Plastic Fantastic


In October 2020 Heatons’ Men in Sheds were approached by Stockport Plastic Shed and asked if we might be able to build them a plastic shredder.

At the time, HMIS were not meeting at the workshop due to Covid restrictions so we, as a group would not be able to build the shredder as intended, but as I had all the tools needed in my workshop at home and had time on my hands I was up for the challenge. So with agreement of the HMIS committee, I decided to take on the project

Stockport Plastic Shed are an organisation that works with the local community to bring people together with the main aim of recycling plastic waste into useful items. During the first video meeting we had with the folks at the Plastic Shed they showed us the website for ‘Precious Plastic’, they specialised in providing advice, parts and machines for reprocessing plastic waste.  Some of the shredder machines shown on the site were fairly basic and did not appear too complex to construct. The site also advertised various companies that could supply the Cutter Box, which is one of the more complex parts of the shredder and could be supplied in either fully assembled or kit form.

The build started in the first week of November with the cutter box, made up of parts ordered from Czechoslovakia. The lead time on the cutter box kit was 3 weeks, so this gave me time to design and build the table on which to mount the shredder and look at how I was going to build a frame to hold the cutter and procure the drive motor.

After investigating various options for the drive motor, I concluded that a motor from a 2-ton hoist would be suitable and with its gearbox meets all the shredders requirements. I finally managed to track down a hoist of the correct capacity this was duly purchased.

The cutter box I ordered a couple of weeks previously arrived in a cardboard box containing around 70 metal parts plus lots of nuts & bolts and with no assembly instructions. Fortunately, there were general instructions on the Precious Plastic Web site and a couple of YouTube videos.

After a few of hours playing with the metal jigsaw puzzle, I managed to produce something that resembled a cutter box, complete with 14 nasty looking rotating stainless steel cutter blades.

The next issue that presented itself was how do you connect together a cutter with a motor/gearbox that can output 400 ft/lbs of torque that are not designed to be connected together.  I had looked on the Precious Plastic website for ideas and saw plenty of comments from people who had struggled with this coupling but no clear way to achieve a robust connection.  Well, this is where I got lucky, when I stripped down the hoist, I found the motor/gearbox output was a 27mm AF hexagon shaft which is perfect for a 27mm x ½” square drive socket. Now all I needed was to get the hardened 20mm ‘round’ drive shaft on the cutter box to take a hexagon socket. 

Well, I won’t go into the finer detail but the process required the use of an engineers vice, 4” angle grinder, a 3D printed 3mm thick ‘U’ shaped plastic shim, 2x combination squares, a spirit level and I must not forget the hammer for the fine adjustment when levelling the shaft in the vice. After about 20 minutes of very careful grinding and shaft rotating I ended up with a perfectly symmetrical 17mm AF hexagon cutter drive shaft that would take a 17mm x ½” drive socket. Using the two sockets, each with the ½” drive, it was now a simple matter of using a short length of ½” square hardened key steel to link the sockets together and complete the drive coupling.

One of the advantages of using the short length of ½” key steel in the coupling is that it provides the weakest link in the drive chain. Therefore, should the cutter ever get overloaded to a point where the extreme power that the motor can supply could damage major/expensive components in the shredder assembly, including motor & gearbox itself, then the 1/2” key steel link will sheer taking any load from the motor & gearbox. The short length of ½” key steel has an overall cost of about £1.50 each and takes less than 10 minutes to replace, 9 of these sacrificial links have been supplied with the shredder in the maintenance kit.

The rest of the mechanical build was pretty straightforward. The main frame holding the cutter and the motor was fabricated using lengths 30mm square section tubing and some 5mm thick sheet steel bar. The frame has welded joints and forms a robust support for the drive motor and cutter that will not twist under the extreme torque force.

The hopper on top of the cutter box was made using 20mm angle bar with clear acrylic side panels. The hopper has a hinged lid guard that will allow the plastic for shredding to be dropped into the cutter but small enough that it will not allow the operator to be able to reach down with their hand into the cutter area.

As the shredder could be used by inexperienced operators, personal safety was a critical consideration. So, to achieve additional safety, the hinged lid is electrically interlocked such that, if lifted, will instantly cut power and stop drive to the cutting blades. The rotating drive shafts were also covered with guards.

Up to this point of the build the shredder had been operated using the push button controls that were supplied with the hoist but for the shredder to be used efficiently, and incorporate the lid safety interlock feature, an electrical control box was needed.

Again, there is no ‘off the shelf’ control box that could be used so the control box needed was designed from scratch. Fortunately, all the parts required to build the control box are readily available. 

Following the control box design, all the parts were procured which included: 240v Digital Current Limiter, 12v DC Power Supply, 2x 12v Relays, 2x 240v 60amp contactors and various wiring, connectors, lamps and switches.

After quite a few hours of assembly and test the control box was finally wired into the shredder mechanism.

The shredder has been thoroughly tested and has successfully shredded a large number of different plastic items. The Plastic Shredder was passed over to the Plastic Shed at the beginning of March 2021 and is currently installed in their Stockport workshop, situated in Merseyway Shopping Centre.

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