Guide on How to Achieve Color Consistency in Plastics

Plastic injection molding has become one of the most important methods of plastic manufacturing. Almost every type of plastic product that is sold as consumer goods, OEM, aftermarket, or even custom parts contain in one form or another, a plastic component. While most of these plastic parts serve a functional purpose, they do have some decorative aspects about them. This is why plastic injection molding products relatively come in similar colors, but rather a variety of designs and finishes that can be fit for distribution in the wider market.


Like all kinds of manufacturing processes, however, there is no such thing as a 100% perfect injection mold. Every ejected product, while they may appear to have no defects about them, may have tiny portions where discoloration or absence of color consistency can occur. This is due to factors that can be both associated with the machine operator, but mainly with the quality and the kind of equipment used.


As you may know, injection molding begins with the injection molding resin pellets made of various thermoplastic. Different resins can be fashioned into pellets, namely ABS, polystyrene, polypropylene, HDPE, LDPE, polyethylene, and many more. Oftentimes these pellets usually arrive in a clear or translucent design, prior to coloration. 


This is due to the fact that not all kinds of colorants may be suitable for a polymer. There may be issues with the mixing, curing agents, or the type of pigmentation used, that can affect the functionality of the pellets once they are molten and take the shape of the mold. 


Keeping all of these in mind, it wouldn’t be surprising to imagine that 100% color consistency may be impossible to achieve. This is due to the number of factors that can affect the quality of the injection molded plastic. But there are a few key things that can be done to achieve at least a high grade color consistency. Continue reading to learn more. 


Choose the Right Colorant

Plastic injection molding resin pellets can typically arrive in the regular translucent appearance, or they may already be pre-colored beforehand using the right colorants. In this manufacturing process, two kinds of coloring methods exist, namely internally and internal coloring.


As the name suggests, internal coloring means that the pigment has already been mixed during the manufacturing for the resin. Compared to other methods, the external coloring method is said to be able to produce a more stable and more uniform appearance on the pellets. 


For external coloring, the pellets can be designed in one of three ways: coating, plating, or printing. This can also be done electrostatically where a finely-pulverized powder is sprayed onto the pellets.


However there are some cases wherein the combined pigment and resin may not really be suitable. This can cause a number of issues in the resin and may even ultimately reduce the efficacy of its properties. For example HDPE is a tough and rigid plastic that has excellent chemical-resistance, moisture-resistance, and strength. If HDPE pellets were to be combined with the wrong kind of colorant, this can create unwanted negative chemical reactions on the pellet, causing it to disturb the polymer structure of the HDPE.


In order for any plastic injection molding product and cycle time to become successful, it’s essential that manufacturers are able to use the right type of colorant that’s suited for the resin. 


Maintain Injection Molding Pressure

Several defects can occur in the injection molding process. Some of them can be extreme to the point that they already affect the shape, form, and functionality of the resulting product. Some defects on the other hand, are fairly minor and have only aesthetic implications on the material.

Since the subject being dealt with in this article concerns the color of the injection molding product, one common defect that’s caused by this is called the flow lines. As said before the flow lines don’t really have a great impact on the shape of the product, but they can be unsightly. These marks are usually formed when the molten resin isn’t able to achieve a consistent cooling after it has already been poured into the mold. This is more common in molds that have varying wall thicknesses.


Another factor that can affect the color has to do with the injection molding pressure, especially when it is applied at a low pressure. To prevent this from happening, injection molding machine operators may make modifications to the design of the mold — i.e. incorporate less sharp corners and smoother areas. This allows the mold to flow at a consistent rate, thereby resulting in a uniform appearance as well. 


Clean the Nozzle

It’s normal for multiple cycle times of various injection molding resins to happen one after the other. This is so that the entire process remains speedy and efficient without sacrificing the quality too much.

The downside with this, is that there may be left over resin pellets that can get stuck inside the nozzle component. When the next batch of pellets have already melted, they can combine with the stuck resin and produce disjointed colors that can be unappealing.


Fortunately, injection mold manufacturers know just enough on how to avoid this from happening — by making sure that the nozzle is properly clean and free from any pellet residue. In this way, every cycle is guaranteed to be free from this defect.


Key Takeaway

Several factors may affect color consistency in plastic injection molding. As discussed in this article, this can range from the type of colorant, the cleanliness of the nozzle, as well as the injection molding pressure. 


As mentioned before, achieving a total injection molding consistency is not possible, but if the measures above have been taken into consideration, then injection molding processes will definitely improve.