To achieve the vacuum by mechanical procedure in a packaging, the caps have to be properly prepared. The use of notches in its wing allows to enable sufficient space to perform the vacuum inside the container. This work describes the different ways of notching the lids.
In the majority of food packaging, the elimination of the interior air of the container is used after filling. The suppression of oxygen is sought, the origin of different processes of degeneration of the content. An effective way to eliminate this air is to generate a vacuum in the free space (head space of the container). There are several ways to achieve this emptiness. For example:
– Fill to the top (without headspace) and hot. When the product cools, it shrinks and a vacuum chamber is obtained.
– Inject water vapor into the head space. It moves to the air and when condensed, when cooled, the same effect is generated.
– By means of mechanical vacuum. For this purpose, the container must be closed in a closed and hermetic receptacle in which an adequate vacuum is generated, therefore also inside the container and then to make the closure.
in this last case we will focus.
The seamers prepared for this work have the closing head integrated in an enclosure as we have said airtight, connected to a vacuum pump. Once the container to be closed is placed, full of product in the closing position, the pump is activated generating the vacuum in the chamber. Arrived the same at the desired value, the closing is carried out. This process is slow, so production is low and equipment expensive. It is only justified for high value products in the market. Typical examples are infant formulas for babies, coffees or certain meat products. In many cases, an inert gas, for example nitrogen, is injected after the vacuum has been applied in order to maintain a certain pressure inside the container in an inactive atmosphere.
In order to achieve a vacuum in the headspace of the filled container, it is necessary to allow the air trapped therein to be easily released. The cover is already placed in the position prior to closing, that is to say with the wing thereof resting on the flange of the container. This arrangement can hinder the extraction of air because said lid is acting as a shutter. The wing of the lid fits over the flange of the body of the container along two circumferences: a) The first defined by the edge of the flange in its contact with the gummed area of the wing. B) The second determined by the fitting of the wall of the lid tray at the base of the flange. Therefore, it is necessary to slightly raise said lid, so that a groove is generated between the wing thereof and the flange.
This effect is achieved by making a series of notches on the wing. Said notches are those that rest on the flange, the rest of the periphery of the wing being elevated and therefore with free passage for the air outlet of the head space. These notches are called “dimples” in English, a designation that is also used in the Spanish-speaking industry.
We now enter into the basic motif of this work, which focuses on the different types of notches (“simple”) that can be used and how to perform them.
CIRCULAR WINES IN THE CENTER OF THE TOP WING
In the center of the wing, a series of small circular embossments of 0.35 mm are made. approximately in depth and 2.4 in diameter. Its number depends on the diameter of the lid, but in any case a minimum of 6 to 60º for caps with a diameter smaller than 100 mm and from 8 to 45º for larger than this value. See drawing no. 1. Therefore, a series of projections appear on the inner side of the wing, although the application of the closing rubber softens its relief somewhat, they are clearly noticeable.
Drawing nº1: Dimensions of a notch on the wing of the lid
When placing the lid on the container flange as a step prior to closing, these relief points are those that rest on the end of the flange. Therefore the wing of the lid is slightly raised on it. In this way it is possible to extract the air contained in the head space, producing the corresponding vacuum.
To help this function, it is convenient that the flange of the container is well inclined, at least 7 ° 30 ‘, so that the contact between the notches and the flange is punctual, just at the end of the latter. Thus, the obstacle to the exit of the air will be minimal.
These notches are made during the die cutting of the lid. For this, the following contributions must be made in the die:
– “Matrix center ring” (Fixed lower part of the die): Make a series of housings that will make a female in the die.
– “Expelling ring” (moving upper part of the die): Mount the same number of small males. See drawing no. 2. This piece should also be provided with a system of guides that ensure that it does not turn or move.
Drawing nº 2: Detail on a die of notching pieces
The “ejector ring” is a piece that under normal conditions maintains a certain clearance to facilitate its movement inside the die. But in this case, being the carrier of the males to mark the points on the wing, it is necessary to guarantee its exact positioning. Hence, you have to add a pair of guide rods. See drawing nº 3
Drawing nº 3: Die with modified ejector with guide rods
This system of generating the notches has the advantage of a proven reliability, since no matter how little relief they have, when resting on the edge of the flange of the container, they guarantee an exit channel. Possibly the most widely used. On the contrary they have the disadvantage compared to other systems, quite complex in the preparation of the die. The realization of the set of male-female couples of drawing is a task that requires a lot of precision. Also the smallness of the males makes easy its deterioration during the work of the die.
MUESCAS IN MEDIA CANE IN ANCHO DEL ALA
Throughout the width of the wing, some recesses are made in the shape of a half-round, in a number similar to the previous case – between 6 and 8, depending on the diameter – and distributed symmetrically. The depth of the same is usually 0.35 mm, ie the same as for circular notches. Its width is about 3 to 3.5 mm. See drawing no. 4. In this way, radial ribs appear over the inner face of the wing that emerge over the closure compound. They perform a function analogous to the case of circular grimaces in terms of the action of performing the vacuum.
The recommendation on the angle of inclination of the flange is also applicable in this case.
Drawing nº 4: Position of a notch in half reed on the wing of a lid
To make these “simple”, it is enough to mechanize the corresponding channels in the upper part of the “matrix center ring”. Its dimensions are indicated in drawing No. 5. It is not necessary to mount any type of male on the “ejector ring”, since the tin plate copies the shape of the channel during the filling of the lid.
Drawing nº 5: Machining of the top of the ring center matrix
This way of forming the vacuum notches has the advantage of simplicity in the preparation of the tooling. Only one piece is affected and therefore, there is no difficulty in aligning with others. The die is not complicated and maintains its robustness. On the other hand, the main drawback detected with this system of “simple” is in the closure of the container. The curl of the lid in the areas of the notches, has a greater height, logical thing to exist in these points less deformation – lower height of bucket. Depending on the type of seamer, these differences of height of curl in the same lid, can generate difficulties in the feeding and even problems in the quality of the closure.
MUESCAS IN THE INTERIOR ARCH OF THE TOP WING
In this case the notches are made in the arc that joins the wing with the tank of the cover, that is to say on the inner radius of the wing. They have an inclined half-round shape. Its depth is 0.6 mm. and its length 1.5 mm approx, both means in its inclined position. In the drawing no. 6 its position is appreciated. Their number can vary from 6 to 16, depending on the diameter of the lid. With this type of notches, unlike the previous ones, the support on the flange is made on the radius of the same, instead of on its edge. Therefore, the final angle of the flange is not very important.
Drawing nº 6: Positioning of notches in the inner arch of the wing
Similar to the previous system, the preparation of the die for its realization, focuses on realization on the “matrix center ring” of machining with the inclination and appropriate measures. In the drawing No. 7 they are reflected.
Drawing nº 7: Detail of the preparation of the “matrix center ring”
Drawing no. 8 shows the working method of the tool and how the notch is formed.
The advantage of this type of “simple” is, like the previous one, the ease in the preparation of the die, because it is once again a single piece – ring matrix center – which must be adequate. Therefore, its simplicity and strength are assured. It does not present significant disadvantages. Only in the visual aspect, after the closure, there are slight marks on the wall of the tray. Also the seamer must be robust to provide the extra effort necessary to crush these notches.
PREPARATION OF THE CONTAINER
To achieve optimal results with any of the types of notches that we have described in the previous points, it is also convenient to adapt the body of the container to the use of lids with this type of notches.
The actions to be taken on the bodies are the following:
– 1.- As already indicated above, it is convenient that the flange of the container is well inclined, at least 7º 30 ‘, so that the contact between the notches and the flange is punctual, just at the end of the latter. Thus, the obstacle to the exit of the air will be minimal.
– 2.- It is normal that the cover fits slightly forced on the body. The best practices advise that the diameter of the lid should be approx. 0.20 mm. larger than the inside of the body (although this value is a function of the size of the container). However, in the containers that are going to be closed to the mechanical vacuum, using notches, this adjustment must be the opposite, that is, there must be a play between the lid and the body of the container. The diameter of the body should be 0.30 mm approx greater than that of the lid. The reason for this is to prevent the lid from acting as a shutter, and to allow air to escape between them. This does not mean that more material has to be given to the development of the body to obtain a larger diameter. The same measure of cylinder development is used as in a normal case, and therefore also the same diameter initially. Its increase is achieved by enlarging the diameter of the flange-tampon tool in this amount, and therefore flare the body at its end in the flanging operation.
– 3.- In some cases, slits are used in the flange of the body as a complement to the “simple” ones. These slits have a depth between 1 to 1.5 mm and a rounded profile. They are positioned radially. They are performed in the flange operation, modifying the flange buffer in an appropriate manner. Their number must always be different from the “simple” so that when the lid is overlaid on the container they never coincide on each other. This solution can replace the proposal in No. 2, since it has the same effect of ensuring that the fit between the lid and the body is not airtight.