You know my “bottle butt test”: cutting though the center “south pole” of a plastic bottle where the injection of plastic has been made and where the plastic is thicker. This is not an easy test for any blades. It’s tricky.
Today, my idea was to test the incredibly sharp edge of my new BushcraftUK knife. The zero grind, the O1 steel, the ultra sharp blade and the confortable handle of that little heavy fixed blade are amazing. For strong whittling or pruning that knife is a king. It’s the sharpest Spyderco I ever own with the Moran FB01 convexed.
Unfortunatly I was not able to cut the bottle through the “south pole”. The blade was stuck at one good inch from it. Sawing did not change anything.
The blade was to thick, it was stuck in the plastic preventing the keen edge to effectively cut.
OK. It’s not the first excellent blade who cannot pass that test.
So I decided to finish that bottle with my Sage II. I have convexed the edge and that little rascal is a aggressive wonder toward wood and cardboard. I love it.
Unfortunatly, the Sage II Blade was stuck at half an inch of the South Pole. This time it was the handle which seemed not comfortable enough to transfer all the force. Despite my strenght (I’m 1m98 and 97kg BTW…) the blade was stuck and would not go further.
OK that bottle seemed really thick. It happens. ALl plastic bottle are not equal. That Cola one was harder than many others.
I decided to finish it off with the Gayle Bradley.
This time I was able to get a confortable grip and to cut perfectly through the South Pole in one attempt.
You feel the M4 edge going through the hard plastic in one push: this was purely amazing and relieving !
Why ? All three knives are razorsharp. But we got here three different geometry and three different destinations.
The Sage II is a “polyvalent EDC”. The Bushcrafter (reviewed soon) is a “versatile wood tool”. The Gayle Bradley is an “hard used folder”.
But only the Gayle Bradley gives enough leverage near the handle (the choil is incorporated in the wide blocky handle). You can apply a lot of vertical force on a very thin hollow ground blade. This is the best recipe against hard plastic.
But here we got three excellent tools designed toward high performance but only one was able to cut hard and deep in the plastic.
I told you my little Bottle Butt test was tricky.
My favorite knives for that test was Xavier Conil’s Pointu. A thin zero full flat ground folder which cut everything like butter ! I have since send it to Sal Glesser for him to test it in Golden…
Later on another bottle I was able to make the Sage 2 pass through the exact center.
It was harder to go through (handle less confortable and different grind) than with the GB but it still was possible.
Eventually (part 2) I was able to change my technic for cutting with the Bushcrafter.
As on a fifth attempt I was stuck again by the thickness of the blade, I have decided to push with two hands.
And this time the razor sharp edge got enough force applied to cut right through the but:
So… for the easiest cutting of the butt of a plastic bottle you’ll need a thick confortable handle and a thin ground blade.
— Update from Surnia, registered member of the SPyderco forums:
“In regards to your plastic bottle tests, plastic bottles are not actually injected from the bottom. They’re made into blanks first which are very thick walled plastic test tube shaped things with the bottle threading already present at the top.
From there, they’re passed to a molding station where the blanks are heated, inserted into the molds, then inflated with air pressure to form fit the mold. If you carefully heat a plastic bottle (cap on, and evenly warm it up slowly… vent the air every so often to continue shrinking it. The air allows it to retain the bottle shape and not shrivel up excessively in one spot) and do it evenly, it’ll eventually get close to the original blank’s shape. It won’t go back to it, but it’ll get within a certain limit…
Best example i have of the blanks are here:
where they’ve used them for other purposes. “