Rotational Molding

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Molding and Manufacturing

The rotational molding process uses centrifugal force and high temperature to fill the mold and form the plastic part or component. The process is ideal to produce large, hollow, one-piece parts.

Sterling Tech operates “Carousel” type rotational molding machines with at least 3 “arms” with multiple molds positioned on each. In its simplest form, one arm is being loaded and unloaded, one arm is rotating or spinning in a gas-fired oven molding the parts, and the third arm is being air or water cooled. Each arm may contain 2,4,7 up to 10 separate molds per process cycle. The machine’s arms move independently from one another allowing a variety of mold sizes with different receipts for heating and thickness.

Each part being molded has its own receipt for production. This includes the amount and formulation of the resin, rotation speed, oven temperature and processing or heating time. Cycle times can be long… often up to 30 minutes. With these longer cycle times, rotationally molded parts typically have run lengths range from just a couple to hundreds of parts with annual order quantities as low as 50 up to thousands of parts.

Ensuring proper product design is critical for manufacturing and assembly optimization as well as the general success of the product itself. Designs and prototypes using 3D solid model rendering and rotolog tooling capability studies are used to help guide customers through the intricacies of the rotational molding process.

With pending ISO 9002 certification and drawing from techniques outlined in Six Sigma, World Class and Lean Manufacturing, Sterling Technologies is dedicated to delivering superior quality. Process controls include rotolog tooling capability studies to optimize product quality throughout the run.

Materials – Resins and Polymers

Polyethylene represents the vast majority of material used in the rotational molding process. These include low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), cross-linked polyethylene (PEX), high-density polyethylene (HDPE), and regrind. Other compounds used are PVC, plastisols, polypropylene, polyvinyl chloride, nylon and polycarbonate.

Resins and polymers are stored and mixed on site. Sterling can make a myriad of adjustments including density, texture and color… even within the same run.

Low Cost Tooling

Since the rotational process uses centrifugal force, not pressure, to fill the mold, rotational molds are relatively low cost when compared to other processes like injection and blow molding. It is important for customers to choose the correct rotational mold for their application. Each type of mold lends itself to different types of products.

The following is an overview of molds available for the plastic rotational molding process.

• Fabricated Molds are used mainly for large parts with simple shape Fabricated from steel, aluminum or stainless steel these molds are lightweight, have a consistently thin cavity wall and are the lowest cost approach when dealing with large molds.
• Cast Molds are the most common type of rotational molds to produce parts varying from very small to large. Cast molds produce parts that require considerable detail or elaborate shapes and they can be modified to integrate design changes.
• CNC Molds are used when extreme precision or extended production runs are required. Because of a higher cost, they are used only in special cases.
• Epoxy Molds use a liquid thermosetting polyester and epoxy material that is formed and cured at room temperature.

The most important thing to consider with all types of molding is the quality of the mold. There is no substitute for an excellent mold. A Production rotational molder can advise you on the type of mold best for your application, and the plastic material most suitable for your product. Sterling Technologies can help you make the right decisions to make your product stand out from the rest.

Part Finishing

Removed from the mold, parts are quickly transferred to finishing where the warm flash is trimmed, assembly holes are drilled and the part is staged for further operation or is wrapped for shipping. If desired, Sterling will “flame” the surface to create a smoother more glossy finish without affecting part quality.

Additional graphic details can be added before or after the molding process. Mold-in-Graphics can be placed inside the mold’s cavity prior to molding. These graphics can be large, colorful and will withstand continued use. Sterling Technologies also provides a range of other graphic enhances including hand painting! Sterling provides:

• Custom trim and cutting operations
• In-mold and post-mold graphics
• Post-mold flaming
• Foam fill and air pressurization
• Air tight pressure testing
• Leak testing

Component or Part Assembly

The Sterling Advantage extends to complete outsourcing of assembly and sub-assembly operations including steel fabrication, assembly, welding and packaging services. Once assembled, we’ll warehouse the parts and drop ship direct to your customers… true one stop shopping.