Plastic-Coated Bearing Materials: Polyurethane vs. Nylon (PA66) – Process Comparison and Selection Guide

1. Introduction

Plastic-coated bearings (also known as polyurethane-coated bearings) are composite bearings made by coating a layer of engineering plastic or elastomer onto the outer ring of a metal bearing. The choice of coating material directly determines the bearing's wear resistance, corrosion resistance, vibration damping performance, and service life. Among the many coating materials, polyurethane (PU/TPU) and nylon (PA66) are the two most popular choices.

Polyurethane is an elastomeric material that falls between rubber and plastic, combining the high elasticity of rubber with the high strength of plastic. Nylon (PA66) is a crystalline thermoplastic engineering plastic known for its high mechanical strength and excellent oil resistance. These two materials differ significantly in properties, processing techniques, and suitable applications. Understanding these differences is key to making the right selection.

【Table 1: Core Properties Comparison of Common Plastic-Coated Bearing Materials】

Dimension	Polyurethane (PU/TPU)	Nylon 66 (PA66)
Material Type	Thermoplastic/thermoset elastomer	Crystalline thermoplastic
Typical Operating Temp	-30°C~80°C (standard); -20°C~120°C (high-temp)	-40°C~120°C (continuous)
Hardness Range	Shore A 60~95 (adjustable)	Fixed, relatively hard
Wear Resistance	Excellent (3-5x life of rubber wheels)	Good
Elasticity/Vibration Damping	Excellent (between rubber and plastic)	Moderate
Oil Resistance	Excellent	Excellent
Water Resistance	Excellent (polyether type better)	Moderate (high water absorption)
Processing Methods	Cast (CPU) / Thermoplastic (TPU)	Injection molding

 

2. Polyurethane (PU/TPU) Processing Technology

 

2.1 Two Types of Polyurethane Processing

Polyurethane can be divided into two types based on processing technology: cast polyurethane (CPU) and thermoplastic polyurethane (TPU) .

Cast Polyurethane (CPU): Liquid polyurethane prepolymer is poured into a mold and cured. Suitable for large, thick-walled, complex-shaped products with high tear strength, but lower production efficiency, suitable for small to medium batch production.

Thermoplastic Polyurethane (TPU): Solid pellets are melted and injected through an injection molding machine. Solid before molding, enables mass production with high efficiency and lower cost. The finished products are transparent, free of burrs, flash, and bubbles, with good wear resistance.

Most mainstream plastic-coated bearings use thermoplastic processing, with materials often sourced from imported BASF TPU, combining the advantages of both CPU and TPU.

 

2.2 Complete Process Flow for Polyurethane-Coated Bearings

The injection molding process for polyurethane-coated bearings is a processing technology that coats polyurethane material onto the bearing or wheel core surface through injection molding equipment, forming a high-strength wear-resistant layer. The process is divided into three stages:

Stage 1: Pretreatment

Metal Substrate Treatment: The bearing outer ring or wheel core is sandblasted (35-mesh steel grit recommended) to increase surface roughness and improve the bond strength between polyurethane and metal. Some processes require chemical etching or adhesive coating to enhance bonding.

Cleaning: After sandblasting, the bearing surface is blown clean with air and then cleaned with solvent to ensure no grit residue remains.

Adhesive Application: A uniform layer of primer is applied to the bearing surface to enhance the bond between polyurethane and metal.

Material Preparation: Select the appropriate polyurethane type based on operating conditions. Polyester type is suitable for wear-resistant applications, while polyether type offers better hydrolysis resistance.

Stage 2: Injection Molding

Equipment Parameter Settings: Use a screw injection molding machine with barrel temperature controlled at 177~232°C (adjusted according to material hardness) and mold temperature maintained at 10~60°C (higher hardness requires higher mold temperature).

Injection and Holding Pressure: Injection pressure set at 20~110MPa, holding pressure approximately 50% of injection pressure, ensuring the polyurethane fully fills and compacts the mold cavity.

Cooling and Demolding: Mold temperature is controlled through a constant temperature water circulation system; cooling time is adjusted based on product thickness to avoid internal stress from rapid cooling.

Stage 3: Post-Processing

Deburring and Finishing: Flash at the parting line is removed by machining or manual sanding.

Performance Testing: Quality is verified through peel strength testing (≥8MPa), hardness testing (Shore A 60~95), and wear testing (DIN abrasion ≤80mm³).

 

2.3 Key Process Control Points for Polyurethane-Coated Bearings

Mold Design: Runner surface polished to Ra0.4μm or below to reduce flow resistance; submarine or fan gates used, positioned at thick-wall sections.

Venting System: Venting slots 0.15mm deep and 6mm wide are provided at the parting line to prevent surface defects from trapped air.

Temperature Management: Barrel temperature uses gradient control (lower at rear, higher at front).

Time Control: The molding cycle consists of filling time (2~5s), holding time (10~20s), and cooling time (15~60s).

Special Note: Plastic-coated bearing processing typically involves placing the finished bearing into the injection mold for coating. Therefore, the bearing's temperature resistance is critical - the bearing seals and grease must withstand temperatures higher than the mold temperature to prevent seal aging or grease leakage.

 

3. Nylon (PA66) Processing Technology

 

3.1 Material Properties

PA66 (polyamide resin), also known as nylon, offers excellent wear resistance, self-lubrication, high mechanical strength, good heat resistance, excellent electrical insulation, good low-temperature performance, self-extinguishing properties, good chemical resistance - especially excellent oil resistance - with continuous heat resistance of 80-120°C. PA66 is primarily processed by injection molding.

 

3.2 Process Flow for Nylon-Coated Bearings

Nylon-coated bearings also follow the process route where the bearing is first manufactured and then placed into the injection mold for coating. The injection molding process requirements are as follows:

Injection Temperature: Feed section 250~260°C, middle section 260~280°C, nozzle section 255~270°C

Injection Pressure: 50~100MPa

Drying Treatment: PA66 has strong moisture absorption and must be dried before processing. If the material is sealed before processing, drying is not required. If the container has been opened, drying in 85°C hot air is recommended. If moisture content exceeds 0.2%, vacuum drying at 105°C for 12 hours is required.

Mold Temperature: For structural components, crystallinity is important, so a mold temperature of 80~90°C is recommended. Thin-walled or long-flow-path parts also benefit from higher mold temperatures to improve strength and rigidity.

 

3.3 Key Control Points for Nylon-Coated Bearings

Drying Control: Nylon readily absorbs moisture and must be thoroughly dried before processing; otherwise, surface defects and reduced mechanical properties may result

Mold Temperature: Higher mold temperatures (80~90°C) improve crystallinity and product strength

Injection Temperature: Nylon's injection temperature is much higher than polyurethane's, requiring higher bearing temperature resistance

 

4. Core Differences Between Polyurethane and Nylon Processing

【Table 2: Polyurethane vs. Nylon Process Comparison】

Comparison Item	Polyurethane (PU/TPU)	Nylon (PA66)
Processing Method	Cast (CPU) / Thermoplastic (TPU)	Injection molding
Injection Temperature	177~232°C	250~280°C
Injection Pressure	20~110MPa	50~100MPa
Mold Temperature	10~60°C	80~90°C
Drying Requirements	Low (TPU low moisture absorption)	Strict (drying above 85°C required)
Pretreatment	Sandblasting + cleaning + adhesive	Sandblasting + cleaning
Cooling Method	Constant temperature water circulation	Natural or water cooling
Post-Processing	Deburring + performance testing	Deburring + performance testing
Production Efficiency	TPU: high (mass); CPU: low (small-medium batch)	High (mass production)

 

5. Selection Recommendations

【Table 3: Polyurethane vs. Nylon Selection Decision Table】

Operating Condition	Recommended Material	Rationale
High wear resistance, heavy load, high frequency	Polyurethane	3-5x life of rubber wheels
High vibration/noise damping requirements	Polyurethane	Good elasticity, absorbs vibration and impact
Humid, washdown environments	Polyurethane (polyether type)	Excellent hydrolysis resistance
Oily environments	Either	Both have excellent oil resistance
Rough environments with stones and sharp debris	Nylon (PA66)	High hardness, impact and scratch resistance
High precision, dimensional stability requirements	Nylon (PA66)	Dimensionally stable through injection molding
Cost-sensitive, mass production	Nylon (PA66)	Lower raw material cost, high production efficiency
High-temperature environments (>80°C)	High-temp PU or PA66	Both have high-temperature grades available

 

6. Summary

Polyurethane and nylon are the two most common outer layer materials for plastic-coated bearings, with significant differences in material properties and processing techniques:

Polyurethane is primarily processed by thermoplastic (TPU) injection molding, with injection temperature 177~232°C and mold temperature 10~60°C. It offers high elasticity, excellent wear resistance, and vibration/noise damping, making it suitable for heavy-load, high-frequency, humid environments, and applications with strict noise requirements.

Nylon (PA66) is processed by injection molding, with injection temperature 250~280°C and mold temperature 80~90°C. It offers high strength, high rigidity, and good dimensional stability, making it suitable for rough environments with stones and debris, as well as applications requiring high precision.

Proper material selection requires comprehensive consideration of the operating environment (humidity, temperature, media), load type (heavy/light), performance requirements (damping/precision/wear resistance), and cost budget to choose the most suitable material.