Plastic Structural Design: Draft Angle

author: Universe plastic

2024-09-06

In plastic structure design, draft angle is a very important parameter in mold design, which refers to the inclination angle between the plastic part and the mold parting surface.

The main purpose of draft angle design is to reduce the friction between the plastic part and the mold and help the plastic part to come out of the mold smoothly. If there is not enough draft angle, the plastic part will easily get stuck when it is demoulded, causing surface scratches, damage or deformation.

Standard draft angle design size

1. Conventional conditions: The draft angle is generally designed to be 0.5-2°.

2. Larger plastic parts or deep cavity parts: It is recommended that the draft angle is >2°.

3. Textured surface: The minimum draft angle can be 0.5-1°.

4. Textured surface: common designs are 1.5-3° or larger. (To prevent the texture from being roughened or damaged, the deeper the texture, the larger the draft angle)

What are the factors that affect draft angle?

1. Wall thickness of plastic parts

Plastic parts with thicker walls will shrink more significantly when cooled, which may lead to increased resistance during demolding. Therefore, thicker wall thicknesses often require increased draft angles to ensure smooth demoulding.

2. Material of plastic parts

Thermoplastics often require larger draft angles because these materials tend to shrink as they cool, increasing contact forces with the mold surface. Thermosetting plastics do not shrink after molding, so the draft angle can be relatively small.

3. Draft direction

The internal structure of the inner surface plastic parts (such as holes, grooves, etc.) also needs to be set with a draft angle to prevent these inner cavity parts from being unable to be smoothly demoulded due to excessive friction after molding. The larger the draft angle of the outer surface, the easier it is to demould, but the appearance requirements need to be considered.

4. Texture and finish of plastic parts

Textures or special finishes on the surface of plastic parts usually increase friction and may cause surface damage during demoulding. In order to avoid this problem, the draft angle should be appropriately increased during design. For every 0.025mm of grain depth, the draft angle needs to be increased by approximately 1°.

5. Parting surface of plastic parts

When designing the parting surface, the symmetry of the draft angle also needs to be considered. If the structure of the plastic part is complex or there are multiple parting surfaces, a corresponding draft angle needs to be designed for each parting surface.

Plastic part problems caused by draft angle design and improvement suggestions

1. Difficulty in demolding

Usually due to insufficient draft angle or shrinkage of the plastic part in the mold. This can be solved by increasing the draft angle or optimizing the cooling process.

2. Surface scratches

Caused by insufficient draft angle or too deep surface texture, increasing the draft angle or optimizing the surface treatment process can reduce the damage.

3. Plastic parts deform during demolding

If the draft angle is too small, the plastic part will be subject to excessive stress during demoulding. The solution is to increase the draft angle or use anti-deformation materials.

Summarize

Reasonable draft angle design can ensure molding quality and reduce friction and wear during production. If there are specific application scenarios or material requirements, the draft angle can be adjusted accordingly.

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