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Process

How it works

Dry, powdered plastic is added to a closed mold which is then rotated through a heating and cooling cycle. The result is a stress-free, robust part with excellent impact strength. As a result of this process, the interior of each part, where the plastic form does not come in contact with a mold surface, is effectively an uncontrolled surface. This means that tolerances cannot be applied to the inside of the part, unless the surface is subject to a secondary cutting or facing operation after molding. Wall thickness can also vary throughout the part, though material can be accumulated in specific areas or thinned in different areas during forming by mold construction features which either encourage heat to the area (thicker walls) or mask to prevent heat (thinner walls).

A feature unique to rotomolding is molded-in metal inserts. These inserts are added by attaching them to the mold prior to adding powder so that they are permanently molded into the final plastic part. These inserts can have a variety of threads or fittings to work with your mating components.

Another feature that can be formed with rotational molding is a kiss-off, where the mold walls are close enough that when plastic forms over them the two walls touch each other, which can add strength to these areas. This feature is achieved through both mold construction and shot weight and can take a little effort to dial in during pre-production part validation. Techniques likes these are what make Robomold’s special brand of precise plastic solutions.

Tolerances

Tolerances

Typical tolerances for a molded part, from tool side, are listed below. Note that tolerances will be affected by your specific part, tool construction, and material. Please let us know what you need from your part, and we will discuss options for achieving:

Tolerances of a molded part: +/- .030” + .005 in/in for every additional inch past 12”

Molds

Mold Construction

Rotomolded tools are typically machined or cast aluminum and can vary vastly in complexity – from a simple 2-part tool to a multi-piece with multiple pull direction pieces. The draft of the part has to match the pull direction of the mold, so that the part can be pulled out after molding. Negative draft details are accommodated by mold construction. Features such as molded-through holes, cores, and baffles can also be achieved with tool construction.

Part texture is a result of the texture of the mold surface, usually achieved through a media blast to the tool wall of varying particle size depending on the desired texture. The mold can also be polished for a smooth finish. Teflon coating can also be applied to achieve glossy finish over whatever texture is selected.

Materials

Material selection

Gemstar specializes in rotomolding polyethylene parts. We have a variety of in-stock materials as well as the ability to source a variety of custom options.

Custom colors are an option for rotomolding, with two different material options to achieve – precompounded or colorant. With the former, the actual plastic is the color that you desire. With the latter, a color compound is added to a natural-colored material and blended. This lowers the overall impact strength of the final part. Gemstar can guide you through your options for colors and help you decide which option is best for your application.

Available materials:

  • HDPE
  • LLDPE
  • Cross-link
  • Specialty properties:
    • NSF/Food/Medical Grade
    • UL rated flame retardant
    • Conductive
    • Custom colors
    • RoHS and REACH compliant

Logos and Labels

Logo and Labeling options

Permanent graphics and logos can be added to rotomolded parts through a few methods. Embossed and debossed logos can be achieved by machining directly into the tool so that they are molded directly into the part. Where color is desired, mold-in graphics are another permanent option. With this option, a graphic is applied directly to the mold wall which then forms into plastic during molding.