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Mechanical Design

The Ins and Outs of Thermoforming

Throughout my design experience, there have been times when I thought I had designed the perfect part to find that I didn’t know the correct process to manufacture it.

As designers, the more we know about how things are made, the better we are at designing new parts. That’s why thermoforming can be a huge asset to add to your toolbox when planning your designs for production. Overshadowed at times by the much more common injection molding, thermoforming is a unique process that can even provide opportunities to create detailed geometry.

Before we get into the nitty gritty of what thermoforming does well, let’s start with the fundamentals and take a look at how thermoforming works. 

 

Thermoforming Basics

Thermoforming begins with heat and a mold. A sheet of thermoplastic is heated up and stretched over a mold to create a part.  Usually, the machine generates enough heat to not fully melt the sheet, but at the perfect temperature to allow it to easily form and shape the plastic. The mold, which can either be female or male, is made of a variety of materials that then contours the hot plastic into shape. Once the sheet has cooled on the mold, it can be trimmed to leave the desired part.

thermoforming-principle
Figure 1: Thermoforming starts with preheating plastic sheet and stretching it over a mold to create the pre-designed shape.

There are two main types of thermoforming: vacuum thermoforming and pressure thermoforming.  Vacuum forming removes air between the part and the mold to pull the material against surface as tightly as possible. Pressure forming adds air pressure to the top surface of the part to push it against the mold.

pressure-thermoforming
Figure 2: Pressure thermoforming starts with the same preheated plastic sheet and vacuum pressure but adds additional air pressure on top of the part to force it against the mold.


When it comes to picking a material for thermoforming, a wide variety of thermoplastics work well. Some of the more common materials include HIPS, PET, and ABS, though other materials like PC, HDPE, PP, or PVC also work. Different thickness sheets can be formed, and the size of parts can be in inches all the way up to feet.

 

When to Use Thermoforming

Right away, it’s easy to compare thermoforming to injection molding, since they are somewhat related. Injection molding uses molten plastic or rubber and injects into a cavity, whereas thermoforming uses flat material and stretches it into a part.

Size is the big benefit for thermoforming versus other processes, as it can allow for larger parts. If you have a very large part that is of uniform thickness, for example, thermoforming is a potential choice. With a large mold using injection molding, more force is required to hold it shut. However, with thermoforming, that's not a concern.

It also does well at making thin-gauge parts. Widely used in the packaging industry, thermoforming can easily create disposable cups, containers, lids, and trays cost effectively. Thin material also allows for more leeway with undercuts.

thermoforming-packaging
Figure 3: Blister packaging like this pill pack is a great example of a thermoformed part made from a stiff clear plastic like PET.


Considerations for Thermoforming

As great as thermoforming may sound, there are a few things to look out for when getting ready to form. First, it’s important to notice corners and how they might change during the molding process. Try and keep a radius on corners and edges so that these areas won’t thin when forming.  

Also consider the depth of a cavity. It can’t exceed a certain limit, as the material has to stretch to create each feature. If the draw is too large, the material will be too thin and won’t be able to form the shape.  A certain amount of draft is also required to ensure the part can be released form the mold.

If there is one side of the part that needs better dimensional accuracy than the other, it’s important to specify this early on, since using a male versus female mold can help bring this out.
 

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