I just found out that most recycling programs request that the caps on 2 liter pop bottles be removed and not recycled. Some quick googling reveals that this is because the caps are a different type of plastic that cannot be processed by the local recycling plant. Some google results even indicate that a capped bottle found at the recycling plant is completely discarded.
So, to simplify the recycling process, why can’t the caps be made of the same plastic as the bottles?
Because they have a different function.
The caps on bottles are made of a thicker, stronger plastic, in order to be screwed on and off multiple times. They would not work very well if they were made of the same thin plastic as the sides of the bottle.
Well, the threads on the bottle itself are made of the same plastic as the rest of the bottle, just thicker. It is subjected to the same stresses as the cap.
Maybe making an opaque, solid color cap requires a different plastic. All the plastic bottles I have seen are transparent and if not clear only lightly colored.
Just a guess though…
The answer to why in business is money.
The bottle is blowmolded from a PET preform. Clear plastics are more expensive to produce because they need to be free from the specks that are hidden in colored plastic.
The cap is injection molded from PP, cheaper per pound than PET.
For the recycling machines they want the cap taken off because it wears out the crushing mechanisms faster with the cap screwed on. They have to squeeze a bottle full of air and pop it vs if the cap is off it just smooshes down.
Just a guess - plastics might gall when they are threaded together, meaning they stick and tear pieces of one another out. That is, the dividing line between the two pieces shifts due to adhesions. This can also happen to metals, and at least in the case of metals, one remedy is to use two different kinds of metal for the two mating pieces, for example two metals of different hardness.
Galling is a function of speed, surface finish, pressure, and lubricity.
High speed, coarse finish, high pressure and low lubricity will lead to galling.
Caps to bottles are an example of low speed, mirror finish, light pressure, and high lubricity.
The thread form on the unscrewing cores of a mold need to be diamond polished to a mirror finish since the plastic shrinks tightly onto the core. The friction would be too great otherwise.
While PP is not teflon, it is a plastic like teflon, and can you think of something that slips better than teflon?
IMHO galling is not a design criteria for closure makers.
I dunno. Why don’t glass bottles has glass caps?
The softer material of the cap lets it form a better seal with the harder bottle material,