When choosing a cardboard should take into account the structural requirements, printing requirements and design. The preservation of the stability of the shape of the packaging from cardboard depends on many factors. Cardboard features anisotropy of properties along the length and width of the sheet. In the machine direction (along the length of the sheet), the preponderance of cellulose macromolecules occurs during the production of cardboard, leading to an increase in strength and rigidity.
The difference between the strength properties in the machine and transverse directions increases with increasing thickness of the cardboard. This phenomenon should be taken into account when boxes designing. The most durable and rigid machine direction of the cardboard (in most folding cartons in the form of tubes) perpendicular to the height of the boxes (match the perimeter of the box), which contributes to maintaining the stability of the shape. This is especially important in case of packaging powdered products, which will "inflate" the box.
From a technical point of view, the important factors are compatibility with the packaging equipment and the ability to glue.
The humidity of the environment exerts a significant influence on the strength and rigidity, and consequently on the stability of the shape of the boxes. Cardboard absorbs moisture well, while its strength properties are significantly reduced, especially in the machine direction. This feature should be considered when choosing finishing operations (lamination, protective varnish), which should not only improve the appearance of the package, but also reduce the moisture absorption of the cardboard.
The characteristics of the cardboard are diverse, and they should be checked for each specific application. The table shows approximate values for selecting the cardboard of the required thickness based on the weight of the product. Express cardboard thickness is taken in thousandths of a millimeter or in microns.
|Weight of the product to be packed, g||Cardboard thickness, µm|
|Up to 200||300–450|
Models of folding cardboard boxes should be viewed from the outside, where the seal will be printed. The dimensions of the cardboard box are indicated in the following order: length, width, and depth, and the length and width are determined with respect to the opening of the box.
Fold is carried out along the groove towards the outer surface of the box. In the conventional method of cutting, only one side of the cardboard is processed, so that all the grooves go in the same direction. If a bend in the opposite direction is required, a perforating knife is used.
All elements can be conditionally divided into main and auxiliary ones. The main elements of the boxes include the front, back, side, top and bottom panels. Text and graphic information is applied to them. Auxiliary elements serve for fixing the main elements of the boxes. These include an glue flaps, upper and lower side flaps, additional fixing elements. The side flap system and the upper or lower panels form a lid or bottom. They can be glued or sealed with various locks. The lid can be glued or repeatedly opened with a locks. Locks are usually performed on the upper faceplate.
The shutter flaps of the lid and the bottom can be of different design: butt-joint, overlapping, hook-locks, gluing. In size, the flaps can be different, the same or with one elongated outer valve. The bottom and lid can be formed by their three or four flaps.
The choice of the design of the bottom and the lid depends on many factors: the type of the product to be packed, its weight, the features of the packaging technology and other stages of the life cycle, including the characteristics of consumption of the product.
When packaging small granular or powdered products, it is advisable to use overlapping overlapping and overlapping pairs of upper and lower panels for the lid and the bottom. The size of the panels is equal to the width of the box. This design ensures maximum bonding area and tightness of glue joints.
When packaging large products for which the tightness and strength of the bottom and cover are not decisive factors, the locking panels and side flaps can be trimmed to save material. Also, the lid and bottom can be repeatedly opened with different lock gates.
For packing, formed by the method of folding a flat workpiece, there is a significant dependence of the strength and material consumption on the ratio of overall dimensions for the same volume.
It was found that the minimum cutting area of the cardboard package will be at a ratio of the sides (length L: width W: depth D) as 2:1:2. The most stable under static and dynamic loads is a cubic package with the ratio of sides as 1:1:1, but the surface area is 12% larger. Optimum strength values are achieved with the ratio of sides for boxes (for groups "A" and "C") as 2:1:3, and 4:3:1 for trays (for groups "B" and "D").
When designing, it is preferable to choose the average dimensions (length "L" 100–250 mm). Too large dimensions lead to a reduction in the strength of the entire package and a decrease in the reliability of the fixation elements. Small dimensions lead to a decrease in the reliability of the dieboard and complicated the assembly of the finished product. The finished drawing of the box must be cut out on the selected material (preferably on the cutting-folding plotter), and make sure that the product is correctly assembled and has the necessary dimensions.