Coloring and grinding for rotomoulding
Coloring and grinding for rotomoulding are two important steps in the material preparation process. The plastic resin starts as granules: round balls of resin that are about 3 mm in diameter. To prepare this material for rotomoulding the granules need to be reduced in size into particles below 600 mu. This fine powder can be blended with pigments or other additives to create specific properties and to colour the resin.
Grinding is the process of reducing the size of the plastic pellets to a powder. This is done using a grinder, which is a machine that uses high-speed rotary blades to chop the pellets into smaller pieces. The grinding process is important because the resulting powder must have a defined particle size distribution as well as specific particle shapes. Good grinding ensures that the plastic powder is of the correct size and shape, so it melts into the solid and consistent wall during rotomoulding. This is necessary to achieve finished products of good quality.
Dry-blending is a process that can be used in the preparation of plastic powder for rotomoulding. It involves mixing the plastic resin with other ingredients, such as pigments, fillers, stabilizers, and UV inhibitors, in a dry form. This can be done using a dry-blending machine, which typically includes a mixing chamber, an auger, and a discharge chute.
The ingredients are added to the mixing chamber, where they are mixed by the auger, and then discharged through the chute. Dry-blending can produce a consistent and homogenous mixture, required for achieving a consistent color and other properties in the finished product. The blended powder can be used for rotomoulding directly.
Compounding is another process of mixing plastic resin with various ingredients such as pigments, anti-oxidants, stabilizers, and UV inhibitors. This is done using a compounding machine that typically includes a mixing funnel, an extruder, and a pelletizer. The plastic resin is added to the mixing funnel and mixed with other ingredients using mechanical agitation. The mixture is then fed into the extruder where it is melted and homogenized under high pressure. The extruded mixture is then pelletized and cooled, forming small plastic pellets. The compounded and pelletized material now is ready for the next processing step which is grinding.
Through compounding, various types of plastic compounds can be created, each with specific properties such as colour and UV resistance, depending on the ingredients used. Compounding is also useful when it comes to using recycled materials. Compounding will homogenize the recycled plastic and prepare it for the next lease of life with the right level of antioxidants and UV inhibitors.
Micropellets, also known as micro-pellets or mini-pellets, are small plastic pellets that can be directly used in the rotomoulding process. Micropellets are also made by compounding granules and then pelletizing and cooling. The big difference with regular compounding is the pellet size. These pellets are much smaller in size than traditional plastic pellets, with a diameter well below 1 mm. There is no need for the grinding step otherwise required to reduce size of granules.
The small pellets have a consistent size and shape and are also more compact: their “bulk density” is lower compared to regular powder. When used in rotomoulding they show very good flow characteristics: the micropellets roll into the finest details of a mould and through the thinnest areas possible in rotomoulding. Overall, micropellets can be beneficial for small, intricate technical mouldings, with high part definition.