4 Types Pretreatment for Plastic Painting

The process of painting plastic materials requires a delicate touch to ensure a durable and aesthetically pleasing finish. Unlike traditional painting on metal or wood, painting plastics involves unique challenges due to their chemical composition and physical properties. To achieve the best quality in plastic painting, it is crucial to apply the necessary pretreatments. This article will delve into four popular pretreatment methods that are essential for a successful plastic painting system: removal of internal stress, decontamination, dust removal, and addressing paint adhesion issues.

Removal of Internal Stress

Plastic products are susceptible to the development of internal stress during the molding and injection process. This stress can lead to stress concentration, which in turn may cause cracks in the plastic parts during the painting process. To mitigate this, it is essential to remove the internal stress before painting.

There are two primary methods for stress removal: flaming and full-face treatment. Flaming treatment is a quick and direct method that is popularly used for automotive parts such as bumpers. It involves exposing the plastic to a flame to relieve the internal stress. Another approach is to heat the plastic part below its heat distortion temperature, typically around 60°C, and maintain this temperature for approximately two hours. This process necessitates the use of large baking lines, making it more suitable for industrial applications.

For a more accessible and scalable solution, a full-surface processing technique can be employed. This involves immersing the plastic in a liquid solution that is capable of eliminating internal stress at room temperature for a period of 15 to 20 minutes. After this treatment, it is imperative to ensure that the plastic is thoroughly dried before it is loaded into the painting machine.

Decontamination

The surface of plastic parts is often contaminated with substances such as oil, hand sweat, and mold release agents. These contaminants can impede the proper adhesion of paint, leading to issues like cracking, blistering, and peeling of the coating. Therefore, a thorough degreasing treatment is a prerequisite before painting.

Plastic parts are typically cleaned using solvents like gasoline or alcohol, followed by a chemical degreasing process. It is crucial to remove any residual chemicals from the product's surface, which can be achieved by rinsing with pure water and then drying. Depending on the condition of the surface, different cleaning agents may be used, each with its own specific process.

High-Pressure Ion Air Removal and Dust Removal

Plastic parts, being insulators, have a high surface resistance, generally around 10^13 Ω. This high resistance makes them prone to generating static electricity, which can attract and hold fine dust particles from the air. These particles adhere to the surface due to electrostatic forces, making them difficult to remove with conventional blowing methods.

To effectively remove dust and neutralize static charges, a high-pressure ionized air stream is recommended. This method not only dislodges the dust particles but also reduces the surface resistance, preventing further accumulation of dust.

Paint Adhesion Promoter

Materials such as nylon and polypropylene (PP) have high surface crystallinity and low polarity, which can result in poor adhesion when applying conventional paints like rubber paint or polyurethane (PU) paint. The weak bond between the substrate and the paint can lead to the paint chipping or peeling off.

To enhance paint adhesion, an adhesion promoter can be applied before the paint. This promoter works by disrupting the molecular arrangement on the substrate's surface, increasing its surface energy, or dyne value. By doing so, it creates a more receptive surface for the paint, thereby improving adhesion and preventing paint from lifting or flaking.

In conclusion, the success of a plastic painting project hinges on the proper application of pretreatment methods. By addressing internal stress, decontaminating the surface, removing dust with high-pressure ionized air, and ensuring paint adhesion, one can achieve a high quality, durable, and visually appealing finish on plastic materials.