Basics of Injection Molding Design

Learn about the various cost elements of injection molding, including mold, and material costs.

Injection molding is a critical manufacturing technology capable of mass-producing thousands of parts. Therefore, an engineer or designer who wants to use this technology will need basic knowledge to fully utilize the technology. This article will help. 

How Injection Molding Works

Injection molding is a mass manufacturing process that shapes melted plastic into the desired geometric shape. This process best fits mass production because it is highly efficient and repeatable. It results in manufacturing complex shapes with a high degree of precision at minimum waste, hence very versatile and cost-effective at scale.

An injection molding machine consists of several critical components:

  • Hopper: This supplies plastic pellets to the machine to ensure a consistent supply of material for melting and injecting. It often contains a drying unit to eliminate moisture from the plastic pellets since it can cause defects in the final product.


  • Barrel and Screw: In the barrel, there exists a screw that rotates about its length to melt and homogenize the plastic, then inject the molten plastic into the mold. The configuration and rotation of the screw are essential in achieving appropriate temperatures where the material melts and excellent flow properties.


  • Mold: This is the part that defines the shape of the last part and is usually made of steel or aluminum. It can be single-cavity, meaning it produces one part per cycle, or multi-cavity, thus producing many parts. Steel is more resistant to wear and the material of choice for large parts, while aluminum is less expensive.


  • Clamping Unit: The clamping unit holds the two halves of the mold together during the injection of the melt and cooling. It applies a very high force to prevent molten plastic from coming out of the mold.


Material Selection

Selecting an appropriate material is one of the most critical choices in the injection molding process.

Generally, three materials with characteristic properties are used to manufacture molded parts: thermoplastics, thermosets, and elastomers.

The selection of material should consider factors such as:

  • Mechanical Properties: These include tensile strength, flexibility, and hardness to meet the part’s functional requirements.
  • Thermal Properties: A material’s melting temperature and thermal conductivity are essential in the molding process and the resultant performance of a molded part in the application environment.
  • Cost: Material cost and performance must be balanced to maintain product profitability.

Knowing the flow characteristics of the selected material can help design a part that fills the mold properly and maintains the intended characteristics.

Not sure which material to choose? Our engineers can help determine the best-fit material for your budget.

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Key Design Considerations

  • Wall Thickness: Achieving uniform thickness throughout the walls is crucial. Distortions and uniform cooling lead to better dimensional stability and structural integrity of the part.


  • Ribs and Gussets: They provide additional rigidity and greater strength while taking minimum toll on increased thickness, a part of the reason for material reduction and cycle time in the process. Appropriate design of ribs and gussets facilitates the removal of weight from the optimal location and improves performance overall.


  • Draft Angles: Where features are designed to come together at right angles, small draft angles will help parts release from the mold more freely. This reduces potential damage and limits cycle time.


  • Fillets and Radii: Transitioning between surfaces should be designed to be as smooth as possible. Reducing stress concentration will improve part strength and increase life. Stress points and potential failure: you want to avoid sharp corners.


  • Mold design: the mold is the heart of the injection molding process. However, it also tends to be quite expensive; some molds cost thousands of euros, depending on the complexity and material used.

Quality Control

High quality can be guaranteed in an injection molding process through the following:

  • Visual Inspection helps identify surface flaws. The areas of concern are scratches, surfaces and variations in color and flash.


  • Dimensional Measurement: Physical geometrical characteristics are determined. This verifies whether a part meets the proposed design using calipers, CMM devices, and optical comparators.


  • Mechanical Testing: Measures and ensures a part’s strength. In most cases, testing is carried out using the tensile test, impact test, and fatigue test.

MakerVerse’s engineering team visually inspects every part. Additional dimensional measurement and mechanical testing reports are available as needed.

Start Your Injection Molding Project

Every engineer and designer interested in producing high-quality parts efficiently must learn the basics of injection molding design. An on-demand manufacturing platform like MakerVerse helps streamline this process.

MakerVerse provides advanced manufacturing technologies, expert guidance, and rapid production speed to bring innovative designs to life. Try for yourself.