Injection molding is used to form the closures. The first stage of the process is mix the color, additives and base resin in a batch. This can be done of variety of ways from a manual barrel tumbling process to an inline mixing on the press with gravimetric scales. Once the mix is blended together it is placed in the hopper of the injection molding machine.
The material is fed into the screw and barrel where the mix is heated as it advanced forward through the screw flights into the injection zone of the screw. At this point the mixture is fully liquid at approximately 430° F. The molten plastic is injected into the mold at 10,000 psi of pressure. The mold is made of many duplicate parts, or cavities. The number of cavities range from 1 to 96. Each cavity is filled at the same time. The molds made from steel and can be as large as 20,000 lbs. They are cooled with water to return the plastic to the solid state.
After the parts are cooled the mold opens and they are unscrewed from the mold. At this point, flip top parts need to be closed. There are a variety of options.
In Mold Closing – A sequence in the mold during part ejection has a cam action that closes the lid while the part is still in the mold. A fully closed part is ejected from the mold and can go directly into boxes. The benefits of this system are smaller foot print and typically lower capital expenditures. The challenges are that it does not allow for secondary inspection and the parts may experience opening force variation.
Robotic Closing – Parts are removed from the robot and are placed into a closing fixture. The parts are all closed within the next cycle time. They are 100 percent inspected and placed into a box. The benefits of this method include more consistent hinge performance, cleaner parts as they are not dropped into the bed of the machine, the cycle time is typically faster and they are fully inspected. The challenges are they are typically more expensive and require a space directly next to the closing machine.
Auxiliary Closing Machines – Parts are dropped onto a conveyor and put into a bowl feeding mechanism. The parts are feed through tracks into a closing machine. As they progress through a variety of stations. They are closed and inspected. These systems have the benefits of allow faster cycle times but do also take up space next to injection molding machine. They can have part contamination from being dropped and have a wide range of closing time that can lead to higher variation in opening force and hinge performance.
The parts are then boxed and palletized for shipment, either automatically or by an operator.