Heating a mold with hot air may seem inefficient and that is the thought process developers have used to ingeniously create oil jacketed rotomolding machines. By circulating a heated fluid in a jacket surrounding a mold, more heat is concentrated directly on the target area and this yields lower costs when heating the materials to high temperatures. For example, polycarbonate materials require a high temperature and this method is considered successful for operating with this material.
This concept has been used on commercial machines such as the Leonardo automated rotomolding machine; this machine has been developed to run tooling which heats and cools the parts using an external jacket containing oil. Shown here is the fully automated Leonardo rotational molding machine (Reference, equiplast):
Disadvantages to using this type of machinery include the high temperature at which the oil is circulated; should there be any leaks, there are safety concerns such as injury or fire. Oil can also leak from rotating joints which also releases fumes and can possibly contaminate the plastic resin.
However, even though these disadvantages are present there are significant advantages with respect to product consistency and production control.
Fixed arm Carousel rotational molding machines are some of the most used machines in the industry. They are developed for high production runs of medium sized parts; Roto Dynamics as well as various other rotational molding companies use a 3 arm fixed carousel machine. Each arm is 120 degrees away from the other and is driven by a central turret. All three arms rotate in the machine as they are fixed to the turret; one arm is in the heating station the other in the cooling station and third arm is at the loading/unloading station shown here (Reference: Wikipedia):
Being able to run various molds of different sizes and material requirements is a major process advantage of the fixed arm carousel machine. Not only can different materials be run on each machine, but molds can easily be changed without having any impact on production times. However, that being said the effect of using different molds and materials creates the need for various optimum cooling, heating and servicing settings therefore dictating the cycle times by the slowest arm. This problem has been overcome by the development of the independent arm carousel machine. Independent arm machine: Independent arm machines solve the problem that fixed arm carousel machines possess; the cycle time is no longer dictated by the arm. These machines have 5 stations and have the ability to run 2-4 arms running independently of one another. The benefits provided by the independent arms include the capability to designate the vacant station as an auxiliary oven, station cooling station or additional loading station. This machine can especially be advantageous to the custom molder who is running a variety of materials and molds of different complexities.
There are several transitional physical phases which the plastic powdered resin goes through when inside a biaxially rotated mold. To be specific there are four terms assigned to these physical phases; in the following order they are, rotating pool, cascading, rimming and collapsing.
In the rotating pool phase, a majority of the molten resin remains in the state of a liquid pool at the bottom of the mold; this tends to occur when the liquid is a low viscosity and at a low rotation speed.
The cascading flow occurs at higher rotation speeds and viscosity causing the liquid pool to rise and get carried over the top. Eventually cascading to the opposite side of the mold.
As the speed and viscosity increases further the liquid is carried over the top side of the mold without any cascading or dripping as such. It is essentially carried over the top and follows through in a centripetal motion. The liquid is now said to be of a steady state.
Solid body Rotation occurs when the rotation speed and viscosity are high enough such that there is no liquid flow. At this stage there is no longer a liquid pool instead there is a residue coating the inside of the mold.
Inherently rotational molding produces uniform wall thickness in the same manner in which thermoformed or blow molded parts are. Single surface processes such as rotomoulding and blow molding do not have as good a wall thickness tolerance as two surface processes such as extrusion.
The usual wall thickness tolerance for a large part would be in the range of +/- 15-20% whereas with a more precise smaller part, the tolerance range is +/- 10-15%. This wide tolerance range in rotational molding, blow molding and thermoforming results in a standard for which minimum wall thickness is provided rather than nominal wall thickness.
As mentioned in a prior post we discussed the use of several methods of decorating the surface of rotationally molded plastic parts. Just to mention a few of them again, we discussed color, logo, metallized surface, mirrored surface, textured surface, in molded graphics, on molded graphics and polyolefin paints. The most commonly used decorative effect is color. When applying color there are a two options, the most comprehensive and widely used method is the use of pigmented or compounded resin; the other being painting.
Roto Dynamics has the ability to match a majority of colors requested; this can range from a pantone color code to a custom color reflecting a pearlescence. Color matching is a process which is conducted in a lab to determine the closest possible match to a sample or color code provided by the customer. Within a relatively short time frame we are able to match the color requested and provide a sample part.
There are a few methods which are feasible for applying paint to rotomolded parts. These include silk screening, pad printing and traditional spraying methods using polyolefin paints. Each method has its own advantages and disadvantages however we will discuss these in more detail in a later.
If you have a company logo or brand that is required on a product, Roto Dynamics can use a computer rendered image and acquire the graphics required to mold in or on to the rotomolded part.