Injection molding design: ten keys to success (II)

The first part of this article detailed five of the ten main design-related parameters that affect the quality of injection molded parts: material selection and consequences, critical tolerance, dent marks, steel safety zone, and gate location. The last five parameters are explored here.

6. Closing Angle

Most people are familiar with the terms "closing angle" and "bypass". These terms refer to the minimum angle between the core and the cavity, which usually creates an opening in the part that would otherwise require a sliding piece or cam. By designing bypass features in the mold, features such as round holes, spring locks, or large rectangular openings can often be molded into the walls perpendicular to the drawing line.

This complex chassis design has many features that may require multiple side actions in the mold, thus increasing cost, maintenance, and cycle time. However, by utilizing a bypass the part is molded in a simple two-piece mold. The modeler has validated the overall concept and proposed parting lines before the design is completed to avoid major designs.

All molders want as many angles as possible between the core and cavity, while designers usually want no or minimal angles in these features. In most cases, the compromise is usually between 3° and 5° minimum. The benefits of discussing these details with the molder or tool manufacturer should not be overlooked. Wasting your time detailing part features in CAD takes a lot of time with lengthy feature trees that are difficult to edit once the part is fully detailed. Some molders will accept a minimum 3° angle, while others may require at least 8° to 10°. Tool life, tool quality, mold steel specifications, and molding material will all affect these details.

7. Draft angle orientation

When we begin to describe a concept in detail and translate it into production injection molded parts, the draft angle must be added to all surfaces. In most cases, the direction of the draft is obvious. However, there exist situations where the draft can be oriented toward the core or cavity. These decisions can affect parting lines, tool design, fit between parts, and cost. In some cases, the location of the parting line may unnecessarily complicate the mold and increase machining costs. Reviewing these details with the molder during development will ensure that the design is optimized for minimal cost and optimal performance when transferred to the molder for production.

8. Textures and Drafts

Experienced designers and engineers familiar with injection molding are well aware of the impact of surface finish on draft angles. A high gloss smooth surface can be ejected from the mold more easily than a rough or textured surface. During the detailing process of a production part, designers must minimize the draft angle or specify texture on the outer surface. For example, core pins and bosses may require 1/2° or less draft to eliminate potential dents. Core pins with a minimum draft should be polished to allow for part ejection. This is also true for ribs or other features that are typically inside the part.

Parts are often designed with features that can be created from the core or cavity side of the mold. This opening (highlighted in blue) can be drafted from either side of the part, affecting tool design and possibly cost. It is recommended to use a molder to verify such features in order to optimize the manufactured part.

On the outer surface, a specific texture is usually etched into the steel to a certain depth. Sometimes a depth texture is specified for the desired effect. Typically, for each mil of texture depth, the outer surface should include a 1° draw, in addition to a 1° starting draw angle. While this basic rule seems simple enough, there may be cases where the texture may have to bleed out on a surface where the draft does not meet these requirements. It is recommended that these requirements be discussed with the molder to ensure that the part meets the aesthetic and functional requirements of the design.

9. Scheduling of critical start-up phases

An important part of the design process includes scheduling key milestones during each development phase. Each project requires that design activities be synchronized with the business plan associated with the product. These activities include trade shows, clinical trials and regulatory compliance, and the final product launch. Close communication with the molder is an important activity to ensure that project stakeholders are able to achieve their goals. Key project milestones directly related to the molder include ordering steel, tool design, machining molds, texturing tools, sample molding, designing and building fixtures, establishing quality standards, and optimizing production parameters. These critical tasks must be planned and coordinated for overall project goals to avoid costly tooling revisions or production delays. Full integration of these activities with your molder is an essential part of overall product development and manufacturing design.

10. Secondary operations and fixtures

Secondary operations and fixtures are often overlooked in budgets or project plans until the final moments of production start-up. Secondary operations such as pad printing, labeling, painting, machining, and adding inserts can have some impact on the design. Certain secondary operations, such as ultrasonic insertion, ultrasonic gluing, and machining, often increase capital expenditures.