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Common Issues in UV Vacuum Plating (Part 2)
Release time:
2026-05-30 16:45
In the production process of UV vacuum plating, poor adhesion is one of the most serious issues affecting product quality. Poor adhesion manifests as weak bonding between the coating layer and the substrate, or between the topcoat and the coating layer, leading to delamination, peeling, or complete flaking. This problem directly results in product scrap, causing material waste and increased costs. Understanding the characteristics and root causes of poor adhesion helps identify potential risk factors during manufacturing.
I. Manifestations of Poor Adhesion
Poor adhesion is primarily manifested as insufficient interfacial bonding strength between the layers of the coating system. When the adhesion between the primer and the substrate is poor, the coating layer peels off the substrate surface in large, intact sheets together with the primer; the peeled-off surface is smooth, with no residual substrate material remaining. Adhesion failure between the topcoat and the coating layer results in the topcoat lifting or delaminating from the coated surface, while the coating layer remains intact on the primer. Adhesion defects between the coating layer and the primer cause the coating layer to detach from the primer surface along with the topcoat, leaving the primer layer undisturbed. In the cross‑hatch test, samples with poor adhesion exhibit extensive coating loss upon tape removal, leading to a low rating.
II. Inadequate substrate cleaning
Incomplete cleaning of the substrate is the primary cause of poor adhesion. During injection molding, plastic substrates often retain mold release agents on their surfaces. These agents are low‑surface‑energy substances that exhibit both hydrophobic and oleophobic properties, forming an isolating film on the substrate surface. When a primer is applied, this barrier prevents direct contact between the primer and the substrate, thereby preventing the formation of effective chemical bonds or mechanical interlocking.
Oil contamination is also a common issue. The substrate surface may be contaminated with oils originating from processing equipment, operators, or the environment. Such oily layers likewise exhibit low surface energy, making it difficult for primers to wet and adhere to them. Dust particles adhering to the substrate surface, while not forming a continuous barrier like release agents, reduce the effective contact area between the primer and the substrate, creating zones of weak adhesion around the particles. For low‑surface‑energy substrates such as PP and PE, the inherent surface energy is already low, further hindering direct wetting and adhesion of the primer. When these materials are subjected to vacuum plating without prior treatment, their adhesion typically remains poor.
III. Improper Curing of the Primer
The curing state of the primer directly affects adhesion. When the primer is incompletely cured, unreacted resin or monomers remain within the coating. These low‑molecular‑weight substances act as plasticizers, reducing the coating’s cohesive strength. As the coating or topcoat shrinks, the primer layer is prone to internal damage, leading to delamination. An incompletely cured primer surface may exhibit tackiness, and this sticky residue can further weaken the bond between the coating and the primer.
Over‑curing of the primer can also cause problems. Excessive curing leads to an excessively high crosslink density in the coating, making it brittle and increasing internal stresses. The shrinkage stresses generated during curing accumulate within the coating; when these stresses exceed the interfacial adhesion strength, the coating may delaminate from the substrate. Furthermore, an over‑cured primer forms a surface that is too dense, hindering the topcoat’s ability to wet and penetrate, thereby reducing interlayer adhesion.
IV. Improper Control of Coating Thickness
The coating thickness of the primer significantly affects adhesion. When the primer is too thin, it fails to provide an adequate bonding substrate. A thin coating has a limited contact area with the substrate and insufficient mechanical strength to withstand the shrinkage stresses generated during the curing of both the undercoat and the topcoat. Moreover, an overly thin primer can lead to localized areas of incomplete coverage, where the coating layer directly contacts the substrate, resulting in even poorer adhesion.
When the primer is applied too thickly, significant internal stresses accumulate within the coating. During crosslinking, UV‑curable coatings undergo volumetric shrinkage; the thicker the coating, the greater the shrinkage and the stronger the internal stresses. Excessive internal stress can initiate failure at the interface or within the coating itself, leading to delamination or cracking. Thick coatings are also prone to bubble formation, and the presence of bubbles further compromises the structural integrity of the coating.
V. Insufficient Surface Energy of the Substrate
The surface energy of the substrate is a fundamental factor influencing adhesion. Low‑surface‑energy substrates such as PP, PE, and PA have weak surface molecular polarity, making it difficult for them to form chemical bonds with the polar functional groups in the primer. During primer application, the coating exhibits a large contact angle on these substrates, resulting in poor wettability and difficulty in spreading to form a continuous, uniform film. Even if coating is achieved under forceful conditions, the interfacial adhesion between the primer and the substrate remains weak, leading to easy delamination under external stress.
During injection molding, some substrates may develop a dense skin layer on their surface. This skin layer exhibits lower surface energy than the bulk material, further complicating adhesion. Additionally, additives such as pigments, fillers, and flame retardants present in the substrate can become enriched at the surface, altering its chemical composition and thereby affecting adhesion.
VI. Interface Contamination
Interfacial contamination between the primer and the coating layer can lead to reduced adhesion. Prior to coating, the primer surface may become contaminated with oil, fingerprints, or dust. These contaminants form a weak interlayer on the primer, causing metal particles to deposit on them and preventing a strong bond from forming between the coating and the primer. Similarly, interfacial contamination between the topcoat and the coating layer can also impair adhesion. During storage or handling, coated parts may adsorb environmental contaminants; if these are not removed before topcoat application, interfacial contamination can result in inadequate topcoat adhesion.
VII. Internal Stress in the Coating Layer
The coating layer develops internal stresses during deposition. In thermal evaporation coating, as metal vapor molecules condense on the substrate surface, temperature gradients and lattice mismatch give rise to stress within the film. When the coating is relatively thick or deposition conditions are suboptimal, these internal stresses can become significant. The internal stresses in the coating are transmitted to the primer layer and the interface, potentially leading to delamination.
The particles in magnetron sputtering deposition possess relatively high kinetic energy, and during deposition they bombard the surface of the primer. This bombardment can enhance interfacial bonding; however, excessively high particle energy may damage the primer surface, thereby reducing adhesion. Moreover, the substrate temperature during the coating process also influences the generation and distribution of residual stresses within the film.
VIII. Conclusion
Poor adhesion is a critical issue in UV vacuum plating that compromises product reliability, with its root causes spanning substrate condition, primer curing, and coating thickness. Inadequate substrate cleaning allows release agents, oils, and other contaminants to impede primer adhesion; low‑surface‑energy substrates that have not been activated are difficult for the primer to wet; improper primer curing—whether incomplete or excessive—alters interfacial bonding; and inadequate control of coating thickness can result in insufficient adhesion, whether the layer is too thin or too thick. Insufficient surface energy is an intrinsic property of materials such as PP and PE; interfacial contamination introduces weak layers both before plating and prior to the topcoat; moreover, internal stresses within the plated layer can also trigger delamination. These issues are often interrelated, and poor adhesion typically arises from the combined effects of multiple factors. Understanding the manifestations and underlying causes of poor adhesion is essential for identifying this defect.
Disclaimer: The above content has been compiled from publicly available sources and is provided for reference only. If any infringement occurs, please contact us, and we will address it promptly.
Bossin Recommended Products – Vacuum Plating | ||
Primer | ||
Product Model/English Abbreviation | Product Name/Product Type | Product Features |
B-113 | Bisphenol A epoxy acrylate | High hardness, high gloss, high fullness, contains 20% TPGDA. |
B-151 | Modified epoxy acrylate | Low halogen, yellowing-resistant, excellent plating performance, and strong adhesion. |
B-160D | Modified epoxy acrylate | Good flexibility, yellowing resistance, and excellent adhesion. |
B-163 | Modified epoxy acrylate | Good flexibility, excellent pigment wetting, and strong adhesion. |
B-165 | Modified epoxy acrylate | Good flexibility and strong adhesion |
B-212A | Aromatic polyurethane acrylate | High cost-performance, excellent plating adhesion, good toughness, and resistant to boiling water. |
B-221 | Aliphatic polyurethane acrylate | Fast curing, resistant to boiling water |
B-268M | Aliphatic polyurethane acrylate | Good flexibility, excellent adhesion, superior plating performance, and strong hiding power. |
B-574C | Polyester acrylate | Low viscosity, low odor, excellent wettability, suitable for LED UV. |
B-619W | Aliphatic polyurethane acrylate | Fast curing, high hardness, excellent toughness, wear resistance, and chemical resistance. |
Intermediate coat | ||
Product Model/English Abbreviation | Product Name/Product Type | Product Features |
B-374 | Aliphatic polyurethane acrylate | Excellent flexibility, good leveling, resistant to abrasion and chemicals, and resistant to yellowing. |
B-601 | Aromatic polyurethane acrylate | High hardness, scratch resistance, chemical resistance, and excellent cost-effectiveness. |
B-6020 | Special functional group acrylate | Resistant to boiling water, excellent color development, and strong interlayer adhesion. |
Topcoat | ||
Product Model/English Abbreviation | Product Name/Product Type | Product Features |
B-221 | Aliphatic polyurethane acrylate | Fast curing, resistant to boiling water |
B-301 | Aromatic polyurethane acrylate | Fast curing, excellent toughness, and good sandability. |
B-302 | Aromatic polyurethane acrylate | Fast curing, high strength, excellent toughness, and good grindability. |
B-368 | Aliphatic polyurethane acrylate | Good toughness, excellent leveling, excellent bend resistance, and excellent heat resistance. |
B-374 | Aliphatic polyurethane acrylate | Excellent flexibility, good leveling, resistant to abrasion and chemicals, and resistant to yellowing. |
B-574C | Polyester acrylate | Low viscosity, low odor, excellent wettability, suitable for LED UV. |
B-601 | Aromatic polyurethane acrylate | High hardness, scratch resistance, chemical resistance, and excellent cost-effectiveness. |
B-6016C | Special functional group acrylate | Easy to apply, resistant to yellowing and boiling water, and improves the appearance of the paint film. |
B-6019 | Special functional group acrylate | Good leveling, excellent wettability, resistant to boiling water, and superior color dispersion. |
B-609 | Aliphatic polyurethane acrylate | Fast curing, high hardness, scratch resistance, and chemical resistance. |
B-615A | Aliphatic polyurethane acrylate | Fast curing, excellent toughness, wear resistance, and chemical resistance. |
B-619W | Aliphatic polyurethane acrylate | Fast curing, high hardness, excellent toughness, wear resistance, and chemical resistance. |
B-6210 | Aliphatic polyurethane acrylate | Low viscosity, chemical resistance, environmental resistance, and dual photocatalytic–thermal curing. |
B-6211 | Aliphatic polyurethane acrylate | Fast curing, high hardness, scratch-resistant, and free of organotin. |
B-919B | Aliphatic polyurethane acrylate | Fast curing, high hardness, excellent toughness, and superior chemical and wear resistance. |
Monomer Recommendation | ||
Product Model/English Abbreviation | Product Name/Product Type | Product Features |
BM2223 (TPGDA) | Dipropylene glycol diacrylate | Good flexibility and low volatility |
BM3231 (TMPTA) | Trimethylolpropane triacrylate | High crosslink density, high hardness, high gloss, and excellent wear resistance. |
BM3235 (PET3A) | Pentaerythritol triacrylate | Fast curing, high crosslink density, high hardness, and excellent chemical resistance. |
BM3380 (3EO-TMPTA) | Pentaerythritol triacrylate | More flexible and less irritating than TMPTA. |
BM6261 (DPHA-80) | Dipentaerythritol hexaacrylate | High crosslink density, high hardness, chemical and wear resistance, and water resistance. |
BM6263 (DPHA-90) | Dipentaerythritol hexaacrylate | High crosslink density, high hardness, chemical and wear resistance, and water resistance. |
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