascinating area of visual communication and instruction  I hope to produce a simple pictorial instruction for using the Kingston Hot Press. Pictures are not ideal for printing and distributing and therefore further work could also be done to produce a schematic visual instruction set that would be sufficient to overcome language barriers. I find this area very interesting and would love to hear from knowledgeable readers who could direct me to further resources.
Final Design and Analysis
|0||Turn on the Kingston Hot Press to preheat. This will save you time later.|
|1||Place down the thin steel sheet.|
|2||Place the release surface onto the thin steel sheet.|
|3||Place several plastic bags on the release surface.|
|4||Place sheets of paper on the plastic bags.|
|5||Place several more plastic bags on top of the paper layer.|
|6||Place another release surface onto the plastic bags.|
|7||Place another thin steel sheet over the release surface.|
|8||Take the thin film mold and slide it between the two press plates of the Kingston Hot Press.|
|9||Using a lever pump the hydraulic press to bring the press plates together. Stop once the pressure gauge indicates an acceptable pressure (Approximately 3000psi).|
|10||Wait approximately 15-30 minutes.|
|11||Very carefully remove the mold, but remember the plates are hot! Peeling off the release you will see the final product (first image is a thin film, second a tile).|
|12||If you are making a film, it is now time to shred the material to make feedstock for the final tile. You can use scissors, or if available a shredder.|
|13||Repeat the previous steps but filling a metal mold as shown on the right instead of making layers with raw bags and paper.|
|14||Finally you have a tile!|
Conclusions and Recommendations
- Logan, R.S. (1982) Instructional Systems Development:An International View of Theory and Practice. Academic Press, New York.