Rotomolding machines all inherently have the same process and function; to rotate heat and cool. Rotationally molded parts can vary in size ranging from 0.05 litres to 10 000litres and this can be one of the main determinants when selecting which machine to use. In this article we will discuss the various commercial rotational molding machines that are available.

Rock and Roll Machines:

The rock and roll machine functions on two axes, the first axis motion consists of a rocking motion and the second a 360 degree rotation about the perpendicular axis. The image below:

Rock and Roll Machines

Reference: Roto moulding world

It is considered that biaxially rotating machines are an updated version of the rock and roll machine and that the rock and roll machine is best suited for parts which are symmetrical along the central axis perpendicular to the rocking axis. However due to the ease of access to the molds on a rock and roll machine, there is a resurgence in the use of the machine.

Similar in nature to the rock and roll machine is a Rocking Oven, shown below:

rocking oven

Reference: thomex.com

A rocking machine is raised above an open oven which is heated by hot air. The molds are secured to the structure which rocks with oven.

Clamshell Machines:

A clamshell machine has a distinct clamshell motion. The oven moves in a clamshell action over the mold and is shown below:

Clamshell Machines

Reference: meadoworksinc.com

A significant benefit of the clamshell machine is the low floor space requirement; the biaxial rotation is identical of that in a fixed turret machine while also having two support arms connected to the horizontal shaft. Molds are mounted as assemblies on the turntables through the shaft. Once the clamshell oven closes, the axle rotates moving the molds in a spherical motion while also rotating the molds about their axis. The required temperature is obtained through heating and circulating air through the machine. For the cooling cycle, cooled air is circulated and a mist of water is also dispersed. At the end of the cycle the clamshell oven opens and the molds are opened for part removal; the molds are charged with resin and the cycle begins again.

Developing a custom plastic product may seem like a daunting task however, it should not stop you from pursuing what can be accomplished with rotomolding. Whether you are converting a product from a costly metal construction or manufacturing a completely new idea, the design capabilities of rotational molding designers are far reaching. When manufacturing a rotomolded part, the first step begins with converting the design from a 3D computer aided design file to an aluminum billet, casted or fabricated mold.

This can be done in several ways the first is to input the CAD file into a CNC machine which has factored the shrinkage. The second is to create a model from which patterns can be made for casting; the method used for creating a model varies. The third and final method of producing a mold is carried out by specialist experienced sheet metal fabricators who bend and weld sheets of metal together to form the required part detail.

Each of these methods has its own advantage and disadvantages for example, CNC billet tools have provide a part with a more precise tolerance range. This is essential when various parts of assembly are rotomolded however, these tools can be costly due to setup and material costs in the CNC process. Sheet metal molds have a higher thermal conductivity which can allow a more efficient cycle time to be run and this can save on overheads in the long run.

Metal material handling carts can be bulky, costly and an eye sore. With limited design capabilities it does seem logical to manufacture these in rotomolded plastic. The ability to add ribs and increase the aesthetics of such a product in 3D CAD software enables us to reduce weight, save cost, increase durability and provide a quality product. Not only does polyethylene not rust but it is a hydrophobic compound which gives its non-porous characteristic.

The concept of adding casters may be interpreted as a financial hurdle however with little to no difficulty inserts are molded in place to accommodate the relevant size thread. This avoids having to create a costly stainless steel metal or wooden frame for the wheels base and also avoids the possibility of rust or rot. There are several advantages gained from using a polyethylene, for example high density polyethylene has a high impact resistance and good environmental stress crack resistance therefore providing the product with the ability to have a long lifespan.

Roto Dynamics Inc. also has the capability to rotomold 4-way fork lift accessible bins for material handling. Whether you are looking for a stackable bin one that requires casters or is fork lift accessible, RDI can make it.

When working with customers who are in the design phase of product development and are unsure about which process to use, we do our best to recommend an alternative process if Rotational molding is not suitable for the product in development.
Take for example Mike. He came to Roto Dynamics with an innovative idea and was almost certain he wanted to rotomold his part. We sat with Mike in a design meeting and discussed all potential processes for his part and which would suit his budget constraints, lead times and product form/function/fit. We came to the conclusion that rotomolding was the correct plastic process to make his product and began the designing the product. Below is an image of the final design:

Rotomolded part:

Full Assembly:

After Mike had received his design, the feedback he received from potential distributors at tradeshows was so good that his output requirement had increased dramatically to 500 parts a week. At this point we suggested another plastic process he could use and this was the final product after moving in this direction:

Rotational molding is an ideal process for manufacturing chassis/ Cases for electromechanical assemblies or products requiring an aesthetically pleasing exterior. The rugged properties of rotomolded plastic products offer good impact resistance, chemical resistance and variety of aesthetic properties such as color or graphics. The added benefits of using rotomolding include design advantage, cost advantage and time advantage.

Design advantage - The ability to use 3D cad drawings in the mold making and molding process allows consistent tolerances to be produced from part to part. When compared to metal fabrication design creativity is easier to express when using the various methods available to plastic designers.

Cost advantage - Metal fabricated chassis are costly when compared to high quality rotomolded plastic parts. The ability to run short batches of parts minimizes setup costs.

Time advantage - Rotomolded parts are all made from a fine resin which is readily available and stocked, therefore lead times on resources are short and 5-8 parts can be run in a single shift.

The advantages listed above are not the only reason why rotomolding should persuade you to use our process; look at the products around you and you will notice that many machine chassis are rotomolded.