Plastic Part

Plastic Part Design Guidelines for Injection Molding

Injection Molding process is best way to manufacture plastic parts for high volumes and low cost. Plastic part design guidelines for injection molding ensures good quality products. 

Injection molding has various advantages compared to other plastic manufacturing process. You can check advantages of injection molding here. In this article we will discuss plastic part design guidelines for injection molding.

Why to Follow Plastic Part Design Guidelines for Injection Molding?

Several factors affect the quality and consistency of molded parts. Even following all of plastic part design guidelines for injection molding does not guarantee best quality. Overall part quality depends on various factors such as mold design, material selection, molding conditions e.t.c But following plastic part design guidelines adds value.

 Although it is not possible to follow all design guidelines. But it is recommended to follow as much as possible to achieve good quality and defects free part.

Uniform Wall Thickness

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

Effects of non-uniform wall thickness ?

After molten plastic is injected, thin section starts cooling first. But when thick section starts cooling. It shrinks and builds stresses near the boundary area between thin and thick sections.

As the thin section has already hardened, it doesn’t yield. Therefore when the thick section yields, it leads to warping or twisting of the part. In some cases it can also cause cracks in parts.

Design Guidelines

In design, it’s not feasible to provide uniform wall thickness everywhere. Following points can help in reducing the effect of non-uniform wall thickness.

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

Boss Design Guidelines

Boss feature is used as a point of attachment and assembly in plastic parts. It consist of a cylindrical projection with holes. Boss designed to receive screws or threaded inserts. 

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

Boss Wall Thickness 

Boss wall thicknesses shall be less than 60 percent of the nominal wall thickness.

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 imposed by self-tapping screws. But this will have bad aesthetics impact.

Radius at Boss Base

Generous radius at the boss base increases boss strength and helps for easy part removal after injection molding.

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

Minimum center to center Distance Between Bosses

Close bosses results in thin areas in between two bosses. Thin area is very difficult to cool and results in injection molding defects. Therefore product quality gets affected.

Thin sections are also very difficult to manufacture and result in tool wear. 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 are provided for the easy removal of part from injection mold.

Minimum Draft Angle in Outer ID = 0.5 degree

Minimum Draft Angle in inner ID = 0.25 degree

Other boss Design Notes
  • 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, As shown above, recess around the base of the boss can be added to reduce the chances of shrinkage

Rib Design Guidelines

Before discussing rib design guidelines. We try to find out why ribs are used?

  • To increase bending stiffness of a part without increasing thickness.
  • Ribs increase the moment of inertia. which increases the bending stiffness.
Bending Stiffness = E (young’s Modulus) x I (Moment of Inertia)
Rib Thickness Recommendation

Recommended rib thickness is 0.5 to 0.75 times of the nominal wall thickness to avoid shrinkage in a part.

Rib Thickness (W) = 0.5 to 0.75 X T

Recommended distance between Two Ribs

To avoid thin section in mould. 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 ensures easy removal of parts from mold. Value of draft angle depends of draft height. Mostly half degree of draft angle on each side of rib is enough for the easy removal of part from mold

Recommended Rib Height

Maximum recommended rib height is less than three times of nominal wall thickness. This is done to avoid large variation in rib thickness. It is recommended to use multiple ribs instead of one very tall rib to increase bending stiffness .

Max. Rib Height (H) < 3 X T

    Sharp Corner Radius

    Sharp corners at rib base results in stress concentration. A minimum generous radius (0.25XT) is recommended to avoid stress concentration after injection molding.

    Rib Intersection

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

    Rib Orientation

    A rib is oriented to provide maximum bending stiffness to the part. Rib orientation depends on part geometry and bending load.

    Sharp Corners in Plastic Parts

    Sharp corners can lead to stresses, limit material flow, and often reduce part strength that can lead to part failure. Therefore it is recommended to add Radii to prevent sharp corners.

    Corner Radius

    Suggested inside radius at corners is 0.5 times the nominal wall thickness and the outside radius is 1.5 times the material thickness. A bigger radius is recommended if part design allows.

    Stress Concentration Factor

    Stress concentration factor at corners depends on radius and nominal thickness. Value of stress concentration factor is high if value of R/T < 0.5. It is recommended to keep R/T values more than 0.5 .

    Fillet Radius

    Fillet radius provides a streamlined flow path for the molten plastic, resulting in an easier fill of the mold.

    Gussets In Plastic Parts

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

    Draft Angle in Injection Molded Plastic Parts

    Before discussing draft angle recommendations we will discuss why draft angle is required?

    Draft Angles are provided (parallel to the direction of part release) to facilitate part removal from the mold. 

    Higher the value of draft angle. Easy will be part removal from injection mold. Industrial designer will always ask for zero draft but mold designers need max possible draft angle.

    Factors Affecting Draft Angle Value

    Following factors affect the value of draft angle:

    • Part Depth
    • Size
    • Mold Finish
    • Plastic Resin
    • Part Geometry
    • Mold Ejection System.
    Draft angle for Textures

    Draft for texturing is dependent on the part design and specific texture desired. As a general guideline, 1.5° min. per 0.025 mm  depth of texture plus normal draft need to be provided for easy removal of parts.

    Other Recommendations
    • Polished mold surfaces require less draft. Parts with many cores may need a higher draft angle.
    • Parts with small ejector-pin contact area need extra draft to prevent distortion during ejection.

    Conclusion

    To sum up, to achieve to quality and durable injection molded plastic products, Injection molding guidelines are followed. Its not feasible to follow all guidelines, But wherever guidelines are not followed its effect has to be brainstormed.

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