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ASH Industries, Inc.

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Rotational Molding: A Once Simple Process Goes High Tech

Rotational Molding

Rotational Molding Process

If you need hollow parts with complex and varied shapes, rotationally molded plastics could be your answer. This unique, rapidly growing process permits the use of materials tailored to your particular needs. It is superior to other molding techniques in the crucial areas of cost reduction, economical runs and part size. Rotomolded products provide lightweight replacement for more traditionally used materials.

Improved plastics technology, advances in mold construction, resins, and machinery have increased the use of a variety of plastics. Versatility of the process allows for product sizes ranging from ping pong balls to large 20,000 gallon vessels. The process is simple in concept. Heat is used to melt and fuse a plastic resin in a closed mold. Unlike most other plastic processes, no pressure is involved. The three-stage process includes loading the resin in the mold, heating and fusion of the resin and cooling and unloading the mold.

After the charged mold is moved into an oven, the mold is rotated on two axes at low speed. As heat penetrates the mold, the resin adheres to the mold's inner surface until it is completely fused. The mold is then cooled by air or water spray or a combination of both while still rotating, lowering the temperature in a gradual manner. The mold is opened, finished part removed and mold recharged for the next cycle. Cycle times vary, from 7 to 60 minutes, depending on part size, material and wall thickness.

In rotational molding, a premeasured amount of plastic material in liquid or powder form is placed in a cavity and the mold is closed. The molding machine then indexes the mold into an oven where the mold and subsequently the plastic is brought up to the molding temperature. As the mold is heated, it is rotated continuously about its vertical and horizontal axes. This biaxial rotation brings all surfaces of the mold into contact with the puddle of plastic material. The mold continues to rotate within the oven until all of the plastic material has been picked up by the hot inside surfaces of the cavity. The mold continues to rotate until the plastic material densities into a uniform layer of melt.

While continuing to rotate, the machine indexes the mold out of the oven and into a cooling chamber. Air, or a mixture of air and water, cools the mold and the layer of molten plastic material. This cooling process continues until the part has cooled sufficiently to retain its shape. The machine then indexes the mold to the loading and unloading station. The mold is then opened and the part removed. A new batch of material is then placed in the cavity, the mold is closed and the process is repeated.

There are several different kinds of rotational molding equipment currently in use, but the three-station layout is most common. The biaxial rotation is usually achieved by a series of gears or chains and sprockets. Small, multiple-cavity molds of the same parts are usually centrally mounted.

Applications

Rotational molding used to be the way we made tanks and simple plastic products. Now it's the way we make some of the most complex plastic parts ever to come off a drawing board...parts that could not be made any other way without the cost being prohibitive.

The process is still designed for the production of hollow, seamless parts, but new engineering developments have brought it as far from simple tank manufacturing as its multiple geared, biaxially rotating molding equipment is from the first wheel.

Today's ability to produce complex and sharp contours has greatly extended the capabilities of rotational molding while giving the design engineer increased flexibility. Products that formerly required thermoforming or specialty metal fabrication to achieve required contours can now be rotomolded with precision. In many cases, parts can be eliminated by designing an integrally formed unit, thereby reducing the cost of additional tooling, secondary operations and assembly.

Industries Served

The rotational molding process is used today in industries such as agricultural, packaging, industrial, material handling, recreational, home/office/school, health/science, automotive, construction, military, electronics, chemical and toys.

In most applications, one rotationally molded part and secondary machining will replace fabricated and molded parts.

The following examples will give the design engineer a better understanding of the rotational molding process and its own unique capabilities.

A fuel cell for the Bradley Cavalry Personnel Carrier was designed to take advantage of every possible void for fuel storage, so its design and contours are complex. It has a cast-in central hub and requires numerous secondary operations. Drilled holes must exactly match backing plates. Nylon was used to prevent fuel osmosis. Weight is 170 pounds.

In another situation, relatively simple units replaced fabricated metal. Lower tooling cost eliminated other methods of plastic molding. Drilling and hole cutting provided a product ready for final assembly.

Three components were formerly fabricated from stainless steel and each component required several parts and assembly. Molding as complete units gave design engineers increased flexibility, provided a more efficient design, and cut costs by up to a ten-to-one ratio.

Pallets function as station-to-station fixtures for equipment during assembly. Complex contours and molded-in inserts were required. LLDPE used is more durable than wood or cardboard, and nonconductive.

Two of six fuel cells are made from cross-link polyethylene. Complex contours are required and secondary operations had to be precise to accommodate fixtures for various access holes, pumps, and sensors.

Another example features a simple, yet brilliant concept. To be marketed through beverage outlets, this unit features double-wall construction, a reversible cover to accommodate large and small kegs, and heavy duty handles to survive many uses. One basic rotationally molded part replaced 36 plastic and stainless steel fabricated parts previously used in a cabinet. The previous cost to paint and plate all the metal cabinet parts costs more than the one-piece rotationally molded design.

In another case, an integral one-piece design replaced eighteen separately fabricated pieces in previous design.

A functional and attractive part, molded from polyethylene in brown and tan, has an aesthetic finish and a durable construction. As a limited run product, rotational molding was the obvious choice cored through holes to accommodate pressure. This one-piece conversion design replaced twenty-two parts.

Advantages

Designers of plastic parts turn to rotational molding to produce small or large parts of unusual shapes that cannot be produced as one piece by other processes.

Relative to their size, rotationally molded parts can have thinner walls than similar parts made by other processes. Rotational molding tends to produce an increasing wall thickness on outside corners of parts, which gives the process a distinct advantage over blow molding and thermoforming since these processes tend to produce thin outside corners.

Rotational molding is a low-pressure process, and the strength required from the molds is minimal. This results in its ability to produce large or complex parts on short notice, using low cost molds.

The low processing pressure involved in rotational molding has the added advantage of producing parts which are relatively stress free, as compared to other high pressure processes. This advantage is especially important when considering large, load-bearing parts in applications which must provide corrosion or stress-crack resistance.

The surface finish and color of rotationally molded parts can be tailored to suit the product's requirements.

Metal inserts or integrally molded in threads are possible with rotationally molded parts.

Reversal parts with closely spaced double walls are common. Many parts are molded with little or no draft angle. With some materials, it is possible to produce parts with undercuts.

When designing a new or conversion product review the advantages rotational molding has to offer before deciding on the manufacturing process. These include:

Material Considerations

Though there is no one ideal resin for today's diverse rotational molding applications, a variety of useful resins have been developed to meet the needs of both designer and molder.

In selecting a rotomolding resin, the three most critical properties to consider are flow, impact, and stresscrack resistance (ESCR). Resin cost, of course, is also a major consideration that varies considerably depending on the type of resin.

Most plastic materials are supplied in a fine powder (35 mesh) which melts more quickly and uniformly than pellets.

MOLDABLE MATERIALS

Material Characteristics

Polyethylene

LDPE - Flexible and tough, easy to process, low strength, excellent chemical resistance. Available in powder form with UV stabilized and FDA approveable grades. Applications include tanks, toys, containers and industrial parts.

LLDPE - Better mechanical properties than LDPE. Higher stiffness, excellent low temperature impact strength and environmental stresscrack resistance.

HDPE - Stiffest of the polyethylenes. Like the other types, it has excellent chemical resistance, environmental stress-crack resistance, easy processability and low cost. Applications include barrier-coated gasoline tanks, toys and furniture.

Cross Linked - Contains a crosslinking agent which reacts with the material during the molding cycle, forming a cross-linked molecule similar to a thermoset. This reaction improves the toughness and environmental stress-crack resistance. Applications include chemical and sewage tanks, trash containers, seats and other products where stress cracking and impact strength are important.

EVA Copolymer - Excellent low temperature flexibility. Available in UV stabilized and FDA approvable grades. Applications include soft toys and blending with other materials to improve impact strength.

Polycarbonate

Excellent mechanical properties including stiffness, tensile strength and creep resistance. Highest impact strength of all rigid plastics. High heat resistance. Can be molded clear. Applications include light fixture globes, snowmobile engine hoods, shipping containers and other applications where high heat resistance and toughness are required.

Nylon

Excellent tensile strength, stiffness and impact strength. High heat resistance so properties are maintained at elevated temperatures. Excellent chemical resistance. Moderate in cost. Applications include military fuel tanks, hydraulic oil and solvent tanks, grain buckets and air ducts.

Polyvinyl Chloride

PVC compounds can be molded in either liquid or powdered form and are moderate in cost and easily processed. They can be formulated to produce parts ranging from flexible to semi-rigid. Applications include balls, doll heads, teething rings, planters, novelty items and flexible bellows.

Polyesters

Rotationally molded thermoplastic polyesters are normally flexible materials which have excellent impact strength. Especially good resistance to long-term contact with water. Intermediate temperature and chemical resistance. Will mold to very thin wall thickness. Applications include tank liners and waterbed mattresses.

SPECIAL ENGINEERING MATERIALS

Today, with the rapid expansion of rotomolding, markets have grown large enough to justify the development of specially compounded plastics that are suitable for the rotational molding process. Following is a list of plastic materials now being used occasionally for the rotational molding of specialty products:

Molded-In Insert

Molded-in inserts can be incorporated into rotationally molded plastic parts. In this process, the insert is mounted inside the cavity and the plastic material molds around the insert to lock it into the molded part.

The best results are achieved when the shape of the insert provides undercuts into which the plastic can flow to lock the insert into the part.

Molded Threads

Inside and outside threads are routinely molded into rotationally molded plastic parts. All types of threads have been molded; however, coarse thread forms of the Acme or modified buttress type, with a thick profile, are preferred for rotationally molded parts.

Threads with sharp profiles such as the American Standard of tapered pipe threads are difficult to produce by rotational molding without bridging over of the tips of the threaded cavities. This results in underfilled parts. When these types of threads must be provided, they should be machined into the part after molding.

The designer must recognize that specifying threads on a part will increase the cost and complexity of the mold and, in some cases, the molding cost.

Decorating

Cosmetic specifications may require light trimming at parting lines or mold shutoffs. Many parts have special decorating requirements such as painting, hot stamping, silkscreening, labeling, etc. other decorative effects such as textured and engraved surfaces can be molded directly into the part.

Outside surface finish can be polished, flame treated, and waxed to achieve glossy effect.

Mold Considerations

All types of rotational molds have several things in common. They are relatively low in cost when compared to other type molds, such as those used for blow or injection molding.

Rotational molds are usually shelltype molds that define the outside shape and surface of the part. These molds do not have internal cores and the inside shape of the surface of the part is determined by the outside shape of the part and the varying thicknesses of the part wall.

The designer of molds for rotational molding must give careful consideration to heat transfer, as the mold must be heated and cooled during the course of each molding cycle. Designing for high thermal conductivity often dictates the choice of material for mold construction.

Cast Molds

Cast aluminum molds are the most common type mold for small-to-medium size rotationally molded parts. Aluminum molds cast from models or patterns are ideal for freeform shapes such as hobby-horses, mannequins or anatomical models and other unusual shapes that would be difficult to machine. Cast aluminum molds also have cost advantages when multiple molds will be required.

Sheet Metal Molds

Sheet metal molds fabricated from steel, aluminum or stainless steel sheets are widely used. They find their widest usage when the size of the parts is large and when only one cavity is required.

Sheet metal molds do not lend themselves to the production of freeform shapes.

Mold Finishes

When a very smooth part is required, the molds must be polished or plated. if, on the other hand, a textured or embossed pattern is desired on the part, this can be achieved by texturing the interior mold surface.

To a great extent, the quality and especially the tolerances on a rotationally molded part, like plastic parts made by other processes, are dependent upon the quality and precision which are incorporated into the mold. There is no substitute for a good quality mold.

Each of the various types of molds which is described here has its own unique advantages and disadvantages. A production rotational molder can advise you as to which type of mold will be best for your particular application.

Prototyping

Since the rotomolding process does not use any pressure, the molds are low in cost. A small one-piece electronics keyboard had tooling costs of $5,000 compared to $80,000 for two injection molds producing two parts which would also have to be joined by sonic welding or otherwise.

Wall thickness and strength is difficult to specify early in a design program. This process allows you to run several different samples for product testing after the tool is complete. Example: one part was tested in 1 1/2 lb., 1 3/4 lb., 2 lbs., and 2 1/2 lbs. weights prior to selection of 1 3/4 lbs.

The lead time on tools is approximately 4-6 weeks on fabricated tooling and 6-10 weeks on cast aluminum tools which require a wood pattern.

If tight tolerance control is mandatory, it is critical to specify the material types prior to building the mold, since the molds are built oversized to allow for shrink factors. However, most of the time initial material testing can be done with parts rotomolded from existing tools of the same size.

Testing

A qualified roto-molder requires all the dimensional inspection equipment necessary to inspect dimensions.

Ultrasonic testing equipment must be available to test for wall thickness.

To assure the end product will achieve the strength requirement, rigid material testing resources must be available. The raw material processing parameters have an effect on the product's strength; therefore continuous in-process testing must be performed on:

Production Rates

Depending on the mold design, wall thickness and material, the number of parts which can be produced by one mold will vary. Using a three-arm machine, the output is controlled by the oven time. Indexing can not take place until the oven cycle is completed. The unit production time, with only one mold used, is three times the oven cycle. To achieve greater output, more molds are required. To determine the number of molds, take your requirements and compare to daily output.

Secondary Operations (Post Molding)

Seldom is the finished part done after the part is rotomolded. All rotomolded parts come out of the molds as hollow parts and as a minimum they require access mounting holes, or, when using double cavity back-to-back tools, the parts must be separated.

The types of secondary operations often necessary are:

The best post molding operations are those that can be eliminated or reduced in complexity. With proper assistance from a rotational molding engineer prior to finalizing the tool design, many of these operations can be incorporated into the molding process such as:

Tips On Purchasing Roto-Molded Parts

For best results a qualified rotomolder should be contacted prior to finalizing any design. He can offer additional tips to reduce the cost or incorporate more than one part in the final design. The following questions should be reviewed:

A custom rotational molder can answer all of these questions for you and offer suggestions which will give you the optimum results.

This technical information has been contributed by
ASH Industries, Inc.

Click on Company Name for a Detailed Profile

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