Rotational molding offers tremendous flexibility both in design and production.

The rotational molding process offers tremendous flexibility both in design and production making it possible to mold very large hollow pieces in virtually any shape, size, color and configuration. The "double wall" construction creates a lightweight piece with good structural integrity.

Rotational molding process advantages include:

  • Uses dual-axis rotation, not pressure, to fill the mold
  • Low tooling cost
  • Flexible design parameters
  • Excellent load bearing characteristics
  • High Impact tolerance
  • Exceptionally strong corner strength
  • Easy wall thickness corrections
  • Quick speed to market

Custom rotationally molded part Click on chart to view it at full size. Rotational Molding Process Comparison Chart

The Sterling Advantage

The Sterling Advantage reflects not only our rotational molding capabilities, but also the dedication and commitment of our team of rotational molders. Guided by process and quality controls the "Can-Do" team spirit ensures customers receive their parts as specified and on time. World class customer service provides constant production updates and open communication with customers. Team leaders from estimating, production scheduling, molding and finishing all work to provide streamlined communications and set proper expectations. The Sterling Advantage... providing leadership in custom rotationally molded plastic parts.

Molding and Manufacturing

Rotational Molding and Manufacturing

The rotational molding process uses biaxial rotation and high temperature to fill the mold and form the plastic part or component. The rotomolding 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 rotomolding 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 receipe 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.

Rotational Molding Process Plant

Ensuring proper product design is critical for manufacturing and assembly optimization as well as the general success of the product itself. Rotational molding 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

Rotational Molding Polyethylene

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 in the rotomolding process are PVC, plastisols, polypropylene, polyvinyl chloride, nylon and polycarbonate.

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

Low Cost Tooling

Custom Rotational Molding Rotationally Molded Plastic Part

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

Rotational Molding Parts Finishing

Removed from the rotational 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 the quality of your rotationally molded part.

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 rotational molding

  • 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

custom rotational molding

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.

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The Molding Process Comparison Chart
Molding Process Injection Reaction Injection Blow Vacuum Forming (vacuuforming) Compression Extrusion Rotational (Rotomolding) Thermoforming Twin Sheet Foaming Selective Laser Sintering Rapid Prototyping Lost Foam Casting (for making molds)
Process Description Heated plastic is injected into mold Plastic is injected into mold and cures from chemical reactions A parison (tubular plastic charge) is attached to a mold and then filled with air. Heated plastic sheet is stretched over mold and suctioned into the form. Heated plastic charge is compressed to shape of the mold. Heated plastic is forced through a die creating a long part Plastic charge is placed in a mold which rotates, bi-directionally, in oven Heated plastic sheet is stretched over mold Method of Thermoforming that welds two plastic sheets into one 3D product Plastic material is heated with a laser until its particles adhere to each other Photosensitive plastic is cured by a laser in layers Part is coated and pressed into sand. Metal (or other material) is poured into depression.
Ideal Purposes Creating small and/or critical tolerance parts Panels, enclosures, housings, automotive parts Bottles, various containers Product packaging, speaker casings, car dashboards, aerospace Automotive parts, textiles, large pattern pieces Tubing, piping, fiber optics Containers, fuel cells, Large/complex products, housing, enclosures, concept products Disposable cups, containers, lids, trays, blisters, clamshells, vehicle door and dash panels, refrigerator liners, utility vehicle beds and pallets Pallets, portable toilets, housings, tanks, air and ventilation ducts, enclosures, cases, toys, flat and transportation-related products Concept or highly specific/critical tolerance designs Concept design Making molds and duplicates of concept designs
Low Mold Cost? X X X X X X N/A N/A N/A
Low Unit Cost? X X X X X
Quick Turn Around Time? X X X X X
High Strength Parts? X X X X X
Complex Part Geometries? X X X X X
Pros Quick turn around and very detailed parts Low mold costs, strong flexible parts Fast, cheap production Flexibility in molding structures Can mold large and intricate patterns, very low cost, Ultra large basic shape production Low cost and quick turn around Very strong, flexible, cheap parts. Several finishing and production options. Complex geometries are possible Strong and flexible parts. Stiff, more structural parts Useful for extremely critical dimensioned designs, no mold costs Excellent for scaled down concept models. Great for making molds and rarely duplicates of parts our of specific metal materials
Cons Weaker parts than other processes, very high upfront costs Long process times, expensive raw material costs Weaker parts, limited geometry Only adept at making shallow parts, processing can be difficult, limited geometry Poor product consistency, heavy flash issues VERY limited geometry Very small tolerances are difficult to form, slower than high-speed processes. Slower processing and very difficult to form complex geometries Limited geometry, additional machines required for various materials, parts aren't as flexible Very high costs, slow process, size limitations Very expensive, fragile, cannot be easily modified, size restrictions Not useful for production and expensive

Q&A with Sterling's Design Engineers

Product Development
Sterling Technologies

How does your involvement with customers generally begin?

Our involvements typically starts from the very beginning. The customer will contact  our sales department to outline the project, and at that point they will set up a conference call or a face-to-face meeting to discuss the project.

Do customer typically submit a drawing right away, or is it just a concept idea?

That all depends on where they are at with their project. Some customers come to us with a fully designed product that we just have to redline to make sure if fits our molding process, others may come to us with a rough sketch of what they want, and some just have nothing at all and we have to start from scratch. I think that is what makes us unique is the fact that no matter where they are in the process we can help them get them product to the finish line.

What kinds of evaluations do you go through to be sure it is engineered properly for rotomolding?

There are a number of things we look for when designing and evaluating a project. We look for sharp corners or even very small radii, in rotomolding typically these type of areas like to trap air/gas bubbles which result in porous edges. Another area we look at is does the part have the proper draft built into it and are there any areas the create an undercut that would prevent the part from being able to be de-molded. We also review the model to make sure that there are no areas that are narrow and deep that may prevent heat transfer in the mold itself. Without heat transfer in the mold the plastic will not form in those areas. The last major thing we look for is the actual interior volume of the mold. There must be enough volume in the mold to accommodate the proper charge weight of the part. A general rule of thumb is 1 cu.ft. of interior volume can hold up to approx. 21 pounds of resin. The charge weight of a part dictates the wall thickness. The same mold can produce parts with different wall thicknesses just by increasing or decreasing the charge weight. So for example, if the part requires 42 lbs of resin to create a part with a wall thickness of 3/16" it would need approx. 2 cu.ft. of interior volume, but the mold only has 1.375 cu.ft. of interior volume. So it would not be able to produce a part with a 3/16" wall thickness. The customer would then have to do one of two things, increase the interior volume or decrease the wall thickness of the part. These are only some of the major things we look for, there could be some other areas that we find on occasionally that need to be modified.

Is there a typical timeframe that you try to adhere to? Two days, two weeks, two months?

Every project is different. It really depends to the scope of work. I have seen some jobs take a day to design and some have taken a lot longer.

Have you had conversations with clients about their experiences with other designers or molders? ...how does Sterling stack up... easy to work with, fast results, etc

I have not heard directly from the customers, but I have heard through other people that our turnaround time for a design is fast and our level of professionalism is top notch.

How involved are you with the actual mold design and specification?

Very involved. We layout how we want the mold to look and how they are to operate one they are complete. We create the complete list of specification for the tooling group.

Sterling Rotational Molding Machine R9

One of Sterling's rotational molding machines producing large plastic parts

Sterling Rotational Molding Machine R9

One of Sterling's rotational molding machines producing large plastic parts

Tooling for Rotational Molding

Tooling for Rotational Molding

Rotational Molding Tool is readyied for production

A new road barrier complex mold is put into production

Rotational Molding Tool is readyied for production

A new road barrier complex mold is put into production

Rotational Molding Cooling Chamber

Rotational Molding Cooling Chamber

Sterling FastTracks Polaris Project

Polaris Accessories

Polaris is the leader in off-road, ATV and Utility vehicles for sportsman and outdoor enthusiasts. Hunters have been a strong and growing customer of their business. To capitalize on this market, Polaris looked to Sterling Technologies to help develop some products for their new line-up.

Scott S., Polaris Team Leader - Engineered Parts & Accessories, had a concept for a gun vault, dog kennel, and cooler package option for the bed of their popular Ranger™ UTV. After doing his research, he contacted Sterling Technologies to explain his vision, review the timeline of project and discuss consumer price points. Within one month, Scott had his final drawings and ordered his tooling through Sterling's OneSource FastTrack program. The tool was online, molding sample quantities just 3 months from that initial phone call.

Scott S.,  Polaris Team Leader - Engineered Parts & Accessories

"Working with the Sterling team from sales through production was a terrific experience. They helped us develop a high-quality, modular system all while helping us maintain desired margins. Our company has a strong reputation for product quality and value and that's what was delivered. I'm sure this is just the beginning of our partnership with Sterling."

Scott S., Polaris Team Leader - Polaris Accessories

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