Now that some of the special features of closures and packaging units have already been presented, the question arises: how are the plastic parts actually manufactured? What we mean is of course the process referred to as injection molding, in our specific case for producing plastic closures and packaging components. Even though some might think that it cannot be all that difficult, the development of the initial concept and then getting series production up and running – and this can involve several hundred million parts a year - takes some doing. Experience has shown that there are lots of people who have absolutely no idea how a closure is made, nor what is needed to make them, or what processes are used. Broadly speaking, three things are necessary when manufacturing a “plastic cap” by injection molding, namely, an injection molding tool (as the mold), an injection molding machine and of course plastic, usually referred to as plastic granulate. All three components are required for injection molding - that’s the process used to bring the plastic into a specific shape. So, what happens in the machine and in the mold to create a perfectly shaped and colored plastic part out of a few crumbs of granulate (or plastic) and a couple of grains of master batch (that’s the additive with the color pigment)?
Mold, barrel, screw – how do they relate to closures?
Although everything looks so simple and uncomplicated when standing in front of the machine, the process behind injection molding is a very sophisticated and demanding one. For those who are not overly interested in technical matters: in the injection molding process, liquid plastic is injected into the molds in the closed tool where it cools down; then the tool is opened and the finished parts fall out. And now with a few more technical details. The machine required for injection molding is made up of two main units, the injection unit which melts and prepares the granulate before injecting it into the mold (thus, injection molding), and the clamping unit which holds, closes and keeps the mold tool closed, then opens it again after injection molding. The injection unit consists of a horizontal barrel containing a so-called screw. We will come back to the significance of the barrel later. The apparatus for filling in the plastic granulate is at one end of the barrel whereas the nozzle that constitutes the transition to the clamping unit is at the other end. Now, returning to the screw and the actual injection molding process: the plastic runs into the barrel as granulate and is conveyed by the rotating screw towards the screw tip and the nozzle. In the process, the granulate is liquified by the hot barrel and mixed with any color pigments needed. Due to the pressure and the screw, the mixture accumulates in front of the still closed nozzle. The result of this dynamic pressure is that the plastic is compacted and homogenized to a pliable mass. At this point, the clamping unit with the tooling plate travels to the injection unit and the molten plastic is pressed through the now open nozzle at high pressure (between 500 and 2000 bar) into the cavities in the mold that give the plastic its shape.
Injection molding in top form: dozens, then hundreds, then tens of thousands, then…
As soon as the hot plastic mass hits the colder steel of the mold, it starts to solidify from the outside in. The material continues to cool down until the core has also reached a state of solidification that allows the plastic part to be ejected - or removed - from the mold. To do this, the tool opens again and the closure (or another item) either falls onto a conveyor belt adjacent to the machine or is removed by a robot system. Having now read about injection molding, you might have got the impression that the process is a long-drawn-out one, whereas in fact the opposite is the case. The cycle - from closing the tool and injecting the plastic, cooling down and re-opening, until the parts are finished and fall out or are ejected – often takes only a few seconds, depending on the size and volume of the part. Since a mold generally has multiple cavities – frequently 48, 64 or even more – it is possible to produce several tens of thousands of closures per hour and machine or tool by injection molding. Here an example: a mold that has 48 cavities and requires five seconds for each injection cycle will repeat this cycle twenty times a minute. As a result, the tool or machine produces 12 times 48 closures per minute, making 576 pieces. In this case, the output is about 35,000 pieces an hour. By extending this calculation to daily, monthly, annual outputs, it adds up to quite an awesome figure!