Rotational Molding is a Process Used to Produce Hollow and Seamless Plastic Parts
We answer a lot of questions related to rotational molding. Here are some of the most frequently asked questions regarding our processes, products and capabilities.
The Rotational Molding process is a manufacturing method that yields hollow plastic parts. Traditional applications for rotationally molded parts include large storage tanks, toys, kayaks, fuel tanks, hydraulic tanks, coolers, storage, and recreational equipment. One of the great features of rotational molding is the ability to integrate several metal parts into a single multifunctional plastic part. This not only eliminates assembly time and cost, but can also result in a more useful part with improved styling and ergonomics.
To manufacture a part, a metal mold is mounted to an arm on a molding machine. The mold is filled with a measured charge of powdered resin and is slowly rotated on two axes in convection oven chamber where it is heated quickly with turbulent hot air. During the early stages of the molding cycle, the powder flows over the interior surfaces of the mold as it falls to the bottom of the mold. As the mold heats and reaches the melt point of the resin, thin layers of melted material are distributed evenly until the powder pool is exhausted. The heating cycle continues to complete the sintering process. The “cure” point is reached when most or all bubbles are eliminated from the melt. The mold and part are then cooled to the appropriate de-molding temperature with a combination of fan blown air and water spray. Finished parts are removed, and molds are prepped and charged for the next cycle.
While the oven residency time is generally 15-25 minutes, molding machines typically have 3 or 4 arms that cycle in succession, yielding production rates in the one part per one to two hour range.
Advantages of rotational molding include design flexibility, ability to mold complex parts, consistent wall thickness, and the ability to produce a wide variety of product sizes and colors at a competitive price. Solar Plastics, Inc. produces molded parts in a variety of configurations, ranging in size from 1 to 10 feet in diameter. Annual production quantities range from less than 100 to 50,000 or more.
Design Flexibility
Rotational molding will provide a relatively uniform wall thickness compared to other hollow part processing methods.
Multi-layer construction for sightline or cost reduction.
One of the great features of rotational molding is the ability to convert large numbers of assembled components into a single functional plastic part. This not only eliminates assembly time and cost, but can also result in a more functional part with better ergonomics.
Designers can easily add molded-in metal inserts, features to strengthen like ribs, kiss-offs, and cores, and texture and styling to enhance appearance.
Tooling Cost Savings
Rotational molding is a low pressure process and molds are primarily a thin hollow shell that defines the outside shape and surface of the part. This results in mold costs much less than other processes
Consistent Wall Thickness
A uniform wall thickness can be maintained to within ± 20 percent, giving rotational molding tighter tolerances than blow molding.
Rotational molding tends to produce an increased wall thickness on outside corners, which adds strength to the part. A distinct advantage over blow molding or thermoforming, which tend to produce thin outside corners. Wall thicknesses can range from 1/32 to 1 inch or more (0.8mm to 25mm).
Metal Conversion
Plastic is lighter weight for better fuel economy and less expensive to manufacture than metal parts.
Stress Free / Low Pressure Process
The low processing pressure involved in rotational molding has the added advantage of producing parts which are relatively stress free, as compared to other high pressure processes. This advantage of the process is especially important when considering large, load bearing parts in applications which must provide corrosion or stress-crack resistance.
Shorter Production Times
Solar Plastics can design, tool and begin production in as little as 3 months. Injection and blow molding can take 6 months or more. Rotational molding can get your product to market faster and more economically than any other process.
Ability to do small runs
Due to less expensive tooling, rotational molding makes it more economical to do smaller production runs.
Corrosion Resistance
Plastic will not corrode and can be designed to accommodate many climatic conditions.
UV Protection
Resins contain additives protect against failure due to exposure to UV.
Chemical Resistance
Typical rotational molding resins have excellent resistance to a wide variety of chemicals making them ideal materials for packaging, storage, and where chemical contact is likely.
Strength
Plastic parts may be designed with materials that are impact and dent resistant. Parts can be reinforced with molded features for added strength.
Economical
Plastic is lighter weight for better fuel economy and cheaper to manufacture than metal parts.
Large numbers of assembled components can be incorporated into a single plastic part. This not only eliminates assembly time and cost, but can also result in a more functional part with better ergonomics and styling.
Aesthetics
Since color is part of the molded piece, no secondary painting operations are necessary. The result: a uniform, maintenance free and cost effective color for your part.
Graphics can be made to your specifications and molded into the part, which protects the graphic from ever coming off.
Design flexibility provides opportunities to enhance cosmetics and styling.
When considering your design for rotationally molding, you will want to address a variety of questions to select the type of plastic to best suit your need. What requirements does the design have? Once your product requirements have been identified, an appropriate plastic can be selected. Please feel free to contact us at Solar Plastics, Inc. for guidance in this process.
The plastics used for rotational molding are a finely ground version of many different plastic compounds including polyethylene, cross-linked polyethylene, polypropylene, and nylon among others. These plastics are often referred to as resins and are grouped into two primary categories: thermoplastics and thermoset resins. Thermoplastics are capable of being repeatedly softened by heat and hardened by cooling (polyethylene, polypropylene, and nylon) and thermosets are a plastics that undergo a chemical reaction when molded that changes the fundamental molecular structure (cross-linked polyethylene) and can not be remolded with additional heat.
Polyethylenes – LMDPE (Linear Medium Density Polyethylene) and LHDPE (Linear Medium Density Polyethylene)
This plastic exhibits a flexible to medium stiffness, fair abrasion resistance, good impact strength, and good warp resistance. It is tough and has excellent chemical and environmental stress crack resistance. Most grades will meet the requirements of FDA, USDA, NSF & UL (Horizontal Burn). Applications include tanks, ducting, toys, containers, playground equipment and industrial parts.
XLPE (Cross-Linked Polyethylene)
This plastic contains a cross-linking agent that reacts with the material during the molding cycle, forming a cross-linked molecule similar to a thermoset. This reaction improves the toughness, environmental stress-crack resistance, and tear resistance. The plastic exhibits good abrasion resistance and excellent impact strength. Chemical and vapor permeation properties make it a good choice for fuel tanks. On the other hand, aromatic hydrocarbons do permeate the surface. This plastic is good for parts that are subjected to cold temperatures, but would not be practical for parts subjected to long-term continuous service at 150 degrees F or higher. Most grades do not meet FDA, USDA, or NSF requirements. Applications include gas storage tanks, chemical and sewage tanks, trash containers, seats and other products where stress cracking and tear resistance is important.
Rotolining
Rotolining is a polyethylene that has been chemically modified to allow the resin to adhere to metals or other resins. This plastic forms a chemical bond to polar materials and can be used to line metal vessels for corrosion and chemical resistance.
Polypropylene (PP)
This material is stiffer than polyethylene and has a higher heat distortion temperature. It is autoclavable and possesses excellent chemical and environmental stress crack resistance. It exhibits low impact strength at cold temperatures. Applications include bio-chemical vessels, chemical shipping drums, radioactive material containers and high temperature air ducts.
Polyurethane Foaming
Solar Plastics, LLC has the ability to perform secondary foaming operations on your parts. The polyurethane foam available is a Class 1 (UL723) system which has an overall density of 1.86 to 2.00 lb/ft3. It has no CFC’s and has a K-factor at -20°F (ASTM C-518) of 0.101 – 0.111.
Nylon 6
This material has excellent tensile strength, chemical resistance, and stiffness. In comparison to polyethylene, nylon is easier to paint and has improved abrasion resistance, wear resistance, and creep resistance. Its high heat resistance (>10,000 hours at 212 degrees F; >1,000 hours at 248 degrees F) enables the plastic to maintain its properties at elevated temperatures. Nylon is especially good with hydrocarbons such as gasoline and oil due to the low permeation rate. Nylon is also chemical and permeation resistant to aromatic and aliphatic solvents. It is not resistant to continuous exposure to water or strong acids. Applications include military fuel tanks, hydraulic oil and solvent tanks, grain buckets and air ducts.
Nylon 11
This plastic also exhibits the advantages that nylon 6 does over polyethylene. In comparison to Nylon 6, this material has a lower stiffness and higher elongation. Applications include hydraulic tanks and air ducts.
Nylon 12
Nylon 12 has similar advantages that nylon 6 does over polyethylene. Its moisture absorption, melting point and mechanical properties are lower than Nylon 6, but it is more easily processed. Applications include heating and air conditioning ducts, gasoline /diesel tanks and chemical tanks.
Color
The most common additives are colorants, dyes or pigments. Coloring comes in two basic forms – dry blended or compounded resins. Dry blended resins start with a natural PE resin in which a coloring agent is mixed. This mechanical mixing process causes the dry blend colorant to encapsulate each grain of the PE resin. This is the lower cost alternative for coloring. Compounding is a process where a coloring agent is introduced into PE resin, they are both melted and then extruded to form resin pellets. This homogenous pellet is then reground into rotational molding PE powder resin. The added processing increases cost, but the intensity (chroma) of the color and the lack of translucency normally associated with plastic parts are greatly reduced. Compounding is especially effective for a part needing a “retail” appearance. We will be happy to help you chose the most appropriate method of coloring for your project.
UV Stabilizers
This item is added to absorb and screen ultraviolet light from the sun protecting the plastic and extending it useful outdoor life.
Flame Retardants
These additives enable the plastic to meet the flame contact requirements established by Underwriter Laboratories (UL) for electrical applications.
Foaming Agents
When heated in the molding process, the foaming agent generates a gas that is trapped inside the molten plastic and causes it to foam. The material then has porous walls that are stiffer but lighter in weight than a solid wall of the same strength. Techniques have also been developed for producing foamed parts with solid un-foamed skins./
Although both processes make hollow parts, rotational molding offers design flexibility and molded part characteristics that are difficult to match in blow molding.
- Complex shapes with deep draws can be rotomolded with consistent wall thickness with relative ease. Rotomolded parts actually have their thickest wall sections in outside corners where durability is often critical.
- Rotomolding tooling can be made to provide undercut and cored features that even if possible would be very costly to tool and process in blow molding.
- In-mold decorating and mold-in inserts are also easily accomplished with rotational molding. Rotational molding allows the designer more freedom to design to part function rather than to the constraints of the process or tooling.
Solar offers many design help resources, and can even complete the design to your specifications. Numerous design manuals, technical bulletins, and other references are available.
- A process and design presentation tailored to your needs can be given at your facility. We can even suggest design seminars presented by SPE and the Association of Rotational Molders. These resources will help you determine suitable geometry, features, and material characteristics.
- Solar’s experienced design team can then review your design and provide direction to fine-tune it to enhance processing, tooling reliability, and part quality. Our designers can also complete a design to your requirements using ProE CAD tools.
A single tool may produce approximately 3,000 to 3,500 pieces per year depending on complexity and wall thickness. Peaks in annual usage may require multiple tools./
Specific chemical resistance information is available upon request. It is important to provide additional information such as temperature, concentrations, density, and how the product is to be used.
Thin wall, hollow molds of two or more sections are most common. These molds are typically cast or machined aluminum, or fabricated from steel or aluminum. Individual mold sections are fitted with frames and a clamping system. The mold is provided with a method to mount it to a rotomolding machine.
- Cast tools, which are most common, are built in four stages. 1) Pattern creation: a full size (plus shrink) model of the part is built from design specifications. 2) Foundry: individual mold sections are produced with a sand casting process from the pattern. 3) Finishing: castings are polished, fit, and framed. 4) Final detailing: machined features, texture detail, and coatings are applied. In some instances, tooling can be CNC machined from the part model, eliminating the need to produce a pattern.
- Prototypes can be produced from a mold that is 50 – 80 % complete.
- Solar’s development engineers will help you determine the most appropriate and cost effective construction method. We also provide coordination services during tool build to ensure tools are accurate, durable, and meet our production needs.
While the actual duration depends on the complexity of the design, and the approval process, the typical development timeline is: 6-12 weeks for tool build, 1-3 weeks for inspected samples, 1-3 weeks for finished part evaluation (total 8-18 weeks).
Typically, production part wall thickness can vary from nominal by +/- 20%, and linear dimensions by a basic +/- .030″ plus .005″ per linear inch of the dimension in question. Additionally, a tooling tolerance of +/- .030″ plus .0025″ per linear inch of dimension must be considered when molds are being built. Tighter production tolerances may be achieved. Our development engineers are available to discuss important features, and how well they can be controlled.
Color can be incorporated by dry blending a customized pigment formulation with ground natural material, or by hot melt compounding prior to grinding. With both methods, a wide range of colors is available. Our design team will evaluate specific requests. Important considerations are: base resin, pigment type restrictions (if any), environment, cost, opacity required, and part surface finish.
Benefits include weight reduction, corrosion resistance, and reduced cost. Rotational molding provides the designer the freedom to address form along with function. Parts can be designed to be more ergonomic, and esthetically pleasing without affecting performance. In many cases rotomolded parts reduce the number of components in an assembly.
Process Overview
How does a rotomolded part compare to a blow molded part?
