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

injection molded part
In the second installment of this two-part article, we explore five more parameters that design engineers should keep in mind when designing parts for injection molding, including shut-off angles and scheduling of critical start-up phases.

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.

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.

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.

9. Scheduling of critical start-up phases

A significant part of the design process includes scheduling of critical milestones throughout each phase of development. Every project requires design activities to be synchronized with business plans associated with the product. These events include trade shows, clinical trials and regulatory compliance, as well as final product release. Close communication with a molder is an essential activity to ensure the project stakeholder will be able to attain his or her objectives. Critical project milestones directly associated with a molder include ordering steel, tool design, machining molds, texturing tools, sample shots, designing and building fixtures, establishing quality standards and optimizing production parameters. These critical tasks must be planned and coordinated with overall project objectives to avoid costly tooling revisions or production delays. Fully integrating these activities with your molder is an essential part of overall product development and design for manufacturing. 

10. Secondary operations and fixtures

Secondary operations and fixtures are often omitted from the budget or project plans until the last moments of production startup. Secondary operations such as pad printing, labeling, painting, machining and adding inserts all will have some effect on design. Certain secondary operations such as ultrasonic insertion, ultrasonic bonding and machining often add to capital expenditures. Technical considerations pertaining to ultrasonic joints and tolerances should be discussed with molders to minimize problems during production. Secondary machining operations may require fixtures as well as affect part design. Good molding partners can point out these subtle details in advance, so when CAD files and documentation are released for production, everyone agrees on the final product and capital investment.

I hope this article has enlightened you on the benefits of partnering with your vendors early in the design process and closely collaborating with them until all final details have been defined in product documentation. Designing for injection molded plastic parts is by far the most challenging of any of the plastic manufacturing processes.

The benefits of a close partnership with your molder and tool maker throughout the design and development process cannot be over emphasized. Agreements between all parties on major design parameters create stronger bonds, build trust and eliminate surprises during the critical moment of production startup. Designs are typically improved and optimized for production. One of the major problems in most product launches is a surprise that no one expected. Honest, critical design for manufacturing analyses throughout the design process leads to a graceful transition to production launch with minimal changes and no unwanted surprises. Everyone can share in the glory of the successful product launch.

About the author

Michael Paloian is president of Integrated Design Systems Inc. (IDS), located in Oyster Bay, New York. He has an undergraduate degree in plastics engineering from UMass Lowell and a master's of industrial design from Rhode Island School of Design. Paloian has an in-depth knowledge of designing parts in numerous processes and materials, including plastics, metals and composites. Paloian holds more than 40 patents, was past chair of SPE RMD and PD3. He frequently speaks at SPE, SPI, ARM, MD&M and IDSA conferences. He has also written hundreds of design-related articles for many publications.

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