Injection molding is the most commonly used manufacturing process for the fabrication of plastic parts. A wide variety of products are manufactured using injection molding, which varies greatly in their size, complexity, and application. The injection molding process requires the use of an injection molding machine, raw plastic material, and a mold. The plastic is melted in the injection molding machine and then injected into the mold, where it cools and solidifies into the final part.
The Plastic Injection Molding Process
Once your plastic injection mold tools are ready, the molding process consists of the following basic steps:
Step 1. The plastic resin comes in raw pellets. These are dried to the right moisture content, and then if necessary they can be blended with pigments or master batch colorants.
Step 2. The dried pellets are poured into the hopper of the molding machine. A reciprocating screw inside the barrel of the machine will transport these pellets towards the mold.
Step 3. Within the barrel, the pellets are mixed and heated until fully molten, forming the liquid resin.
Step 4. The heated mold closes automatically and resin, under high pressure, is injected through a gate and into the mold cavity.
Step 5. The mold is cooled to solidify the part inside.
Step 6. The mold opens, and the part is ejected to begin a new cycle.
There are four ways injection molding for plastics can be done:
1. Compounded Colors
3. Solvent Coloring
4. Dry Pigment Mixing
Plastic Injection Molding using Masterbatches
The resin supplier makes Masterbatches colors. A masterbatch can be bought in standard or custom colors, but these plastic pellets are heavily pigmented, 50% color to 50% resin. They are not ready to injection mold as-is; instead, they must be mixed with uncolored plastic to achieve a ratio of 2% color by volume. With a minimum order of one ton, this masterbatch now represents 20 tons of moldable plastic, thus only making sense when producing large quantities of finished parts.
What if you only want a few thousand parts to start out with and you’ve got a custom color?
First, the client and the compounder must agree on the color to be used to get a near match to the sample. And “near” is the operative term here. Getting a close visual match with your color sample is the best that can be done, even when using standard visual and testing conditions. You can then opt to have a small batch of plastic pre-mixed by the molder, using one of two additional methods.
Here a pigmented oil is mixed with a solvent. This solvent is sprayed over uncolored pellets, thoroughly coating them just before injection. In this method, it’s difficult to control the ratio of color to plastic precisely.
Dry Pigment Mixing
The last method is similar, but uses dry pigment powder directly mixed with plastic pellets in a hopper, again with a ratio of 2% pigment by volume. For some engineering plastics, the pellets must be dried first before adding the pigment. This mixing in the hopper is also an approximation – most of the dry pigment will stick to the plastic pellets (which is what you want), but some will also stick to the walls of the hopper and related plumbing, thereby diluting the concentration of color. Any water vapor in the atmosphere will also enter into the mixture the moment that it’s taken from the dryer. All told, this method is expedient if there is a small volume of a custom color – too small to have it compounded for you. But the price of suitability is that there are too many variables to provide the assurance you’ll get precisely the color you want.
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