Beverage & Packaging Sealings - 19'th Edition: Plastic Closures :Myths and Facts about 1-piece vs 2-piece Caps

In previous edition we have covered how a single-piece cap works. Single piece caps are generally produced from HDPE material. Today we will cover 2-piece or double-piece closures.

First thing first: 2-pc caps are generally(if not exclusively) used for Sparkling beverages/CSD (Carbonated Soft Drinks) application. So the application domain of 2-pc cap generally covers the beverages having internal pressure inside the bottle. Other beverages application like packaged drinking/ mineral water & sensitive beverages including Juices (both Hot-Filling & Cold Aseptic Filling); have been fine with 1-pc caps.?

SACMI?, since beginning, has been supplying technology for both 1-pc as well as 2-pc caps; without favouring one type of cap over other. For this very reason & unlike Other technology suppliers who supply just 1-pc cap lines(hence vociferously favouring one type over other);?SACMI?has always been promoting a balanced and unbiased opinion backed by brutal facts, when it comes to this hotly debated topic of 1-pc vs 2-pc beverage caps.?

Before to go deeper into this subject (and in order to arrest any 1-piece vs 2-pc argument in the nip, which has always been blown out of proportion by alternative technology suppliers), let me highlight the fact that with advancement in HDPE materials, 1-piece HDPE caps are well accepted even in Hot and challenging geographies with extreme environmental conditions. And these days, most of CSD closure lines being shipped out of?SACMI?premise are going with SACMI 1-pc CSD molds.?

Let us start with a basic and an innocent questions:?Why do we need a 2-piece cap(which of course is comparatively costlier) when we already have 1-piece caps at our disposal??And the answer lies in following explanation:?

The Fundamental principle of packaging sealing demands that ONE out of two mating surfaces (one being bottle neck & other being closure sealing zone, in this particular case of beverages) has to be softer than other; in order to have an effective sealing. HDPE(widely used for 1-pc closure manufacturing), by it’s intrinsic nature, is softer than the PET; hence is an ideal material to seal PET bottles. However, CSD’s (carbonated Beverages) having an internal pressure inside the bottle, imposed another demand on the closure : & that is, a Good mechanical strength in order to retain the pressure and to avoid doming of closure top panel. You can see the doming effect (rounded/ spherical closure top) in below picture:?

And to find a fine balance between these two demands from HDPE material (being softer, at the same time, being mechanically stronger) is not easy(if not impossible in the past; but nowadays due to advancement in Polymerization process, we are close to this fine balance). And this need became the genesis for the development of 2-pc closures in the past.?

And this fine balance became even more difficult to achieve, when we had to deal with Hot climatic conditions (tropical countries with bad road conditions – that is a recipe for more internal pressure). Let me explain it in this way:

All global CSD brand owners, fill a certain Gas volume (amount of Carbon Di Oxide) into their Sparkling beverages. So far as the closure performance is concerned, the challenge comes from the pressure inside. And following the ideal Gas equation PV = nRT; for the same gas volume, pressure will be more for higher ambient temperature in Hot and Tropical geographies. Again during transportation, if roads are uneven and bumpy, then there will be more shaking of filled bottles that will lead to more pressure build-up. For these reasons, a closure for hotter climatic geographies (like India) has to be more sturdy compared to the cooler geographies like the ones in northern hemisphere.

So in a 2-pc closure which is made of 2 different materials; we are extracting two independent properties from 2 different materials – good mechanical strength from a harder PP (Polypropylene) + Good sealing properties from a soft lining material. Here below is a sketch/ diagram showing how a 2-pc closure seals a PET Bottle neck finish.

To be continued in next addition of news letter...........