Roto molding or rotomolding, also known as rotomoulding, plastic rotational molding or rotational molding, is a plastic formation process used to shape raw plastic material into hollow, usable products.
Quick links to Rotomolding Information
Types of Thermoplastics
It involves the use of a rotational mold and nearly any thermoplastic. Some of the thermoplastics most commonly molded into plastic products using roto molding include: high density polyethylene (HDPE), low density polyethylene (LDPE), nylon, polyethylene terephthalate (PET), polycarbonate and polyvinyl chloride (PVC). Used to create both small products in large quantities and large products in small quantities, roto molding is known to create plastic formations that are uniform in both quality and precision wall width. The products it can produce are very diverse indeed, including items like plastic construction cones, plastic tanks and even plastic flamingos. Also, rotomolding can be used to create both standard and custom stock shapes.
Rotational Molding Process
Typically, roto machines consist of molds, an oven, a cooling chamber and mold spindles. The molds are mounted on the mold spindles, which allow the molds to rotate. Most roto molds are made out of aluminum or stainless steel. To begin the rotational molding process, manufacturers first stack raw plastic material into a half a mold until it is full. Once full, they attach the mold’s other half and then seal it. Once sealed, the plastic inside the mold is placed inside the oven and heated until it becomes either malleable or molten, depending on the requirements of the piece. At this point in most other thermoplastic forming or molding processes, like injection molding or extrusion, the manufacturer would apply heat and/or pressure to shape the molten plastic. In roto molding, however, the mold is instead rotated by the mold spindles upon which it is mounted. Depending on the type of the equipment being used, the mold may be rotated either in a single direction only or biaxially. The choice of which equipment to use depends upon what kind of shape or shapes a manufacturer would like to form. Either way, once the plastic has been rotated inside the mold long enough, it will have coated its interior, effectively taken on its shape. Once this has happened, the mold stops rotating and is quickly cooled down either inside the cooling chamber, by an external water jet cooling system or perhaps by another method. After it has been cooled, the plastic is ejected from the mold, having taken on its shape. Once a part has been roto molded it can either be prepared for its shipment to customers or it can be sent on for secondary processing like surface treatment or cutting. This depends entirely on the shape and requirements of the new part’s intended application. However, secondary processes and other forming processes are often used in conjunction with roto molding because the demand for plastic shapes is so large and varied.
Types of Rotomolding Machines
Roto molded parts can be made on one of several different types of roto molding machines. These machines include: rock and roll machines, clamshell machines, shuttle machines, vertical or up and over machines, carousel machines and swing arm machines. Each model offers something different.
- Rock and Roll Machines
- Specially designed with small heating chambers to create long, narrow parts.
- Clamshell Machines
- Actually a type of rock and roll machine, but they are distinguished by their use of a single primary rotational arm, though they usually also have support arms on their ends. Most other roto molding machines use two or more arms.
- Shuttle Machines
- Typically have two arms that work independently, moving the mold back and forth in between the heater and the cooler as it turns biaxially. Some shuttle machines do, however, have only one arm. In this case, the arm moves the mold horizontally from the heater to the cooler as it spins.
- Vertical Rotational Machines
- Smaller than most other rotomolding machines; they are considered small to medium in size. They are valued for their energy efficiency, which is born out of their compact heating and cooling chambers.
- Carousel Machines
- One of the most commonly used rotational molding machines. Carousel machines may have up to four arms and six stations, and they come as either fixed or independent equipment. Typically, fixed arm carousel machines consist of three arms that are required to move together, while independent arm carousel machines have between three and four arms, all which move separately from one another. Both types of carousel machines are available in a wide variety of sizes.
- Swing Arm Machines
- Can have up to four arms, all of which move bi-axially. The arms, which do not all have to be engaged at the same, are mounted on the respective corners of the machine, where they swing in and out of the oven.