What is Slip and why is it used in Plastic Caps

Whenever we validate a new cap, we always need to consider what slip ratio is required. As we use various materials to manufacture caps, and the bottles will also vary in the base resin, so the COF (Coefficient Of Friction) between the bottle and cap will vary widely.

Not enough slip and the cap will bind on application and not fully seal the pack. Too much slip and the cap may slip in the capping chuck so not tight, or worse the high slip will allow the cap to jump the thread so it can never be applied tightly. Generally we control slip 3 ways.

  1. The material selection of the cap and the bottle (base resins) and the resultant friction between them.
  2. The inherit slip in the colour additive (master batch)
  3. The addition of a slip agent.

Material selection for us is in the cap is generally Polypropylene, High Density Polyethylene or blends of Low density Polyethylene. The base resins have differing functional benefits. PP has a very good memory, is quite rigid and is used in caps with live hinges to give them a long hinge life, but PP is not naturally slippery so it is not recommended for use on a PP bottle. In contrast, HDPE is naturally very slippery and flexible, but it is not good for a hinge application. This also needs to be considered with multi piece closures such as a push pull sports caps where the 3 parts will need to move in relation to each other during the products life.

               Colour master batches are made from many different base materials to give the desired colour. This can often require different addition rates of the colour concentrate to give the depth of colour. Rates can be from 1% to as much as 5%. Different colours will have widely varying slip values they impart on the end caps

                There are 2 kinds of slip additives we use. Erucamide and Behenamide. Both these slips are food safe. Erucamide is made from a fatty acid amide combined with ammonia and has a high slip and is quite cheap. Behenamide is formed from Behenic acid and has a lower slip function but is widely used on Still water applications as a much lower tendency to taint products like unflavoured waters.

                These slip agents can bloom to the surface of closures over time increasing the slip ratio and leaving a slight white film on the surface of the closure. Once slightly heated this film will disappear. One way to understand the amount of slip within a batch of closures is to take a full carton of closures, open the lid and try to push your fist to the bottom of the carton. If your fist moves easily to the bottom of the carton, the caps have a high slip ratio. If it takes a lot of force and you struggle to get your fist to the bottom of the carton, the caps have a very low slip ratio.