What Is the Difference Between Thermoforming and Compression Molding?

In plastic manufacturing, thermoforming and compression molding are two common methods. Both use heat and molds to shape plastics, but their processes, applications, and finished product characteristics are quite different. Here’s a simple guide to help you understand and choose between them.

What is Thermoforming?

Thermoforming begins by heating a plastic sheet until it becomes soft and pliable. The sheet is then stretched or vacuumed over a single-sided, open mold. Once cooled and hardened, the shaped part is removed, and excess material is trimmed away.

Key points:

Ideal for thin-walled parts
Uses simple, open molds
Fast production with lower mold costs
Common applications: packaging, trays, and simple plastic products

What is Compression Molding?

Compression molding uses a measured amount of plastic material, often a reinforced thermoset resin called a "charge," which is placed into a heated, multi-piece closed mold. High pressure presses the material into the mold until it hardens. The result is a thick, strong part that usually requires little finishing.

Key points:

Suitable for thick, strong, and precise parts
Uses complex, closed steel molds
More time-consuming and higher mold costs
Common applications: automotive parts, aerospace components, industrial fittings

Key Differences Between Thermoforming and Compression Molding

The main differences between thermoforming and compression molding lie in the material used, mold type, production process, and finished part characteristics.

Material and Shape

Thermoforming works with pre-made plastic sheets, which are heated and shaped to create thin, simple parts. In contrast, compression molding uses measured amounts of plastic or reinforced resin, known as “charges,” to produce thicker, stronger parts that can withstand higher mechanical stress.

Molds

Thermoforming relies on open, single-sided molds, which are easier and cheaper to make. Compression molding, however, requires complex, multi-piece closed molds made of hardened steel, designed to withstand high pressures and ensure precision in the final part.

Production Speed and Cost

Thermoforming is generally faster and more economical, especially for high-volume production of simple shapes. Compression molding is a slower and more expensive process, due to the time needed to heat, press, and cure the material, as well as the higher cost of durable molds.

Finishing and Precision

Thermoformed parts often require trimming or additional finishing because the process starts with a larger sheet than the final shape. Compression-molded parts, on the other hand, are typically ready to use straight from the mold, with high precision and minimal post-processing.

Which One Should You Choose?

If you need a fast and economical way to produce simple, thin plastic parts, thermoforming is the right choice. However, if you need strong, precise parts and have a larger production scale, compression molding is the better option.