Background Introduction

RRotational molding is a pressure-free, low-energy plastic forming process widely adopted for manufacturing large hollow products such as water tanks, fuel containers, playground equipment, and industrial bins. The process works by heating and slowly rotating a hollow mold, causing plastic powder to uniformly melt and adhere to the inner mold surface before cooling into a seamless finished product.
The granulation stage is the critical preparation step that determines the quality of rotational molding powder. Powder flowability, particle size distribution, and additive dispersion directly influence melt behavior, wall thickness uniformity, and the mechanical properties of the final molded part. WANROOETECH’s complete production line integrates every stage from raw material intake through finished powder delivery — including precise material selection, masterbatch and additive dosing, accurate weighing, multi-stage mixing, water ring granulation, and controlled grinding. Each stage is engineered to work in seamless coordination, creating a precision manufacturing system that transforms raw polymer into production-ready rotational molding powder.

Selection of raw materials

The selection of raw materials is the primary step in laying the foundation for performance. Roll molding granulation mainly uses polyethylene (PE), including high-density polyethylene (HDPE), low-density polyethylene (LDPE), and linear low-density polyethylene (LLDPE). Its excellent chemical resistance, low-temperature toughness, and processing flowability make it the preferred material. In some scenarios, materials such as polypropylene (PP) and nylon (PA) are also used. The raw materials need to meet the requirements of uniform particle size, high purity, and low impurities to ensure the formation of a stable melt during melting and extrusion, and to avoid impurities affecting the flowability of the powder and the surface quality of the product; At the same time, resin with a melt index (MFR) of 2-10 g/10min should be selected based on the thickness of the product to balance the melting rate and molding cycle, providing a stable foundation for subsequent processes.

Color masterbatch and additive precise dosing system

For different product requirements, the production line supports precise addition of multi-component additives such as color masterbatch, UV stabilizer, flame retardant, anti-static agent, lubricant, etc. By combining a twin-screw weight loss feeder with a liquid metering pump, dynamic proportioning of solid and liquid additives is achieved (with an error of ≤ 0.3%), and preliminary mixing is carried out through a pre mixing bin to ensure uniform dispersion of additives and substrates. The system supports the storage and one click switching of over 200 formulas, meeting the production needs of small batches and multiple varieties.

Measurement and weighing system

Accurate material proportioning is the technical foundation of consistent granulation quality. The process demands raw material ratio accuracy within 0.5% tolerance, and our integrated weighing system achieves this through three layers of dynamic control.
The loss-in-weight feeder continuously monitors the mass of each additive hopper and adjusts screw speed in real time to maintain exact dosing rates for trace components. For main raw materials, a screw metering device paired with variable frequency drive control delivers continuous, surge-free material flow at precisely calibrated rates. The complete system is governed by a PLC automation controller with HMI touchscreen interface that records every batch’s proportioning data with timestamps and lot numbers. Full batch traceability eliminates recipe drift, supports quality audits, and ensures that every production run replicates the last with identical material composition.

Multi stage mixing process

Achieving uniform additive dispersion is the central challenge in rotational molding powder production. WANROOETECH addresses this through a three-stage mixing sequence that progressively intensifies blending while managing thermal history.
Stage one employs a high-speed batch mixer with rotating blades operating at 1000 to 1500 rpm. The intense shear forces distribute color masterbatch and solid additives throughout the resin pellets, breaking up initial agglomerates and creating a homogeneous dry blend. Stage two moves the pre-mixed material into a twin-screw extruder where the polymer enters the molten state. Co-rotating screws generate controlled shear and elongational flow that further disperses additives at the molecular level while simultaneously devolatilizing trapped moisture and low-molecular-weight contaminants. Stage three passes the molten extrudate through a cooling mixer that rapidly drops temperature to prevent thermal degradation and pellet clumping, producing a stable, free-flowing feedstock ready for the granulation cutter.
This progressive mixing approach eliminates additive agglomeration, ensures color consistency across every batch, and guarantees that functional additives such as UV stabilizers and flame retardants are evenly distributed throughout each individual particle.

Water ring granulation system

The water ring granulation system achieves efficient granulation through an integrated process. The system integrates three steps: melt extrusion, water-cooled ring cutting, and particle drying. The mixed material is melted by a single/twin-screw extruder and then formed into a strip like melt through a die; The high-speed rotating cutter cuts through the melt, while the water ring jet cooling rapidly solidifies the particles to avoid adhesion; Centrifugal dehydrator removes surface moisture from particles to ensure dryness ≤ 0.5%. The particles produced by this process are cylindrical or spherical in shape, with a particle size of 2-5 mm and excellent flowability, and can be directly used for rotational powder processing.

Grinding and screening system

In the rotational molding process, the fine processing of particles by the grinding machine is the core link to control the particle size of the powder and improve the quality of the product. Rotational molding requires the use of plastic powder with a mesh size of 200-800. Therefore, the grinding machine needs to efficiently grind the raw material particles to the target particle size range, and rely on multi-stage vibrating screens to achieve graded screening, ensuring that the powder particle size distribution strictly meets the process window. This process not only optimizes the uniformity of the powder, but also further improves the flowability and dispersibility of the powder through modification treatment, providing stable raw materials for subsequent molding.

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