Injection molding design: The 10 keys to success, part two

In part one of this article, Michael Paloian detailed five of 10 major design-related parameters that affect the quality of an injection molded part: Material options and consequences; critical tolerances; sink marks; steel safe areas; and gate location. The final five parameters are explored here. 

6. Shut-off angles

Most readers will be familiar with the terms “shutoff angle” and “bypass.” These terms refer to the minimum angle between the core and cavity, which typically creates an opening in a part that would otherwise require a slide or cam. Features such as circular holes, snap locks or large rectangular openings can usually be molded in walls perpendicular to the line of draw by designing features for a bypass in the mold.

injection molded part injection molded part

This complex chassis was designed with many features that could have required multiple side actions in the mold, thereby increasing cost, maintenance and cycle time. However, the part was molded in a simple two-part mold by utilizing bypasses. The overall concept and proposed parting lines were verified with the molder before the design was finalized to avoid major redesigns. 

All molders want as much angle between the core and cavity as possible, whereas designers typically want no angle or minimal angle in these features. The compromise usually lies between a minimum of 3° to 5° in most cases. Benefits of discussing these details with a molder or tool maker cannot be over emphasized. Many hours will be saved before you waste your time detailing part features in CAD with lengthy feature trees that are difficult to edit after the part has been fully detailed. Some molders will accept a 3° minimum angle, while others may require a minimum of 8° to 10°. The longevity of the tool, tool quality, mold steel specifications and materials being molded all will affect these details. 

7. Draft angle orientation

When we begin detailing a concept and transforming it into a production injection molded part, draft angles must be added to all surfaces in line of draw. In most cases the draft orientation is obvious. However, there are instances where the draft can be oriented toward the core or cavity. These decisions affect parting lines, tool design, fits between parts and cost. There are instances where the location of the parting line could unnecessarily complicate the mold and increase tooling cost. Reviewing these details with a molder during the development process will ensure that the design has been optimized for minimal cost and optimal performance when it is transferred to the molder for production.

8. Texturing and draft

Experienced designers and engineers familiar with injection molding are well aware of the effect surface finish has on draft angles. High gloss smooth surfaces can be ejected from a mold much easier than a rough or textured surface. There are numerous instances during the detailing of production parts where designers must minimize draft angles or specify textures on exterior surfaces. For example, core pins and bosses may require a ½° draft or less to eliminate potential sink marks. Core pins with minimal draft should be polished for easy part ejection. The same is true for ribs or other features that are typically internal to a part.

Injection molding

Parts are often designed with features that could be created from the core or cavity side of the mold. This opening (highlighted in blue) could be drafted from either side of the part, affecting tool design and possibly cost. It’s advisable to verify features such as this with the molder to optimize the part for manufacturing.

On external surfaces, specific textures usually are etched into the steel to a certain depth. Deep textures are sometimes specified for a desired effect. Generally, exterior surfaces should include 1°draft for each mil of textured depth in addition to a starting draft angle of 1°. Although this basic rule appears straightforward, there are instances where the texture may have to bleed off on surfaces where the draft cannot comply with these requirements. It is advisable to discuss these requirements with a molder to ensure that the parts comply with the aesthetic and functional requirements of the design.

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