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Annealing Plastics: A Key Process for Ensuring High-Quality Plastic Parts


Plastics are ubiquitous. You can find them in just about everything—from utensils, wires, and piping to bicycle helmets, automobile airbags, and child safety seats. These materials are popular with manufacturers because of their production advantages.

Plastics are lightweight, low-cost, corrosion-resistant, and easily moldable. They’re also versatile and low density. These advantages make them an almost ideal material for producing manufactured parts.

Generating high-quality manufactured plastic parts, though, has its challenges. Annealing plastics in an industrial oven can help you overcome these challenges and elevate part quality. It also offers additional benefits that streamline working with plastics.

But you might have assumed that annealing was solely for metals. That’s not the case; plastics can also undergo annealing. Heat-treating plastics in an industrial oven can help you address manufacturing challenges specific to plastics and provide a competitive edge.

Advantages of Annealing Plastics

Plastic parts can contract and expand over time when used in various applications. Annealing exposes the part to this aging process in the extreme, making the material more resistant to cracking and surface defects. Annealing creates:

  • Tighter tolerances
  • Increased dimensional stability over time
  • Enhanced material rigidity and wear resistance
  • Improved flatness control  
  • Better chemical resistance
  • Increased ability to handle machined stresses

Put simply, annealing alters the materials’ internal properties without affecting its structural integrity.

Annealing Plastics Reduces Internal Stress

So, how do internal stresses occur?

Manufacturing parts from plastics often involves heating the plastic first and then injecting it into a mold cavity (injection molding) or extruding it on a surface (3D printing). The part is then allowed to cool and solidify into its final shape. However, the cooling process is usually rapid and non-uniform.

Because plastics are poor conductors of heat, cooling can result in the formation of internal stresses during the manufacturing process. Additionally, internal stresses in plastic can be introduced when drilling deep holes, reducing uneven thicknesses, or cutting screw threads. So can machining a molded part if you use a poorly designed cutting tool, that can overheat the material locally, thanks to excessive cutting and feed speeds.

Creating internal stresses makes plastics more susceptible to:

  • Cracking
  • Excessive wear
  • Poor chemical resistance
  • Diminished overall performance

It’s worth noting that some plastics, such as polycarbonate and polysulfone, are more prone to the formation of internal stresses than others. However, annealing plastics serves as an effective solution to eliminate these internal stresses caused during the manufacturing process.

Types of Plastic Heat-Treating Methods

Two types of heat-treating methods for plastics exist. Both methods are effective in relieving internal stresses in plastic parts.

  • Normalizing: This method increases plastics’ resistance to stresses and creates a uniform structure. With normalizing, the material is heated to a specific temperature and then allowed to cool at a controlled rate, which alters its properties. Manufacturers use normalizing to get a calculable microstructure, enhance a plastic’s mechanical characteristics, and improves the material’s ability to withstand substantial impacts.

  • Annealing improves a plastic’s physical and chemical properties, increases the material’s ductility, and boosts wear resistance. Plus, it makes the material more malleable. Annealing produces a predictable microstructure, just like normalizing. This heat-treating method is ideal for plastics that require stress relief created during production, after machining, or where a material would crack after post-polishing operations.

You can use annealing as a stand-alone process or combine it with other techniques, such as injection molding or extrusion.

Steps in Annealing Plastics

The process for annealing plastics is relatively similar to that of annealing metals— except you have to make sure you heat the plastic in temperature ranges that fall between the material’s glass transition temperature and melting point. Annealing is critical to the quality and use of the plastic—especially if you intend to use the part or parts in high-precision, industrial, or structural applications.

The annealing process is relatively straightforward. First, place the plastic part in a plastics annealing oven or a heating chamber that supplies constant heat. The heat’s temperature depends on the plastic.

Next, determine how long you want to anneal the material, adjust the temperature accordingly, and secure the part or parts in the heating chamber. Remember each type of plastic has different heating requirements and compositions. 

Finally, when you’ve finished heating the part in the chamber, let it cool at room temperature. Operators need to ensure the cooling rate isn’t higher than 0.5° C. That ensures that the plastic’s crystalline structure remains stable. This process also ensures that the part isn’t damaged and has no cracks.

Tips on Annealing Plastics

Three techniques for annealing plastics exist:

  • Batch annealing, the most common technique, occurs in a batch oven with forced convection. Many manufacturers use this technique, but it requires a batch-type process and takes longer because of the restricted airflow.

  • Conveyorized forced hot air annealing (CFHA) is a continuous annealing process where you place the plastic parts on a conveyor belt moving through an oven. This technique is faster than in the batch oven and, because of the continuous character of the technique, preferable in an industrial setting.

  • Infrared annealing also uses a continuous oven approach, but its heating method differs from CHFA. The technique heats plastic parts using energy transmitted by infrared radiation instead of hot air. While this is the fastest of the three techniques, it’s not always uniform. Shadowed portions of the part, for example, heat up slower.

Annealing plastics at a low temperature is preferable for enhancing a material’s strength. Annealing at a high temperature, on the other hand, is preferable for enhancing a material’s thermal resistance. Since machining plastic parts can cause cracking, you may want to anneal a machined piece to ensure you’ve eliminated this defect.

Annealing Differs for Different Plastics

The annealing process differs for all plastics, especially when machining is involved. Engineered plastics like Ultem and Torlon, for example, benefit significantly from post-machining processes. Annealing Torlon can take as long as seven days using special industrial ovens. Other plastics, like PEEK, need more immediate annealing steps to maintain tight tolerances and flatness.

Plastics require different annealing temperatures and conditions:

  • Polycarbonate: 250°F(121°C) for as short a time as possible, determined through experimenting with test data
  • Ultem: slowly brought up to 400°F(204°C) and then held for two hours
  • ABS materials: heat deflection temperature at 264 psi (170°F to 240°F) and held for 1-2 hours.
  • Super resins: Annealed at 400°F(204°C) for 4 hours

The environment and temperature maintained, both during and after the annealing process, and cooling rates are critical.

Key Features of Annealing Plastics Ovens

You can use just any industrial oven for annealing plastics. Instead, you need a high-performance industrial oven with exceptional heating consistency to get the most out of the process. It needs to be designed specifically for annealing, be high-performing, offer a variety of chamber sizes and temperature capabilities, and feature consistent temperature uniformity flexibility.

Plastic annealing ovens find use in a broad range of industries, including technology, aerospace, transportation, electronics, general manufacturing, and medical device manufacturing. Specialized annealing ovens enable manufacturers to tightly control and monitor the heating and cooling process. That allows for proper recrystallization to occur within the materials.

The materials are annealed at the eutectic point and then allowed to cool. Annealing softens the materials so you can shape or cut them easily and increase their strength and ductility. Annealing, however, isn’t needed in all plastic molding. Some stresses can be eliminated without the annealing process, depending on the design and materials.

Annealing Plastics Make Sense

Annealing plastic makes sense for manufacturers looking for a competitive edge. Annealing improves dimensional stability, boosts crack resistance, and enhances a material’s wear and thermal resistance, among other benefits. Plus, it boosts part value and quality.

Key considerations when annealing plastics are:

  • type of plastic
  • annealing medium (air, water, or oil)
  • annealing temperature
  • part thickness
  • holding time.

Some plastics may need a nitrogen-based atmosphere during annealing because they’re susceptible to surface oxidation at high temperatures. A few experimental trials can tell you the ideal temperature and time required for annealing a part.

Heat-treating plastics in a plastics annealing oven explicitly designed for the process provides the best results. But you may have to work with an experienced oven manufacturer to tailor your plastics annealing oven to your needs. Finding the right oven can help you create a profitable competitive advantage.

Despatch manufactures plastic tempering and plastic annealing ovens in batch and conveyor styles. If you need more information about our ovens, from design to installation and maintenance services, please call us at 952-900-6635.