Cap & Liner Assembly Processes

Author:  Avery Hallett - Applications Engineer II 

Common methods of assembling liners and caps together: 
• Press Fit 
• Glue Fit 
• Ultrasonic 

Press Fit- A liner is manually or mechanically pressed into a cap using friction to help hold the liner in place. Liners are typically slightly oversized, compared to glue fit or ultrasonic, to allow for an interference fit. Some caps are specifically designed for press fitting. These caps will have a retaining ring or bead which has a smaller inner diameter than the base of the cap where the liner sits. In this instance, the liner is pushed past the retaining ring so that it is held in place along the area surrounding the outer diameter of the liner. 

Glue Fit- Glue is dispensed onto the inside of a cap before a liner is pressed into place. The glue helps keep the liner from falling out. Various glues are available depending on the material(s) being used. The glue can be dispensed in different ways to better fit the product, such as center dispensing, edge dispensing, or custom point dispensing. In any scenario, the liner needs to be pressed into the cap to distribute the glue along the designated area of the liner surface. The glue then needs time to dry for best adhesion. The timing for the glue to dry will vary based on amount used, type of glue, and the cap and liner materials. 

Ultrasonic- A liner is placed into a cap and welded together using an ultrasonic frequency to keep the liner from falling out. An anvil will press and hold the liner in the cap while a horn tuned to a specific frequency makes contact with the top of the cap. The frequency is then sent through the horn and into the cap. This frequency will shake the cap rapidly. While this is happening, the anvil is holding the liner in place, creating friction between the liner and the cap. This friction will cause a melting between the cap and liner, fusing them together. After a very short amount of time, the materials will have cooled into a solid bond. For this to occur, the liner requires a backing material to contact the cap. This backing material needs to be the same material as the cap material for a bond to occur. Unlike materials have different melt points which make the bond either unreliable or non-existent. This holds true even if attempting to bond a homopolymer to a copolymer with the same base resin. Custom horns and anvils can be made to fit various cap sizes and shapes. The custom anvils and horns can also focus the energy in select areas of the cap which can be beneficial if you have an open top cap or a large surface area.

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