The Complete Guide to the Plastic Injection Molding Process (2023)

Plastic injection molding is a highly versatile and widely used manufacturing technique that enables the production of intricate and complex plastic parts in large quantities. This article will provide a comprehensive guide to understanding the plastic injection molding process, including material selection, mold design, injection, cooling, ejecting, and finishing.(Click here)

Plastic injection molding has revolutionized the manufacturing industry, enabling the production of a variety of plastic components used in industries as diverse as automotive, electronics, medical devices, and consumer products. By injecting molten plastic material into a custom mold, manufacturers can create complex shapes and precise dimensions with high efficiency and precision.

Plastic injection molding is a manufacturing process that involves melting plastic or resin pellets and injecting the molten material into a mold cavity. The mold is usually made of steel or aluminum and is precisely designed to produce the desired shape of the final product. Once the plastic material cools and solidifies within the mold, it is ejected, resulting in a finished plastic part.

material choice

The success of the plastic injection molding process is highly dependent on choosing the right material for the intended application. These are the necessary steps to choose a material:

1: Identify the requirements:Understanding of specific product requirements such as mechanical resistance, chemical resistance and environmental factors.

2: Select the type of plastic: Select the appropriate plastic material based on properties such as flexibility, rigidity, transparency or heat resistance. Common types include polyethylene, polystyrene, polypropylene, and ABS.

3: Consider fillers and additives:Determine if the material needs additional fillers or additives to improve properties or reduce cost.

4: Material tests:Test materials to assess their performance under various conditions, ensuring they meet required specifications.

mold design

The mold design phase is crucial to ensure the production of high-quality plastic parts. Here are the steps involved in mold design:

1: Product overview:Review product design and specifications to determine mold requirements, including parting line, closure, and ejection mechanisms.

2: Mold material selection:Select the appropriate mold material based on factors such as expected production volume, part complexity, and cost considerations.

3: Design of the mold cavity:Design the mold cavity to exactly match the desired shape of the final product. Consider factors such as step angles, wall thickness, and surface finish.

4: mold tools:Create a mold tool, which includes a mold base, core, and cavity inserts. Use precision machining techniques to achieve optimum mold accuracy.


The injection stage consists of injecting molten plastic material into the mold cavity at high pressure. These are the steps involved in the injection process:

1: Fixing:Close the mold well to ensure the exact reproduction of the part and avoid material leaks during injection.

2: Injection unit:The plastic material is melted and homogenized in the injection unit, which consists of a cylinder, a screw and a heated nozzle.

3: Injection speed and pressure: Control injection rate and pressure to correctly fill the mold cavity without causing defects such as air traps or sink marks.

4: Cooling time:Allow sufficient time for the plastic material to cool and harden within the mold. Cooling time depends on factors such as part thickness and material properties.

Cooling is a critical stage in plasticsinjection molding process, where the molten plastic material solidifies and takes its final shape. Proper cooling ensures dimensional stability, minimizes internal stress, and prevents defects in the finished part. Here is a complete step-by-step guide to the cooling process:

1: Diameter and arrangement of channels:The diameter and arrangement of the cooling channels are carefully determined to ensure uniform cooling throughout the entire mold cavity. Factors such as part thickness, material type, and expected cycle time affect the dimensions and layout of the slots.

2: Calculation of cooling time:The calculation of the cooling time includes consideration of factors such as the melting temperature of the material, the temperature of the mold, the heat transfer coefficients and the thermal diffusivity of the plastic material. Simulation software or empirical formulas can help estimate cooling times.

3: Monitoring of the cooling process:Temperature sensors integrated in the mold or external control devices are used to measure the temperature of the mold during the cooling phase. This data helps assess the efficiency of the cooling process and identifies any imbalances or cooling problems.

4: Cooling Efficiency Optimization:Adjustments can be made to optimize cooling efficiency. This may include modifying the design of cooling channels, using conformal cooling, or implementing advanced cooling techniques such as turbulent flow or heat pipes.

5: Opening and ejection of the mold:After the cooling phase, the mold is opened and the ejection system removes the solidified part. The part must be adequately cooled and hardened to withstand the pullout forces without bending or damage.


The ejection phase of the plastic injection molding process consists of removing the solidified part from the mold cavity after the cooling phase. The correct ejection techniques and mechanisms are essential to avoid damage or deformation of the parts. Here is a complete step-by-step guide to the eviction process:

1: Design of the ejection system:During the mold design phase, an ejection system is built into the mold to facilitate removal of the solidified part. The design takes into account factors such as part geometry, undercuts, and pullout force requirements.

2: Passers or expulsion plates:Ejector pins or plates are commonly used in an ejector system. These components apply force to the part, ejecting it from the mold cavity. The number, size, and position of the stripper pins or plates depend on the shape and complexity of the part.

3: Selection of the ejection mechanism:Depending on the part design and stripping requirements, different stripping mechanisms can be used, such as stripping plates, sleeves, or hydraulic cylinders. The mechanism must provide enough force and control to eject the part correctly.

4: Expulsion Order: The ejection sequence is programmed in the control system of the injection molding machine. Determines the ejection time and order of multiple pins or ejector plates, ensuring a smooth and synchronized ejection process.

5: Calculation of the ejection force:The ejection force required to remove the part is calculated based on factors such as the surface area of ​​the part, the material properties of the part, the friction between the part and the mold, and any additional features such as notches or threads.


The final step of the plastic injection molding process involves post-processing procedures to improve the final appearance and functionality of the molded parts. This guide provides a step-by-step overview of the finishing process:

1: Flash and spout removal:Flash is the excess material that protrudes beyond the parting line of the mold, while the channel is the channel through which the molten plastic entered the mold. The first step in finishing is to remove burrs and burrs with a cutting or cutting tool.

2: Elimination of the borders:If the molded part has sharp edges or irregularities as a result of the mold design or injection process, deburring is performed to smooth or remove these imperfections. Deburring tools or techniques such as drilling or grinding are used for this.

3: Surface treatment:Depending on the desired surface finish, additional procedures such as sanding, polishing or texturing can be performed. These techniques enhance the look, feel, and texture of the final piece.

4: Paint or coating:If you want additional protection or color, the next step is to paint or coat the piece. This can be done using techniques such as spray painting, powder coating, or electroplating, depending on the material and requirements of the part.

Plastic injection molding is a highly versatile and efficient manufacturing process that enables the production of complex plastic parts with precision and repeatability. By following the steps outlined in this guide, manufacturers can ensure successful performance of the plastic injection molding process, from material selection to finishing. Emphasizing the importance of each step in the process will result in high-quality plastic parts that meet your desired specifications.

Q: What industries benefit from plastic injection molding?

Plastic injection molding is used in industries such as automotive, electronics, medical devices, consumer products, etc.

Q: Are there any restrictions on injection molding?

Plastic injection molding has certain limitations, such as high initial tooling costs and longer setup times for complex molds.

Q: What types of plastic materials are commonly used in injection molding?

Common types include polyethylene, polystyrene, polypropylene, and ABS, among others.

Q: How long does the plastic injection molding process take?

The time required for the entire process depends on factors such as the complexity of the part, the design of the mold, the type of material and the cooling time. It can vary from seconds to minutes.

Q: Can injection molding accommodate multiple part sizes?

Yes, plastic injection molding can produce small or large parts, depending on the size and capacity of the injection molding machine.

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