A very good friend of mine helps look after the blow moulding section of a dairy milk processing plant. After I told him about my reprap project he offered to get me some samples of the granular HDPE that they use to make milk bottles.
This is what he brought me.
The two bags on the left have either 60% or 50% regrind added to the mix of virgin resin granules. The regrind comes from the recycling of flashing, which is the excess plastic left over from blow moulding. The three bags on the right are virgin resin granules only.
If you click on the image you can get a much closer look.
I’m told that if too much regrind is added to the mix it can effect the quality of the milk held in the bottles. My best guess is that the repeated melting of the plastic causes a very small fraction to start to break down and thus reacts with the milk which over time spoils it. Although he did make a point of saying you cant tell by looking at a fresh bottle how much regrind it has in it. Its important to remember that there is no way to sort the regrind and so the same bit of plastic may have been heated, extruded to form flashing and then ground again multiple times.
The plastic is extruded at 190 degrees celcius.
Something important worth noting is that although the regrind chips varied in size a lot, none of it is ever larger than the original virgin granules. The virgin granules are all around 4mm in size.
I’m told they order the HDPE granules by the shipping container at a cost of around $1 a kg. This just goes to show the economy of scale. Keep in mind this is food grade HDPE that has strict quality controls and must be safe for use in milk bottles.
All up he brought me 1.6kg of granules and said he can get as much more as I need at no cost as these are sample bags which are used for quality control then discarded.
With this bit of fantastic news my next task is to come up with a way to extrude it into a usable filament.