Does cutting plastic dull a knife?


Even though plastic can be very soft and cutting it with a knife may sound like a walk in the park, but have you ever wondered if it dulls the blade in the process? How can something so soft dull something that hard?

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Any type of plastic can dull knives, even if they are very soft. Certain types of plastic like shrink wrap are much more abrasive, and therefore, they will get your knife to dull extremely fast.

You ordered a package from Amazon, and it just arrived. After opening the box to get your newly bought knife, you are left behind with a box full of plastic bubble wrap. So what to do with it? You can try and repurpose it, or maybe cut it in pieces to throw it away?

For me, after ordering so many packages online, I do not have any room for more bubble wrap. So I’d rather cut it, to be honest with you, and simply get rid of it. However, cutting with any knife won’t do. First, bubble wrap is a pain to cut, and secondly, it’s materials make it hard to cut with a thick blade.

So if you, by coincidence, do not have a good sharp knife at home, you might end up with a problem. But let’s assume you have a good knife, does it affect it’s sharpness if you use it to cut this plastic? I mean, is that possible? As mentioned in many other articles, the moment you use your knife to cut anything, you dull it in that process, period! Even if this material is super soft, it does not matter.

Of course, the harder or abrasive the material, the worse it is for your knife in the long run. Also, did you know that even if you don’t use your knife, it still dulls without use? But that’s not what we are talking about today, let’s focus on plastic and see what it actually does to your knife.

Why do plastic dull knives?

As mentioned earlier, just like with anything else, using your knife to cut something will dull it in the process. Even if you are cutting super thin paper, it still can dull your knives. So how is all of this possible? I have gone through about it in detail in this article, but just to have the short version.

All of it is basically happening on a microscopic level. Even though you might not feel any difference after using it once or twice, your knife has become less sharp after using it even one time. If you could use a microscope and check the edge of your knife, you’d see that its made out of thousand tiny little metal teeth.

Each time you cut something, they start to misalign. But you might wonder how this is possible as metal is harder than plastic or at least very soft plastic. Well, because these super tiny teeth are actually fragile on their own, even the most delicate material can move them to the right or to the left, which basically means they are misaligning.

A misaligned edge is also known as a dull edge. So you can imagine what is happening if you are cutting something even harder or abrasive, right? Yep, it dulls super fast. So, there is no escape to it, I am afraid.

Your knife, whether you like it or not, will dull even if you are very careful or not use it all. So what you should be doing to make sure it stays sharp, is to maintain and taking care of it properly.

Different types of plastic different kind of reaction.

Not all plastics are made equal, and the same can be said about knives. 

The general rule one should not forget is, the thicker the blade, the harder it will be to cut through the plastic. So it means you need a super sharp knife to have an easy time cutting through, for example, that bubble wrap you received in that package you ordered.

If you are unlucky, the bubble wrap might be made out of good material, which means you’d have a harder time cutting through it. But if it’s low quality, you might get away with a less sharp or thick just a knife. 

Let’s go through some examples:

Polyethylene terephthalate: Also known as PET, these are the bottles of water and coke you buy from any store. You probably have seen videos going around on youtube where they show you how to repurpose these plastic bottles.

Well, these are one of the worst types of plastic for your knife. They are super hard and abrasive and can be quite thick even. So I instead use a foam cutter. It’s super easy and straightforward. Believe me, you’ll thank me later for this.

Shrinkwrap: This is one super sturdy plastic. I remember when I was a kid, I used to hate this plastic, why? Well, my toy was wrapped up in it, and it was so sturdy I just couldn’t cut through it. I used scissors instead and could finally reach my newest edition of G.I. Joe figurine.

If you try to cut with a serrated knife, it will just snag on it, and with a plain edge knife, its a disaster for the edge, and can even be dangerous as it can slide if you force it too much. Do this often enough, and you might end up with a cut in your hand.

The only thing I might say is ok to do is to use the tip of your knife to open a hole and that it, after that, use a box cutter for the rest. 

Bubble wrap: One of the most common plastic you can find in online ordered packages. Also, one of the most addictive plastics out there. Haven’t you been in that place where you just want to pop all those bubbles and couldn’t stop?

Bubble wrap is often made out of polyethylene (LDPE) film with a shaped side bonded to a flat side to form air bubbles. Depending on the quality, you might have a hard time cutting through this material, and in this process, dull your knife even more. It is, however, suggested to use scissors instead. 

What about cutting foam?

I moved not so long ago, and I had ordered a lot of things online for my new house. At some point, we had one room full of foam, there was so much of it, and it had piled up so fast we didn’t even realize.

Cutting them seemed an easy task, but we had to use our kitchen knife to get the job done. Only to find out later how much it had dulled because of it. The reason? Due to the combination of air fraction and polyurethane. Let me explain it better. At the knife’s edge scale, that foam is, locally, either polyurethane or air.

Cutting through the air is obviously easy, but when it comes to that small fraction of polyurethane, that’s actually quite severe. So, even though you’re putting only a few newtons of force on the knife, at the edge, those few newtons are concentrated on only a few teeth.

The best solution for this? Use a foam cutter! Please don’t use a kitchen knife as I did. In case you don’t have a foam cutter, use an old knife or dulled knife you don’t sharpen anymore. 

Plastic cutting boards are a different story.

Since we are talking about plastic, let’s also talk about plastic cutting boards. Although they are made out of plastic, this is a whole other story when it comes to this cutting board. Firstly you should always use the right cutting board for the correct type of knife.

But plastic cutting boards are not as bad as most think they are. This type of cutting board is actually in some cases (depending on what you wish to cut), better than a wooden cutting board. The reason is that you need to be able to score on the surface, and this is actually what a knife needs to be able to do. 

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