Thermoforming
I’m back to share with you even more ways to form plastic parts! This week, we’ll delve into the details of thermoforming and how exactly plastics are formed using this method. Thermoforming can be broken down into two main categories: formed by air pressure and formed by tool compression. Methods using air pressure include vacuum forming, pressure forming and twin sheet molding. Then compression molding has many variants involved as well but all tend to be simply called compression molding.
Vacuum forming is a very commonly used low cost method of forming simple shapes. The process works by taking a thin sheet of plastic, heating it up until it becomes pliable, draping it over a mold or form, and then pulling a vacuum from underneath to suck it on tight to the mold thereby causing it to form tightly over the mold and taking on the curves and edges. This is a very low cost and effective method of forming plastic. You might recognize these plastics from packaging for electronics or consumer goods.
Check out the video below to see how vacuum forming works:
Pressure forming is just like vacuum forming, except rather than pulling just a vacuum through little holes in the mold, you pull both a vacuum and add pressure to the other side. This allows for even more detail and fine lines. With this process you can form logos or words with good clarity. In this method you can also add texture to the tool to provide a nice surface finish.
Twin Sheet Molding
Twin sheet molding is a very similar process to pressure forming. In this particular method, two sheets of plastic are inserted into the mold. Then a vacuum is pulled in both the upper and lower part of the cavity and air is forced between the sheets. The sheets take on the shape of the two mold halves while leaving a hollow in the middle. The edges of the sheet are pinched together in the tool and end up joined thus completing the hollow cavity.
The final step for all these manufacturing methods is to trim off the excess material. This can be done as a secondary with a die that simply chops the edge or with a waterjet machine. Another method is to build into the mold a mechanism and fixture that cuts the excess material off automatically before the part is demolded. All three methods produce good edge quality and can be used for A-surface parts.
For all these methods, prototype molds can be made very low cost from urethane or ren board materials to produce low quantities of parts. Or they can be made from aluminum or steel to produce millions of parts. Production tooling is also very cost effective because it includes air lines only. The tooling does not have complicated cooling lines or injection manifolds with gates. However, because of this it is a slightly longer cycle time and more raw material is used, so the piece price is higher than injection molding. There are also numerous design limitations. Vacuum forming is a great option for packaging your final product or for creating bulk packaging trays for shipping individual components to your final assembly location. But it can also be great for A-surface parts or anything that is large but doesn’t have undercuts.
Many of your common thermoplastic materials are supplied in sheet form and are suitable for these types of manufacturing. You can go as thin as .12mm all the way up to 20mm thick. The method can be used to create packaging for the tiniest chips to large advertisement housings. There are a number of design concessions that need to be made for any of the pressure forming methods. When the plastic is drawn to form a depression the plastic stretches thin and you get thinner walls. So that needs to be taken into account when designing your part to ensure you can manage the thinner walls. There is also a limit to how deep a draw you can pull on a plastic sheet before getting molding deformities and surface imperfections.
Compression molding can use thicker sheets and can also be used with more advanced materials like composites. Compression molding uses an A side and B side of a tool which come together in a press and compress the sheet material into the desired shape. Both the material and the mold are heated to ensure the material is pliable enough to fill in the features of the mold. Often for complex shapes the sheet goes through a pre-forming stage, and then is shaped into its final form in the compression mold. These tools are more expensive as the mold is heated and cooled through the compression cycle and therefore is more complicated to build. Whereas with vacuum forming only the sheet material is heated until it becomes pliable.
Compression molding has numerous advantages over vacuum forming because with the heated mold and high pressures you can force the sheet compound into very complex shapes. You can add ribs and other structural features in addition to deep draws. However just as with vacuum forming you cannot add undercuts.
When it comes to the materials you can use, it is very similar to vacuum forming in that any thermoplastic material can technically be made into a sheet and used for compression molding. But the biggest advantages lie in the advanced materials that can be used. You can add glass fibers or even carbon fibers for increased strength of the part. You can also add glass and carbon fiber to injection molded plastics. But they tend to be short fibers and its hard to control the direction in the molten plastic. With sheet molding compound you can often have much longer fibers and can control their orientation better. This leads to improved strength of the product. I suppose in this way compression molding is the middle ground between composite layup and injection molding.
What makes compression molding interesting is the wide range of materials that can be used in essentially the same process. You can use plastic sheet, carbon fiber with resin, carbon fiber with epoxy, etc. You can even insert carpet or textile with a plastic sheet and mold them together to form a part!
The video below shows the process of compression molding with a high carbon fiber content epoxy part.
That concludes sheet molding manufacturing methods! Be sure to check in next week when we review all the other methods of manufacturing plastics.