In powder coating production, "poor coating curing" is one of the most troublesome problems—either the coating adhesion is poor, peeling off with a light scrape; or the impact resistance and flexibility fail to meet standards, making it impossible to pass quality inspection. Many people, when encountering this situation, mistakenly believe it's due to poor powder coating quality, but this is not the case. Poor coating curing usually stems from two main causes: firstly, an unreasonable powder coating formulation design; and secondly, inadequate control of the coating process. Today, we'll break down these causes one by one and provide targeted solutions to help you avoid detours in production.
1. The Powder Coating Itself
If the powder coating formulation design is flawed, even with excellent subsequent process control, it's difficult to produce a qualified coating. The core issues mainly focus on two aspects: Firstly, an unreasonable match between the resin and the curing agent. Resin and curing agent are like a "couple"—only when properly matched can a perfect cross-linking reaction occur, forming a strong coating. If the resin's reactivity is incorrectly selected—for example, a resin requiring high-temperature curing is paired with a low-activity curing agent; or the type and amount of curing agent are unsuitable—either the curing reaction will be insufficient or too vigorous, leading to a significant reduction in the physical and mechanical properties of the coating film, resulting in incomplete curing and cracking.
Secondly, improper filler addition. Fillers can enhance the hardness and wear resistance of the coating film, but everything in excess is harmful. If the filler's mass percentage or volume concentration is too high, it will directly affect the cross-linking reaction between the resin and the curing agent—like too many "bystanders" crowding into a "chemical reaction party," making it difficult for the resin and curing agent to fully contact, ultimately leading to incomplete curing of the coating film and reduced adhesion and flexibility. Therefore, when choosing powder coatings, it is essential to choose reputable manufacturers and ensure that their formulations have undergone rigorous testing, that the resin and curing agent are properly matched, and that the filler addition ratio is scientifically sound, thus avoiding curing problems from the outset.
2. Coating Process
More often than not, poor coating curing is not due to powder quality issues, but rather to improper process control during the coating process. The following five common problems are pitfalls that many manufacturers have encountered, so please pay close attention:
2.1 Baking Temperature Not Target
Many people believe that as long as the oven temperature is set to the value required by the powder, everything will be fine—but in actual production, the "no-load temperature" and "full-load temperature" often differ significantly. Under no-load conditions, the oven easily reaches the design temperature; however, once the workpieces are fully loaded (full load), if the oven's total heat supply is insufficient, the actual temperature may not reach the standard required for powder curing. Especially in northern winters, where workshop temperatures are already low, if the oven's insulation is poor, and the workpiece has a thick wall and high heat capacity (such as large metal components), the low temperature before entering the oven means more heat is needed for heating, making it easier to experience a situation where "the surface temperature is sufficient, but the internal temperature is insufficient," leading to incomplete coating curing. There are two ways to solve this problem: one is to directly increase the oven's heating power to ensure sufficient heat is provided at full load; the other is to appropriately extend the baking time if the temperature is insufficient, allowing the workpiece enough time to complete the curing reaction.
2.2 Insufficient Baking Time
This is a common mistake made by many novice manufacturers: they start calculating the baking time the moment the workpiece is placed in the oven. However, the "baking time" required for powder coatings actually refers to the time after the workpiece has fully reached the set baking temperature, followed by continued heating—the initial heating phase is irrelevant. For example, the thicker the wall of the workpiece and the greater the number of pieces, the greater the heat capacity, and the longer it takes to reach the set temperature. With a fixed oven length and conveyor speed, the effective curing time is compressed. If the effective time is insufficient, the cross-linking reaction of the coating cannot be fully completed, and the physical and mechanical properties will naturally fail to meet requirements. It is recommended to use an "oven temperature tracking meter" to conduct a test: under full-load production, monitor the temperature change of the workpiece in the oven in real time, accurately calculate the true heating time and effective curing time, and then adjust the baking temperature or conveyor speed accordingly to ensure that each batch of workpieces is fully cured.
2.3 Poor Surface Preparation of the Workpiece
The adhesion between the coating and the workpiece depends entirely on the surface preparation. If the workpiece surface has oil, scale, rust, or other contaminants that haven't been properly cleaned, or if the phosphating film is of poor quality and has a layer of phosphating solution residue on the surface, it's equivalent to adding a "separation layer" between the coating and the substrate. In this case, even if the powder curing reaction is complete, the coating cannot adhere firmly to the workpiece. Not only will the adhesion be poor, but the impact resistance will also be greatly reduced, and it may peel off with the slightest touch. The key to solving this problem is to strictly control the quality of surface preparation: degreasing and rust removal must be thorough, the phosphating process must be standardized, and the workpiece surface must be clean and have appropriate roughness to provide a solid "adhesion base" for the coating.
2.4 Non-Standardized Inspection Conditions
Sometimes, the coating itself cures very well, but fails performance testing—this may not be a problem with the product, but rather a problem with the inspection conditions not meeting standards. According to industry standards, coating performance testing must be conducted at specified test temperatures and coating thicknesses. If the actual coating thickness is thicker than specified, or the test chamber temperature is lower than the standard temperature, the coating's impact resistance, flexibility, cupping properties, and adhesion will be worse, making it less likely to pass the test. Conversely, when the coating is thinner and the test temperature is higher, these indicators are more likely to meet the standards. To accurately judge coating quality, testing must be conducted strictly according to the conditions specified in the product standard (or conditions close to the standard). From a quality control perspective, testing can even be conducted under more stringent conditions—such as slightly lowering the test temperature and increasing the coating thickness—to more comprehensively expose potential problems and ensure that the products leaving the factory are flawless.
2.5 Over-baking
Many people believe that "baking longer is safer," but in reality, excessively high baking temperatures and prolonged baking times can lead to "thermal aging" of the coating: not only will the color change (e.g., yellowing or darkening), but the physical and mechanical properties such as flexibility and impact resistance will also decrease significantly, resulting in problems such as brittleness and cracking. For example, some light-colored powder coatings may exhibit noticeable yellowing when baked at temperatures exceeding 50°C. On the other hand, for workpieces requiring high flexibility (such as agricultural machinery parts), excessive baking can cause the coating to lose elasticity and become prone to breakage upon impact. Therefore, baking parameters are not "the higher the better" or "the longer the better," but rather must be precisely matched to the requirements of the powder coating.
3. Summary
The curing quality of powder coatings is the result of the combined effect of the "inherent formula" and the "acquired process"—a reasonable formula is fundamental, and precise processing is key. To avoid poor curing, firstly, choose powder coatings with mature formulas and stable quality; secondly, focus on controlling the baking temperature and time, using an oven temperature tracker to accurately test the actual parameters under full load to avoid "insufficient temperature" or "over-baking"; simultaneously, strictly treat the surface of the object to be coated to build a solid "foundation" for coating adhesion; finally, standardize performance testing conditions to ensure that test results accurately reflect product quality. During production, it is recommended to classify the objects to be coated: based on the workpiece's wall thickness, size, and heat capacity, rationally arrange the number and arrangement of hangers, and then adjust the baking parameters accordingly. Through multiple trials and optimizations, a standardized production process can be formed, which can stably produce well-cured and compliant coating products, reducing waste and improving production efficiency.
