Plastic Part Design Guidelines for Injection Molding Plastic Parts

Injection Molding is a widely used plastic manufacturing process to manufacture plastic parts in large volumes and at low cost. It has various advantages compared to other plastic manufacturing processes. Following plastic part design guidelines for injection molding ensures good quality manufactured parts. In this article we will discuss various plastic part design guidelines for injection molding.

Plastic part design plays a very important role in ensuring injection molded plastic part quality. If plastic part design is as per guidelines, Relatively better injection molding window can be achieved. Therefore more variation in the injection molding process can be accommodated.

But following all plastic part design guidelines for injection molding does not guarantee best quality parts. Therefore it is always recommended to also control mold design and injection molding process to manufacture quality plastic parts.

Following factors affect the quality and consistency of injection molded plastic parts.

  • Plastic Part Design
  • Plastic Material
  • Process Parameters
  • Injection Molding Machines
  • Injection mold Design

Plastic Part Design Guidelines

Although it is not possible to follow all design guidelines because of product design constraints. But it is recommended to follow as much as possible to achieve defects free plastic parts. If the plastic part design is not as per guidelines. It is recommended to do simulation studies (mold flow analysis for plastic parts) to reduce the risk.

Following injection molding plastic part design suggestions have a huge impact on manufactured plastic part quality.

  • Uniform Plastic Part Wall Thickness
  • Plastic Boss Design. 
  • Rib Design Recommendation.
  • Sharp Corners
  • Gussets
  • Draft Angle

1. Uniform Wall Thickness in Plastic Parts

Uniform wall thickness in injection molded parts ensures molten plastic is not forced through varying restrictions during injection molding. Therefore it allows the injection mold cavity to fill more easily.

How Uniform Wall Thickness Will Improve Plastic Part Design?

When molten plastic is injected into injection mold with variable part cross-section. The thin plastic section starts cooling first.

But when the thick section starts to cool. Part shrinks and stresses are built up near thin and thick section boundary areas. Because the thin section has already hardened, it doesn’t yield. Therefore when the thick section yields, it leads to warping, twisting, shrinkage or void formation in the part. High stresses can also cause cracks in molded plastic parts.

Design Guidelines for non-uniform Plastic Part Wall Thickness

In design, it’s not feasible to provide uniform wall thickness in all sections. Therefore following points are recommended to reduce the impact of non-uniform wall thickness.

Gradual change in injection molded plastic part thickness is recommended.

Recommendation for Non-Uniform Plastic Part Thickness

  • Gradual change in plastic part wall thickness.
  • Avoid filling molten plastic from thin to thick sections.
  • Remove plastic from thick sections (coring).
  • Analyse flow of molten plastic in thinner sections.

2. Plastic Boss Design Guidelines on Plastic Parts

Boss feature is used as a point of attachment and assembly in plastic parts.
Boss Design Recommendations in Plastic Parts

Boss features in plastic parts are designed to receive screws, threaded inserts or guide other parts. It consists of a cylindrical projection with holes. 

During product life-cycle bosses are subjected to various loads. Therefore Boss design in plastic parts should be as per guidelines to ensure boss strength and avoid injection molding defects in plastic parts.

Boss Wall Thickness

Recommended boss wall thickness is 0.6 times of nominal wall thickness of the plastic part.

Boss Wall Thickness (A) = 0.6 x T (Nominal Wall Thickness)

Note: Boss wall thickness can be increased to increase its strength. For example, in case of high stresses produced while using self-tapping screws. But this will have an impact on part aesthetics.

Radius at Boss Base

Small radius at the boss base increases boss strength and also helps in part ejection during injection molding.

Recommended Radius at the base of boss feature (R) = 0.25 to 0.5 times of nominal wall thickness

Minimum center to center Distance Between Bosses

Small center to center distance between two boss features in plastic part design can result in the formation of a thick section. Thick sections are very difficult to cool and result in injection molding defects. Therefore it is recommended to maintain minimum spacing between bosses.

Minimum Center to center distance between Bosses = D1 + 2T

Where:

D1 – Boss Max Diameter

T – Nominal wall thickness

Draft Angle for Boss feature

Draft angle in injection molded parts helps in easy removal of parts from injection mold. Its Value depends on surface finish and boss height. Mostly 0.5º to 1º draft is recommended in boss section.

Other boss Design Requirements
  • Chamfer on top of the hole is provided for the good lead in of fasteners.
  • Boss strength can be increased by providing gussets at the base or by using connecting ribs to nearby walls.
  • If the boss-wall thickness is more than recommended thickness. As shown above, Recess around the base of the boss is provided to reduce the probability of shrinkage

3. Reinforcing Rib Structure Design in Plastic Parts

Ribs in plastic parts increase the part moment of inertia by increasing the bending stiffness. Therefore Ribs are used in plastic parts to increase bending stiffness without increasing plastic part thickness. 

Bending Stiffness = E (young’s Modulus) x I (Moment of Inertia)

Ribs increase the moment of inertia. which increases the bending stiffness.
Rib Design Recommendation in plastic part
Rib Thickness Recommendation

In plastic part design recommended rib thickness is 0.5 to 0.75 times of the nominal wall thickness to avoid shrinkage in injection molded parts.

Rib Thickness (W) = 0.5 to 0.75 x T

Recommended distance between Two Ribs

To avoid thin sections in injection mold. Recommended minimum distance between two ribs is two times of nominal wall thickness.

Distance between two ribs (X) > 2 x T

Draft Angle in Ribs

Draft angle in ribs ensures easy removal of injection molded plastic parts from injection mold.

Minimum Draft Angle in Outer ID = 0.5 degree

Minimum Draft Angle in inner ID = 0.25 degree

Rib Height

Maximum recommended rib height is less than three times of nominal wall thickness. It is required to avoid large variation in rib thickness. To increase bending stiffness, Multiple ribs are recommended instead of one large rib.

Maximum Rib Height (H) < 3 x T

Sharp Corner Radius

Sharp corners at rib base results in stress concentration. A minimum radius equal to 0.25 times of nominal thickness is recommended to avoid stress concentration after injection molding.

Rib Intersection in injection molded Part

Coring out rib at rib intersection is recommended to avoid excessive sinking on the opposite side of the rib.

Rib Orientation Recommendation

A rib is oriented in such a way, it provides maximum bending stiffness to the part. Rib orientation depends on part geometry and bending load.

4. Sharp Corners in injection molded Plastic Parts

Sharp corners in plastic parts can lead to high stress, restrict material flow, and reduce part strength. Therefore it is recommended to add Radius in plastic part design for injection molding. Recommended inside corner radius in plastic pat design is more than 0.5 times the nominal wall thickness. Whereas recommended outside radius is inside radius plus nominal wall thickness.

Sharp corners are not recommended in plastic parts.

Stress concentration factor at plastic part corners depends on its corner radius and nominal wall thickness. 

If the ratio of the radius and thickness is less than 0.5. Stress concentration factor is considered high. Therefore it is recommended to keep R/T values more than 0.5 

5. Gussets In Plastic Parts

Gussets are used in plastic parts to increase part strength in the required section. But the location of gussets prevents direct venting in mold steel. Therefore Gussets in plastic parts should be designed in such a way that it should not create any venting or filling problems.

6. Draft Angle in Injection Molded Plastic Parts

Draft Angles are provided parallel to the direction of part release. It helps in easy removal of plastic part from injection mold

Higher the value of draft angle. Easy will the removal of part from injection mold. Industrial designers will always ask for zero draft but mold designers need max possible draft angle. Polished plastic parts require relatively small draft angle compared to matte finish parts.

Draft Angles are provided parallel to the direction of part release for easy removal of part from injection mold.
Factors Affecting Draft Angle Value

Following factors affect the value of draft angle:

  • Feature Depth
  • Feature Size
  • Mold Finish
  • Plastic Material
  • Part Geometry
  • Mold Ejection System.
Draft angle in Textures

Minimum (1.5° + Part nominal draft angle ) per 0.025 mm texture depth is recommended for easy removal of parts

To sum up, Injection molding plastic part design guidelines are followed during plastic part design to ensure part quality and ease of manufacturing. It’s not feasible to follow all design guidelines. But wherever guidelines are not followed, It is good to reduce the risks by performing simulation studies.

We will keep updating injection molded plastic part design guidelines. If you need any support or have questions on plastic part design. Write us in the comment box.

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