Color unevenness is one of the defective appearance caused by the main materials, colorless pellets, colored masterbatches, and grinding materials that are returned. The color of the surface is not uniform, but a muscular and marble pattern appears. It has a characteristic that it is not noticeable in a dark color, and it stands out in light colors.
Color unevenness has different areas for each shot. It occurs randomly, regardless of the molded part. In addition, color unevenness may occur during continuous molding.
The cause of color unevenness in injection molding is roughly divided into two types: the front pier and the molding conditions.
There are three factors that become uneven color due to the front stage processing process. The measures for each are as follows.
A mixture (tumbler) is a device in which the main material and master batch are put into the drum tank and mixed by rotating them mechanically. Used to mix raw materials. The mixture (tumbler) is a type in which the internal blades rotate and the drum tank itself rotates.
The mixing time depends on the type, specific gravity, grade, mixing amount, master batch magnification, etc. If the mixing time is not enough, it may cause color unevenness because it does not mix sufficiently. Set the mixing time sufficiently and check out if it is mixed well before putting it in.
The material may stick to the wall due to static electricity, causing a mixture unevenness causes color unevenness. If you absolutely stick, spread it with an air gun.
When mixed with a certain percentage of grinding materials, color unevenness may occur due to the heat deterioration of the crushed material return. The runners are crushed and returned, among the molded molded products and runners. The crushed material that has repeated this return will deteriorate (performance deterioration) and uneven color. In particular, the mold with a high runner ratio should be adjusted to the composition ratio without returning all crushed.
The risk of uneven color unevenness differs for each transportation method of mixed materials.
.To suck the ingredients mixed with tumblers in a hopper on the molded machine, use a single -type loader. Due to the specific gravity of the main material, crushed material, and MB, the material sucks the hopper while separating. Even with raw materials that are firmly blended with tumblers, this separation may change the composition ratio. It is improved by shortening one suction time and minimizing the mixture ratio due to separation.
.Introducing a measuring type compounding machine is effective in preventing the compounding material separation using a transport machine. The measuring type combination machine is a device that weighs the main material, crushed material, and MB as the ratio and mix. By installing it on a molding machine, you can supply evenly mixed materials.
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Color mottling is one type of surface defect that occurs when the main material, which are colorless pellets, and the color-adding masterbatch and returned ground material do not mix well. The surface color is not uniform, showing streaks or a marbled pattern. It is less noticeable in dark colors and more noticeable in light colors.
Color mottling occurs randomly and can appear at different places with each shot. It may also suddenly appear during continuous molding.
The causes of color mottling in injection molding can largely be divided into two categories: pre-setup processes and molding conditions.
A mixer (tumbler) is a device that mechanically rotates to mix the main material and masterbatch in a drum tank. The type of mixer may have rotating blades inside or the drum tank itself may rotate.
Mixing time varies depending on the type of material, specific gravity, grade, mixing amount, and masterbatch ratio. Insufficient mixing time can cause color mottling. Set adequate mixing time and visually confirm the mix before insertion.
Material can stick to the walls due to static electricity, causing uneven mixing and mottling. If sticking occurs, use an air gun to forcibly disperse the material.
When manufacturing products by mixing a certain ratio of ground material, thermal degradation of returned ground material can cause color mottling. Especially in molds with a high runner ratio, do not return all the ground runners; adjust the mixing ratio instead.
The temperature setting of the heating barrel's compression section is crucial for preventing color mottling. Avoid excessive temperature settings; typically increase the temperature by 5°C to 10°C from the hopper side towards the nozzle. Overheating can lead to other defects such as burns or silvers.
Increasing back pressure can improve mixing. Low back pressure results in lower resin density, which can worsen the mixing state and make color mottling more likely. Higher back pressure improves resin density and mixing state. However, high back pressure increases the measuring time to the completion value, so adjust the back pressure and screw rotation speed accordingly within the cooling time.
Introducing a mixing screw or nozzle can mechanically improve color mottling. A mixing screw performs better in mixing compared to regular screws and allows mixing without excessive heating. A mixing nozzle can be switched out depending on the product, offering a cost-effective solution.
We have summarized the causes, factors, and countermeasures for color mottling in injection molding. It's a defect that can arise from both pre-setup processes and molding conditions. By systematically testing each factor, improvements can be expected. Aim for high-quality molding techniques without overlooking any instance of color mottling.
