Injection molding design: 10 critical quality design considerations

Michael Paloian | Aug 04, 2019

There are thousands of designers who design injection molded parts but there is an elite group within this large community who can actually design parts for injection molders. Injection molded product design evolves through many phases of development before all the parts are ultimately documented and released to a molder for production. This last step in the development process is the most critical, since design changes or corrections can no longer be made without significantly adding cost or project delays.Unfortunately, plastic part design mistakes will be uncovered only after first article parts are inspected and evaluated by the project team. Even with today's sophisticated mold flow simulation, 3D CAD interference checks, rapid prototyping and numerous other development tools, it is impossible for anyone to predict every potential problem for an injection molded part. However, there is a very simple, low-cost method for minimizing potential problems and virtually ensuring perfect parts. It's called partnering with your molder, which is the focus of this article.

It doesn't matter how well you think you know how to properly design parts for injection molding—you should always form a close partnership with your preferred molder as early in the design process as possible. Every molder has his or her own tooling preferences and techniques for molding parts, which can have a significant effect on part design. These subjective preferences can influence any of the following major design-related parameters affecting an injection molded part:

It's often difficult for designers/engineers to develop this relationship early in the design process, since the selection of a molder is often postponed until the design is completed and released for formal quoting by the purchasing department. In addition, many molders will not provide any input until they are assured the project will be awarded to them. This stalemate precludes designers from following these recommendations, often resulting in unacceptable delays or cost overruns because of tooling complexity or long cycle times. These policies are not cost effective in the long run, since they significantly reduce the efficiency of developing a product. However, there are some simple solutions for solving this paradox.

The first solution typically used by larger companies is to generate a short list of preferred vendors based on an extensive analysis of experts within their staff. This limited group of three to four preferred molders and tool makers are typically accessible to engineers throughout the development because of their mutually beneficial business arrangements. Smaller companies can select one or two viable molders early in the process by establishing a good-faith business relationship. This informal handshake agreement requires both parties to be mutually honest about the estimated costs and terms of ultimately doing business with one another. Although there are no guarantees, an alliance could be developed as molders and designers share their knowledge throughout the design development process.

It should be noted that designing a quality injection molded part requires a designer to be knowledgeable about all the fundamental design parameters associated with injection molding and to be highly skilled in the art. The molder/designer partnership is not intended to be an internship program—it is supposed to optimize handoff of the final design to production with few or no changes. If completed successfully, final production parts typically are cost effectively molded precisely to specifications for the following reasons.

1. Material options and consequences

Materials are often specified early in the design process and should be mutually agreed upon by both parties. Sometimes molders may purchase large quantities of specific resins at major discounts. These discounts can be passed on to customers. For example, if a designer can specify an ABS grade that matches the properties of one purchased in large quantities by a molder, many tens of thousands of dollars can be saved. A designer may discover certain high-performance resins may not be ideally suited for a molder due to viscosity, high glass content or crystallinity. A resin may be chosen for specific physical or chemical-resistance properties but may be very difficult to mold or maintain specified tolerances. Molders should be in agreement with specified resins and overall part requirements, since they will be required to actually mold the parts.

2. Critical tolerances

Although designers should always provide generous tolerances whenever possible, there are many times tight tolerances must be maintained for fit, function or appearance. These images illustrate design details in a set of injection molded parts that were required to comply with reasonable, but tight-fitting, tolerances to attain cosmetic and functional requirements. The molder was included in the design reviews to interject his comments and commitment to maintain the specifications.

One of the greatest challenges for any designer faced with designing an injection molded part is providing enough clearance in the design for tolerance variation. Tolerance variation depends upon several variables, including materials, process control and tool design. Acceptable tolerance ranges in a design will vary greatly from one molder to another. It's imperative that designers discuss reasonable critical tolerance specifications with a molder and consider options for possible mold revisions, if required. This may require certain design features to be intentionally designed with extra clearance, which will later be tightened by removing steel from the mold. No one wants to add steel with welding to remedy interference problems. Molders may offer a number of suggestions for maintaining tight tolerance control, including post machining, fixturing and gate locations.

3. Sink marks

Experienced designers are always faced with the challenge of avoiding sink marks in injection molded parts. Although the recommended maximum wall thickness at the base of a rib or boss should be less than 60% that of the perpendicular face wall, some molders prefer 50% or less. It should be noted that this is a guideline and not a guarantee that the part will be acceptable to the QC department.

Avoiding sink marks on cosmetic surfaces is always a challenge during the design development of injection molded parts. Molders are always reluctant to guarantee a cosmetic surface will be devoid of any sink marks if ribs or bosses are added to the opposite side. The challenge is compounded when the ribs and bosses include draft. This ribbing detail is a great example to illustrate this point. Close collaboration with your molder might lead to simple solutions like minimizing draft, rib heights or adding other features to eliminate sinks.

Cosmetic surface imperfections are dependent on gate location, tool quality, nominal wall thickness, material, additives, surface finish, color and viewing angle. Production problems can be avoided by clearly establishing acceptable surface quality with the molder well before any of these decisions are made. Reputable molders will provide honest expectations and backup plans well before production starts. Molders may suggest eliminating all features on the inside of a part, while others may suggest special coring techniques.

4. Steel safe areas

When we are designing injection molded parts, we’re often faced with details requiring tight tolerances such as snap fits, alignment features or interlocking parts. It's easy to perfectly align and match these features in CAD, but it's not that easy to repeatedly produce them during production. Details that cannot be confidently reproduced by a molder are often designed "steel safe." For the benefit of those not familiar with the term, steel safe means the design feature is detailed with enough clearance to allow a tool maker to easily machine away steel in the mold to tighten up the clearances after initial test shots are molded. Most molders prefer these precautionary measures to avoid welding material back into the mold, which is then later machined.

Welding always compromises tooling quality, is expensive and delays production startup. Close collaboration with a molder or tool maker early in the design process will minimize revisions in your design, enabling both of you to agree on critical dimensions that should be made steel safe and on the amount of clearance to include in the design. Typically, these cooperative, well-planned decisions add little or nothing to the tooling budget and have a minimal effect on production launch. Conversely, some molders want parts designed exactly as expected and don't want added clearance. That's why close communication with your selected molder is important.

5. Gate location

Gate location ideally should be specified by a designer, molder and tool maker. Gate location is critical to virtually every attribute of an injection molded part. It affects appearance, warpage, tolerances, surface finish, wall thickness, molded in stresses and physical properties, to name a few.

Some designers use mold flow simulations to dictate gate design and location. I think that's great if the molder agrees with their recommendations. I disagree with designers who insist that their gate recommendations must be maintained without compromise. In either case, close collaboration with a molder throughout the design cycle will ensure that the gate will not adversely affect part performance, appearance or fit. Molders are also willing to advise designers about the type of gate and features that may have to be added to the part geometry based on gate design. Molders also will offer trade-offs between different types of gates, including fan gates, edge gates or sprue gates.

Read part two of this article here.

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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