Abstract

Below is a modified abstract submitted for the Mech 461 presentation. Page content will be updated over the coming weeks.

Rapid prototypers allow for quick and accurate fabrication of products or scale models and are a useful production and design tool. Recently an open source model, the RepRap, has been developed which can be built for under $1000, greatly expanding the potential user base of rapid prototypers. It could feasibly be used as a small-scale manufacturing or sustainable development tool. The RepRap's plastic feedstock is one area where cost can still be reduced. A device was created by Web4Deb which turns waste plastic into a growth medium for plants. His device has been modified to create feedstock for the RepRap printer. A description and analysis of the design is presented including component properties, testing procedures and extrusion results. The success of this device would further enhance RepRap affordability by reducing operating costs. Filament production could also offer an alternative income source. In addition, it enables in-home plastic recycling with a usable byproduct. This has implications in the field of waste management as in-home recycling could avoid the greenhouse gas emissions and economic costs associated with municipal recycling programs by bypassing waste collection and transportation.


Project Summary

As part of the Queen's University Mechanical Engineering course, Mech 461, I will be working on a device to convert household plastic waste into usable material to be fed into small scale rapid prototyping machines, such as the RepRap, RapMan and Fab@home machines. Below is the official description for this project as advertised on the Mech 461 website. [1].

Introduction

Traditionally, 3-D printing has been used for rapid prototyping, where good tolerances, durability and fast print times dominate the user requirements. For this reason, commercial rapid prototyping machines are used in many industries to make custom parts for design-stage products and are able to perform operations such as printing a working ball bearing using overhangs and two material deposition methods. Recently, the development of open-source rapid prototypers, such as the RepRap, have made rapid prototyping inexpensive enough to be accessible to home users and potentially useful for open source appropriate technology (OSAT). Commercial printers excel at rapidly producing high-tolerance representations of complex parts; however, they are far more expensive ($5000-$200,000) than the ~$1,000 RepRap. Also as proprietary rapid prototypers generally have proprietary feedstocks, they are also extremely expensive ($1/in3 to $4/in3), while ABS plastic, often used for the RepRap (www.reprap.org), is strikingly less expensive at $0.032/in3. The goal of this project is to push open source rapid prototypers even further – to use waste plastic as a feedstock rather than preformed ABS filament.

Scientific and Engineering Background

It is has been recently proposed that open source 3D printers could be used to drive sustainable development [2] . For this to become realistic it is critical that feed stocks be developed from locally-available materials in order to prevent the erosion of cost advantages for local production. Using locally-available materials for fabricating OSAT not only ensures the community in question will be less dependent on foreign assistance if there are problems with the technology, but it also creates a sense of empowerment as technology is not handed out as a form of charity furthering dependence on foreign aid. This can be accomplished through the use of feedstocks created from waste products (e.g. plastic bags or bottles) or through the use of available local materials such as bio-polymers. The sheer abundance of plastics in household waste (including bags, bottles, food and entertainment packaging) is a reality in most non-rural communities, but this waste can be reused. A process could be used to create a waste-plastic filament for use in the RepRap. Thus a plastic extruder, which could heat the plastics and extrude them as a filament that can be used by the 3-D printer, is necessary. It should be noted that producing filament feedstock is a challenge as the diameter has to be precise and the filament must be very round (not oval) or the extruder will produce poor quality parts or jam as has been often encountered when RapMan owners buy replacement filament locally. A hopper-designed extruder may not be as dependent on the size properties of the material and also reliable advances in using pellet feed stock combined with a pelletizer is another option that can be explored.

Research Objective

The objective of this research project is to design, build and test an extruder for the RepRap that can take polymer waste as a feedstock.


Literature Review

Please follow this link to access the full Waste Plastic Extruder: Literature Review. The review explores a range of literature on the subjects of open source rapid prototyping technology, waste plastic handling and recycling, material properties and extrusion technology, among others.


Design

A description of the design, including detailed assembly instructions can be found below.

Extruder

The design for the waste plastic extruder is heavily influenced by an extruder developed by "Web4Deb" (online username) which extrudes HDPE for use as a growth medium in aquaponics. Details of this design can be found at the Web4Deb's blog and on the device's RepRap wiki page. It was decided this this design would be used as a base and modified to produce 3mm filament for use with the RepRap or RapMan 3D printer.

The extruder consists of 3/4" (inner diameter) piping divided into three sections; a gearing section, a hopper section and a heating section. Ground plastic is fed into the hopper and transported to the heating section by a 3/4 - 17" ship bore wood auger, inserted through the piping and turned using a windshield wiper motor sourced from a local auto wrecker. The auger is driven from the motor using a sprocket and chain drive system with a 2:1 gear ratio to decrease speed and increase torque. The gearing section of the extruder body houses the auger and also provides support for a thrust bearing used to counter axial force encountered during transportation of the ground plastic. Plastic is pushed into the heating section where it is gradually melted and forced through a die for the production of 3mm plastic filament.

Main Body Assembly

Heating Section

The heating zone is constructed using 14 gauge nichrome wire secured in place with heat resistant Kapton tape. Furnace cement was applied between the piping and the nichrome because uninsulated wire was used, resulting in short circuiting through the pipe at high temperatures. This step can be avoided in future by using insulated nichrome wire. Insulated wire is available at higher gauges (smaller diameter) but I was unable to find any insulated wire below 18 gauge. The furnace cement proved to be a good solution as it prevented short circuiting, but still allowed heat to be transferred to the extruder barrel. Temperature testing with a contact thermocouple found interior temperatures generally lagged behind the temperature of the nichrome wire by about 30oC (i.e. when Nichrom was at 215oC, the interior temperature was found to be 185oC). Kapton tape would also have prevented short circuiting through the barrel, however Kapton tape is only rated to 250oC. This would pose a problem when extrudring polymers such as ABS, which would require a heating section temperature of atleast 260oC.

Fourteen gauge wire was chosen in an effort to create the heating zone with minimal power requirements. At present writing, Nichrome wire temperatures of 225oC have been achieved with 75W of power (15V, 5A). Temperature can be adjusted by changing the current passing through the Nichrome wire. Increasing the current increases the wire temperature. Currently the section has only one uniform heating zone, however, provisions were made for two heating zones should more gradual heating of the plastic be found to be beneficial. Many commercial processes use gradual heating to ensure material is evenly heated [3]. This website has specifications on the resistance of each gauge of wire and the current required to heat each gauge to various temperature thresholds. [4] Data from this site was used to make initial design calculations.

Presently, the heating section is uninsulated however adding insulation would likely improve performance. Heat up time when the machine is first started could be reduced and higher temperatures could be achieved in the extruder barrel. Fibreglass/kevlar wraps could be used as insulation, as could household fibreglass batting.

Parts Lists

Several further modifications and custom parts are included in the design. A shaft extention was added to the windshield wiper motor to increase the shaft diameter and provide adequate space for mounting of the sprocket. A collar was created to fit between the shaft and gearing section hold the auger in place and allow for the mounting of a thrust bearing. Each of these custom parts is included in the drawings below.

The extruder was constructed with a combination of custom designed and stock materials. Stock materials are summarized in the list below, along with links to their supplier.

Part
Quantity
Example Supplier
3/4”x17” Ship bore auger
1
Irwin ]
Nichrome wire, 14 ga.
1-1/4 lb roll
McMaster-Carr ]
Sprockets - Part#: H40B12x1/2 and H40B24
2
Ringball Corporation ]
Chain - Part#: 40-1R
2 ft
Ringball Corporation ]
Needle thrust bearing and washers - Part #: NTA815 and TRA815
1 bearing, 2 washers
Koyo ]
3/4” piping
min. 16”
Order online ]
1/2” sheet metal
1 sheet, 1ft x 1ft
Metals Depot ]
Furnace cement
100 mL
Bomix Pyromix from BMR ]
Alligator clips
10
The Source ]
Corner brackets
10
Local hardware
Kapton tape
1 – 3/8” x 36 yard roll
Uline ]
Threaded rods
2
Order online ]
Windshield wiper motor
1
Used on Ebay ]
Office shredder
1
Staples ]
Arduino Uno
1
Arduino from RobotShop ]
Arduino compatible USB cable
1
Phidgets, RobotShop]
Plywood
variable
Local hardware
Wood
variable
Local hardware
Fasteners
variable
Local hardware
Copper wire
2 ft
McMaster Carr ]


Custom parts were fabricated in the Queen's University, Department of Mechanical and Materials Machine Shop. Special thanks to Mr. Andy Bryson and his team for their help in fabrication. Most of the parts were manufactured with the use of welding, cutting and drilling equipment. In some cases, a lathe was also required for fabrication. STL files for each of the custom parts can be found below, as well as manufacturing drawings of individual components. The files show the dimensions used for this particular extruder and were chosen based on precedent Web4Deb extruderand to fit other components (i.e. the auger). Optimization of extruder dimensions has not been investigated.

Waste plastic extruder: Files

Plastic Grinding

In order for plastic bottles to be converted into viable feedstock using this method, they first had to be ground into small pieces. A number of methods were investigated for this purpose.

Following the work of students at Delft University, a number of common kitchen appliances were tested for their grinding ability [5]. A food processor, coffee grinder and blender were tested, with the blender proving to be the most efficient. In agreement with the DelftU group's work, it was found that the blender worked best when water was added to keep the machine cool and contain the plastic being ground.

This solution was not efficient enough for grinding in large quantities however, as it required too much time. In addition, the ground up plastic had to be dried before it could be used in the extruder. A more efficient method was found by using a Staples(R) brand office shredder which was designed for the shredding of credit cards and compact discs. This proved to be much more time and energy efficient and avoided the use of water. A used shredder was found at a local Value Village for $24.99.

Testing has shown that this method is sufficient. Some problems were encountered with thicker plastic bottles, as the shredder was not able to fully cut them into small pieces. The cutting edge of the auger was still able to handle most of these specimens after they were passed through the shredder. In order to ensure the machine would run smoothly and extrude at a constant rate, scissors were used to cut the largest pieces into smaller bits before they were placed in the hopper.

The full plastic preparation method was as follows:

  • washing of plastic bottles
  • removal of labels and lids
  • cutting into manageable pieces for the shredder
  • shredding

Handle and lids were not used as they couldn't be fed through the shredder. The cutting and shredding steps were alternated to ensure that the shredding machine did not overheat as occurred under continuous shredding over a 15 minute period. Intermittent shredding over the course of an hour did not cause any problems with the shredding machine.

Future work should focus on a shredding device which will not require any cutting of the bottles and which can produce smaller plastic chips to be fed to the hopper.

Testing

Testing of extrusion and development of a working 3mm filament feedstock is ongoing. Updates to come.

Future Work

Grinding Device

  • creation of a low cost, domestic scale grinding device.
  • should grind bottles into small pieces (area < 1 cm2)
  • no pre-cutting required
  • can accept handles, lids, etc.


See Also

References

  1. Pearce, J. Project Description: Design and Testing of a Waste Plastic Extruder for Open Source Rapid Prototyper. Mech 461. [1]
  2. J. M Pearce, C. Morris Blair, K. J. Laciak, R. Andrews, A. Nosrat and I. Zelenika-Zovko, “3-D Printing of Open Source Appropriate Technologies for Self-Directed Sustainable Development”, Journal of Sustainable Development 3(4), pp. 17-29 (2010). | http://www.ccsenet.org/journal/index.php/jsd/article/view/6984
  3. Rosato, Dominick (1997). Plastics Processing Data Handbook (2nd Edition).. Springer - Verlag. (Online)
  4. http://www.wiretron.com/design.html
  5. . Braanker, G.B., Duwel, J.E.P., Flohil, J.J., & Tokaya, G.E. (2010), "Developing a plastics recycling add-on for RepRap 3D Printer". (Online) Available: http://reprapdelft.files.wordpress.com/2010/04/reprap-granule-extruder-tudelft1.pdf (June 30, 2010).
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