11 Things to Consider When Designing Plastic Parts for Injection Molding

Injection molding stands firmly as the foundation for modern manufacturing industries, allowing the mass production of complex plastic parts. However, having top-notch quality plastic parts and their smooth production requires a planned and detailed structure of their design.

For instance, choosing the right material, surface finish, draft angles, etc. These elements are important for giving a classy outlook and finish to the plastic parts that are produced by the injection molding process. 

Let’s delve into the top 11 considerations we should keep in mind before designing plastic parts.

Essential Elements to Consider

Some important elements that you should keep in mind for the right design are given as follows:

1. Selection of Material 

Selecting the right material can be an easy job or a complex one, depending on your requirements. A few things which are to be kept in mind are:

• Heat resistance: Is your equipment resistant to extreme temperatures? Such kinds of things should be considered and kept in mind in order to choose the suitable material for your production process.
• Flexibility or rigidity: Check the mechanical loads the part is going to face and choose the right material. Will it be flexible or rigid? Such questions should never be skipped.
• Chemical compatibility: Chemicals and solvents can destroy your part. Make sure to study its chemical compatibility and choose the right material that is resistant to degradation. A bit of carelessness can affect your overall effort and money.
• Visual: Lastly, consider the visual appearance of the part and choose the material that would give it a smooth appearance and proper finish.

2. Taper Angles

Taper angles are tapered features that are incorporated into the sidewalls of the plastic part. They are responsible for the effortless removal of the molded component from the mold. They are crucial for two reasons:

• Lesser friction: Taper or draft angles reduce friction between the solid plastic and the mold walls, which further prevents any kind of damage to the equipment. 

• Deeper parts require more draft: Deeper parts, such as containers, require steeper draft angles for better and easier demolding.

3. Ensure Consistent Wall Thickness

During injection molding, inconsistent wall thickness can cause sink marks (depression) and warping due to uneven cooling and shrinkage. In order to have a high-quality part, consider the following points:

• Optimize design: Focus on the design and conduct an in-depth analysis of where the wall thickness can be streamlined. 

• Use ribs and cores: Utilize ribs and cores in the right manner. Where ribs can add strength without excessive thickness, cores can remove unnecessary materials in thicker sections.

Follow a consistent wall thickness and have a top-quality product with minimal or no defects.

4. Parting Line

A parting line is termed an imaginary line where the two halves of the mold come together to create the part cavity. A planned and detailed placing of the parting line plays a major role for several reasons:

• Reducing material wastage: Placing the parting line has a huge benefit in ensuring very less wastage that needs to be removed after the molding process. 

• Flexibility of design: With the help of parting lines, you can easily mold your part in whatever shape you want. The right place of lines would give you design freedom.

Discuss the parting line and placement with your injection molding partner for both manufacturability and design intent.

5. Avoid Undercuts 

Undercuts are features that cause a reverse slope on the part that looks like a hook or a cavity with an overhanging lip. They pose challenges during the demolding process:

• Mold complexity: Due to undercut features, the easy method of pulling part directly from the mold becomes complex. Therefore, sliders and lifters are needed to complete the process, which messes with the overall mold cost. 

• Prone to damage: Ejecting parts with undercut features is difficult and highly prone to overall damage to the part of the mold.

Try to redesign the part to cut off undercuts. This would allow smooth processing and reduce the overall cost of the process.

6. Filets and Radii

Sharp corners present on the part act as the stress concentration points, which increases the risk of getting cracked under a high-pressure zone. Filets and radii can be used as their rescuers:

• Improves strength: By replacing the sharp corners with filets and radii, the stress on the edges can be evenly distributed across the part and enhance its structural integrity.

• Amplified moldability: Filets and radii facilitate plastic flow during the injection molding process, which improves the whole process.

Adding files and radii gives you a more unique and manufacturable design. 

7. Desired Surface Finish

We want a desired surface finish. This is because it has a major role in enhancing or degrading the parts’ final look. It influences both its functionality and visualization. Here’s a reason how it’s done

• High gloss finish: A polished mold surface is needed to have a high gloss finish on the part, which would increase the cost of the mold. Therefore, it is mandatory to ensure everything goes within your budget.

• Textured finish: A textured mold surface creates a matte or textured finish on the part, which is a bit cheaper as compared to a gloss finish. This would be in your budget. 

Choose your desired surface look and budget and simultaneously consider these things

8. Gating and Ejection

The gating is where the material enters and fills part of the cavity. It is important to find out where you want to put the gate and work on installing it. Some questions to consider:

Can I put a gating marker where I can see my gate? Does the substance flow from the location I specified?

Check the marks on the release pins on the bottom of the casting. If eject entries are not allowed, you will need to explore alternative removal methods.

9. Additional Processes

Injection molding is not the last step in the production process. Often, certain parts require additional processes such as:

• Machining and finishing: Holes, threads, or surface textures are some examples that require a machining or finishing process even after the entire injection molding production process is complete.

• Assembly: Form and fit or other functions require additional assembly or additional parts.

10. Cost Analysis

The entire injection molding process is affected by several factors. These are given below:

• Material selection: Choosing the right material is very important because it influences the overall cost of the process.

• Complexity of parts: A complex design requires a complex mold. This would add complexity and affect your overall budget.

• Mold design: Features such as undercuts and special molds add to the overall complexity and cost of the mold.

All of these factors would make the mold easier to use.

11. Manufacturability-focused design

In manufacturability planning or DFM, designers and injection mold masters collaborate. It has two phases:

• Early discussions: On-site discussions are important to understand the obstacles and problems that arise during the project process.

• Iterative designs: Designs can be modified based on input from the designer. This would facilitate the smooth running of the entire process.

Conclusion

We hope this article will help you in the future while designing plastic parts for injection molding. Just make sure to focus on the material selection and keep in mind your set budget. 

All these elements are important as well as useful for you to have a smooth and easy production of the plastic parts.

However, if you are confused about how to set the right budget, it will be wise to explore the local market. You can interact with other experienced designers as well. Remember that the right design and choice will help you start a successful journey from a piece of sheet to an original master copy.

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