Beverage & Packaging Sealings: 17’th Edition: Insights into Crown Caps manufacturing Process.

Crown Cork is the first and historic name of this metal cap. During earlier days of last century, when Plastic Lining materials were not yet invented. At that time Crown liner discs were produced from tree-bark. This tree-bark was mainly coming from “BARK OAK” trees found in southern Europe/ Mediterranean countries. Hence the name Crown-cork.

Let us come back to the topic of Crown manufacturing process:

The process starts with steel (either #TinCoated or #TFS). There are 2 ways for a Crown manufacturer: they can either buy pre-cut sheets already stacked on a pallet(sheet already cut to their specific dimensions/size) Or to buy it in the form of a steel-sheet rolls. In second case (roll-form), they need to cut and stack the individual sheets on a pallet.

Second operation is #Lithography: involving both #Coating & #Printing processes. Most of Indian customers are using lithographic machines from Crabtree of Gateshead. Coating is the process of applying ink in the metered-way on the entire surface of #SheetMetal; whereas Printing is the process of ink transfer to selected areas of sheet metal as per print design. These two processes have 2 different stations/equipment named as #CoatingMachine & #PrintingMachine that are followed by an #Oven. Oven is used to bake(drying/curing/hardening of inks) the printed sheets. Pre-cut sheets are fed to Coating/ Printing stations and then passes through the oven. Temperature & sheet-residence time in oven are two important factors to get good #quality of printed sheets.

First Pass of the sheets (assuming that sheet surface is free of impurities, particularly any oil or grease – otherwise they need to be washed first) through Coating machine is called #Sizing. Sizing process is an application of an adhesive layer which is sand-witched between metal and ink. Basically it has very good affinity to both: Metal & Ink; hence provides strong ink adhesion onto metallic surface. So sizing essentially improves printability of metal sheets and ensures that ink is not peeling off/coming out of metal surface(of crowns top surface) due to aberrations. Then the metal sheets are passed through printing machine as many times as the number of #colours/print-designs. As I said earlier, for every coating or print, sheet has to pass through a long oven that may be fired either through petroleum products or Gas or electricity. After receiving all the prints, sheet again goes to coating station in order to apply a transparent #Varnish layer on top of print. Varnish is the topmost layer on the Crown cap. Purpose of this varnish layer is to protect prints against scratches during handling and transportation.

In the last pass; printed sheets are passed through coating machine in order to apply a lacquer on the other side of the sheet (inner side of crown-cap). This lacquer is an adhesive layer sand-witched between metal surface and plastic Liner. This layer is important to retain(stick) the liner inside the crown cap. Generally lacquering is done in the last in order to avoid any ink/varnish transfer to the lacquer side of the sheet during printing/varnishing.... any print/varnish transfer to the lacquer side of the sheet is called - Patching, and this results into non-adhesion of liners into the Crown-Caps.

Next is “Punching & Forming” operation wherein a Crown is produced from the printed sheet; followed by Lining operation wherein a plastic liner if formed/molded within the crown itself. For these 2 operations; @SACMI is the only company supplying turn-key solutions and very high speed lines (6,000 crowns per minute) to produce Crown Caps all across the globe.

Punching/Forming presses: Here printed sheets are fed into SACMI press as an input, while well-formed Crowns are received as output. a blanked metal sheet skeleton is the by-product as waste/scrap. Following images can explain how this conversion happens:

In above picture, Metal sheet has entered into the die-set & Die-set is in open phase, just before punching/blanking operation.

Above picture represent Die-Set in closed condition. Blanking punch has entered into blanking die. And a circular disc has been cut.

While Blanking tools remains closed, forming punch enters into Forming die - hence Crown formation takes place.

In above pic, die set is opened & a well formed crown cap is being ejected out of die-set.

Three high points on blanking die cutting edge(marked with black marker-ink in picture), meets the the flat cutting surface of cutting punch first. And then it is scissoring action onwards till the lowest point of wave shape cutting edge of die meets cutting punch surface.

Next is the Lining Operation. Let me try to explain it through following sketch:

Above sketch is Showing how liner is formed as a result of in-shell molding of liner into Crown(crown used as a die whereas a forming punch forms the liner)

·?????? SACMI Crown Lining machine works on the principle of #CompressionMolding - the same technology we use to mold plastic closures. The only difference here is the fact that in Crown-Lining machine molding forces is provided by a mechanical cam (whereas the same come from Hydraulic pressure in SACMI CCMs - continuous Compression Machines).

·?????? process starts with an extruder. at the nozzle of extruder, we cut small balls of molten plastic and place them into the crowns at the first turret of lining machine. Then with the helps of a transfer wheel, Crown with molten plastic ball inside, goes into next turret, where it is placed over a mandrel. Mandrel (hence crown with molten plastic ball) goes up following a Cam and presses the crown against the fixed Forming punch. In this way a liner is molded inside the crown cap.

An important feature of first transfer wheel worth mentioning here is that, it provides R-F Heating of crown cap, hence softening the lacquer material which acts as an adhesion for liner to remain attached to metal. Here are the images of Crown before & after Lining operation, for your ready reference:

Above picture shown inside of Crown before Lining operation.

Above Picture shows a Crown cap with liner, after Lining Operation

After the molding turret, Crown goes for online quality control. In earlier days, SACMI lining machines used to have a dedicated turret for this purpose. After the molding turret, the crown with fully formed liners, goes to Q.C. turret through a transfer wheel. There the #Sealing #Integrity of liner was checked with Vacuum. Any leakage in the vacuum converted/corresponded to an electrical signal by transducers. This signal goes to lining machine PLC which sends a command to the #FailSafe rejection system, that removes defective crown from the exit belt. Hence SACMI ensures a 100% inspected and good crowns going into the boxes. Thanks to it's in-house #VisionSystem expertise and deep knowledge of #ArtificialIntegence, SACMI has now done away with this elctro-mechanical method of Liner Checking, which has now been fully replaced with #CVS (#CameraVisionSystem).

From the lining machine Crowns goes into a Cooling Conveyor, where they are cooled before to enter into boxes (to avoid any condensation, hence rusting issues later). At the end of Cooling Conveyor, there is a counting device which ensure correct counting of crowns packed into a box/container.