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To finish that series I’d like to share some insights on the postproduction of the materials. First and foremost we try to reduce it to a minimum as despite a very pedantic extraction we produce micro plastic particles (more on that to follow soon). Ideas on that so far is to use clean tools and a separate extraction for working with plastic in order to remelt the chips.
PP and HDPE work well with almost any tool and edges can be easily cleaned by shaving them with a knife. PS however is a bit more demanding. A good result was to use lowest speeds possible and cooling where possible, e.g. with a water cooled metal chop saw or spiral milling bits with compressed air cooling. In general metal tools are the right choice here, e.g. spiral drill bit instead of forstner bit.
In general I found it handy to heat up the surface to smoothen it and to get rid of discolorations from the mold.
maybe my favorite so far and a true advantage of plastic! I extruded a very thin round profile (ø10mm) from PP to get some proper dowels. A simple pressure fit didn’t maintain at all, so heat needed to be involved. After shrinkage, the dowels got a diameter of 9.5mm, I cut them into pieces and chamfered one side with a pencil sharpener. Then I put them into a drill at highest speed and pressed it with the chamfered tip into a 7.5mm hole. Friction will melt and bond the materials together, you can even see a mixed color ring around the dowel. Make sure to let the material cool down before removing the drill from the dowel to ensureproper bonding! Then just trim of the excess (and make sure to recycle it!) and give it a smooth finish with a sharp chisel or knife.
In the second example I combined it with a mortise joint, the last example uses the same technique to join a beam and a sheet, both incredibly strong. Both for the sake of a circular economy and a better bonding make sure to use the same resin throughout all pieces!
mortise and tenon
third example is the only one made from PS and it’s another level of difficulty. The material itself is pretty strong and useful for structural parts of furniture, but machining is a hell of lot more difficult as it melts fast and sticks to the blades and bits. In terms of the joint it’s crucial to give it enough tolerance as there is no such thing as flexibility with PS. A good way is to avoid hammering the pieces together, it will result instantly in cracks. The heat gun did a good job in bonding together! In terms of strength, the torque is insane!
As mentioned before by @michael95 we dug a bit deeper into the topic of joinery the last weeks at PPHQ. @s2019, strength testing is definitely on our agenda to generate a reliable database for future projects. Up until now, we focused on an experimental approach, as well because I’m not specifically proficient in scientific testing. So in case you have any suggestions for setup and method let me know and I’ll conduct some tests on our prototypes.
Anyways I’d like to share some of the approaches that seem promising to you, see photos attached from left to right:
works pretty well with the HDPE sheet used here, as it provides enough flexibility to get a tight pressure fit without cracking. It’s pretty important to make a small notch on the tenon’s side to prevent it from slipping through the hole and relieving the pressure. Besides that a straight forward process with jigsaw and chisel. The esthetic advantages are pretty obvious, but as well in terms of strength it’s convincing (again, quantitive data is about to follow).