During the thermoforming process, heat is applied to a sheet of plastic to make it more workable so that it can be placed into or onto a mold and take on the desired shape. Thermoforming has evolved to incorporate a wide variety of sheet structures and materials, and customers today expect elegant, tailor-made parts that can meet high industry standards.

There are some critical factors to consider when planning the thermoforming process for a production run.

Differences between Thermoforming Types

Thermoforming includes a wide range of processing methods. Some of the most common ones are:

Vacuum forming: A fast and cost-effective process, which is ideal for high-volume projects. In this process, plastic sheets, placed in a mold frame, are heated to the point where they become flexible. As the sheets are pressed into the mold, a vacuum pulls the air out. 

Pressure forming: As with vacuum forming, the sheets are framed in a mold. With pressure forming, however, an additional part presses the sheet into the mold as the vacuum removes the air. This results in a more detailed product.

Matched mold forming:  In this option, a two-sided mold forces the polycarbonate sheet to take on a specific shape. While this process is highly detailed, the tooling costs are higher due to the additional pieces required.

Hot line bending: During hot line bending, a narrow, heated strip, such as a hot wire or an electric heater, is used to soften the length of the sheet. Heat can be applied to one side or both sides of the polycarbonate sheet, depending on the thickness and duration. By using hot line bending properly, you can produce a product with almost no thickness loss at the bending area.

The vacuum-forming process, however, results in stretching and loss of thickets.

Free forming: Rather than using a mold, high air pressure is used to blow the heated polycarbonate sheets into a dome shape. The process is commonly used to create skylight domes and lighting fixtures.

Thin Gauge Thermoforming Vs Heavy Gauge Thermoforming

Packaging, containers, displays, and other applications that do not require loads to be carried are usually made of thin gauge plastic. On the other hand, heavy gauge thermoforming is used to make more constructive elements.

Polycarbonate sheets with a thickness of 5.0mm or less can easily be thermoformed using a one-sided heater machine. However, for demanding applications involving complex products or thick gauge polycarbonate sheets, using two-sided heated machines is recommended. Whether you use pressure molds, vacuum-forming or hot line bending, this recommendation applies.

During the thermoforming or vacuum forming process, the sheet’s thickness may become uneven. For such cases, UV protection layer thickness and structural integrity are key considerations. In order to maintain a certain level of robustness and to avoid weak points, consider the complexity of the shape and the thickness reduction. Plan your product with all these considerations in mind, and choose the right thickness for the processed sheet.

Thermoforming UV Protected Sheets

Solid polycarbonate sheets are a popular choice for thermoforming due to their excellent strength, clarity, and durability. When thermoforming polycarbonate elements for use in an outdoor environment, it is crucial to use UV-protected sheets. Some polycarbonate sheets are only UV-protected on one side. Plan the thermoforming process so that the UV-protected side faces the source of UV radiation. Sheets with an especially thick UV protective layer can be used for demanding applications. This allows for higher draw ratios.

Why and How Much to Pre-Dry

Pre-drying is necessary to ensure that the sheet has low residual moisture content so it can be formed properly. A low residual moisture level prevents bubbles from forming and molding. To separate the sheets when drying, a “clamp and hang” system is recommended.

If bubbles appear in the thermoformed product, either the sheets were not properly pre-dried or they were overheated. If this happens, you might try thermoforming at a lower temperature or pre-drying for a longer period of time or at a higher temperature. Pre-drying times may vary depending on local conditions. You can test whether your sheets are well dried and prepared by cutting a 50mm x 50mm slice of your pre-dried sheet and baking it at 180-190°C. After five minutes, if there are no bubbles, the sheet is dry. If bubbles do appear, the sheet still needs further pre-dying.

Pre-Drying General guidelines*

Preparation and preheating of the mold

Mold preheating shortens your start-up time and prevents induced stresses. Use the minimal vacuum pressure necessary to obtain the required shape. Excessive vacuum pressure may result in suction marks.

Thermoforming Sheets Masked with PE Foil

There are two aspects to consider regarding masking foil: How easy it is to remove the film after thermoforming, and whether the film leaves marks.

In order to avoid leaving marks on the fabricated product, make sure the masking material is monochromatic and label-free. 

Some masking foils made from polyethylene (PE) may not be suitable for thermoforming. It is crucial to choose a brand that uses PE masking engineered for thermoforming. These maskings are pre-drying compatible and will peel off easily after the fabrication process has been completed.

 

Contact us for more details, or Download our Complete Technical Guide for Solid Polycarbonate Processing