What are the principles and classifications of rotational molding?

What are the principles and classifications of rotational molding?

What are the principles and classifications of rotational molding? Rotomolding is a process for hollow molding thermoplastics. Its principle is as follows: resin is added to the mold, which is then closed. The mold is heated and continuously rotated along two perpendicular axes, causing the mold to rotate/roll in three dimensions. The resin, under its own gravity, evenly fills the mold cavity and gradually melts until it is completely melted and evenly adheres to the inner wall of the mold cavity. Heating is then stopped and the cooling process begins. Once the product cools and solidifies, it is demolded to obtain the desired seamless hollow part.

During rotational molding, the mold's rotation speed is not high, and the product molding depends entirely on the natural flow of the resin, so the product has almost no internal stress and is not prone to defects such as deformation and dents.

Rotomolding can be divided into three categories based on the resin form used.

1. Dry powder rotomolding, typified by polyethylene resins. This is the most widely used type of rotomolding. Because the resin is transformed from powder to molten without external force, air trapped between the powder particles forms bubbles in the melt, which then grow and detach from the melt. The complete removal of these bubbles directly impacts the physical and mechanical properties of the finished product.

2. Rotomolding of PVC paste resin, commonly known as slush molding in the industry, primarily produces hollow, soft PVC products.

3. Monomer polymerization rotomolding: The resin monomer enters the mold cavity, where it polymerizes under suitable temperature conditions and with the help of additives, allowing it to flow and form. This process is currently relatively unused. The rotomolding process has a wide range of applications, and can be used to form seamless hollow products as small as ping-pong balls, as large as yachts or assault boats, or chemical storage tanks with a volume of hundreds of cubic meters. The wall thickness of the product can be adjusted from less than 2mm to several centimeters without changing the mold. Complex structural products can be manufactured in a single step. Inserts or other components can be added to the product according to design requirements.

Rotomolding molds are simple to manufacture and inexpensive, typically costing only 1/4 to 1/3 the cost of other molds of similar size. Rotomolding molds are particularly suitable for large-scale products, enabling edge thickening to ensure edge strength for large-diameter products. Furthermore, rotomolded parts are flash-free, minimizing material waste. However, the disadvantages of rotomolding include: a relatively long production cycle, the need for pre-grinding of the material, extensive manual labor, and high labor intensity.

Using polyethylene powder rotomolding as an example, the entire process can be divided into four steps.

1. Feeding: The mold interior is cleaned, the material is metered in, and the mold is closed, preparing for subsequent rotomolding.

2. Molding: The mold and rotary system enter a designated thermoforming station, and vertical rotation is activated, causing the mold to rotate in three dimensions. The mold is then heated externally (using hot air circulation, electric heating, or open flame heating) to the set process temperature (260-320°C). This temperature is maintained for a specified period of time to ensure complete adhesion of the melt to the mold cavity walls, achieving a uniform thickness, and eliminating air bubbles. 3. Cooling Process: Maintaining the mold's three-dimensional rotational state, transfer to the cooling station and cool the mold and part using cold air, spray, or other methods to achieve complete solidification and finalization.
4. Demolding Process: Stop the mold's rotation, transfer to the loading/unloading station, open the mold, and remove the part. Clean the mold interior and prepare for the next production cycle.

For pigment products used in rotationally molded parts, particular attention should be paid to properties such as heat resistance, dispersibility, light resistance, weather resistance, and safety.

1. Heat Resistance: Rotomolding is a non-forced molding process. It requires excellent melt flow properties.
Heat conduction is a natural, unidirectional transfer method for the resin, without friction or shear heating. Bubbles in the melt require sufficient time to naturally accumulate and break. Therefore, the processing temperature is much higher than other molding processes for similar resins. Similarly, the operating time at these high temperatures is also much longer than with other processes. If nitrogen is not used to shield the mold cavity during rotational molding, oxidation at high temperatures must also be considered. Therefore, the selected pigments must possess excellent heat resistance; otherwise, product quality cannot be guaranteed.
2. Dispersibility: As can be seen from the rotomolding process, if the resin system is not pre-mixed, but simply dry-powdered (currently adopted by many manufacturers), the pigment powder particles are not effectively dispersed throughout the entire process. Coarse and difficult-to-disperse powder particles can cause defects such as color specks, affecting product quality.
3. Light and Weather Resistance: Rotomolded products are widely used outdoors, especially for yachts, piers, and large toys, which require vibrant colors. Therefore, pigments used in these products must possess excellent light and weather resistance.
4. Safety: A large number of consumer products and toys, particularly children's toys, come into direct contact with the human body. Therefore, the safety of pigment products is crucial.