In injection molding process of plastic parts, draft angle is a key design element. It directly affects whether plastic part can be smoothly ejected from mold, as well as product quality and production efficiency.

 

Factors Affecting Draft Angle
Product Surface Characteristics and Precision Requirements: If plastic part has a smooth surface, high dimensional accuracy requirements, and low shrinkage, a smaller draft angle, such as (0.5°), should be selected.
Because of smooth surface and high precision requirements, a smaller draft angle better ensures product's appearance and dimensional accuracy, while low shrinkage allows for smooth demolding with a smaller draft angle.
For example, plastic shell of a precision optical instrument has a smooth surface and extremely high dimensional accuracy requirements, with relatively low shrinkage; a draft angle of (0.5°) can be selected.
Product Size: For taller and larger plastic parts, a smaller draft angle needs to be selected based on actual calculations.
For large-sized products, excessively large draft angles can affect the overall dimensional accuracy and appearance.
Through practical calculations, it's possible to minimize impact of draft angles on product dimensions while ensuring smooth demolding.
Plastic Shrinkage Rate: Shrinkage rate of plastic part significantly affects selection of draft angle.
Plastic materials with high shrinkage rates should use larger draft angle values.
This is because a high shrinkage rate means greater clamping force between product and mold surface during cooling. A larger draft angle helps overcome this clamping force, allowing for smooth demolding.
For example, some crystalline plastics have relatively high shrinkage rates, so draft angle should be appropriately increased compared to amorphous plastics.
Part Wall Thickness: Thicker plastic parts experience increased molding shrinkage, so a larger draft angle should be used.
Thick-walled plastic parts experience greater internal stress during cooling and shrinkage, making them prone to tightly adhering to mold surface. A larger draft angle can effectively prevent demolding difficulties caused by shrinkage.
For large plastic boxes with relatively thick walls, draft angle needs to be appropriately increased.
Plastic Material Types: Characteristics of different plastic materials determine different requirements for draft angles.
For transparent parts, due to material properties, draft angle should be increased to avoid scratches during demolding.
Generally, draft angle for PS material should be no less than (2.5° - 3°), draft angle for ABS and PC materials should be no less than (1.5° - 2°).
For example, common transparent PS plastic cups require a relatively large draft angle to ensure that surface is not scratched during demolding.
Appearance Treatment: For plastic parts with textured surfaces, sandblasting, etc., sidewalls should have a draft angle of (2° - 5°) depending on depth of texture.
The deeper texture, the larger draft angle should be.
Because textured or sandblasted finishes increase friction between plastic part and mold surface, deeper textures require a larger draft angle to ensure smooth demolding.
For example, for mobile phone casings with a textured finish, a deeper texture necessitates a draft angle close to 5°.
Product Structural Characteristics: When structure is designed for interlocking, draft angle of interlocking surface is generally 1° - 3°.
This draft angle setting ensures stability of interlocking structure during injection molding and facilitates subsequent demolding.
For reinforcing ribs and cavities, draft angle selection varies depending on height.
For reinforcing ribs less than 3mm high, draft angle is 0.5°; for ribs between 3 and 5mm high, it is 1°; and for others, it is 1.5°.
For cavities less than 3mm high, draft angle is 0.5°; for cavities between 3 and 5mm high, it is 1°; and for others, it is 1.5°.
This is to take into account impact of height of reinforcing ribs and cavity on demolding difficulty, and to reasonably set draft angle to ensure smooth demolding.
Determining Direction of Draft Angle
Inner Hole: Generally, inner hole is based on smaller end, conforming to drawing, and draft angle is obtained from direction of expansion.
This design ensures the dimensional accuracy of inner hole, and during demolding, as plastic part is ejected from mold, inner hole size gradually expands, facilitating smooth demolding.
Outer Shape: Outer shape is based on larger end, conforming to drawing, and draft angle is obtained from direction of contraction.
This allows outer shape of plastic part to gradually shrink during demolding, facilitating its ejection from mold cavity, while ensuring that outer dimensions meet design requirements.
Relationship between Draft Angle and Tolerance
Generally, draft angle is not included in tolerance range of plastic part.
This is because draft angle is mainly to meet demolding requirements, its size variation has a relatively small impact on key dimensional tolerances of plastic part's function and assembly, so it is considered separately and not confused with dimensional tolerances of plastic part.
Draft Angles for Special Parts
Outer Shell Surface: Draft angle for outer shell surface is greater than or equal to (3°). As directly visible part of product, a larger draft angle facilitates rapid demolding with minimal impact on shell's appearance, while ensuring the overall strength and stability of product.
Non-Outer Shell Features: Draft angle for all other shell features, except outer shell surface, is (1°) as standard.
Maintaining a relatively uniform standard draft angle, while ensuring smooth demolding, facilitates mold design and manufacturing, and improves production efficiency.
Determining draft angle for plastic parts requires comprehensive consideration of factors such as product's surface characteristics, dimensions, material shrinkage, wall thickness, surface treatment, and structural features.
A well-designed draft angle not only ensures smooth demolding and reduces risk of damage during demolding process but also guarantees dimensional accuracy and appearance quality, improving production efficiency.
Below are design principles for draft angles of plastic materials.

 

In actual design and production process, this knowledge needs to be applied flexibly according to specific product requirements, and draft angle design needs to be continuously optimized in order to achieve the best production results.