Injection Molding is a very commonly used plastic manufacturing process. It is used to manufacture plastic parts in large volumes and at low cost. Plastic part design guidelines for injection molding ensures good quality manufactured parts.
Injection molding has various advantages compared to other plastic manufacturing processes. In this article we will discuss plastic part design guidelines for injection molding.
Why to Follow Plastic Part Design Guidelines for Injection Molding?
Following factors affect the quality and consistency of injection molded parts.
- Part Design
- Plastic Material
- Manufacturing Process Parameters
- Molding Machines
- Injection mold Design
Following plastic part design guidelines for injection molding does not guarantee best quality parts. Therefore it is always recommended to control the entire manufacturing process to get quality parts.
Although it is not possible to follow all design guidelines. But it is recommended to follow as much as possible to achieve 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 ?
When molten plastic is injected into injection mold with varying cross-section. The thin section starts cooling first. But when the thick section starts cooling. Part shrinks and stresses are buildup near thin and thick section boundary area.
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. High stresses can also cause cracks in parts.
In design, it’s not feasible to provide uniform wall thickness in all sections. Therefore following points are recommended to reduce the effect of non-uniform wall thickness.
- Gradual change in wall thickness.
- Avoid filling of molten plastic 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 for guiding and assembly of plastic parts. It consists of a cylindrical projection with holes. Bosses in plastic parts are designed to receive screws or threaded inserts.
During product life-cycle bosses are subjected to various loads. Therefore Boss design guidelines are followed 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.
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 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.
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 bosses results in thick section. Thick sections are very difficult to cool and results in injection molding defects. Therefore it is recommended to maintain minimum spacing between bosses.
Minimum Center to center distance between Bosses = D1 + 2T
D1 – Boss Max Diameter
T – Nominal wall thickness
Draft Angle for Boss feature
Draft angle in injection molded parts helps in easy removal of part from injection mold. Value of the draft angle depends on surface finish and plastic part height. For example, glossy finish requires small draft angle compared to matte finish.
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, As shown above, recess around the base of the boss can be added to reduce the chances of shrinkage
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 sections 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 injection mold.
Minimum Draft Angle in Outer ID = 0.5 degree
Minimum Draft Angle in inner ID = 0.25 degree
Recommended 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.
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.
Sharp Corners in Plastic Parts
Sharp corners can lead to stress, limit material flow, and often reduce part strength that can lead to part failure. Therefore it is recommended to add Radius to prevent sharp corners.
Recommended inside corner radius is more than 0.5 times the nominal wall thickness. Whereas recommended outside radius is inside radius plus nominal wall thickness.
Stress Concentration Factor
Stress concentration factor at corners depends on corner radius and nominal wall thickness.
Value of the stress concentration factor is high if ratio of the radius and thickness is greater than 0.5. Therefore it is recommended to keep R/T values more than 0.5
Gussets In Plastic Parts
Gussets are used in plastic parts to increase part strength in that section. 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
Draft Angles are provided parallel to the direction of part release. It helps in easy removal of 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.
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.
- Polished plastic parts require relatively small draft angle compared to matte finish parts.
To sum up, Injection molding design guidelines are followed to ensure part quality. It’s not feasible to follow all design guidelines.
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