Designing Rotationally Molded Parts

Rotomolded part design is more of an art than a science. There are not nearly as many guidelines as for injection molding. There are some general recommendations, but nothing specific.

Rotomolded parts must be hollow due to the nature of the process. Many shapes can be achieved. Some examples are agricultural storage tanks, trash carts, buoys, highway barriers, and large children's toys like riding horses, slides, and playhouses. Stiff rotomolded materials like polycarbonate can compete with fiberglass and resin parts like truck caps and snowmobile housings.

Use Draft

Draft all surfaces that are parallel to the mold opening direction. For rotationally molded parts, 3° draft is common. Click here for the draft section for injection molding (it is not all applicable, but it explains the concept).

Radius All Sharp Corners

As in any part design, avoid sharp corners. Break all sharps with radii (click here for details). There are no specific guidelines for how large a radius is necessary, but bigger is usually better. Corners that are too sharp may lead to blow holes and porosity because the powder cannot flow easily into the corner.

Carefully Design Large, Flat Sections

Large, flat sections should be avoided because they tend to warp. Use a rib or dome structure as shown in Figure 1 to stiffen a wall.

Minimize Wall Thickness

Wall thickness variation in a rotomolded part is less than that in blow molding and thermoforming, but not as good as injection molding (variation of +/- 10% is common). Wall thickness for rotomolded parts is usually between 0.3" to 1.2" (much thicker than other processes). Since cooling and mold heating take up such a long part of the cycle, the design should use the minimum wall thickness allowable for functionality. This is crucial for thermally sensitive resins. For example, a 0.3" increase in wall thickness of a Nylon 6 resin will increase heating time by about two minutes. This extra time could cause the plastic to degrade, as well as increase manufacturing cost. Thermal sensitivity of the resin can limit the wall thickness.

Avoid Thin Features

Thin boss features should be avoided because of filling problems. For example, conventional ribs cannot be used, so the entire wall must be corrugated, as shown in Figure 1. For this reason, it is also difficult to add bosses for fasteners. Before any complicated features are added, consider if the powder will be able to flow into the channel.

Avoid Undercuts

Undercuts should be avoided if possible, because they add cost and complexity to the mold, and add to the loading and unloading time. Click here for the undercut section for injection molding (although it is not all applicable, it explains undercuts and the associated ejection problems). External threads are possible to mold, but they should be large rounded threads (V shapes should be avoided because they may not fill).

Miscellaneous

Metal inserts can easily be used if needed. They can be placed in the mold during the loading stage, and the plastic will fuse around them during the heating stage.

Sometimes, to increase the part's stiffness, an internal foam core is added into the part's hollow section during a secondary operation.