ROTATIONAL MOLDING

Rotational molding is a unique process of molding that was first applied in 1946 due to the availability of liquid plastics. In 1961, pulverized polyethylene was introduced allowing an expansion of more demanding rotational molding applications. Rotational molding applications are formed using a combination of important components such as a mold, resinous material, heat, spinning axes, cooling chamber and load and unloading chambers.

Rotational molding, commonly known as rotomolding, is a process that enables people to mold hollow products by using a particular rotational motion. Besides producing hollow products, rotational molding has other important advantages. By using this manufacturing process, products can be molded as one piece, which eliminates the time needed to assemble two or more parts. As long as the material inserted in the mold remains consistent, rotational molded products tend to have farley equal wall thickness, everywhere except in the corners. Wall thicknesses can be up to 1/2" thick with uniformity being approximately .015" with tolerances being +/-.010". The products corners are almost completely stress free, due to the added material that cakes up in the corners.

This particular molding process, rotational molding, derives from the machine itself. The machine, is one with arms that hold a mold in place and rotate it slowly until a material (resin) inside the mold is properly shaped and conformed to the mold. This rotating is what allows the melted resin to become evenly distributed to all parts of an aluminum or steel cast mold.

Depending on the rotational molding machine used, a mold can be rotated by one arm or three to four arms. The arms are spun on a major and a minor axis. The major axis usually revolves four times to every one of the minor. (See illustration below) Machines with fixed-arm turrets complete each process in the same amount of time. Meaning that heating and cooling are completed in the same amount of time. Independent-arm machines allow for one arm to stay stationary, while the others are still in motion.

The arms of the machine allow the mold to be rotated on two axes along with moving the mold through different stages in the process. The needed stages in rotomolding process include, the oven, cooling chamber, and the loading/unloading stations. Smaller machines, such as the one used to conduct this research (See Fig. 1.0), are not equipped with a cooling chamber or a loading/unloading station.

(Fig. 1.0 Ball State University's rotomolding maching)

After the rotational molding stages are complete, it is the final hollow products that compliment the whole process. Hollow objects are indeed used for all sorts of everyday tasks. "Rotational molding (also called rotomolding) enables manufacturers to produce medium- to large-scale hollow plastic components ranging from 6-in.-diam containers to 20,000-gal tanks.)"¹ Below are a few examples of what can actually be manufactured using this process.

Trionic Rotational Molding: http://www.trioniccorp.com/rotation.htm

Creating a hollow object may seem difficult, but rotational molding machines enhance our ability to do so. Even though the rotational molding process is not a simple one, due to the different stages in the process, the rotomolding machines allow us to produce such products. According to Rotational Molding website, There are five categories of rotational molding machines, and they are as follows.

clamshell

independent cart

rock-'n'-roll

shuttle

turret

Rotational Molding: http://www.devicelink.com/mddi/archive/96/04/013.html

What actually takes place in the rotational molding process can be explained in a few easy to remember steps. This of course is dependant upon the machine, mold, material, and other parameters that must be accounted for.

Determine the product to be produced by selecting the proper mold.

Determine the proper amount of resin to be inserted into the mold.

Mold is placed in an oven.

Oven temperature is set.

Mold is rotated on two axes for a predetermined time.

Machine is turned off, the mold is taken out of oven .

Product goes through a cooling process.

Product is removed from the mold.

A good way to visualize the actual rotating process is to image two fixtures rotating at a 90 degree angle, at the same time. See Figure 1.1)

(Fig. 1.1 Processes Of Manufacturing, R. Thomas Wright, Pg. 89).

(This film displays a rotational molding machine in action-Fig. 1.2)

(Hover your mouse over the picture to see movie)

(Fig 1.2 Movie of Ball State University's rotational molding machine)

As the machine runs, an evenly distributed amount of resin is spread to all parts of the heated mold, due to the rotation of the two axes. Two axes are used, instead of one, to ensure a consistent amount of material is distributed throughout the mold. The axes are perpendicular to one another and rotate simultaneously, meaning that each axes rotations per minute (rpm) are consistent with one another. As the resin liquefies, it is evenly distributed throughout the molds interior wall. It is the rotation during cooling, not during heating, that distributes the material. The amount of resin used in the mold will determine the wall thickness of the finished product.

The wall thickness of rotationally molded products is a concern for those involved with the rotomolding industry. This concern comes from an economic standpoint. If excess material (resin) is used for a production run, several hundred thousand dollars can be wasted, therefore, a products wall thickness is very important. This process can be expensive if there is a large amount of wasted material involved. Although, if done right, there is hardly any left over waste material. Usually, rotational molding is a cheap and economical way to produce products. The production rate is low, but since equipment and production time are low too it is an efficient way to mold a product. The wall thickness of any rotationally molded product can be determined by the amount of resin that is placed inside the mold at the beginning of the process.

The resin may be liquefied or in a solid state. The liquefied resin is known as a polyvinyl chloride plastisol. The dry fine powder is known as a polycarbonate. Experimental test runs are conducted to determine the proper wall thickness for a product that is to be mass produced. Usually, an educated estimate of how much resin to use can be made before test runs are conducted. This estimate comes from prior experience by the experts in this field. Since mass production is an expensive process, determining the proper amount of resin is extremely important before a series of products are to be produced. Along with wall thickness, the finished products exterior finish is important as well. If a products exterior finish doesn't have to be smooth, then a textured finish may be used.

The Steps Taken To Complete A Rotational Molded Product For This Research Project Are As Follows:

Pick the mold that is desired for production.
Open the two halves of the mold.
Spray both halves with non stick spray.

Fill one of the two halves full of low density polyethylene.

Secure the mold by tightening two bolts, opposite one another, with wing nuts.

Place mold into the center of two plates in the rotational molding machine.
Tighten the top and bottom plates against the mold using wing nuts that are attached to the plates.