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How to Thermoform Paperboard


Regardless of the type of thermoforming machine used to thermoform paper plates or trays, the process is dependent upon three interrelated parameters which are inversely proportional to some degree; heat, dwell and pressure.  Thermoforming 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 paperboard thermoforming press to deform the fibers of the paperboard and steam iron them into the shape of a plate or tray.  In addition, as the paper is drawn into the die, the diameter or original outside dimensions of the blank are reduced.  This causes overlaps of paperboard to occur in the plate or tray creating wrinkles or pleats on the sidewall and rim of the paperboard container.  The heat, dwell time and pressure supplied by the die and the paperboard thermoforming press, steam iron these pleats into the container 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 or tray 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.  It is also extremely important that the paperboard contain between 8.5% and 13% moisture by weight in order to be soft and pliable for thermoforming and in order to provide the catalyst for welding together the pleats.  Too little moisture and the paperboard will simply fracture or the resulting paperboard container will lack strength and rigidity.   

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 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 in the closed position can be decreased 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 thermoforming machine 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 thermoforming machines 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 or tray 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 thermoforming 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 thermoforming paper plates and trays remains the same regardless of the machine type.  The moistened paperboard is cut to a predetermined shape and creased by the diecutting machine.  The creases or scores are strategically placed in the areas where controlled wrinkles called pleats will be formed.  This pre-creased, cut blank is stacked, then the stack is put into the blank feeder in the paperboard thermoforming machine.  The blank feeder in the paperboard thermoforming machine then picks one blank at a time per lane, and drops the blank onto the draw ring surrounding the male forming die via gravity, guided by an stainless steel channel (blank chute), and is centered between a heated female cavity die on the top reciprocating platen, and a draw ring and heated male punch die on the bottom stationary bolster plate.
   
The heated female die descends via the top reciprocating platen in the paperboard thermoforming machine and contacts the paperboard resting on the draw ring in order to provide tension to the material as the male die pushes it into the female cavity.  The purpose of the draw ring is to hold the paperboard tightly against the draw pad on the top female cavity die to control the formation of wrinkles in the container as the material slips into the heated female cavity.  The draw ring forces the excess paper into the scored areas and ensures that neat, even gathers are made, following the precreased lines.  The more tension applied to the paperboard through springs or air cylinders under the draw ring, the neater the controlled wrinkles.  However, too much tension and the male punch will be pushed through the paperboard, causing fractures and tears.  It is also extremely important that the material have enough moisture in it to make it soft and pliable.  This level of moisture can range from 8.5% to 13% by weight depending upon the type of material and the geometric shape of the container being made.


 After all of the paperboard has drawn into the heated cavity, the press extends the female cavity die solidly into the male punch die and allows the paperboard thermoforming 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 or pleats of board 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, finished plate or tray is ejected onto the conveyor where it is automatically stacked ready for inspection and packing.

 

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