Putting The Right Spin On Rotational-Molding Designs

John Fawcett
Fawcett Design Inc.
Kent, Ohio
www.fawcett.com

Rotational molding, often known as rotomolding, is usually described as a plastic process suited for forming large hollow parts. This description was accurate 20 or 30 years ago but does not fully describe the process today. Rotationally molded products are a good fit for a wide range of industries. The most common are toy and juvenile products; industrial, agricultural, and chemical tanks; recreational and sporting goods; and material-handling parts such as totes, pallets, and bins.

In their simplest form, rotomolding machines have three arms that rotate between loading/unloading stations, heating ovens, and cooling chambers. Molds made from either sheet metal or cast aluminum mount on each arm. A premeasured amount of powdered or liquid plastic goes in half the mold. The mold then closes and the arm moves it into the heating oven.

Inside the oven, the molds simultaneously rotate around two right-angle axes. Heat fuses the resin into uniform layers on mold surfaces. The amount of resin added controls wall thickness. The rotating molds then move to the cooling chamber. As a combination of air and water cools the molds the plastic solidifies.

Parts made from different grades of polyethylene (PE) dominate the rotomolding market. The most common grade of PE is linear low density (LLDPE). Others include high-density (HDPE), cross-linked (XPE), and ethylene-vinyl-acetate (EVA) copolymers. Polyvinylchloride (PVC) was the original material used for rotational molding and is probably the second most common material. It can be either liquid or powder and comes in a wide variety of durometers. Thermoplastics such as nylon, polycarbonate (PC), or polypropylene (PP) generally give better heat resistance, tensile strength, and stiffness than PE.

Rotomolding is known for providing design flexibility, low-cost tooling, and stress-free parts. Its disadvantages, however, include higher part cost, fewer material choices compared to other processes, and slower production.

DESIGN REQUIREMENTS
Because most rotomolded products use PE, the following guidelines are geared toward this family of resins. Nylons, PVCs, and PCs will need minor modifications.

Wall thicknesses of 0.125 to 0.25 in. cover most PE applications and should be specified with nominal dimensions. Tolerances generally should be on the order of ±20% depending on part size and shape. Tighter tolerances are possible but will boost part cost. Often designers can get a feel for reasonable wall thicknesses by examining similar rotationally molded parts.

Part design and mold details affect wall thickness tolerances. Any detail that affects heat transfer into the mold will influence wall thickness in that area. Thinner walls result from thicker mold walls, deep recesses, and shielding from mounting frames. Deep and sharp V-shaped recesses, for example, serve to form trim-lines in parts letting sections of the part be easily removed.

Conversely, mold areas with thin walls form thicker-walled parts. Ditto for those with protruding part details. Of course it's possible to adjust the wall thickness of a rotomolded part even after you've fabricated the mold. This lets designers refine strength and cost after producing samples. As with most plastic processes, it takes longer and costs more to make parts with thicker walls.

In most cases, the walls of the mold must be made with a slight angle, called draft. Tapering the sides of the mold so the top is slightly larger than the bottom makes for easy part removal. The amount of draft, however, depends on whether the surface is an inside or an outside wall. Draft may be unnecessary on the outside wall of a large hollow part. But molders often recommend a 1° draft if it will not degrade the part function. Deep texture on an outside wall does, however, require more draft as do parts having holes and recesses near an outside wall. These features can otherwise keep the part from shrinking away from the mold.

Holes or recesses that create inside walls demand greater draft angles of perhaps 5°. This compensates for the part shrinking onto the mold core instead of away from it. As would be expected, greater depths or more recesses and holes demand larger draft angles. Material shrinkage also affects draft. PE, for example, is somewhat slippery and needs less draft than other materials. And draft angle should increase as material stiffness rises.

Corner radii are important in these parts. Their primary function is to let material flow around corners. Inside corners will have different radii than outside corners. They should be at minimum 1⁄8 and 3⁄16 in., respectively. Sharp corners are generally unacceptable. Sharp outside corners create bridging across the feature and holes in the material. This is a cosmetic blemish that also weakens the part. Sharp inside corners generate thin, weak walls. But sharp corners can be useful for creating lettering and logos having details less than 0.030 in. deep. Two situations require larger radii. Walls thicker than 3⁄16 in. need a minimum outside radius equal to the wall thickness. The minimum inside radius should increase by the same amount. Corner angles below 45°, may need as much as a 0.5-in. radius. However, smaller-than-normal radii find use in special situations. For shallow recesses or steps, designers often blend outside and inside radii together forming an “S” curve. This has the added advantage of built-in draft. Compared to other plastic processes, rotational molding requires larger tolerances. Low pressures used in rotomolding tend to produce parts that shrink and move freely during cooling. Process controls also are apt to be less sophisticated than other forming processes.

Variables such as the quality and accuracy of the pattern contribute to wider tolerances. The shrinkage value assigned when making the pattern may impact tolerances as well. Actual part shrinkage varies with part design, wall thickness, and processing parameters used to make the part.

Tolerances for rotationally molded parts are generally given as a percentage of the dimensions. A wide tolerance is around ±2%, while tight tolerances are on the order of ±0.5%.

SPECIAL REQUIREMENTS
Large flat surfaces are a major problem with rotomolded parts. If possible, flat surfaces should be broken up by adding design details such as steps or recesses. This helps reduce part distortion. When large flat surfaces are unavoidable, a slight crown added to the surface often keeps the surface from oil-canning or bowing-in. Adding heavy textures to flat surfaces makes a part look better if it distorts.

Rotationally molded parts with parallel walls require careful design con-siderations. Molds with closely spaced walls restrict material flow and create thin part walls. Small parts having walls 1⁄8 in. thick or less need a minimum 5⁄8 in. from outside wall-to-out-side wall. Extremely large parallel walls often need at least 1 in. between them. As walls get thicker the minimum distance between them must rise as well. In general, 1 in. will suffice, except for extremely thick walls over a large distance.

Design features called kiss-offs are also important. They form when two walls of a part come together in small circular or oval-shaped areas. Kiss-offs make parts stiffer. They are used individually or in a series or pattern and outperform ribs in many cases. The distance between the outside walls of the kiss-off should be 1.5 to 1.75 times the part wall thickness. This creates a small gap in the mold. It is better to start with a small gap and increase it as necessary. Kiss-offs must be spaced at least 2 in. apart. This lets material flow through and around them, so walls on the rest of the part keep uniform thickness.

Kiss-offs, however, cause a blemish on the opposite wall. To make surfaces look better without losing the strength advantage from kiss-offs, molders use the "almost" kiss-off. An almost kiss-off leaves a slight gap between the two walls. A good starting point is to use spacing equal to twice the wall thickness plus ¹ /8 in. No blemish is formed because the two walls don't join. This design is almost as strong as a regular kiss-off because the opposite wall only need flex ¹ /8 in. before the kiss-off supports it. The technique often finds use in playground slides where regular kiss-offs tend to leave a slight hump in the part.

Rotational molding tolerates designs with undercuts (inwardly or outwardly projecting walls that are parallel to mold parting lines) better than most plastic-processing techniques. Molded undercuts often require that parts be forcibly bent or twisted to be demolded. But, because the molded material is free to shrink away from mold walls, it is possible to design parts that shrink completely away from the undercut. This makes removal easy.

In general, undercuts equaling half the shrinkage are acceptable for an unrestricted part. It's best to avoid under-cuts on mold cores because the material shrinks onto the core and the plastic must stretch to demold the part.

3D MODELING
Productivity has been boosted in many areas by 3D modeling, and is particularly useful for rotationally molded products. But, despite this, most tooling patterns are still being made by hand. Designers often use 3D CAD packages to create drawing views and cross sections that simplify the job of building a pattern.

Rapid prototyping is made possible by 3D data files. Rotational-mold prototypes often are made in several pieces and fastened together to avoid size limitations of prototype equipment. Scale models can help check fit and function and aesthetics when parts are too large even to be prototyped as pieces.

Likewise, 3D modeling lets designers quickly calculate exact part weights. Solid models are useful for determining the amount of material needed to mold the part. Such calculations tell whether the volume of powered plastic needed will actually fit inside the mold. The part center-of-gravity also comes out of the 3D model. This information is useful where parts move or rotate during use.

CNC machines have been around for many years, but the rotationalmolding industry didn't use them much until 3D models came along. Now patterns are machined directly from the 3D data. They are as durable as typical handmade wood patterns and more accurate. CNC may be the only option for making patterns of complex and highly contoured parts. But traditional wood patterns are still best for large but simple patterns.

It's also possible to CNC an aluminum mold directly from the 3D data, entirely eliminating a pattern. CNC molds are well established for injection and blow-molding industries, but are relatively new to rotational molding.

Automotive and aerospace industries use CNC molds because of the complexity and tight tolerances of their designs. Machined molds often retain tolerances of ±0.010 in. over a 48-in. span compared to tolerances of ±0.030 ipf of mold length for castings.

Unlike injection or blow molds, rotational molds must be machined on both the inside and outside to maintain even wall thickness, which is typically ^{1/4 to 3} /8 in. Most machined molds have shallow cross sections. However, molds with deep cross sections often use multiple piece assemblies to help reduce the amount of metal that must be removed.

Machined aluminum molds are tougher and denser than their cast aluminum counterparts. This gives them better parting lines and wear resistance. In addition, machined molds have no porosity so they accept any texture regardless of its depth and often take 25 to 50% less time to make.