Probably the most important and also the most overlooked parameter in thermoforming of paperboard is the moisture content of the material. Although the paperboard or corrugated thermoforming machine is capable of controlling the three important parameters of heat, dwell and pressure, the extremely important parameter of paperboard moisture can only be controlled by the converter prior to the material reaching the paperboard or corrugated thermoforming press. Without the proper amount of moisture in the paperboard, the material will be brittle, will not fold properly into pleats, will most likely fracture and tear, and will not hold its shape.
Paper is made up of millions of fibers similar to small, microscopic straws. Under normal dry conditions, these fibers behave as if they were sticks of wood because in reality, that is what they are. Therefore, when you try to bend a stick of wood into a shape and then release the tension on it, it will spring back to its unbent, straight condition. Or, if you put too much stress on it and bend it too far, it will break into two pieces. That is exactly what happens when you try to thermoform dry paperboard. If you are trying to thermoform a very shallow container that is easy to thermoform such as a paper plate, the paper will simply try to spring back to its original flat blank state as soon as you release it from the die. However, if you apply too much stress by trying to thermoform it into a deep or complex shape, the dry paper will simply break.
Now, if we take that same stick of wood and soak it in water for a period of time, then try to bend and shape it using heat and pressure, the wood will easily conform to the shape we are attempting to make. This is exactly how luthiers (makers of wooden stringed instruments such as guitars and violins) bend the sides of their instruments. They soak the wood in water, then use heat and pressure to form the sides of the guitar or violin. After the wood has dried in the correct shape, the heat and pressure are removed and the wood stays in the desired shape.
This is exactly the same concept we apply in the thermoforming of paperboard or corrugated materials. The fibers that make up the sheet of paperboard or corrugate must be moistened to the point that deformation can easily occur without breaking the fiber as it is bent in the forming die to make the shape of the container.
The amount of moisture in the paperboard or corrugated sheet is measured as a percentage of the weight of the sheet when it is dry. Different types of paperboard and/or corrugated sheets require different amounts of moisture in order to render them formable into containers. Also, the more complex the shape or the deeper the container being thermoformed, the more moisture will be required. Recycled materials require more moisture than virgin materials. Generally, the range of moisture by weight will be between 8% and 13%, depending upon these factors. High quality, simple, shallow paper plates made from solid board can be made at 8% moisture. Containers of 3cm to 5cm in depth made from recycled corrugated materials may require up to 13% moisture. Gralex Inc. has over 35 years of experience in thermoforming these materials and can assist in determining the correct level of moisture given the material and the geometry of the container.Moistening the material is not as simple as spraying some water on the sheet, then expecting it to behave any differently than a dry sheet. As with the wood used to make wooden stringed instruments, it takes time for the water to soak into the fibers of the paperboard and make them soft and pliable. When water is simply sprayed onto the sheet prior to thermoforming, it will sit on top of the sheet then will be flashed off as steam when subjected to the heated forming die before it ever gets a chance to soften the fibers. Therefore, it is important that the material achieve the target moisture percentage at least 24 hours prior to thermoforming, depending upon the amount of sizing that is present in the material. Highly sized sheets such as folding carton or liquid carton materials may take up to 72 hours before the moisture can penetrate throughout the sheet, soften the fibers, and reach equilibrium.Too much moisture can also be detrimental to the process. When too much moisture is present, the water has a tendency to break down the bond between the fibers, making the material weak and easily torn. Essentially, too much moisture will result in a drastic reduction in tensile strength of the material, thereby making it impossible to form. Another issue with material that is too wet is de-lamination.
When the material is subjected to the heat in the forming die, too much steam is created and when it exceeds the ability of the vents in the die to dissipate it, it literally begins to blow the material apart causing bubbles within the layers of the paperboard or corrugate. If the material has a coating on one side to enhance the end use characteristics of the tray, there is a good chance that this excess steam will also soften the coating and cause it to stick in the forming die. Application of moisture can be effected by several different methods. For solid boards there are systems available on the market that will spray, coat, or submerge the material in solutions made up of mostly water with a few chemical agents to retard microbial growth and enhance water absorption. However, by far the best means of adding moisture to solid boards is via LAS or a Liquid Application System that utilizes a hydrophilic roller running in reverse to the web direction. Many of these systems use a closed loop computerized feedback system to monitor and automatically adjust the moisturizing level. This particular system forces the water deep into the sheet under tremendous hydraulic pressures thereby reducing the time necessary for the water to soak into the sheet and fibers.
For corrugated sheets, it is best to simply not dry the sheet as much at the end of the corrugating process. This is contrary to what all corrugated machinery operators are taught as they believe it will result in a weak sheet. However, our goal is not to make a strong corrugated sheet, but it is to make a pliable corrugated sheet that can be thermoformed into a corrugated tray or container. Corrugated sheets can also be sprayed after the corrugating process to return the correct amount of moisture to the sheet, but it needs to be remembered that the sheets must sit in storage for a period of time for the moisture to penetrate all layers of the material and reach equilibrium.
Even the best system of moisturizing paperboard or corrugated sheets is no good if common sense is not applied throughout the process. In other words, if you moisten the material to the required 10% moisture for a given application, then allow the material to sit in a warehouse unwrapped and subjected to a dry atmosphere, the material will soon dry out to the ambient condition of 5%-6% moisture, negating all of your efforts. Therefore, it is imperative that once the material has been moistened to the proper level, it must be wrapped in plastic or stored in a humidity controlled atmosphere in order to maintain its moisture. Even a stack of unwrapped precut blanks sitting at the paperboard thermoforming press for a few hours will lose their moisture and will therefore be difficult if not impossible to form.