Guide to Choosing Resins for Rotational Molding

Imagine designing an outdoor water storage tank that must endure harsh sunlight, heavy rain, and ensure the safety of the liquid inside. Choosing the wrong material could lead to product deformation, cracking, or even complete failure, resulting in significant financial losses. In the field of rotational molding, the selection of raw materials is critical, as it directly determines the product's performance, lifespan, and cost-effectiveness. This article delves into the characteristics, applications, and key considerations for selecting resins in rotational molding, helping you create superior products.

Rotational molding, also known as rotomolding, is a manufacturing process used to produce hollow plastic products. It involves injecting plastic powder or liquid into a mold, which is then rotated along two perpendicular axes while being heated. This causes the plastic to melt and evenly coat the inner walls of the mold. After cooling, the desired shape is obtained. Due to its low cost, flexibility, and ability to produce large, complex shapes, rotational molding is widely used in tanks, containers, toys, furniture, automotive parts, and more.

Resins used in rotational molding are primarily divided into three categories: thermoplastic resins, thermosetting resins, and epoxy resins. Each type has unique properties and applications, and the choice depends on specific requirements, production processes, and budget constraints.

• Thermoplastic Resins: These resins soften when heated and harden when cooled, a process that can be repeated multiple times. Thermoplastics are the most commonly used materials in rotational molding, with polyethylene being the dominant choice.
• Thermosetting Resins: Once heated and cured, these resins form an irreversible hardened structure. They offer excellent heat resistance, chemical resistance, and mechanical strength but are more challenging to process.
• Epoxy Resins: A high-performance thermosetting resin, epoxy is known for its superior adhesion, chemical resistance, and electrical insulation. Its use in rotational molding is relatively limited, primarily for specialized applications.

According to a study by LyondellBasell, polyethylene accounts for 84% of the rotational molding market. Its popularity stems from the following advantages:

• Lightweight: Polyethylene is a low-density material, reducing product weight and transportation costs.
• Cost-Effective: Its production costs are relatively low, lowering overall product expenses.
• Corrosion Resistance: Polyethylene exhibits excellent resistance to chemical corrosion, making it suitable for containers storing corrosive substances.

Polyethylene is further classified into low-density polyethylene (LDPE) and high-density polyethylene (HDPE).

• Low-Density Polyethylene (LDPE): Offers flexibility and elongation, ideal for products requiring some degree of pliability, such as hoses and toys.
• High-Density Polyethylene (HDPE): Provides higher strength and rigidity, suitable for load-bearing products like tanks and containers.

Beyond polyethylene, other thermoplastics such as polycarbonate, nylon, polyvinyl chloride (PVC), polyester, and polypropylene make up the remaining 15% of the rotational molding market.

As the rotational molding industry evolves, an increasing variety of resins are being used. Different resins significantly impact product strength, elasticity, vibration resistance, and other properties. Below is a detailed comparison of commonly used rotational molding resins:

• Advantages: Excellent chemical resistance, abrasion resistance, fungal resistance, mechanical strength, durability, and low cost.
• Applications: Tanks, containers, traffic barriers.

• Advantages: High rigidity, heat deflection temperature, and resistance to chemical and environmental stress cracking.
• Disadvantages: Lower impact strength at low temperatures.
• Applications: Automotive parts, medical devices.

• Advantages: Superior chemical resistance, abrasion resistance, impact resistance, fatigue resistance, and performance retention in extreme temperatures.
• Disadvantages: Higher molding difficulty and cost.
• Applications: Boat hulls, pipes, tanks.

• Advantages: High transparency and rigidity.
• Disadvantages: Lower impact strength, requires advanced processing, prone to moisture absorption, less suitable for rotational molding.
• Applications: Light covers, safety helmets.

The Melt Flow Index (MFI) measures a resin's fluidity, indicating how easily it melts and flows under specific temperature and pressure conditions. A higher MFI value signifies better fluidity, while a lower value indicates poorer flow. Selecting a resin with the appropriate MFI is crucial for product quality and production efficiency.

MFI serves as an approximate measure of resin molecular weight or chain length. High-MFI resins have shorter molecular chains and lower molecular weight, while low-MFI resins feature longer chains and higher molecular weight.

• For rotational molding, the ideal MFI range varies by resin type, typically between 80°C and 273°C. For example:
• Polyethylene (PE): 110°C–135°C
• Polypropylene (PP): 180°C–220°C

Density, measured in grams per cubic centimeter (g/cm³), is another vital factor in resin selection. It influences mechanical strength, dimensional stability, tensile strength, impact resistance, chemical resistance, water absorption, and thermal conductivity.

Rotational molding resins are categorized by density:

• Low-Density Resins: Below 0.925 g/cm³; used for less rigid products like toys.
• Medium-Density Resins: 0.926–0.940 g/cm³; suitable for self-supporting parts requiring heat resistance and rigidity.
• High-Density Resins: 0.941–0.959 g/cm³; enhance rigidity and reduce wall thickness.
• Ultra-High-Density Resins: Above 0.960 g/cm³; rarely used in rotational molding.

Polyethylene is the most widely used resin in rotational molding. Its lightweight, durability, and versatility make it ideal for containers, insulators, tanks, and boxes. Key polyethylene types include:

• High-Density Polyethylene (HDPE): For impact-resistant products like crates.
• Low-Density Polyethylene (LDPE): For flexible products like hoses and tanks.
• Cross-Linked HDPE: For chemically resistant tanks and containers.

Advantages of polyethylene in rotational molding:

• Low Cost: Economical to process and manufacture.
• Weather Resistance: Excellent corrosion, UV, heat, and cold resistance.
• Ease of Molding: Adaptable to various shapes.

Polyethylene is typically used in powdered form, micro-pulverized to improve flow during melting. While it offers low-temperature flexibility and impermeability, it can shrink and degrade under UV exposure—issues mitigated with additives.

Beyond polyethylene, other resins like polyurethane and epoxy are used for specialized applications:

• Polyurethane: Highly resistant and flexible, yielding strong, durable products.
• Epoxy Resin: Easy to process, with excellent coating and chemical resistance.

Selecting the appropriate resin is paramount for achieving high-quality rotational molding products. By understanding the properties and trade-offs of various resins, manufacturers can optimize their choices for specific applications. Key factors like MFI and density must be carefully considered to ensure optimal performance and durability.