Annealing is the process of heating the workpiece to relieve internal stresses and enhance its properties.
But is it applicable to plastics? What is the process involved in annealing plastics?
Annealing plastic involves heating a plastic part to reach half its polymer melting temperature and maintaining it for a while before cooling it back to room temperature. The treatment relieves the part from internal stresses and provides better resistance to crazing and more excellent dimensional stability.
This article discusses plastic annealing in detail by going through the process, its types, purpose, and things to consider when annealing plastics.
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What is Plastic Annealing?
Plastic annealing is the process of heating plastic workpieces to alter their internal properties without affecting their structural integrity.
In a nutshell, plastic annealing primarily requires heating a plastic part to a specific temperature between its glass transition (Tg) and melting point (Tm).
Heating a plastic component at the glass transition temperature allows its polymer structure to relieve the internal stresses developed during its fabrication.
The part is held at that temperature for a suitable time and then allowed to cool to complete the annealing process.
But the question is, how do internal stresses develop, and why is it important to relieve these stresses?
Manufacturing of plastic parts generally involves heating the plastic, followed by injecting it into a mold cavity (injection molding) or extruding it on a surface (3D printing).
The plastic material is then allowed to cool and solidify, taking the desired shape.
The cooling of the heated part is usually non-uniform and rapid, owing to the plastic's poor thermal conductivity, resulting in the disorientation of its polymer structure.
This disorientation in the internal structure leads to the formation of internal stresses and non-uniform shrinkage, which can affect the dimensional accuracy of the part.
Apart from that, the localized heating caused due to improper speeds and feeds during the machining of the plastic parts also develops internal stress within the parts.
Machining operations such as drilling deep holes, cutting screw thread, and unevenly reducing thickness contribute to forming internal stresses in a plastic part.
These internal stresses can cause the workpiece to warp, affect its dimensional accuracy, or even result in cracks on the surface of the plastic workpiece by enhancing its brittleness.
Therefore, after machining, the plastic workpiece is annealed to relieve its internal stress and avoid the part from failing under load.
How Long Does it Take to Anneal a Plastic Workpiece?
Annealing plastics generally involves three steps: heating the plastic, holding it at that temperature, and then allowing it to cool.
As a result, the time taken to anneal plastics depends upon the type of plastic, the size of the plastic workpiece, and the internal stresses developed in it due to previous operations.
Generally, plastic annealing involves heating the workpiece gradually at a rate of 30-50℃ (80-100℉) per hour.
Air-annealing plastic workpieces require around 4 hours to gradually heat the workpiece to its annealing temperature.
After heating the workpiece, the workpiece is held at this temperature with a holding time of around 30 minutes per 1/4" of thickness, i.e., a 1/4" thick workpiece requires 30 minutes of holding time, while the holding time doubles to 1 hour for 1/2" thick workpiece.
The cooling rate of the workpiece depends upon the properties required in the workpiece.
If your application requires excellent ductility, it is advised to gradually cool the workpiece at the rate of around 5-20℃ per hour, whereas, if ductility is not the decisive factor, you can quench the workpiece in water for rapid cooling.
Therefore, annealing a thick plastic workpiece can take almost a complete day to execute the process and attain the refined workpiece.
However, certain plastics such as Polyamide and ABS require a comparatively longer holding time, ranging from 24 hours to a few days, increasing the time taken to anneal them.
How is Plastic Annealing Done? 3 Methods
Annealing plastic involves heating plastic parts above their glass transition temperature.
Generally, hot air is used as the medium for heating the plastic workpieces, without deforming them.
The plastic workpieces are placed in an enclosed chamber, such as an oven, and subjected to hot air, which raises the temperature of the workpiece above its glass transition temperature.
The parts are held in the oven at the desired temperature for a specific time before turning the oven off.
After that, the heat source is turned off and the part is allowed to cool at approximately 10ºC per hour until it reaches room temperature.
Therefore, annealing plastic consists of three major steps: heating the part, holding the temperature, and gradual cooling.
| Annealing stages | Purpose | Method |
|---|---|---|
| Heating the part to its annealing temperature | Allow the material to soften to release internal stress. | Heating the part in a suitable medium at a rate of ~30-60 ºC/hour |
| Holding the part at the annealing temperature | Ensure uniform temperature distribution to avoid hot and cold spots. | Hold the part for 2-4 hours based on its thickness. |
| Gradual Cooling of the part to the ambient temperature | Allows the polymer chains to move and recrystallize for enhancing the part's strength. | Cooling the part gradually at a rate of ~6-30 ºC/hour |
However, the optimal annealing conditions of a part depend on its fabrication methods like; injection molding, extrusion, and 3D printing.
Method of Annealing for Injection Molded Parts
| Parameter | Optimal Value |
|---|---|
| Annealing Temperature | 10 - 20℃ higher than the operational temperature |
| Holding Time | around 3 hours |
You can anneal the injection molding parts at a temperature of approximately 10 - 20℃ higher than their expected usage temperature.
Heating the part higher than 20℃ of its expected usage temperature can lead to several unwanted effects, such as discoloration or browning.
Generally, the recommended holding time for annealing injection molding parts is 3 hours but can vary depending on its thickness.
Annealing an injected molded part is not always required, as it will increase the cost with no significant effect on the properties of the workpiece.
For example, performing injection molding using pre-heated molds relieves the internal stresses of the workpiece and refines the grain structure, eliminating the need to anneal the product.
However, this increases the cost of the injection molding process, and is only suitable for applications where relieving internal stresses is always required.
Therefore, for standard injection molding parts (using cold molds), annealing is opted as a secondary process that is performed only when high strength and low internal stresses are desired.
Method of Annealing for Extrusion Parts
| Parameter | Optimal Value |
|---|---|
| Annealing Temperature | Around 145℃ |
| Holding Time | Around 24 hours |
Extrusion of plastic is generally performed to produce rods and plates that are generally thicker than injection molded parts.
As a result, these thick rods and plates require higher annealing temperature and holding time than injection molded parts.
The reason behind it is that owing to the high thickness, the time required for the heat to reach the center of the part is usually high.
For example, when annealing an acetal copolymer extruded rod with a diameter greater than 100 mm, the annealing temperature is about 145℃, and the holding time can be almost 24 hours.
Method of Annealing for 3D Printed Parts
| Parameter | Optimal Value |
|---|---|
| Annealing Temperature | Half the melting temperature |
| Holding Time | 2 - 24 hours |
In the case of 3D printing, as soon as the plastic filament leaves the printer's nozzle, it undergoes rapid cooling, and the part retains the amorphous structure of polymer chains.
Therefore, annealing 3D printed parts require a higher annealing temperature, which is around half their melting point.
After heating, a holding time of 2 to 24 hours is recommended, based on the thickness of the part, followed by gradual cooling.
Types of Plastic Annealing
Based on the type of material, its cross-section, fabrication method, and production scale, the annealing process varies from one application to another.
| Type of Annealing | Suitable Material | Part Cross Section | Fabrication Method | Production Scale |
|---|---|---|---|---|
| Oil annealing | Nylon resin | Thick plates>6mm and circular cross-sections with a diameter of 100 mm or more. | Extrusion | Batch to mass scale |
| Air annealing | All plastics except nylon | Best for parts with thickness ranging from 1 mm – 10 mm | Injection molding, 3D printing, machining. | Small to batch scale |
| Inline annealing | LLDPE, PVC, CPVC, PEX | Thick plates and pipes | Extrusion | Batch to mass scale |
| Infrared annealing | All plastics | - | - | Batch to mass scale |
Oil Annealing
Oil annealing is a type of annealing in which oil is used as a medium for heating the plastic workpiece.
It is one of the most effective plastic annealing technique but involves higher operational and maintenance costs.
Oil annealing is generally preferred for annealing nylon resin-based plastics.
Some of the commonly used oils include purified paraffin, hydrocarbon oil, or waxes.
It requires an electric heater to heat the oil and prepare the annealing bath for the plastic workpiece.
Different types of plastics require different heating rates, however, the holding time and cooling rates generally remain the same.
| Plastic Materials | Heating rate | Hold time | Cooling rate |
|---|---|---|---|
| Nylon 6 | 4 hours to 148°C | 30 minutes per 1/4” thickness | 10°C per hour |
| Nylon 6/6 | 4 hours to 176°C | 30 minutes per 1/4” thickness | 10°C per hour |
| PET-P | 4 hours to 177°C | 30 minutes per 1/4” thickness | 10°C per hour |
Before immersing the part, the oil is usually preheated to 80°C to avoid sticking over the part’s surface.
Following that, the temperature of the equipment is raised based on the part material.
After annealing, Trichloroethylene is usually used to wash the part to remove oil stains.
Air Annealing
| Plastic Materials | Heating rate | Hold time | Cooling rate |
|---|---|---|---|
| Delrin® acetal homopolymer | 4 hours to 160°C | 30 minutes per 1/4” thickness | 10°C per hour |
| Ardel® polyacrylate | 10°C per hour to 166°C | 30 minutes per 1/4” thickness | 10°C per hour |
| Hydex® 4101 PBT-P | 4 hours to 149°F | 30 minutes per 1/4” thickness | 10°C per hour |
Air annealing is the most common method and is usually performed in an air circulation oven.
In this type of annealing, the air surrounding the workpiece acts as the medium for heating it.
However, for applications where a high surface finish is required, the air is replaced with nitrogen gas to avoid oxidation of the workpiece.
Air annealing is more versatile as compared to the oil annealing method and the equipment is easy to maintain.
Therefore, air annealing is most commonly used for DIY applications.
Inline Annealing
Inline annealing is usually incorporated in extrusion systems dedicated to batch-scale production.
It involves a specialized heating system that is integrated with the extrusion system in such as way that the extruded workpiece is simultaneously annealed during the process.
This process is generally suitable for industrial applications.
Apart from plastics, this method is also used for annealing wires in industrial applications.
Infrared Annealing
Infrared annealing uses infrared radiation to heat the workpiece and alter its grain structure, thereby annealing it.
The IR annealing is the fastest of all the methods and a typical annealing cycle ranges from 60 seconds to 6 minutes based on the part thickness.
Despite its advantages, it is not suitable for DIY or small-scale applications because of the costly equipment and safety requirements.
Plastic Annealing for DIY Applications
Boiling Water Method
Boiling water method for annealing plastics uses water as the medium for heating the workpiece.
In this process, the workpiece is placed in boiling water for around 10 minutes to ensure that the part reaches the desired temperature and is evenly heated.
This process is best suited for 3D-printed PLA parts and usually tends to increase the stiffness of the part.
Sous Vide Method
In this method, the PLA parts are vacuum sealed in a plastic pouch and subsequently placed in a water bath with temperatures ranging from 50℃ to 80℃.
The holding time can vary from 2 to 48 hours, depending upon the thickness of the part.
This type of annealing treatment imparts better thermal resistance but is usually ineffective in enhancing the mechanical properties.
Things to Consider when Annealing Plastics
| Plastic Material | Annealing Medium | Annealing Temperature (°C) | Part thickness (mm) | Holding time (min) |
|---|---|---|---|---|
| Polycarbonate (PC) | Air | 125-130 | 1 | 30-40 |
| Polypropylene (PP) | Air | 150 | <6 | 15-30 |
| Polyethylene(PE) | Water | 100 | >6 | 60 |
| Nylon | Oil/water(with surfactant) | 130 | 12 | 15 |
| ABS | Air | 80-100 | 3 | 16-20 |
| Polystyrene(PS) | Air or Water | 60-70 | <6 | 30-60 |
| Polysulfone(PS) | Air or Water | 160 | <6 | 60-180 |
The annealing temperature and the holding time depend upon the factors like the thickness of the part and the medium of annealing (air, water, and oil).
For better accuracy, it is always recommended to decide the annealing temperature and time required for your part based on a few experimental trials.
Moreover, some plastic materials also require a nitrogen-based atmosphere during annealing as they are susceptible to surface oxidation at high temperatures.
Cooling of the part must be gradual, and the optimal cooling rate depends on the part's thickness.
To facilitate gradual cooling of your part, the best practice is to switch off your oven post the holding time and leave the part within until cooled.
Gradual cooling allows uniform alignment of the grains, thereby enhancing the strength of the material.
Annealing medium also plays a crucial role and must be selected based on the part's material and thickness.
For instance, the liquid medium provides a faster heating rate than air because of its higher rate of energy transfer via conduction.
Material like nylon is prone to oxidation on prolonged heating in an air medium, hence, an oil or water medium is preferred.
Purpose of Plastic Annealing
Improves Dimensional Stability
Plastic parts made out of materials like POM, PA, and PBT undergo huge volume changes post-solidification, which results in uneven shrinkage and dimensional instability of the parts.
Annealing helps recrystallize polymer chains of such parts for better dimensional accuracy and enhances their life.
Improves Crack Resistance
Usually, the rapid cooling rates involved in the plastic fabrication method lead to inheriting residual stresses in the parts, making them vulnerable to cracking.
Annealing can be beneficial in releasing such stresses for better practical use of such parts.
Enhances Mechanical Properties
Annealing a plastic part increases its density while enhancing its tensile strength, making it suitable for high-load applications.
The annealed parts usually are stiffer and provide better resistance to the stresses that can cause failure.
Improves Wear and Thermal Resistance
Recrystallization of the part’s polymer chains after annealing increases their thermal stability and wear resistance.
As a result, annealed plastic is suitable for electrical shielding in outdoor applications.
Final Thoughts
Annealing a plastic part enhances its ability to withstand loads, imparts durability, and improves thermal resistance.
Low-temperature annealing must be preferred for enhancing strength, whereas high-temperature annealing must be done when higher thermal resistance is desired.
Furthermore, machining a plastic workpiece can lead to internal stresses that can cause cracking of the workpiece, making it important to anneal the workpiece and eliminate the risk of cracking.
However, annealing plastics with metal inserts can result in cracking of the workpiece, and therefore, should be avoided.
Frequently Asked Questions (FAQ)
What is Conveyorized Forced Hot Air Annealing (CFHA)?
CHFA is a forced hot air-based continuous annealing process. The process uses a conveyor belt for moving the workpieces through an oven designed like a tube.
What is the glass transition temperature and annealing temperature of regular PLA used in 3D printing?
The glass transition temperature of regular PLA is 60℃ to 65℃ and its annealing temperature usually ranges between 55℃ to 70℃.
Why is air annealing not recommended for nylon resin?
Air annealing causes discoloration in the nylon resin which is not desirable.
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