The Thermoforming Process
Thermoforming is a process to form plastic material by the use of a mold and can be done as an inline or offline process. Thermoforming is known for making what we call “blister” packages. Thermoforming can be intermittent or continuous motion. Some of the most common packages that use this method are yogurt cups and the blister packs that contain medication. These packages can also be used to hold other products like electronic devices and toothbrushes. No matter the type of product being packaged, the thermoforming process is similar for all. The primary process involves heating a film to the optimum temperature, forming cavities, loading the product, sealing the backing film, and trimming off the unwanted edges. Usually, the film is supplied off of a roll, but it is possible to receive sheets of the material.
The process of thermoforming typically begins with a roll of film passing between the upper and lower heaters that soften the film. It is possible to use a heater on only one side, but this can make for an uneven heat distribution and cause problems in the forming process. It is critical to have an even heating distribution in the thermoforming process. If the film has multiple layers, it is imperative to heat both sides of the film. Optimal temperature is imperative because if there is excess heat, the film will become too soft and could even melt. If the film is too soft, it can overstretch, resulting in a film so thin that it is unusable. Once the film has been heated to the optimal temperature, it heads to the forming station. The forming station contains an upper and lower die to form the package to the correct shape. It is most common for the female die to determine the final shape of the film, but it is possible for the male die to have this job.
The forming process first starts with the film being positioned over the dies and then they are closed. Once closed, compressed air is blown over the top of the film and a vacuum is in place at the bottom to suction the film to the mold. Having a vacuum at the top and bottom is possible. Some packages are large and need more assistance to form to the mold properly. When forming large packages, a plug can be added to the male section to provide even distribution. Depending on how the plug is made, it can also help add extra thickness to areas through selective film distribution. It is common for the dies to be cooled with water, which in turn helps cool the film to set it.
Once the package is formed and cooled, it is ready to be filled with the desired product and is moved to the filling and loading station. There are many different techniques that can be used to fill the package. Sometimes the product will fall into the cavity with ease, for example, pharmaceutical pill capsules. In this case, the product can be flood filled. Flood filling uses a feeder to place a hopper of product on the film; brushes can be used to help guide the product into the cavities. It is important to think about the product and the best approach for filling. Liquids are going to need liquid filling systems, while other products may be manually loaded.
Once the product has been placed in the package, it needs to be sealed before it can be sent to the distribution line. These packages are commonly sealed with foil, plastic, or a paper backing film. The sealing film is placed over the package and enters a sealing station. Sealing is always dependent on time, temperature, and pressure. Sometimes a tab is desired to allow the package to be peeled back for easy opening. This is done by relieving part of the platen.
At this point, the package has been formed, filled and sealed and all that is left is removing the excess material. To do this, the package moves to a cutting station where knives or slitters are used to remove the unwanted material. There is little waste with thermoformed packages since it is just the edges that are removed. The excess is normally wound on reels and guided to waste bins. Sometimes the packages are die cut to allow a rounded edge and this excess material is removed from the machine by winding it into a take-up reel. This is not ideal as the waste is bulky, but it can be fed into a chipper that chops the material into a waste bin.
If you need to form plastic into a three-dimensional package, thermoforming is one of the least expensive methods. And while it has a few drawbacks, they pale in comparison to the advantages of using thermoforming for blister package production.
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