You can get consistent color in rotomolding.

Rotomolders may assume the pigment is to blame for color variation, but it's only one of many variables to be controlled.

Increasing sophistication in rotational molding has raised the quality expectations of customers in industries such as toys, recreation, automotive, and consumer products. Uniform color is a critical requirement in many of these types of applications. But rotomolders whose prior experience involves mainly production of large tanks and containers, where little or no color is used, may be unprepared to meet strict color specs. A significant number of rotomolders are unfamiliar with the origins of problems such as color shifts, swirls, bleeding, and more severe color-related problems such as part warpage and loss of physical properties.

From a coloring point of view, the rotomolding process isn't very forgiving, unlike injection molding or extrusion, where residence times are relatively short and the process can impart a moderate degree of mixing action. Rotomolders typically operate with temperatures over 500 F and cycle times over 15 minutes, creating a harsh environment for the materials used. And rotomolding is further limited by no shear on the material in the mold.

Rotomolders often assume the pigment is to blame whenever color problems occur, but that is only one part of the picture. The first step in attacking color variation is to understand the sources of color variability that come from the resin, pigment and other additives, the mixing process, and molding conditions.

WHAT'S IN THE RESIN?

The primary resin used in rotomolding is LLDPE. These polymers are produced with metallic catalysts and neutralizers, whose residues can affect color. All LLDPEs are copolymers and the comonomers can also cause color shifts.

Any time you change resins you may be introducing a material of slightly different density, which can affect color. Also, materials suppliers use different additive systems (antioxidants, uv stabilizers, flame retardants, impact modifiers, release agents) that influence color as well. For these reasons, it is important to provide your colorant supplier with a sample of the resin to be used if the application has critical color specs.

PIGMENTS & ADDITIVES

Because of the harsh thermal requirements of rotomolding, only about 30 pigments among the 200 or so used in plastics are suitable for this process. For cross-linked products, the list of available pigments is even shorter. Rotomolders therefore need to be aware of the inherent limitations of certain pigments, and they must provide their color supplier with as much information as possible on part design, blending procedures, resins used, cycle times, and temperatures.

While inorganic pigments are generally less reactive and less prone to color shifts than organic ones, problems can occur with both types under some conditions. Inorganic pigments typically maintain consistent color over a wider processing range than organic pigments. Inorganics also tend to be more opaque, usually have good outdoor light stability, and are relatively easy to disperse. Often they are also less expensive and resist migration or bleeding.

However, inorganics by themselves are frequently not as clean and bright as organics, which limits the range of colors that can matched. Inorganics are generally used at higher loadings, though without a negative effect on physical properties of the finished parts. In cross-linked resins, certain metals in inorganic pigments can react with the peroxide to limit the degree of curing that takes place. That can be another source of impact problems. (You may be able to offset impact problems with a longer cycle or higher process temperature if your product and economic can tolerate them.)

Until recent years, cadmium pigments were among those most widely used in rotomolding. They are clean, bright, stable to heat and light, relatively inexpensive, easy to disperse, non-bleeding, and usable at high levels that do not harm impact strength. They do not interfere with cross-linking. But both cadmium and lead pigments have fallen under a regulatory shadow and often cannot be used. Since there are no other inorganic pigments that offer the popular bright yellows, oranges, and reds, we are forced to look to organic types for help.

In general, organic pigments are strong, bright, clean, and translucent (rather than opaque) and have reasonable heat and outdoor light stability. On the other hand, organics are often expensive and difficult to disperse, and they can shift color over a range of process temperatures. Some of them can cause warpage problems. Some will bleed. And their smaller particles tend to be more susceptible to static electricity, resulting in color swirls. Peroxide cross-linking agents will react with certain organic pigments and cause a large shift in color.

As noted, rotomolding resins can contain a wide variety of additives, some of which can affect color. Another important additive where color is concerned is the antistatic agent supplied by the color producer to reduce color swirls. Post-molding antistats are designed to migrate to the surface of the product over longer periods of time to prevent attraction of dust and dirt to the finished product.

Some pigments, especially organics, tend to migrate with antistats and end up on the surface of the part where they can rub off, for example, on a child's clothes.

MIXING IS CRITICAL

In rotomolding the colorants, additives, and mixing procedure strongly affect cycle time and physical properties of the part. With dry-color mixing - the most widely used coloring approach in rotomolding, the processor must measure colorant very accurately. Adding as little as 15 g excess dry color to 99 lb of resin (0.04%) may cause a 50% reduction in impact strength, while a drop of 15 g may reduce opacity by 30%. What's more, pigments may affect warpage. Colors that contain relatively high levels of titanium dioxide will require extended cycle times.

To increase consistency, improve quality, and reduce rejects with the use of dry color, you will need a high-intensity mixer. High-intensity mixers generate heat by friction. These high-shear mixers allow the molder to use temperature as the control parameter rather than time. That way a molder knows adequate shear (work) is put into the material. In addition, the high-intensity mixer must be filled to the appropriate level. If the mixer is too full, the batch will not blend properly. If it is not full enough, the mixer will whip air into the blend and thus reduce shear.

FINE-TUNE YOUR PROCESS

During initial powder deposition in the oven, the smallest particles are the first to adhere to the mold surface. These fine particles are particularly high in pigment-to-resin ratio. Anything that disrupts uniform distribution of this fine-particle fraction will result in localized concentrations of pigment on the exterior surface of the part. Static charges caused by the powder flowing over the mold surface are a prime culprit. People often mistake static swirls in rotomolded parts for a sign of poorly distributed pigment. Static swirls, however, usually appear in the same area on the outside wall of finished parts - never on the inside wall.

The abusive time/temperature conditions of rotomolding can cause the same lot of colorant to give good results with some parts and poor results with others. This is most likely to happen if you try to run different parts on the same arm of the machine. The conditions necessary to fully fuse some parts may "overcook" others and change the color. You may have to consider moving the tool to a different arm or to another machine altogether - or else see if you can put back the cycle time or temperature without undercuring the other parts.

A major process variable whose effect on color consistency is often overlooked is the tool finish. Variations in gloss or texture between mold surfaces can result in a perceived color change. This factor can be very important in assemblies of mating parts of the same color.

Mold releases, whether sprayed on or applied as a semipermanent coating, can alter surface finish and cause what looks like a color shift.

COMPOUNDED VS. DRY COLOR

Precolored compounds eliminate many sources of error and color variability. There is no weighing or handling of pigment and no mixing. Static swirl is no longer a problem. In cases where high opacity or superior physical properties are required, precolor may be the only option.

On the other hand, precoloring adds another heat history to the resin, pigments, and additives. The molding process will degrade them even more. Precolor thus may impose a narrower process window. Dry color is also the lowest-cost approach to coloring, and it adds far less inventory burden than precolor.

For best results with either precolor or dry color, you should share process and resin information up front with your color supplier. You also might ask whether the color supplier has a rotomolding machine in its lab to duplicate your process conditions.