How to ensure the bonding strength between PU material and steel balls during the manufacturing process of PU bearings

In the manufacturing of high-performance polyurethane (PU) deep groove ball bearings, a crucial step directly determines their reliability and service life: the bond strength between the PU material and the internal steel balls. This bonding is more than a simple physical encapsulation; it involves a complex chemical and process engineering process designed to ensure that the steel balls remain firmly embedded in the PU raceways, preventing separation, displacement, or slippage, even under high-speed rotation and load.
If the bond is not strong enough, the steel balls may become loose, move, or even fall out during operation, leading to bearing failure. Therefore, ensuring flawless, professional bonding is a key indicator of a PU bearing manufacturer's technical strength.

Material Selection: Laying the Foundation for Bonding from the Start
The primary determinant of bond strength is the inherent properties of the PU material. Not all polyurethanes are suitable for bearing manufacturing; their formulations must be carefully designed to balance a variety of properties:

Chemical Adhesion: To achieve a strong bond with the steel ball surface, PU materials are often enhanced with specific chemical additives, such as isocyanate functional groups. During the curing process, these functional groups chemically react with the microstructure of the steel ball's surface, forming covalent or hydrogen bonds and achieving a molecular-level connection. This is far more robust than simple physical encapsulation.
Physical Property Matching: The PU material's hardness (Shore A or D), elastic modulus, and wear resistance must match the characteristics of the steel ball. If the PU is too soft, even with a strong bond, it won't effectively restrain the steel ball; if it's too hard, its inherent vibration and noise reduction benefits will be lost. The optimal formulation design strikes a balance of properties while ensuring sufficient bonding strength.
Low Shrinkage: PU undergoes a certain amount of volumetric shrinkage during the curing process. Improperly controlled shrinkage can generate internal stresses, potentially leading to microcracks at the interface between the PU and steel ball, weakening the bond. Therefore, choosing a PU formulation with low or controlled shrinkage is crucial.

Surface Treatment: Enabling Perfect Bonding
As the load-bearing core of a PU bearing, the surface condition of the steel ball has a crucial influence on bond strength. Even the best PU formulation won't achieve effective bonding if the steel ball surface is unclean or inactivated. Therefore, before PU injection molding or casting, the steel balls must undergo rigorous surface treatment:
Ultrasonic cleaning: First, the steel balls undergo multiple ultrasonic cleaning steps. Using a specific cleaning agent, contaminants such as oil, dust, and fingerprints can be thoroughly removed from the steel ball surface. These contaminants form a physical barrier, severely hindering direct contact and chemical reactions between the PU material and the steel ball.
Activation: Simply cleaning is not enough. To enhance the affinity between the PU material and the steel ball surface, activation treatment is typically performed. For example, plasma treatment or chemical activators can introduce polar functional groups such as hydroxyl or amine groups onto the steel ball surface. These functional groups react with the isocyanate groups in the PU material, forming strong chemical bonds and significantly improving bonding strength.
Drying: After activation, the steel balls must be thoroughly dried immediately. Any residual moisture can react with the isocyanate groups in the PU material, generating bubbles. This not only affects the curing quality of the PU but also creates voids at the interface, severely weakening bonding strength.

Process Control: Ensures a Precise and Stable Bonding Process
Perfect materials and surface treatment are only prerequisites; precise process control is the key to achieving stable, high-quality bonding:
Temperature Control: The injection or casting temperature of the PU material must be strictly controlled within the process window. Too low a temperature results in excessive PU viscosity and poor fluidity, making it difficult for the PU to fully penetrate the tiny gaps between the steel balls, resulting in uneven coverage. Excessive temperatures can cause the PU material to prematurely cure or even degrade, impacting final performance. Furthermore, the preheating temperature of the steel balls must be precisely controlled to avoid internal stress caused by temperature differences during the PU curing process.
Pressure Control: During the injection molding process, appropriate injection pressure ensures that the PU material fully fills the mold, completely envelops the steel balls, and compacts them, eliminating any potential air bubbles and ensuring close contact between the PU and steel balls.
Curing Time and Temperature Profile: PU curing is a chemical reaction, and its strength depends on the combined effects of time and temperature. During the manufacturing process, the prescribed curing time and temperature profile must be strictly adhered to. Typically, the curing process is divided into multiple stages, from low-temperature pre-cure to high-temperature post-cure, each step is designed to ensure sufficient cross-linking of molecular chains to achieve maximum bond strength and optimal physical properties.