Ferry Industries Inc., Stow, Ohio.

The machinery, materials and methods used in the rotational molding industry have all evolved through the years to become more exacting and better controlled than in the past. As industry and customer standards have become more stringent, the rotational molding process is further challenged to compete with other plastic processes. New technologies must be incorporated to meet customer requirements.

Ferry Industries provides modern high-intensity mixers that will improve the appearance, quality and economies of rotomolded parts through efficient, uniform mixing of a base resin with colorants and additives. High-intensity mixing can maximize color usage, reduce scrap and “borderline” parts, enhance customer satisfaction, and result in higher profits for the molder.

Dry color is the most commonly used material for coloring the resin in rotational molding. The powder consists of several pigments and additives that are mixed together with the resin before molding. The object of high-intensity mixing is to uniformly cover the larger resin particles with the smaller color particles. More uniformly distributed color particles will allow less pigment to be used, will result in fewer inherent stresses, increase batch-to-batch consistency, and provide richer color for the finished part.

Mixing Graph Time

Current practice in the industry for molder prepped colored material requires that batches are made up in advance by putting the color and resin together into a mixer bowl and mixing it for 1 to 4 minutes with a high-intensity mixer, or 20 to 40 minutes with a low-intensity mixer. The color difference (DE) is an indication of how well the color matches a given color standard. Generally, the human eye cannot detect a color difference value of one unit or less. The colored material is then removed from the mixer and either stored for use later or moved into the molding area for immediate use.

Rotational molders are adopting high-intensity mixing technology to improve this process and realizing the following advantages:

  • Reduced mixing times compared to low intensity mixing.
  • Optimized raw material (color) usage due to fully developing the color.
  • Improved physical properties of the final part by reducing stress risers.
  • Reduced clean-out times for color changes.
  • Tighter color tolerances from batch to batch that reduce off-color parts.


High-Intensity Mixing

High-intensity mixing can be distinguished from other types of mixing by the speed and design of the tool that is used to create the mixing action. High-intensity mixers have a tool tip speed of approximately 35 feet per second. The tools are also designed to create a specific flow-path of the material in the bowl that causes even impingement of all the particles in the mix. This results in complete dispersion, promoting a consistent, high-quality mix from batch to batch.

The tool tip-speed is critical. It must be fast enough to provide the shear necessary to break up the agglomerates of color (color particles stuck together) and cause the color particles to be totally dispersed and evenly cover the resin particles. Color, as supplied by the manufacturer does have some agglomerates. The agglomerates need to be broken up to be able to make full use of the material. When the color particles cover the resin particles evenly, through high-intensity mixing, the molded part color is at its full strength.

Mixing Graph Speed

Experiments have demonstrated the impact of tip speed on the quality of color in the final part. In this experiment, a variable-speed laboratory size high-intensity mixer was used to mix the dry color into the resin. Samples of equal amounts of ingredients were prepared at the three tool tip speeds shown. The material was then molded and samples of the molded parts compared for their color. The graph indicates that a better match is made as the speed of the mixer tool increases over the range and that the increased speed caused the agglomerates to break up and coat the resin particles more evenly.

Another important aspect of tool design is that it must be able to move the material within the bowl so that total dispersion is accomplished. Once the color particles have been divided into their smallest parts they can disperse and coat the resin particles when coming into contact with them.




Optimum impingement and dispersion come about when the materials form a vortex while mixing. The vortex is the flow path that the material takes while being moved in the bowl during the mixing process. The mixer tool must be specifically designed to cause the material to move within the bowl as shown.

Mixing Graph Fill

Additionally, the amount of material in the mixing bowl is critical to achieve the desired vortex. Too much material in the bowl will not permit sufficient movement of the material to create the vortex. Too little material in the bowl causes the material to float above the tool preventing equal shear and dispersion. The optimum fill level of the bowl will range from 50 to 85% full, but is empirically developed since it differs based on the materials being mixed. Each mix will have its own acceptable range of fill for the various recipes being used.


High-Intensity vs Low-Intensity Mixing

Low-intensity mixers can distribute color throughout the resin but, since no shear is created, the agglomerates still exist and complete color development is not possible. Additionally, the mixing times of low-intensity mixers are significantly longer than high-intensity.

Since the high-intensity mixer breaks up the agglomerates completely, all the color can be used to coat the resin. You can see the difference in the accompanying photograph. The dark, rich colored, molded sample was prepared using material that had been mixed using a high-intensity mixer. The lighter, pale color sample was mixed using the same proportions of resin and color in a low-intensity mixer.



The high-intensity mixing process is therefore very efficient. Since the color is at full strength, less dry color is required to achieve the same final color, resulting in significant cost savings to the molder.

Repeatability from batch to batch is more consistent with high-intensity mixing because the agglomerates are fully dispersed and the resin coated evenly. Low-intensity mixing will not break up all of the agglomerates, so the degree of dispersion will vary for each batch.

The mechanical properties of the final part are improved when there is consistent and complete dispersion of the dry color. Breaking up the agglomerates and evenly dispersing the colorant reduces the possibility of stress risers in the final part.

Selecting a High-Intensity Mixer

When selecting a high intensity mixer, two obvious considerations are that it have the proper blade design and be powerful enough to produce the shear required to break up the agglomerates and create a uniform coating action of the particles. Beyond that, three other considerations are typically evaluated:

  • Batch size required.
  • Distribution of material and automation requirements.
  • Ease of cleaning.


Stationary mixers are a most economical style. This style of mixer is generally located in a central part of the molding facility. After mixing, the material is distributed to the molding machines. Sizes of mixer bowls range from 200- to 1200-liter capacity for producing batch sizes of 175- to 1100-pounds. Options and features to consider include temperature and/or timer controls, as well as special wear coating on the tools to extend tool life.

To promote fast and easy cleaning, the surfaces in contact with the materials being mixed should be stainless steel and highly polished. The shaft sealing system should prevent material from leaking and protect the bearings.



High-intensity mixing is a technology available to rotational molders that allow a competitive edge over other plastic processes. For additional information contact Ferry Industries, Inc., +1-330-920-9200 or sales@ferryindustries.com