There are two predominant methods of thermoforming that can be used to create shapes out of plastic substrate: free blowing and molding. The overwhelming majority of Replex mirrors are created by free blowing.

Free Blowing: Vacuum Pressure vs. Air Pressure

During the free-blowing process, substrate sheets are heated to a point of malleability. A bubble is then created and regulated by stretching the sheet through the application of either compressed air or a vacuum to one side of the heated sheet.

We most often create the necessary pressure differential through the use of a vacuum chamber, which allows for very effective process control. The main reason we recommend vaccuum pressure instead of air pressure is safety.

The danger lies in the fact that if a sheet is not hot enough when it is stretched—it violently breaks. During the free-blowing method, all the flying projectiles would be contained within the steel vacuum chamber. If a sheet explodes using the air pressure method, however, hot plastic shrapnel is dispersed uncontrollably.

Benefits of Free Blowing

In the free-blowing process, no solid object ever makes contact with the hot substrate, so there is no chance that tiny particles could contaminate the finished product.

Also, the output is extraordinary. Depending on the size and thickness of the product, our machines can typically produce 10-20 cycles per hour. Most cycles last just three minutes. The entire system is automatic and computer controlled. Process settings can easily be changed for each job, and are very repeatable over calendar months and years. When the finished products are smaller, the tools can contain multiple cavities and production can therefore be increased dramatically. For example, a sixteen cavity tool running at 15 cycles per hour produces 240 parts per hour with only one operator.

Molding: The Best Option for Non-Spherical Shapes

The single drawback of free blowing is that it’s hard to make non-spherical shapes accurately. In those cases, a mold has to be precisely machined to the exact shape that the customer wants (it will actually be slightly larger as the part will shrink during cooling).

The substrate is then heated and drawn into the mold, with a vacuum applied to extract all the air. When the mold is removed, the plastic retains the correct shape.

Drawbacks of Molding

It is all but inevitable that particulate matter on the surface of the mold, texture in the surface of the mold, as well as complex thermal effects will imprint the heated plastic. For most thermoforming applications, this wouldn’t be a big deal because the imprints are incredibly small, and most thermoformed parts are not optically critical parts. But with mirrors, those tiny blemishes become exaggerated and visible once the work is finished.

Although quality plastics manufacturers have developed proprietary technology and techniques that minimize this effect, the process is still slower and requires extra steps to maintain strict product integrity. Free blowing with vacuum-induced pressure differential is safe, fast, efficient, and affords manufacturers the highest surface quality.