Sealing aluminium caps over a rubberised stopper or septa has been and still is the preferred way to seal a vial.  I have tested crimp seals to a pressure of well over 15 bar without leakage in a test rig, you would be surprised how big a bubble you can create from the seal bulging through the centre hole in the cap.  “Don’t try this as it was done in controlled conditions”. When fitted properly once a crimp seal is on, that is where it stays and it doesn’t have to be crushed tight to make a good seal.  Many people over-crimp a cap which puts unnecessary pressure on the tooling and can distort the seal.

Crimp cap materials
Historically over-seals (caps) have been made from aluminium because it is light, easy to cut and easy to form, plus when the correct grade of material is used there is less spring back than with steel.  Spring back is and has always been a problem in metal forming.  You need to break down the structure somewhat to make it stay where you put it and where possible you can over bend it to make it stay where you want it to, however when you push against a solid object this is not possible. That is why a rubberised stopper or septa of a reasonable thickness is so important, the seal must be able to cope with the spring back of the aluminium when formed under the head of the container and still maintain a tight seal between the septa/stopper and the container.

Preperation and inspection of parts to be used
The viability of a good seal is also determined by the surface the seal sits on.  There must not be any irregularities such as mould joints or marks that lead across the joint where leakage could flow.  This is often found on plastic bottles when the top has not been properly finished and in some cases there is misalignment between the two halves of the bottle where the mould has been misaligned.

Crimp cap sealing methods
There are a number of ways to form an aluminium cap under the head of the vial or container. The word “crimp” is perhaps what most people will know as what the hairdresser will do to your hair.  However, this relates to the corrugated effect used by metalworkers to reduce available metal.  You can only push so much metal into itself before it gets very hard and thick; so the crimp affect is another way to reduce the amount of available metal especially with steel caps such as beer bottle tops. The serrations are also found on spray cosmetics pump caps where the metal is pushed inwards where it either has to be pressed into itself or corrugated.

This term crimp or crimping although not entirely accurate has passed over to sealing by the hand tool method where the finished crimp is smooth and not corrugated i.e. shrinking the metal by pushing it into itself. Using aluminium makes it easier to do this and the finished product looks cleaner and is smoother.

The above is based on my 50 years experience of metal manipulation of which 25 years has involved the manufacture of crimping tools.  Please contact us for advice on which of our vial crimping tools is most suitable for your application.

Ken Marshall
Director of Engineering
Laboratory Precision Ltd. ©