Regardless of the type of system used to form paper plates or trays, the process is dependent upon three interrelated parameters which are inversely proportional to some degree; heat, dwell and pressure. Forming and setting a paper plate or tray into the desired shape with an acceptable amount of rigidity is very similar to steam ironing a pair of trousers or a shirt. The heat from the dies combine with moisture in the paperboard and the pressure and dwell afforded by the press to deform the fibers of the paperboard and steam iron them into the shape of a plate. In addition, as the paper is drawn into the die, the diameter of the blank is reduced. This causes overlaps of paperboard to occur in the plate creating wrinkles or pleats on the sidewall and rim of the plate. The heat, dwell time and pressure supplied by the die and forming press steam iron these pleats into the plate and somewhat “weld” the layers of paperboard in the pleat together to provide structural rigidity.
These parameters are inversely proportional to a certain degree. For example, if one of the parameters is reduced, then in order to maintain the same quality plate it is necessary to increase the other two parameters. However, in order to keep the production speed as high as possible with as little waste as possible, it is necessary to determine the best balance between the three parameters.
The heat transfer from the die to the paperboard can only be controlled by the material of the die, the temperature of the die, or the degree of contact between the die and the paperboard. Although each of these can be controlled, they can only be varied to a certain degree. Because paperboard is extremely abrasive, it is very important that the die be manufactured from a material hard enough to withstand millions of cycles of paperboard drawing over it under pressure. Unfortunately, most materials that give up their heat easily are not very abrasive resistant and therefore quickly wear causing the fit between the die and paperboard to degrade. If the temperature of the die is increased too high, the paperboard or the coating on the paperboard will soften and stick to the die creating a jam up in the machine. The fit of the die to the paperboard is dependent upon wear of the dies, consistency of the paperboard thickness, and expansion and contraction of the die due to normal temperature fluctuations in the course of production. However, if this heat transfer can be increased, then dwell time and thus speed can be increased providing the pressure remains constant. If the heat transfer is decreased, then the dwell must be increased and thus the speed decreases, once again provided the pressure remains constant.
If the press system can provide for forming pressure variation through force increases or decreases, then there is one more variable that can be adjusted to compensate for changes in dwell or heat transfer. While most press systems have some type of pressure adjustment, it is of course limited and can only be increased so much before there is risk of overtaxing the machine or dies. Furthermore, tests have proven that pressure on the paperboard can only be increased to a certain point before plate rigidity is decreased due to crushing of the paper fibers themselves. When increasing the pressure, it is very important that the wrinkles in the paperboard caused by the forming process are very carefully controlled so as not to create large overlaps that can result in compression fractures of the paperboard. Although the details may vary to some degree, the overall concept of forming paper plates and trays remains the same regardless of the machine type. The moistened paperboard is creased by the creasing section of the machine in the areas where controlled wrinkles will be formed while the blank is still in the web. The web is then indexed a predetermined amount, centering the creased paperboard between a male punch and female die cavity in the cutting die. A reciprocating platen pushes the male punch through the die cavity, shearing the registered pre-creased paperboard and urging it to fall through the die, creating the blank. This pre-creased, cut blank falls to the female forming die cavity via gravity guided by an aluminum channel (blank chute), and is centered between a male die and a draw ring on the top, reciprocating platen, and the heated female cavity on the stationary bolster plate.
The male die with the draw ring in it's extended position, descends and the draw ring contacts the paperboard first to provide tension to the material as the male die pushes it into the cavity. The purpose of the draw ring is to hold the board tightly against the draw pad on the bottom die to control the formation of wrinkles in the container as the material slips into the heated female cavity. For smooth scored plates and trays, the draw ring further forces the excess paper into the scored areas and ensures that neat, even gathers are made, following the precreased lines. For fluted plates, the excess material is forced into the flutes creating a ridge like appearance. For fluted plates it is essential that the flutes be designed to use up the excess material and thereby prevent excessive wrinkles from forming. The more tension applied to the paperboard (through springs, brakes, or air cylinders), the neater the controlled wrinkles. However, too much tension and the male punch will be pushed through the paperboard, causing fractures and tears.
After all of the paperboard has drawn into the heated cavity, the press extends the male die solidly into the female cavity and allows the press to build pressure either by springs, hydraulics or pneumatics, and holds the product under this pressure and heat for a period of dwell time. This allows the moisture in the board to turn into steam and escape through vents in the die, "setting" the product in the shape of the die. Essentially, the controlled wrinkles of board or flutes are steam pressed into the product, imparting structural integrity. Therefore, it requires a combination of moist paperboard, good creasing, heat, pressure, and dwell to form good product.
The press then opens, activating the plunger (ejector) and the finished plate is ejected onto the conveyor ready for stacking, inspection, and packing.
