How Does CNC Injection Molding Work?

CNC injection molding is an innovative manufacturing process that allows you to create complex plastic parts at a low cost. The process used by ITAR-registered injection mold builders can produce intricate parts, but mold design changes can be costly. As a result, achieving the best possible results is critical. Following a few guidelines can help you avoid some common problems that can occur with this process. Ensuring that your part’s walls are of the same thickness can help avoid warping when the part cools.


CNC injection molding begins with a thermoplastic raw material fed through a hopper into a heated barrel with a reciprocating screw. This heat weakens the Van der Waals force between the raw material molecules. As a result, it reduces the viscosity and allows the polymer to flow with the injection unit. The screw also helps mix the thermal and viscous distributions of the polymer, which minimizes the time required to heat the material. Once the material has cooled, the part is removed from the mold, and the process is repeated. Compared to CNC machining, the injection molding process has less waste than CNC. Again, it is because the initial mold setup time is longer than that of CNC machining, but once the mold has been completed, there is virtually no waste. CNC cutting, however, creates a substantial amount of waste material. This waste material must be scrapped or melted down to make another block.


CNC injection molding is a process used to produce complex parts with fine tolerances. The process can produce parts that are up to 0.250 mm thick. Depending on the part, a variety of materials can be used. Fiberglass and glass fibers are commonly used for plastic parts, and a colorant is often used to enhance the part’s appearance. Some common colorants are black, blue, green, and red.

Choosing the right material for CNC injection molding is an important decision. The right materials are necessary for a quality end product. If a part is too thick or too thin, the tool may not be able to produce it. In addition, it may have a lower-quality finish or be prone to a poor visual finish.


CNC injection molding costs are based on several factors, including material costs and mold upkeep. This cost is also dependent on the part rate and plastic weight. Therefore, the overall cost of CNC injection molding will depend on these factors and the estimated scrap. Additionally, the cycle time will determine the cost per part. The time required to complete an injection molding process accounts for about 60 percent of the total cost.

One way to reduce the costs of CNC injection molding is to reduce the number of parts. Although this may seem like a no-brainer, it’s best to get input from all stakeholders. By reducing the number of parts, you can cut down on the cost of each mold cavity. While this can lower the cost per part, it will reduce your profit margin.

Typical Parts

The CNC injection molding process requires several parts. The first component is the mold. Molds are constructed of steel parts called cores, designed to be durable and sturdy. Once the mold is completed, the tooling will begin machining the plastic part. Next, the mold will be equipped with a clamping unit, which includes two separate clamping plates. The clamping unit will close and open to allow the plastic part to be injected into the mold cavity. The plastic part is then cooled and solidified. The plastic part will then be ejected from the mold with minimal draft.

CNC molding machines can produce a wide range of parts. These parts can have a variety of sizes, depending on the design. Some can be as small as 0.5mm in thickness. However, some parts cannot be manufactured using this process because of their size. It is due to the size limitations of injection mold machines.


CNC injection molding requires tight tolerances to ensure a consistent result. Therefore, parts should be uniform in wall thickness to avoid uneven shrinkage, which can cause deformities. If thick walls are not possible, support ribs or gussets are used to provide a uniform thickness. Parts with uneven wall thickness will deform and interfere with assembly.

Tolerances are also required for locational tolerances of holes in parts. If one part is out of tolerance, another will be out of tolerance. CNC molding engineers must control dimensioning throughout the manufacturing process to minimize this problem. Most molds are machined to a tolerance of +/-0.127 mm, but some parts require tighter tolerances (0.005mm or less). CNC precision machining adds to the overall cost, however. Therefore, designers should consider which tolerances are essential for a particular part’s design and which are merely a luxury.


The injection speed of a CNC injection molding machine affects several properties, including the degree of molecular orientation, shrinkage, and joint line development. A high injection speed improves the strength and surface quality of the molded part. High injection speeds also decrease the risk of stress peaks and notch formation.

CNC machines can also achieve extremely high precision and tolerance levels. The process is controlled by a computer and involves fewer variables. The manufacturing process focuses more on product specifications, minimizing the chances of common defects. This type of molding can produce parts faster than manual injection methods, and it can be more cost-effective than manual manufacturing methods.

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