Plastic Part Design Guidelines – SMLease Design

What is injection molding?

Injection molding is a process in which solid thermoplastic resin pellets are melted, injected into a mold, and then cooled back to a solid state in a new form.

During injection and cooling stages of molding process, several factors that may affect the quality & consistency in manufactured part. Although it is not possible to follow all design guidelines, but we always try to hit the bull eye quality & defects free part.

Before moving further we need to understand this is just a guide, even following all of design rules will return best quality. Overall part quality depends on lot of factors e.g. mold design, material selection, molding conditions e.t.c

Uniform Wall Thickness

Part cost and molding cycle time will be reduced if part have a minimum possible wall thickness, as long as that thickness is consistent with the part’s function and meets all mold filling considerations.

How Uniform wall thickness helps in achieving good product quality?

Parts with uniform wall thickness allow the mold cavity to fill more easily since the molten plastic does not have to be forced through varying restrictions as it fills.

What will happen if wall thickness is not uniform?

If the wall thickness is not uniform, thin section cools 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. But when the thick section yields, it leads to warping or twisting of the part, which, if severe enough, can cause cracks.

Points to Consider

In real world for product designers it’s not feasible to provide uniform wall thickness everywhere. Following points need to be considered for a good product design from uniform wall thickness prospective.

  • Change in wall thickness should be gradual.
  • Avoid wall thickness variations that result in filling from thin to thick sections.
  • Plastic need to be removed from the thick area (coring), which helps to keep wall sections uniform, eliminating the problem altogether.
  • Flow of molten plastic need to be considered in thinner sections.

Boss Design Guide in Plastic Parts

Boss feature is used in plastic parts as a point of attachment and assembly. It consist of a cylindrical projection with holes designed to receive screws, threaded inserts. During product lifecycle bosses are subjected to loadings not encountered in other sections of a component.

Boss Wall Thickness 

Wall thicknesses for bosses should be less than 60 percent of the nominal wall thickness 

A= 0.6 X T

A – recommended boss wall thickness

T – Plastic part wall thickness

Note: If the boss is not in a visible area, then the wall thickness can be increased to allow for increased stresses imposed by self-tapping screws.

Radius at Boss Base

Generous radius at the base of the boss is provided for strength and ample draft for easy part removal from the mold.

R = 0.25 to 0.5 X nominal wall thickness

R – Radius at the base of boss feature

Minimum Distance Between Bosses

Very Close bosses results in creating thin areas in between two bosses which are hard to cool that can affect the quality of the product.

Thin sections are very difficult to manufacture and result in tool wear. So it is recommended to maintain minimum spacing between bosses

Minimum Centre to centre distance between Bosses= D1+2T

D1 – Boss Max Diameter

Draft Angle for boss feature

Draft angle are provided for the easy removal of part from mould

Minimum Draft Angle in Outer ID = 0.5 degree

Minimum Draft Angle in inner ID = 0.25 degree

Notes

  • Chamfer on top of the hole is provided for the good lead in of fasterners.
  • Bosses are strengthened by providing gussets at the base or by using connecting ribs attaching to nearby walls.

If the boss-wall thickness is more than recommended, Recess around the base of the boss can be added to reduce the chances of shrinkage

Rib Design Guide in Plastic Parts

First question that comes to mind:

Why Ribs are Used?

  • Ribs are used to increase the bendi ng stiffness of a part without adding 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 should be 0.5 to 0.75 times of the nominal wall thickness to avoid any shrinkage.

W= 0.5 to 0.75 X T

 

 

Recommended distance between Two Ribs

To avoid thin section in mould, distance between two ribs should be two times of nominal wall thickness

X > 2 X T

 

Draft Angle in Ribs

Draft angle of half degree is provided on each side for the easy removal of part from mold

 

Recommended Rib Height

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

H < 3 X T

 

Sharp Corner Radius

Radius Sharp corners at the base of ribs can result in stress concentration. A generous radius (0.25 X T) need to be provided to avoid sharp corners.

Rib Intersection

Coring out rib intersection is a good option to avoid excessive sinking on the opposite side of the rib.

Rib Orientation

A rib can be oriented in such a way as to provide maximum bending stiffness to the part. By paying attention to part geometry, designers must consider the orientation of the rib to the bending load otherwise there will be no increase in stiffness.

Sharp Corners

Radii should be added to prevent sharp corners as corners can lead to stresses, limit material flow, and often reduce part strength that can lead to part failure.

Corner Radius

At corners, the suggested inside radius is 0.5 times the material thickness and the outside radius is 1.5 times the material thickness. A bigger radius should be used if part design allows.

 

Stress Concentration Factor

Stress concentration factor at sharp corners varies with radius for a given thickness. Value of stress concentration factor is high for R/T < 0.5, but for R/T values over 0.5 the concentration lowers. So minimum 0.5XT corner radius need to be provided.

 

Fillet Radius

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

Corner Radius and mould Manufacturing

This is not always required to add radius at all sharp corners. Some-times adding radius make it difficult for the mold designers. Mold manufacturability also needs to be considered

Gussets

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

Draft Angle Design Guide

Why Draft Angle is Required

To facilitate part removal from the mold, Draft Angles are provided on product features such as walls, ribs, posts, and bosses parallel to the direction of release from the mold.

Ideally higher the value of draft it will be easy for removal. Industrial designer will always ask for zero draft but mold designers need max possible draft angle

Factors Affecting the value of Draft Angle

  • Value of draft angle for a given part depends on the depth, size mold finish, resin, part geometry, and mold ejection system.
  • Polished mold surfaces require less draft. Parts with many cores may need a higher amount of draft.
  • Parts with small ejector-pin contact area need extra draft to prevent distortion during ejection.
  • The depth of the texture or letters is somewhat limited, and extra draft needs to be provided to allow for part removal from the mold without dragging or marring the part.
  • 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.
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