In order to receive the needed surface finishing, plastic products need various pretreatment including cleaning, heating and blasting etc. Flame treatment is a popular solution in many plastic painting lines. However it’s not the solely choice according to the exact plastic components. In this article we’ll briefly introduce the pros and cons of flame treatment and another solution called annealing.
What is Flame Treatment?
Flame treatment is a surface treatment technique performed on plastics before painting. It uses flame to briefly heat the plastic surface at high temperatures to improve wettability and adhesion. This treatment effectively removes oil, release agents, and other impurities from the plastic surface, while also increasing the surface’s polarity, thereby improving paint adhesion.
Principles of Flame Treatment
Flame treatment uses a high-temperature flame to transfer energy to the plastic surface, vaporizing oil and impurities. Simultaneously, the ions in the flame react with the plastic surface to form a layer of charged polar functional groups, increasing its surface energy and its ability to absorb liquids.
Advantages and Disadvantages of Flame Treatment
Advantages:
- Environmentally friendly and clean
Flame treatment does not produce harmful dust, making it a relatively environmentally friendly surface treatment method. - Improving Adhesion: Flame treatment significantly improves the adhesion of plastic surfaces, resulting in a more durable coating.
- Easy to Operate: Flame treatment equipment is simple and easy to operate, suitable for plastic parts of various shapes.
Disadvantages:
- Technical Requirements: Precise control of flame temperature, gas selection, gas flow rate, and treatment time is required to ensure consistent and stable treatment results.
- Environmental Requirements: Flame treatment requires a high environmental performance and requires fire prevention measures to prevent fire accidents.
- Thermal Effects: If the flame remains on the workpiece surface for too long, the workpiece may burn, leading to accidents. Flame treatment is not suitable for materials that are not heat-resistant.
Annealing Treatment
Why Annealing?
Uneven plasticization in the barrel or inconsistent cooling rates within the mold cavity often lead to uneven crystallization, orientation, and shrinkage, resulting in internal stress in the product. This is particularly prominent when producing thick-walled products or those with inserts. The presence of internal stress can degrade mechanical properties during storage and use, leading to silver streaks on the surface and even deformation and cracking. In actual production, the most effective way to address this problem is to anneal the product.
Annealing Methods
Annealing involves placing the product in a constant-temperature heated liquid medium (such as hot water, mineral oil, glycerin, ethylene glycol, liquid paraffin, etc.) or a hot air circulation chamber for a period of time, followed by slow cooling to room temperature. The heat from annealing accelerates the relaxation of macromolecules in the plastic, thereby eliminating or reducing residual stress in the product after molding.
Different Plastic Annealing Treatment Conditions
| Type of Plastic | Treatment Medium | Thickness /mm | Temperature /℃ | Time /min |
| ABS | Water /Air | — | 60~75 | 16~20 |
| PS | Water /Air | ≤6 | 60~70 | 30~60 |
| ＞6 | 70~77 | 120~360 | ||
| PMMA | Air | — | 75 | 16~20 |
| POM | Air | 2.5 | 160 | 60 |
| OIL | 2.5 | 160 | 30 | |
| PP | Air | ≤3 | 150 | 30~60 |
| 4~6 | 60 | |||
| HDPE | Water | ≤6 | 100 | 15~30 |
| ＞6 | 60 | |||
| PC | Oil /Air | 1 | 120~130 | 30~40 |
| 3 | 120~130 | 180~360 | ||
| >6 | 130~140 | 620~960 | ||
| PET | Nitrogen furnace | 3 | 130~150 | 30~60 |
| PBT | Nitrogen furnace | 3 | 130~150 | 30~60 |
| PA6 | Water | >6 | 100 | 25 |
| PA66 | OIL | 3~6 | 130 | 20~30 |
| Water:
Potassium acetate
(1:1.25) |
3~6 | 100 | 120~360 | |
| PA1010 | Water | 6 | 100 | 120~360 |
| PPO | OIL /Air | 3~6 | 120~140 | 60~240 |
The annealing temperature is generally controlled to be 10-20°C above the product’s operating temperature, or 10-20°C below the plastic’s heat distortion temperature. Excessively high temperatures can cause warping, while too low temperatures will not achieve the desired annealing effect.
The annealing time depends on factors such as the type of plastic, the temperature of the heating medium, the shape and thickness of the product, and the required precision. The optimal time is to eliminate internal stress in the product. After annealing, the product should be cooled slowly to room temperature. Rapid cooling may re-introduce internal stress. For thin-walled products, the cooling rate is 50-60°C/hour; for thick-walled products, the cooling rate is approximately 10°C/hour or less.
