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,
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:
Material options and consequences / Critical tolerances / Sink marks /Steel safe areas / Gate location / Shut-off angles / Draft angle orientation / Texturing and draft / Scheduling of critical start-up phases / Secondary operations and fixtures
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.
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 considerationspertaining 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