About Rotational Moulding

Rotational moulding, known also as rotomoulding or rotocasting, is a process for manufacturing hollow plastic products. For certain types of liquid vinyls, the term slush molding is also used. Rotational moulding has particular advantages in terms of relatively low levels of residual stresses and inexpensive molds. Rotational moulding also has few competitors for the production of large (> 2 m3) hollow objects in one piece. Rotational moulding is best known for the manufacture of tanks but it can also be used to make complex medical products, toys, leisure craft, and highly aesthetic point-of-sale products.  Click here to read Professor Roy Crawford’s quick guide to rotational moulding.

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Design & Construction

Rotational Moulding makes a huge range of products possible.  With good design and quality construction, parts are fit for purpose, aesthetically pleasing and made to last!

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Sustainability

The most popular material used in rotomoulding, polyethylene is a highly recyclable plastic that is commonly ground up and used in new products at the end of its life.  If you are interested in rotomoulding in the environment, click here to find out more.

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Equipment & Materials

Rotational moulding (or rotomolding) uses different kinds of machines and materials to produce a part.  Find out more about the types of machinery and materials to help you source the perfect combination for your project.

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More Information

Need to know more?  If you would like to discuss any aspect of the process, how to find a quality rotomoulder in your area around the world or what you need to consider when working with moulders, click here for more!

 

Working With Rotomoulders

If you have a product you would like manufactured by rotational moulding there are some simple steps to help you get the most out of the consultation process with your manufacturer. We have included some FAQ’s here to help you:

Do I need to use a professional designer?

Not always.  Some rotational moulding companies have in house design staff; others have strong partnerships within the professional design industry that you can utilize.  Rotational moulding design is a highly specialist area and using a designer who is knowledgeable about the process could save you thousands of dollars in mould design fabrication and manufacturing.

How far along with my product development should I be before I talk to someone?

You don’t need a fully realised design to begin speaking to manufacturers; in fact they can often provide feedback and tips that are specialist to the process to improve your design by eliminating fabrication or simplifying the design.  It’s always better to speak with moulders early in the process to ensure you don’t waste time and money.

Why do quotes vary from moulder to moulder?

ARMA recommends you obtain 3 quotes from moulders during the development process.  Remember it shouldn’t only be about price and what seems too good to be true usually is.  If there is a substantial difference in pricing always ask the company what their price includes.  You may find one company has factored in a realistic price for warranty service for example and another hasn’t included anything.  There may be several excellent reasons for variations in different quotes to help you compare them accurately ARMA has a guide on working with moulders which you can Download.

Should I ask for a contract to supply?

ARMA recommends you only proceed when you have a written contract to supply which includes all the details of your project.  ARMA members will have their own contract or can access the recommended contract through the association.  As a guide your contract should include the amount of items to be made how much you have agreed to pay if the articles will be guaranteed to be fit for purpose if they include a warranty and who will be responsible for warranty claims.  You can contact ARMA for a copy of a contract template if your  moulder doesn’t have one or download an example here.Contract to Supply & Conditions Template

What if I am buying a load of containers or other products from a manufacturer?

Even if you are buying products out of the stock held by a rotational moulder you should have a contract that specifically details who will be responsible for honouring any manufacturer’s or “voluntary” warranty on the products.

Find out more about Warranties in NSW >  (for consumers)

Find out more about Warranties in NSW > (for Businesses)

Find out more about Consumer law changes in NSW >

Can I ask for product insurance or take it out myself?

It’s an unfortunate reality that in business today not everyone will survive in the long term.  If you have decided to offer an additional warranty (voluntary warranty) to sell your products you may want to ask your moulder or your insurer about product insurance.  This will protect you from claims should your manufacturer go out of business before any additional warranty period has expired.

What if something goes wrong with the product?

The legal implications of failure will differ depending on where your company is based.  In some areas (such as New South Wales) the warranty liability remains with the retailer of the product not the manufacturer.  In other areas this may not be the case but the best way to ensure everyone is clear about what will happen if a product does fail is for the details of this to be included in your contract to supply.  ARMA recommends you take independent legal advice before you agree to or sign any legal document to be sure you understand the risks.

 

Regardless of which country or area in which you operate it is a business owner’s responsibility to ensure they are aware of and comply with all relevant legislation in relation to the products they sell and the statements they make to get that sale.

Decoration

Materials for rotational moulding come in a rainbow of colours and so decoration may not alwaysbe necessary however to make the most of your product and to focus your consumers on your company’s brand there are some fantastic decoration options for rotomolding.Moulders can even mix colours to make your part look like stone marble or just a funky blend of bright colours.

Clever use of the materials can give you everything from a stone effect to a lace like finish on parts of the product.

Many products include colourful permanent graphics that are added either during or after the moulding process.The design of graphics is only limited by your imagination and the parameters of the part you are manufacturing.They become a permanent feature of the product.Graphics can also include individually individual serial numbers to help track your products and match production records.

Products can also be decorated with special spray paint type products.Normal paint doesn’t adhere well to polyethelyne.These speciality paints allow you to create unique individual designs or similar designs with difficult shapes.

If aesthetics aren’t vital for your product you can still decorate or mark your part using metal plates or imprints.

No matter what your decoration requirements rotational moulders have a great solution to suite your needs.

Design

The lessons learned here are only now being transferred to other technologies. In addition special types of features such as kiss-off points have been developed by rotational moulders to enhance the load carrying capacity of relatively thin walled shell-like mouldings.

Part designers must adjust to the generous radii and relatively coarse surface textures imposed by the process. Furthermore the process tends to be labour intensive and until recently the technical understanding of the process lagged behind those of other processes such as blow moulding and thermoforming. Part of the reason for this is that unlike nearly every other manufacturing method for plastic parts the rotational moulding process relies on coalescence and densification of discrete powder particles against a rotating mould cavity wall an effect that is extremely difficult to model accurately.  Another part of the reason is that the process has not attracted academic interest in the same way as other processes such as compounding extrusion and injection moulding.

To add strength to the part designers can consider including kiss-offs foaming internal ribs or double walls. Most moulds use only two parts; however some designs may make multi wall moulds necessary.  Product designers commonly use Finite Element Analysis to ensure a product is fit for purpose.  FEA relies heavily on the information provided by material suppliers to the Engineers performing the testing.

Designing for rotational moulding is a specialist area and requires a thorough knowledge of the process and its effect on the final part.  We have a great PDF resource for download full of information to help your designer get familiar with the process and another PDF for adding stiffness to your rotationally moulded part.

Fittings and Inserts

Plastic or metal inserts can be used and the correct choice will depend on the on the dimensional tolerance of the part the wall thickness and the design of the mould itself. During manufacture metal inserts can overheat and affect the final part so in some cases it may be necessary to shield the inserts from heat to ensure the material surrounding it does not overcook.

It is also possible to design in handles which will form from the material or add handles in a similar way to inserts add metal plates to mark the part with information for identification and add threads. All of these types of specialist design areas rely on the correct flow of the powder material to form properly with a crisp clean part so good design and a high quality mould are essential however the additional cost of the mould is usually more than offset in producing a one-step part with minimal post production work necessary.

Many other types of fittings can be added post production and these may include threads taps or faucets screws and other features to aid in assembly of the final part.

How It Works

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Design

Great rotomoulded products begin with great design.  Our unique process of low stress part formation relies on material flowing around the mould and that means design has to start well before the first mould is constructed.  Knowledgeable designers can use the process to add strength, function and beauty.  Rotomolding commonly uses computer aided design and a predictive test called finite element analysis to ensure parts are fit for purpose and will perform their function and last the stated life cycle.  Want to know more about designing for rotational moulding? Check out our Designer’s Guide To Rotational Moulding By Glen Beall or buy one of our books on rotomoulding.

Fill

The first stage of the process is to place the material, ground into either a powder or granule form, into a hollow or shell like mould.  Various materials are used in rotomoulding (sometimes even called rotacasting) and the type of material used will depend on the attributes required in the final part.  Material can be chosen for strength, rigidity, flexibility, surface finish, chemical resistance or many other parameters.  A good rotational moulder will be able to offer a range of materials to suit your product and help you to access high quality moulds that will produce the best possible product at the end of the process.  You can find out more about the materials of rotomoulding here.

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Heat & Rotate

Almost all rotational moulding uses heat to sinter (or partly melt) the material in the mould.  This can be done in a range of ways using different machinery.  Open flame, means the moulds are rotational over a flame open to the air, some machines include ovens that rotate the moulds inside and other machinery heats the moulds themselves.  Room temperature vulcanising materials that don’t require any heat to create a part are also available but less common.  Parts are rotated either on one or two axels during the process to cover the walls of the mould and create the part.  These rotations can be programmed to ensure walls are thicker or thinner depending on the requirements for the end product.  The material needs to be heated enough to allow any bubbles to pass through the walls and create a solid part.  If you would like to know more about the machinery & moulds of rotomoulding follow this link.

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Cool

The final stage of the process is to allow the part to come out of the mould and cool properly.  Most plastic changes as it cools and controlling this part of the process can add even more strength or other features to the final product.  Cooling can be achieved either by allowing ambient air to circulate around the mould, spraying the mould with water or very fine mist.  Cooling for rotomoulding is a specialist area, if you’d like to more consider buying our ARMA Guide to Cooling.

Release

Once cooled to the optimum temperature in the mould, the rotomoulded parts are taken out of the mould.  Most parts will continue to cool for some time after being removed from the mould and may require being placed on special frames to ensure their final parameters are exact.  At this stage value adding usually takes place and could include simple trimming to the addition of various kinds of fixtures and fitting, graphics or decoration to produce the final part.  If you are “hooked” and want to know more, check out the detailed pages on the site on the rotomoulding process.

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Machinery

Whether the cycle takes place in or out of the oven the rocking action that is necessary to cover the internal surface of the mould wall is usually described as either bi-axial or rock ‘n roll. Bi-axial rotation moves the mould through any combination of full rotations of two axis and is unlimited in the recipe that allows the part to mould properly. Rock ‘n roll is limited rotation on two axis and the angle of the rock is vital for long narrow parts to ensure the powder pool covers the entire inside surface.

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Carousel type machines usually use three or four arms on which the mouldsare mounted. These arms rotate the parts bi-axially and move from station to station on the machine being loaded heated cooled unloaded and loaded again ready for another cycle. These machines are excellent for energy efficient high production manufacturing. Carousel machines usually require the largest amount of available floor space.

Shuttle type’s machines will usually have two carts on either side of the oven on which are mounted arms which carry the moulds.These carts will move into and out of the oven in turn. While one cart is in the oven heating the other cart can be unloaded and loaded again ready for the next cycle. Shuttle machines usually require a smaller amount of space in the factory but may not produce as many parts as carousel machines.

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For certain applications open flame machinery can offer an effective return on investment. Cycle times are usually much longer on open flame equipment and parts can be more affected by ambient temperature. For some parts where placing the part in an oven would not allow even one more cycle per shift open flame equipment is a popular alternative.

Clam shell machines are usually single arm machines that run one part at a time. These machines are more efficient than open flame machines but may not produce as many parts as the multi arm machinery available. Clam shell machines need even less factory space than shuttles.

Materials

There special resins have been made available the material prices are high due to the development costs that are passed through to the user and the additional cost of small-scale grinding of the plastic.

Rotational moulders currently use several different materials for rotational moulding however most of the products made using the process use polyethylene in a powder or pellet form. In different parts of the world this material is supplied in either a pellet form which is then ground into powder coloured and prepared to specification by the rotational moulder or the powder is supplied to the moulder ready for manufacture. Apart from a huge range of colours powder may also include a flame retardant uv resistance or a range of other specific formulations.

Currently polyethylene in its many forms represents about 85% to 90% of all polymers that are rotationally moulded. Cross-linked grades of polyethylene are also commonly used in rotational moulding. PVC plastisols make up about 12% of the world consumption and polycarbonate nylon polypropylene and other materials make up the rest.  PE for the process can be pulverised into a powder at room temperature. Normally powder particles vary from less than 150 microns to about 500 microns. The particle size distribution is vital to the appearance of the final product. PE has to flow easily during the process which is measured by a Melt Flow Index value. High MFI material will have a low viscosity and usually for rotomoulding an MFI in the range of 2-8 is desirable depending on the product specifications.

PE’s for rotational moulding come in various densities including Low Density (LDPE) Linear Low (LLDPE) Medium (MDPE) and High (HDPE). The density of the material will have an effect on strength stiffness chemical resistance and impact vulnerability.

Rotomoulders perform a series of tests on materials including dry flow, bulk density, wall thickness, drop impact and others.  If you would like more information about the tests, what they tell you and how they are performed, check out the ARMA Material Testing Guide.

Moulds

Large simple moulds are generally made from fabricated metals. For highly detailed parts where a high quality finish is necessary either cast metals or nickel electroformed moulds are used.

The process doesn’t use pressure to form the part so the mould itself needs only to have enough strength to support itself. As the material cools it naturally shrinks away from the surface of the mould; and mould design must take this into account to create an accurately size final part.

Most moulds are manufactured in two pieces however for complex designs three and four piece moulds can be used. The area where the pieces of the mould connect to each other is known as the parting line and it can include a complex curvature. The parting line is vital because the mould sections must remain tightly clamped together during the heating and cooling of the part. Careful handling of the mould is necessary to ensure the parting line is not damaged.

Moulds are usually mounted to frames in halves to allow them to be placed on the machine. In some cases multiple moulds will be placed on the same frame known as a spider. This can provide substantial savings in demoulding and charging the moulds reducing cycle times.

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Moulds must be vented to allow heated air to flow freely in and out during the process. If the venting tube is too small or poorly designed its possible for the mould to be subject to pressure and either explode or implode. The vent tube must also be deep enough to project through the powder pool so material does not flow out of the pipe during the rotation. A good guideline is that the vent tube should be 13mm for each 1m3 of mould volume.

Moulds can also be treated on the surface to provide a range of different surfaces on the final products and the outside of the mould can be treated by various methods including adding pins roughening the outside and using black paint to increase cycle times.

Release

It’s important to have some adhesion so the part is properly formed; however a part sticking too long to the mould wall can be damaged or ruined so it needs to separate easily from the mould at exactly the right.

Types of releases include permanent systems that are added to the mould by specially treating it reactive & conventional chemical systems and hybrid systems.

Sustainability

Release systems may work for only one cycle or for many cycles at a time and they can affect the shrinkage rate of the part and the quality of the finish. Release agents build up over time and the rate of the build-up depends on the type of release being used. Moulds need to be regularly cleaned to remove all trace of build up when using temporary systems.

Plastics are constantly adapting and changing to society’s needs.  These innovations in products systems and technologies help contribute to a more sustainable world.

Plastics make an immense contribution to the environmental sustainability through their energy saving potential and intrinsic recyclability and energy recovery options. Economically plastics form an important part of the economy and are a major export product. Socially the plastics industry is a major and inclusive employer with an attention to training and education.

Environmental Sustainability

Plastics have a very good environmental profile.  Only 4% of the world’s oil production is used for plastics and much less energy is used to produce it compared to other materials.  Plastics are durable yet lightweight and thus save weight in cars aircraft packaging and pipework.

Economic Sustainability

Plastics products are very cost effective to produce and provide the ability to make single moulded components in complex shapes. This significantly reduces costs associated with assembly and the use of fixtures.
·         Plastics products also provide whole life cost savings:
·         Reduced fuel use in transit due to their lightweight.
·         Reduced maintained requirements for example PVC windows do not require painting.
·          Reduced energy required for heating when plastics insulation is used.
·         Reduced food and consumer goods wastage due to the unparalleled protective properties of plastics packaging.
·         The plastics industry is a leader in research and development and innovation.
·         Plastics materials and applications are constantly developed and improved
·         Significant industrial research and development is supported by a University infrastructure of polymer or polymer engineering departments (including Polymer – IRC)
·         Polymer Innovation Network (PIN) formerly Faraday Plastics plays a leading role promoting innovation. 

Social Sustainability

Completing the final pillar of sustainability plastics are also socially sustainable. The plastics industry is socially inclusive and offers a wide range of worthwhile careers with considerable room for career development progression and training. The industry is very attentive to training.
·         Plastics companies are geographically widely distributed and provide jobs to people in all areas of the world.
·         The plastics industry has a superb health and safety record and is well regulated.
·         Plastics make a major contribution to safety the used plastics in cars for example reduce weight and allows for the addition of safety features such as airbags. Furthermore plastic foams such as EPS and EPP provide the necessary shock absorbency to be used in life saving devices such a bicycle helmets.
·         Plastics are essential in modern day healthcare. Plastics products are used in surgery healthcare products pharmaceuticals drug delivery systems and medical packaging.
Essential medical applications include:
·         Blood bags
Fluid bags
Sharps waste handling
Heart and Lung bypass sets
Blood transfusion sets
Blood vessels in artificial kidneys
Surgical gloves
Catheters
Endotracheal tubing

There are some great videos about recycling plastic on YouTube.  We’ve included the links here to help you research what is commonly done now and what may be achievable in the future!

Express Recycling & Plastics     How Post Consumer Plastics Are Recycled     Recycling Plastic into Oil

What happens when Plastic is Recycled     Recycled Plastic for Chinese Railways     Bridge Made From Recycled Plastic Supports Tank

Recycling Rotomoulded PE

The materials are widely recycled using technology available around the world. While most of the recycled material goes into the blow moulding industry more and more products are being developed to put rotational moulding material back into rotationally moulded (or molded) products such as water systems bunding pallets cattle troughs and other “non potable” applications.

These classifications of the material are very important for recycling so they can be classified in the right density range and used appropriately in different applications.  The majority of rotational molding resins is in a density range of 0.933 g/cc to 0.945 g/cc which puts them in the classifications of MDPE and HDPE.  The majority of blow molding recycle is in the range of 0.940 to 0.960 g/cc with a fractional melt index.  It is very easy to blend roto grades into blow molding grades; however the reverse is not so easy and can lead to very poor performance.

The density classifications are (ASTM 4976) g/cc

• Ultra-Low Density    ULDPE   < 0.910

• Low Density             LDPE    > 0.910 – 0.925 (also LLDPE, low pressure)

• Medium Density      MDPE    > 0.925 – 0.940

• High Density             HDPE    > 0.940 – 0.960

• High Density             HDPE    > 0.960

The Recycling Process

Put simply the recycling process involves factory scrap or products which have been in the environment and have broken down being cut up into manageable pieces which are ground down into even smaller pieces (shown below as recycled granulate). These pieces are then melted new UV stabilizers or other chemicals and colorants are added and then the material is pelletized for use in blow moulding film making or injection moulding or ground into the fine powder required for rotational moulding (molding).

Recycled Materials Are Widely Available

An extensive range of plastic resin grades colours and quantities both prime and recycles in either granules or rotational moulding (molding) powders can currently be provided.

Using recycled resin is both environmentally and economically responsible. Recycled resins generally cost only 60% as much as prime resin. Grades commonly offered include HDPE LDPE MDPE LLDPE PP GPPS. HIPS and ABS. Supplied in multicolour black and natural (limited) in 25kg or bulk bags. Providers can also provide resins with specific additives e.g. antioxidants UV stabilizers process aids and colourants to customer specification.

Our Favourite Sustainable Projects

https://www.rotationalmoulding.com/wp-content/uploads/2017/02/plasticbits.jpgThere’s some fantastic products currently being made from this recycled material and some market leading manufacturers helping consumers to recycle even large products such as water tanks. Managing products from inception through recycling and into new forms is known as product stewardship and there’s an increasing amount of companies ensuring their stewardship is of the highest calibre.

Australian industry embraces sustainable design By Kate Tilley

 

Plastics News Correspondent

MELBOURNE AUSTRALIA (November 17 3:10pm ET) – The Australian plastics industry is embracing an initiative by one of the nation’s environmental agencies to promote sustainable product design and recycling.

https://www.rotationalmoulding.com/wp-content/uploads/2017/02/plasticpellets.jpgIndustry peak body the Melbourne-based Plastics and Chemicals Industries Association (PACIA) has joined forces with Victorian Government agency Sustainability Victoria to promote the concept of design for sustainability.

PACIA Chief Executive Officer Margret Donnan said design for sustainability shorthanded to D4S recognised that the environmental benefits and impacts of any product are “locked in” at its design stage.Donnan said a new program D4S with Plastics involves developing and publishing on the PACIA website 12 sustainability guides for between now and March 2009.She said that the guides will be used as reference material by designers and manufacturers to ensure sustainability is taken into account over the full lifecycle of a product – from the materials used to make it to its potential for recycling.

https://www.rotationalmoulding.com/wp-content/uploads/2017/02/plasticsheets.jpgTopics covered by the guides will include the design of building products and furniture plastics recycling packaging design degradable polymers and the implications of a carbon-constrained economy.”This program is important because plastics and chemicals are at the hub of every production chain ” Donnan said.”In addition product design will increasingly become the biggest opportunity to deliver significant improvement in our response to pressing challenges like climate change and water scarcity.”

Manager of Sustainable Products and Services with Sustainability Victoria Diana Gibson said because the D4S guides will be published on the Internet they will be accessible by designers and those involved in the plastics industry throughout Australia.She said the D4S with Plastics program highlights the importance of collaboration across every stage of the product chain.”Our research shows many groups are involved in making decisions about a product’s design including manufacturers suppliers retailers brand owners recyclers consumers and designers ” Gibson said.”All these groups have the power to influence the design and therefore the sustainability of a product ” she said.

https://www.rotationalmoulding.com/wp-content/uploads/2017/02/plasticfittings.jpgGibson said the design guides will highlight the long-term environmental and economical benefits of D4S.Examples include products being wrapped in lighter packaging so they are more efficient to transport products such as durable plastic water tanks that enable us to conserve water and shorter-life products that have been assembled in such a way that they can be recovered and dismantled for recycling ” she said.

For more information on this article including D4S with Plastics see: Pacia Quick Start Documents

 

More Information

If you would like to find out more about recycling in the rotational moulding industry click on any of the links below for more information on projects that other companies are pursuing:

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