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How to design plastic injection molded parts?

Views: 10     Author: Site Editor     Publish Time: 2023-10-19      Origin: Site

Plastic Injection Molded Parts Design 

Plastic injection molding is a versatile and cost-effective method for manufacturing plastic parts. As a product design engineer and mentor, your knowledge in this field is invaluable. Now, it's time to share your expertise with students and teach them how to design plastic injection molded parts effectively. In this article, we'll cover the key aspects of this process, ensuring that students gain a comprehensive understanding.

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Outline Of The Article

1. Introduction
6. Injection Molding Process
2. Understanding the Basics of Plastic Injection Molding
7. Cooling and Ejection
3. Materials Selection
8. Tolerances and Surface Finish
4. Design Considerations
9. Prototyping and Testing
- Wall Thickness

- Ribs and Bosses

- Draft Angles

- Undercuts

- Parting Lines

10. Cost-Efficiency
11. Sustainability in Plastic Injection Molding
12. Quality Control
13. Case Studies
14. Common Mistakes to Avoid
5. Gate Placement
15. Conclusion

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Introduction Of Plastic Injection Molding

Plastic injection molding is a manufacturing process that involves injecting molten plastic material into a mold cavity, where it cools and solidifies to form the desired part. This method is widely used for creating a variety of plastic products, from everyday items to intricate components for industries like automotive and healthcare.

Understanding the Basics of Plastic Injection Molding

Before diving into the design process, students need to grasp the fundamentals of plastic injection molding. They should understand how the process works, the types of plastics used, and the advantages it offers over alternative manufacturing methods.

Plastic Materials Selection

Select a suitable plastic material based on your part's requirements. Common options include polypropylene, ABS, polycarbonate, and more. Consider factors like strength, rigidity, heat resistance, and cost-effectiveness.

Plastic Injection Molding Parts Design for Manufacturability:

To optimize manufacturability, follow these guidelines:

1. Wall thickness of plastic parts

Maintain consistent wall thickness throughout the part to prevent inconsistencies, warping, and shrinkage during cooling.

2. Plastic parts' ribs and gussets

Include ribs or gussets in your design to enhance structural integrity without adding excessive material.

3. Draft angles of plastic parts

Add draft angles (typically 1-2 degrees) to vertical faces to enable easy part ejection from the mold.

4. Plastic molding parts radii and fillets:

Use rounded edges and corners to avoid stress concentrations and promote smoother flow of material during molding.

5. Undercuts of plastic parts

Undercuts in the context of molding refer to features or shapes in a design that prevent the smooth ejection of a molded part from the mold cavity. Undercuts can complicate the molding process because they may require additional steps or modifications to ensure successful mold release.

6. Parting Lines

Parting lines are the lines or seams on a molded part where the two halves of the mold meet. Proper identification and management of parting lines are essential to minimize imperfections in the final molded product.

7. Tolerances of plastic parts: 

Specify appropriate tolerances to ensure the part functions as intended. Tighter tolerances can affect cost and manufacturability.

Plastic Injection Mold Gate Placement and Runner Design:

The position of the gate, where the molten plastic enters the mold, significantly impacts the final part's quality. Determine the location of the gate (the point of entry for molten plastic) and runner system. Proper gate design is crucial to achieving even filling, minimizing defects, and maintaining part strength. Common gate types include edge gates, sub-gates, and hot runners.

Plastic Injection Molds Design

Plastic Injection Mold Design:

Collaborate closely with mold designers to ensure the mold's effectiveness and longevity:

1. Plastic Mold Cooling Channels: 

Strategically place cooling channels to control temperature during the molding process, reducing cycle time.

2. Ejection System of Plastic Injection Mold:

Design an efficient ejection system (e.g., ejector pins) to release the part from the mold.

3. Plastic Molding Mold Venting: 

Incorporate venting channels to allow air to escape during injection, preventing air traps or burns.

4. Parting Line: 

Position the parting line (where the mold splits) to minimize the need for post-processing.

Plastic molding

Plastic Molding Parts Design for Assembly:

If your product consists of multiple plastic components, consider design features to facilitate assembly, such as snap fits, living hinges, or dovetail joints.

Mechanical Considerations:

For moving components, ensure proper functionality and longevity. Analyze the part for potential issues such as interference, binding, and wear. Use simulation and testing to verify performance.

Surface Finish:

Determine the required surface finish. This will affect the choice of mold steel and any secondary processes like polishing, texturing, or painting.

Testing and Prototyping:

Create prototypes to validate your design. Conduct functional tests, check for fit and form, and evaluate manufacturability. Adjust the design as necessary based on testing results.

Regulatory Compliance:

Depending on your industry and product application, ensure that your design complies with relevant safety and environmental regulations.

Cost Analysis:

Perform a cost analysis to identify areas where you can reduce material usage, cycle time, and tooling costs while maintaining quality.


Create comprehensive design documentation, including 2D and 3D CAD files, tolerance specifications, and material requirements. Accurate documentation is critical for communication with manufacturing partners.


Maintain open communication with your injection molding manufacturer. Their expertise can help you fine-tune the design for cost-effective and efficient production.

Continuous Improvement:

After production begins, gather feedback and monitor the performance of your plastic parts. Use this information to make continuous improvements in future design iterations.

In summary, designing plastic injection-molded parts is a complex process that requires a deep understanding of material properties, molding techniques, and manufacturing constraints. Collaboration, prototyping, and a commitment to improvement are keys to success in this field. Stay informed about the latest developments in materials and processes to create sustainable and efficient designs.

Injection Molding Process

Detail the stages of the injection molding process, from heating the plastic to injecting it into the mold and cooling.

The injection molding process involves several stages that transform plastic pellets into a final molded product. Here's a detailed breakdown of each stage:

1. Plastic Material Preparation:

The process starts by selecting the appropriate plastic material. Plastic pellets, typically made of thermoplastic polymers, are carefully measured and prepared for the injection molding machine. The pellets are usually dried to remove any moisture that could affect the quality of the final product.

2. Heating: 

The plastic pellets are then fed into a hopper located at the top of the injection molding machine. The pellets are heated and melted inside a barrel using heating elements and the friction generated by a rotating screw. The temperature and speed of the heating process are carefully controlled to achieve the desired molten consistency.

3. Injection: 

Once the plastic is fully melted, the injection molding machine advances the screw forward, pushing the molten plastic towards the mold. The mold has a cavity and a core, which determine the shape of the final product. As the molten plastic is injected into the mold, it fills the cavity, taking on its shape. The injection process is precise and controlled to ensure proper filling of the mold.


After the mold cavity is completely filled with molten plastic, the cooling phase begins. Cooling the plastic solidifies it, allowing it to retain the shape of the mold once it is removed. Cooling is typically achieved through a combination of cooling channels within the mold and external cooling methods, such as air or water spraying. The cooling time may vary depending on the size and complexity of the part, as well as the type of plastic used.

5. Mold Opening and Ejection: 

Once the plastic has cooled and solidified, the mold opens to reveal the molded part. The mold halves separate, and an ejection system, such as pins or ejector rods, push the part out of the mold. The part is then collected or dropped into a conveyor for further processing or inspection.

6. Post-Processing: 

Depending on the specific requirements of the part, additional post-processing steps may be needed. This can include trimming excess material, smoothing rough edges, applying surface finishes, or assembling multiple parts together.

The injection molding process is highly automated and requires precision in each stage to achieve consistent and high-quality molded products. It is widely used in various industries for manufacturing a wide range of plastic parts and products.


In summary, designing plastic injection-molded parts is a complex process that requires a deep understanding of material properties, molding techniques, and manufacturing constraints. Collaboration, prototyping, and a commitment to improvement are keys to success in this field. Stay informed about the latest developments in materials and processes to create sustainable and efficient designs. 

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Q1. What is plastic injection molding?

A1. Plastic injection molding is a manufacturing process that involves injecting molten plastic material into a mold cavity to create various plastic parts and components.

Q2. Why is material selection essential in plastic injection molding design?

A2. Material selection is crucial as it determines the part's properties, performance, and suitability for the intended application.

Q3. What are draft angles, and why are they important in injection molding?

A3. Draft angles are tapering features on a part that allow for easy ejection from the mold. They are essential to prevent part sticking and damage.

Q4. How can sustainability be integrated into plastic injection molding design?

A4. Sustainability can be achieved through material choices, energy-efficient processes, and recycling initiatives.

Q5. What quality control measures should be taken in plastic injection molding design?

A5. Quality control involves monitoring the manufacturing process, inspecting parts for defects, and ensuring consistency in the final products.

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