Plastic caps are often made by dip molders; the process involves using a mold, or mandrel, and molten plastic to create an exact fit for a cap or plug. The mandrel is lowered into the molten polymer, commonly PVC or plastisol, and sits in the plastic material for a specified amount of time. The longer the mandrel stays in the liquid, the thicker the wall of the cap will be. It is then removed slowly to prevent irregularities on the surface of the cap, and is allowed to cool and harden.
Some plastic coatings can be heat treated as well to achieve the desired material results. After hardening, caps are removed from the mold and rarely require secondary finishing processes. High precision is required in plastic cap manufacturing because caps are used to seal containers and therefore need to fit tightly. Automatic dip machines aid in keeping process consistency, and also ensure that labor costs are kept relatively low as most of the process can be done by machine.
Using precisely controlled ovens, dip speeds, dip times and withdraw speeds, manufacturers can custom design caps according to color, texture, wall thickness, material hardness and length. Caps, plastic plugs and plastic closures can be round, tapered, pull tab or vinyl hex, allowing for a wide variety of uses and applications. As they are so commonly used, and as many plastic containers requiring closures are produced in standard sizes, warehouses may have a wide range of stock plastic caps that are ready for immediate shipping to use with those standard containers.
Threaded caps can be made which are twist-on and provide a tighter seal, sometimes even a water-tight seal. Other containers might require a tamper-proof capability, and plastic caps can be formed to provide this too. Due to the nature of plastic dip molding, minimal material is wasted in manufacturing as the molten polymer which is not used in the forming of the caps or plugs is left behind in the dipping container. This characteristic of dip molding makes it an economical choice for manufacturers and is a reason dip molding is a popular method for forming many plastic objects.