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    Page 1 of 16Putting The Right Spin On Rotational-Molding Designs | Machine Design

    2/28/2011http://machinedesign.com/article/putting-the-right-spin-on-rotational-molding-designs-0518

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    Buckling Analysis with FEARead Lee Teschler's Latest: On Your Mark, Get Set, ExportLatest score...ardy22 - 0Latest score...rkhattar - 0World's Smartest Design Engineer 2010 Winner -- Steven Ho

    Putting The Right Spin On Rotational-MoldingDesigns

    May 18, 2000

    Stephen J. Mraz

    New developments let rotational molding compete with injection or blow molding.

    Printer-friendly version

    John FawcettFawcett Design Inc.Kent, Ohiowww.fawcett.com

    Computer-generatedcolorrenderingshelprotationalmoldersfinalizedesigns.Theuseof3Dmodelsboostsproductivityandhelpscreatedrawingviewsandcrosssectionsthatsimplifythejobof

    buildingpatterns.

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    Computer-generated color renderings help rotational molders finalize designs. The use of 3D modelsboosts productivity and helps create drawing views and cross sections that simplify the job ofbuilding patterns.

    Apremeasuredamountofpowderedorliquidplasticis

    placedinmoldhalf.Themoldisclosed,transferredtotheheatingoven,andthenontothecoolingstation.Duringtheentireheatingandcoolingprocess,themoldssimultaneouslyrotatearoundtworight-angleaxes.

    A premeasured amount of powdered or liquid plastic is placed in mold half. The mold is closed,transferred to the heating oven, and then on to the cooling station. During the entire heating andcooling process, the molds simultaneously rotate around two right-angle axes.

    Typicalcarouselrotationalmoldingmachine

    withthreearmsthatrotatebetweenloading/unloadingstations,heatingovens,andcoolingchambers.

    Typical carousel rotationalmolding machine with three arms that rotate between loading/unloadingstations, heating ovens, and cooling chambers.

    Page 3 of 16Putting The Right Spin On Rotational-Molding Designs | Machine Design

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    Kiss-offsincreasepartstiffness.Theyareusedindividuallyorinaseriesorpatternandwilloutperformribsinmanycases.Thealmostkiss-offgivesnearlythesamestructuralstrengthwithoutaddingablemishontheoutsidewall.

    Kiss-offs increase part stiffness. They are used individually or in a series or pattern and willoutperform ribs in many cases. The almost kiss-off gives nearly the same structural strength withoutadding a blemish on the outside wall.

    RotationallymoldednewspaperboxesmanufacturedbySteelCity,Youngstown,Ohio,unliketheirmetalcounterpartsretaintheirgoodlooks(nochippedpaintorscratches),remainrustfree,andare

    nearlydingproof.

    Rotationally molded newspaper boxes manufactured by Steel City, Youngstown, Ohio, unlike theirmetal counterparts retain their good looks (no chipped paint or scratches), remain rust free, and arenearly dingproof.

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    Rotationalmoldinggivesaportablefueltankaheavy-dutyhandlewhichprovidesgoodloadcontrolandasecuregrip.MadebyTempoProducts,Cleveland,thetankshavea29-galloncapacityandarecolorcodedfordifferentfuel.

    Rotational molding gives a portable fuel tank a heavy-duty handle which provides good load control

    and a secure grip. Made by Tempo Products, Cleveland, the tanks have a 29-gallon capacity and arecolor coded for different fuel.

    Footballenthusiastsnowsitonrotationallymoldedbenchseats.DesignedbyDant

    ClaytonCorp.,Louisville,Ky.,theplasticseatswereinstalledin1999inthe

    Football enthusiasts now sit on rotationally molded bench seats. Designed by Dant Clayton Corp.,Louisville, Ky., the plastic seats were installed in 1999 in the Citrus Bowl, Orlando, Fla. Normallywith a volume requirement of 65,000 one would consider the blow-molding process. But the projectrequired seven different sized seats each in three different colors. The blow-mold tooling would have

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    been more expensive and the production runs for each size/color combination would have been smallfor the blow-molding process.

    Rotationallymoldedproductsareagoodfitforawiderangeofindustries.Productionrunsupto10,000unitsmaybesuitableforrotationalmolding.

    Rotationally molded products are a good fit for a wide range of industries. Production runs up to10,000 units maybe suitable for rotational molding.

    Rotational molding, often known as rotomolding, is usually described as a plastic process suited forforming 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/unloadingstations, heating ovens, and cooling chambers. Molds made from either sheet metal or cast aluminummount on each arm. A premeasured amount of powdered or liquid plastic goes in half the mold. Themold 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 intouniform layers on mold surfaces. The amount of resin added controls wall thickness. The rotatingmolds then move to the cooling chamber. As a combination of air and water cools the molds theplastic solidifies.

    Parts made from different grades of polyethylene (PE) dominate the rotomolding market. The mostcommon 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 theoriginal material used for rotational molding and is probably the second most common material. It canbe 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, andstiffness 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 REQUIREMENTSBecause most rotomolded products use PE, the following guidelines are geared toward this family ofresins. 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 withnominal dimensions. Tolerances generally should be on the order of 20% depending on part size and

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    shape. Tighter tolerances are possible but will boost part cost. Often designers can get a feel forreasonable wall thicknesses by examining similar rotationally molded parts.

    Part design and mold details affect wall thickness tolerances. Any detail that affects heat transfer intothe mold will influence wall thickness in that area. Thinner walls result from thicker mold walls, deeprecesses, and shielding from mounting frames. Deep and sharp V-shaped recesses, for example, serveto 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 partdetails. Of course it's possible to adjust the wall thickness of a rotomolded part even after you'vefabricated the mold. This lets designers refine strength and cost after producing samples. As withmost 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 sidesof the mold so the top is slightly larger than the bottom makes for easy part removal. The amount ofdraft, however, depends on whether the surface is an inside or an outside wall. Draft may beunnecessary on the outside wall of a large hollow part. But molders often recommend a 1 draft if itwill not degrade the part function. Deep texture on an outside wall does, however, require more draftas 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 compensatesfor the part shrinking onto the mold core instead of away from it. As would be expected, greaterdepths 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 angleshould 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 18 and 316in., 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 letteringand logos having details less than 0.030 in. deep. Two situations require larger radii. Walls thickerthan 316 in. need a minimum outside radius equal to the wall thickness. The minimum inside radiusshould 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 addedadvantage of built-in draft. Compared to other plastic processes, rotational molding requires largertolerances. Low pressures used in rotomolding tend to produce parts that shrink and move freelyduring 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 varieswith 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. Awide tolerance is around 2%, while tight tolerances are on the order of 0.5%.

    SPECIAL REQUIREMENTSLarge flat surfaces are a major problem with rotomolded parts. If possible, flat surfaces should bebroken up by adding design details such as steps or recesses. This helps reduce part distortion. When

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    large flat surfaces are unavoidable, a slight crown added to the surface often keeps the surface fromoil-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 withclosely spaced walls restrict material flow and create thin part walls. Small parts having walls 18 in.thick or less need a minimum 58 in. from outside wall-to-out-side wall. Extremely large parallel wallsoften 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 togetherin small circular or oval-shaped areas. Kiss-offs make parts stiffer. They are used individually or in aseries 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 isbetter 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 uniformthickness.

    Kiss-offs, however, cause a blemish on the opposite wall. To make surfaces look better without losingthe 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-offsupports it. The technique often finds use in playground slides where regular kiss-offs tend to leave aslight hump in the part.

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

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

    3D MODELINGProductivity has been boosted in many areas by 3D modeling, and is particularly useful forrotationally 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 thejob 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 modelscan help check fit and function and aesthetics when parts are too large even to be prototyped aspieces.

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

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

    Automotive and aerospace industries use CNC molds because of the complexity and tight tolerancesof their designs. Machined molds often retain tolerances of 0.010 in. over a 48-in. span compared totolerances 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 crosssections. However, molds with deep cross sections often use multiple piece assemblies to help reducethe amount of metal that must be removed.

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

    A LITTLE HISTORYIn the past, many rotationally molded parts were developed with little or no documentation. A woodpattern was made to build a cast mold. Much of the design review process took place using the woodpattern.

    The finalized pattern was the basis for the first mold. A design review took place after sampling runsand also looked at processing issues. It was common practice to view this first mold as a prototype.

    Revisions took place on the original wood pattern, from which a second mold was made. This step-by-step process was adequate because tooling was inexpensive and few products required rapidexecution.

    Process comparisons

    DESIGN FLEXIBILITYLARGE PARTS

    Injection and rotational molding1.Blow molding2.Vacuum molding3.

    TOOLING COST

    Vacuum molding1.Rotational molding2.Blow molding3.Injection molding4.

    PART COST/HIGH VOLUME

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    Injection molding1.Blow molding2.Rotational molding3.Vacuum molding4.

    LARGE PARTS

    Rotational molding1.Blow and vacuum molding2.Injection molding3.

    Want to use this article? Click here for options! 2011 Penton Media Inc.

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    I want to produce artificial

    On April 17th, 2009 mahmood (not verified) says:

    I want to produce artificial tree , planet and branches can i make them with rotational moulds?

    reply

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