Value Range and Geometric Calculation of Injection Mold Draft Angle

Geometric Formula of Draft Angle

Geometric Relationship Between Draft Angle and Draft Height

The geometric relationship between draft angle and draft surface height is tan θ=X/H;

Among them, θ = draft angle; H = draft height; X = reduced wall thickness (or tilt deviation).

In theory, the larger the draft angle, the easier it is to demold, especially for draft surfaces that are higher (deeper) and have larger areas. This is because the clamping force on the core or cavity is greater, and a larger slope must be designed to facilitate demolding while maintaining the same ejection force of the injection molding machine.

However, according to the above geometric relationship, it can be seen that when H remains constant, the larger θ, the larger X.

Recommended Draft Angle for Different Product Surfaces and Features

Draft Angle for Appearance Surfaces

1. For the appearance surface, the larger the X, the more changes there will be in the appearance surface, even to the extent of deviating from the intention of the ID, and the deviation in appearance dimensions will be significant. Therefore, if the ID allows, the larger the appearance surface draft, the better. If the ID does not allow it:

• a. High gloss surface, with a draft angle of ≥ 1 ° to prevent the risk of scratching, and the maximum value should be taken if possible;

• b. Sun pattern surface, draft angle ≥ 3 ° (depending on the type and depth of the pattern surface, generally, for every 0.001mm depth, a draft angle of 1–1.5 ° is required, based on the draft degree data provided by the sun pattern company).

Draft Angle for Rib Planes

1. For the bone plane (rib surface), the larger X, the smaller the width C at the top of the rib, making injection molding more difficult. Therefore, the rib plane should be designed as shorter as possible, and the draft angle can be larger. If unavoidable, at least ensure that the value of X is generally ≥ 0.2, and the top width C is not less than 0.6.

Draft Angle for Screw Boss Columns

1. For screw columns, the inner hole of the screw column belongs to a type of structural feature with dimensional accuracy requirements. In order to ensure the tolerance range, the draft angle of this type of structural feature will be very small or even no draft angle. The mold structure here requires very small roughness or even polishing, and the nearby ejector pin needs to be designed reasonably.

The inner hole of the screw column can be ejected with a sleeve ejector, so there is no need for core pulling. If using a regular ejector pin, the draft angle should be designed, and the height of the screw column should not be too high. The angle value should be kept at (0.5 °–1.0 °), and the draft is based on half of the screw engagement depth L, as this ensures good screw engagement. Avoid the top diameter of the hole being too large and the bottom diameter being too small, which may cause the screw to loosen at the top of the hole and tighten at the bottom, thereby introducing high stress.

Draft Angle for Other Internal Surfaces

    For other internal surfaces, the draft angle is set at a median value of 1 °, and the specific value needs to be determined based on the height and roughness of the draft surface. At the same time, attention should be paid to the changes in wall thickness after draft to avoid forming defects.