Plastic Thermoforming

A plastic thermoformed part is a plastic component created using the thermoforming process. Thanks to advances in machine technology and innovative materials, plastic thermoformed parts are becoming increasingly attractive for various applications. The low tooling costs and quick project lead times make thermoforming an especially interesting manufacturing method.

How is plastic deformed during thermoforming? The forming process in thermoforming works as follows:

1. Heating: In the first step, the thermoplastic is heated to the specified forming temperature.
2. Forming: During the forming process, the semi-finished material is drawn into the desired shape. The material is clamped into a frame and placed on the mold.
3. Cooling: Once the semi-finished material touches the mold, the cooling process begins. The material's surface cools due to contact with the aluminum surface of the mold.
4. Separation: Once the thermoformed part has cooled, the demolding process takes place. Cutting methods such as punching or milling are used to remove any leftover material.

The thermoforming process usually only takes a few minutes. The speed and duration of the cycle times depend mainly on the thickness of the starting material. When the plastic is thin, the thermoforming process can be performed more quickly.

The longest part of the process is heating the plastic sheet. The actual thermoforming process, however, only takes seconds. Afterward, the formed part is cooled and hardened for a few minutes.

formary offers various tooling options for prototypes and series production in plastic thermoforming.

Prototype tooling:

• 3D printed tools for individual nests
• Full-area 3D printed tools
• Ureol tools for partial segments
• Full-area Ureol tools

Production tooling:

• Production tooling made of aluminum with partial milling of individual nests for component fitting tests
• Full aluminum production tooling

You can learn more about tooling configurations, special components, and prototype options here.

The difficulty of thermoforming plastic depends primarily on the temperature at which the material can be deformed, the size of the temperature window, how much the material sags when softened, how well it can be stretched, and how much it shrinks during and after processing. The required machine equipment and tooling technology also play a role, as materials that are formed at high temperatures require more complex cooling in the mold.

Properties of plastics

High-impact polystyrene (HI PS), commonly referred to as PS in thermoforming, is considered the reference plastic because it has very good thermoforming properties in the mentioned areas. On the other hand, polypropylene (PP) is difficult to thermoform due to the high shrinkage of 2% (the thermoformed part loses 2% of its volume after cooling) and the significant sagging, making it challenging for thermoformers. In some cases, additional machine components are required to counteract the sagging.

Almost all common thermoplastics can be used in plastic thermoforming. It is important that the material is extruded in the form of sheets or films, as pre-extruded material is used in thermoforming.

Types of plastics

Plastics are divided into amorphous and semi-crystalline thermoplastics, which differ in their structure. While amorphous plastics soften and can be shaped at lower temperatures, semi-crystalline plastics remain solid longer. The melting of the crystalline structures only occurs at high temperatures and quickly, meaning the thermoforming window is small. This makes processing semi-crystalline plastics more challenging by nature.

There are also distinctions between standard and engineering plastics, with temperature resistance being a key factor. Additionally, compounds and multi-layer films made of various plastics are used, extruded in layered forms.

Plastics used for thermoforming

Common plastics include amorphous thermoplastics such as:

• ABS
• PS
• PMMA
• PC

And semi-crystalline thermoplastics such as:

• PP
• HD-PE
• A-PET
• PET-G
• PVC

formary uses function-oriented tolerance design in plastic thermoforming, meaning that tolerances are derived from the product requirements.

Especially in automation projects, the tolerances for thermoforming must be clarified from the beginning. If required tolerances cannot be met in plastic thermoforming, we will notify you early or work on alternative options.

Overview of tolerances in plastic thermoformed parts:

• Length measurements (ISO 2768-m):
  • 30 to 120mm: +/- 0.3mm
  • 120 to 400mm: +/- 0.5mm
  • 400 to 1,000mm: +/- 0.8mm
  • 1,000 to 2,000mm: +/- 1.2mm
• Length measurements (ISO 2768-c):
  • 30 to 120mm: +/- 0.8mm
  • 120 to 400mm: +/- 1.2mm
  • 400 to 1,000mm: +/- 2.0mm
  • 1,000 to 2,000mm: +/- 3.0mm
• Hole diameter:
  • 0.15mm for holes <25mm
  • 0.25mm for holes between 25 and 125mm
• Slits:
  • 0.25mm for holes <25mm
  • 0.5mm for holes between 25 and 125mm
• Milling:
  • 0.5mm for holes >125mm

More about Tolerances

To press the heated plastic onto the mold, which the thermoformed part should take the shape of, two techniques are available: vacuum forming and compressed air forming.

Vacuum forming

In vacuum thermoforming, a vacuum pump creates a negative pressure that pulls the material onto the mold. With smaller forming pressures of up to 1 bar, this method is mainly used to produce simpler thermoformed parts that must be manufactured at low cost, as it typically involves cheaper tooling and simpler machines.

In compressed air thermoforming, the material is pressed against the mold using compressed air. Much higher pressures of up to 10 bar can be applied. Due to the shorter necessary heating and cooling times (due to the higher pressures), faster cycle times can be achieved, lowering the per-piece cost. This method also allows for the forming of more complex geometries.

Compressed air forming process

In addition to high production volumes, compressed air forming is particularly suited for high mechanical and optical requirements, such as clarity and material distribution with technical plastics like polycarbonate. Pre-printed parts are also much easier to form with compressed air. 

Alternatives to plastic thermoforming in plastics processing are 3D printing and injection molding.

Put simply, 3D printing techniques are used for smaller quantities, while injection molding remains an attractive option for large quantities in terms of price.

Historically, plastic thermoforming has therefore been positioned between 3D printing (small quantities or prototypes) and injection molding (large quantities). However, new and optimized techniques and materials are making plastic thermoforming attractive in completely new areas that many companies are not familiar with. 

Plastic thermoformed parts can undergo various surface treatments to prepare them optimally for use. formary offers the following surface treatments in plastic thermoforming:

• Polishing
• Painting
• Printing
• Metallizing
• Galvanizing

Learn more about surface treatments.

Various cutting methods can be applied in plastic thermoforming:

• Punching in plastic thermoforming
  • Roller punch
  • Band steel punch
  • Shear cutting tool with die and part
• Milling in plastic thermoforming
  • Multi-axis milling
  • Water jet cutting
  • Laser cutting

For punched edges, burrs (edges, fibers, or small splinters) can occur. If a smooth edge contour is required for the final product, additional deburring can be done. Deburring is either performed manually with a deburring tool or automatically using robotic arms after the milling process.

Learn more about separating.

Yes, plastic thermoforming offers many possibilities for assembly or adding additional components to the thermoformed part.

• Welding in plastic thermoforming
  • Rotational welding
  • Ultrasound welding
  • Vibration welding
  • Heating element welding
  • High-frequency welding
  • Induction welding
• Bonding in plastic thermoforming:
  • When bonding in plastic thermoforming, the correct adhesive must be chosen for the specific thermoplastic. Our material suppliers often provide assistance here.
  • It is important that surfaces are free of particles before bonding. Some thermoplastics with anti-adhesive surfaces, such as PP or HD-PE, may require chemical surface treatment before bonding.
• Screwing in plastic thermoforming:
  • Due to the low strength of plastics, larger diameters should be used. Special plastic screws are recommended.