Technically Speaking 2: Plastic Bag Dispensers

​About Getting Technical Updates

Part of our mission is to educate people. To help people see the problem and potential of plastic. To help people access the potential of this material we need to give them the tools and the knowledge to be a part of the solution. This update marks the first in a series of on-going technical deep-dives summarising our experiences so that others can learn from them.

Getting Technical Issue 2 : Bag Dispenser Builds

Martin was asked by Green Lancaster if we could create dispensers for plastic carrier bags so that bags could be returned by customers of the Lancaster University's Students Union Central shop and then re-used by other customers. The scheme would involve building a large wooden drop-off point into which customers would deposit bags, and two, smaller, wall-mounted, recycled plastic boxes from which customers could take a recycled bag from. This is the first physical product we've ever made so we discussed this with our Design Co-Director Lillie. Lillie collaborated with designer, Mike Soper*

For more of Mike's amazing work, check his portfolio out here!

The Initial Design

​The idea was to combine flexible 2 mm sheets of LDPE with rigid 5 mm sheets of PP in order to build the proposed design. The thick PP sheets would create solid back panels allowing the piece to be secured to a wall, and solid front panels which would include a recycling logo routed from the material. Flexible LDPE panels would be joined to the PP sheets and would form curved side sections which would feature a semi-circular opening at the base to allow bags to be removed from the dispenser.

Martin made PP and LDPE sheets for the job and I got started on testing out what was possible with the LDPE. I definitely got the easier job here! :D

LDPE Sheet Material Work

LDPE is a great material to work with. 2 mm sheet material has unusual characteristics which make it simultaneously challenging and satisfying to work with. I wanted really accurate cuts to make sure the wrapped sheets were square and had a professional finish. For this reason I wanted to avoid hand cutting if possible. It took me a while but I managed to dial in my lasercutter (a Glowforge Basic) to cut these sheets (150 speed, Full Power, 3 passes in case you wanted to know). 

The downside of lasercutting LDPE is that because it has such a low melting point it tends to melt in the heat of the beam and then flow back into the cut before solidifying. Essentially, the material self-heals to a point. With enough passes sufficient material is vaporised by the beam and you get a sort of dotted line where the cuts are complete and the parts can by popped out of the surrounding material. Melted plastic forms a raised ridge around the edge of each cut but this is soft like candle wax (it even smells like candle wax), and can be scraped away with a razor blade to burnish / clean up the edge.

​Joining LDPE to PP (or anything else for that matter) can be frustrating. The issue is that the LDPE and PP are  Polyolefins (polymers based on Olefins (Alkenes) - hydrocarbons containing double carbon-carbon bonds). This double bond makes them chemically stable and unresponsive to solvent glues e.g. superglue or solvent welds. They also lack porosity and permeability which mean they can't wick moisture away from the contact point. As a result, wood glue which relies on moisture being drawn away to leave the binder behind, never dries. The smooth surfaces of the polymers mean that even epoxy resins and contact adhesives find little to bind to. Chemical bonding of such polymers usually requires treating the surface of the material to activate it first. We will investigate thermal welding at a later date as well as there are some interesting opportunities here. For now, we resorted to a more traditional method: screws!

The laser cutter creates easy to use pilot holes, meaning we don't have to measure or position those and the material is easy to counter sink with a traditional countersink. Black, plasterboard screws might be brittle but they give a nice contrasted finish to the showy colours of the LDPE. They're also strong enough to hold the material in place even when the LDPE is subject to rough and repetitive movement. As an aside, this flexibility suggests that LDPE could be used as a living hinge if required.

One of the amazing things about this material is that whilst it will perform a 180 degree bend with a very small radius (a 2.5 mm thick section gets your a radius of between 12.5 - 16.5 mm), if you score it very gently with a razor blade it will snap with the slightest bend. The snap will follow the score exactly, straight and clean edged for the most part*. Anything not scored will remain attached and be tough to tear away, even if it's just a few mm across. 

*Note: I tried to replicate this last finding last night and had trouble so I'll be investigating why this was an issue - perhaps temperature or depth of the score.

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PP Sheet Material Work

​Whilst I investigated the LDPE, Martin had been working on the Polypropylene top and bottom sections. These were cut from a sheet of 5 mm PP made from black coat hangers from Lancaster University's Don't Ditch It Donate It project which reclaims spare materials from students finishing university and attempts to re-use or recycle them. 

The original design called for a large sheet with a complex shape cut out and a recycling logo engraved from the centre. At this point in time (and with a tight deadline) we were unable to get hold of a suitably sized CNC router to fabricate these parts. Instead we revisited the design and reduced the complexity until it was something we had equipment to fulfil. The revised design included two almost semi-circular sections of PP at the top and bottom. The top section would have a hole through which bags could be added and the both sections would be attached to a laser cut wooden backboard. A laser cut LDPE sheet would be wrapped around the front with an opening at the front for customers to remove bags from. The LDPE would be screwed to the backboard too.

We opted for two layers of PP at the top and the bottom so that we had enough surface to screw into and secure the sections to the backboard. Martin jigsawed the parts from a large sheet and finished them with sand paper. The one thing to note when working with thermoplastics is that any form of friction, be it sanding or cutting, creates heat and heat melts plastic. Jigsaws tend to create large pieces of plastic swarf which need to be trimmed. Reducing the feedrate of saws and sanding discs when working with the material should help with this.

Again, binding two layers of the plastic sheet together proved difficult despite leaving them clamped overnight with solvent weld. I think this kinds of tasks, where we're looking two pieces of the same type of plastic together would benefit from thermal welding as this does not diminish the ability to recycle the product later due to type mixing.

Our first test assembly highlighted a couple more things we needed to adjust. Firstly we had gaps between the LDPE and PP sheet but without some sort of chemical bond we could not find a way around this issue. Secondly, we had a rough finish on the PP top section where the deposit hole had been drilled. To mask this rough edge we fabricated a small wooden washer. Finally, we laser engraved a sign at the top of the piece as a call to action to catch customer's eyes.

We're pretty happy with the final result. It's not the original design we'd hoped for but we learned a lot about what the materials are capable of and how we can work with them.

Follow-up Work

• We're looking for a large format CNC on which to cut the originally proposed design as we'd like to know whether we can engrave a complex shape like the recycling symbol from the 5 mm PP sheets.
  
• We also want to know if the PP sheet can be bent over a 90 degree angle if we create kerf cuts. This is a technique often employed in working with plywood and we think it might.
• Finally, we need to investigate thermal welding of PP to PP and LDPE to LDPE. This would allow us more options in constructing complex multi-material products.