Frequently Asked Questions


How does rotomolding compare with injection molding and blow molding?

What types of products most effectively use the advantages of rotomolding?

What materials are ideal for rotomolding?

Why are these ideal for rotomolding as opposed to other materials?

How does the rotomolding process work?

Why has rotomolding grown so popular so quickly?

How much does tooling cost to develop and how long will it last?

How many pieces are produced per cavity in a 24-hour period?

What is mold maintenance and who pays for it?




Q: How does rotomolding compare with injection molding and blow molding?

A: In comparison to injection and blow molding, rotational molding can easily produce both large, small precision and non-precision parts in a cost effective manner from a tiny medical bulb to a 16 foot kayak and up. Tooling for rotomolding is less expensive than injection molding because the process does not require any pressure to be held and rotomolding tooling is far less expensive than blow molding because there's no internal core to manufacture. Since there is no internal core, minor changes can be easily made to an existing mold, enabling product enhancements and line extensions to be considerably less expensive to develop and faster to bring to market.

In addition, Rotomolded parts are formed with heat and rotation, but not with pressure. Therefore, molds don't need to withstand the high pressure of injection molding.

Costs for product conversions from one material to another are reduced because rotomolding permits the replacement of heavier, often more costly materials with lighter, less expensive materials and does not require extreme or expensive tooling changes to switch materials. This advantage makes rotomolding as cost effective for one-of-a-kind prototypes as it does for large production runs.

Rotomolding also offers significant design advantages in comparison to injection molding and blow molding. With proper design and engineering, parts that are assembled from several injection or blow molded pieces can be molded as a single part, eliminating expensive fabrication costs and streamlining the manufacturing process.

Rotomolding also has a number of inherent design strengths, such as consistent wall thickness and strong corners that are virtually stress free. If additional strength is required, reinforcing ribs can be designed and molded into the part.

In the hands of skilled engineers, rotomolding delivers the product the designer envisions without the limitations of other processes. With counsel from Interpak engineers, designers can select the ideal material for their application, including materials that meet FDA or USDA requirements. Weather resistant, anti-static, dielectric and other properties may be specified with the use of a host of additives.

Inserts, threads, handles, undercuts, flat surfaces that eliminate draft angles or fine surface detail are all designed in from the beginning. Designers also have the option of multi-wall molding that can be either hollow or foam filled. Sales and marketing teams always appreciates the range of colors, permanent, molded-in logos and graphics available.



Q: What types of products most effectively use the advantages of rotomolding?

A: Complex products with odd and unique configurations and products used in demanding applications where there is no room for error are especially well suited to rotomolding. Fuel tanks and fuel filler necks can be molded in one, uniform piece eliminating the potential for dangerous leaks and spills.

Oversized items such as spiral-shaped playground equipment, bulk, specialty and chemical material handling containers, kneeboards, toys and recreational and sporting equipment are also well suited. In addition, any products that need to withstand marine and outdoor environments are well suited to rotomolding. Dock floats, marine fuel tanks and other products with a seemless design are virtually impervious to attack from the salts and corrosives of the ocean and harbor while agricultural containers easily stand up to the harsh conditions of the blistering sun and extreme cold.



Q: What materials are ideal for rotomolding?

A: Cross-linked polyethylene (PE); linear low density polyethylene (LLDPE); high density polyethylene (HDPE); Polyvinyl chloride (PVC);



Q: Why are these ideal for rotomolding as opposed to other materials?

A: Cross-linked polyethylene (PE) excels in cold temperatures and won't break on impact in extremes of up to 20 degrees below zero F. LLDPE may be used in a diverse array of applications at relatively low cost. HDPE is used in agricultural chemical and other outdoor applications for its ability to withstand a wide range of elements. PVC is ideal for applications that demand flexibility such as medical tubing, bags and cases.



Q: How does the rotomolding process work?

A: The rotational molding process starts with a quality cast or fabricated mold. The mold is placed in a rotomolding machine that has a loading, heating, staging and cooling area.

Several molds are placed on the machine at the same time. Pre-measured plastic resin is loaded into each mold, then the molds are moved into the oven where they are slowly rotated on both the vertical and horizontal axis. The melting resin sticks to the hot mold and coats every surface evenly. The mold continues to rotate during the cooling cycle so the parts retain a uniform wall thickness, if that is desired.

Once the parts are cooled, they are released from the mold. The rotational speed, heating and cooling times are all controlled throughout the process and may be adjusted based on the design of the product.



Q: Why has rotomolding grown so popular so quickly?

A: Rotomolding offers a relatively low initial tooling investment and permits very short lead times. It also offers a relatively low cost in small-to-moderate production runs. Taken together, these characteristics are quite an advantage to companies that need to bring new products to market quickly at low cost and with little up front investment to minimize risk.



Q: How much does tooling cost to develop and how long will it last?

A: Tooling prices vary based on the complexity and size of the design. Interpak uses 3-D computer modeling and CAD in developing the tooling to speed the process and reduce costs. The lifespan of tooling depends on the intensity of its production runs. If maintained properly, tooling lasts for decades.



Q: How many pieces are produced per cavity in a 24-hour period?

A: Amounts vary based on the type of project and the machine loading but typically range from 15 to 30 pieces per 24-hour period. Our engineers are able to schedule the precise number to be molded in a given time period and offers JIT delivery to ensure only the needed amount of product is molded and delivered.



Q: What is mold maintenance and who pays for it?

A: The intense heating and cooling process of rotomolding puts considerable stress on the molds. That's why it makes sense to develop a high quality mold from the very beginning. The better the mold, the more able it is to withstand many long production runs. Over time, every mold does require maintenance, which consists of reworking framing. Most rotomolders charge for mold maintenance but Interpak includes it as an accommodation.



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