1. Material choosing

Materials are often specified in the early design process and should be mutually agreed by both parties. The 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.

2. Sink marks

Designers are always faced with the challenge of avoiding sink marks in 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 part designers 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. Cosmetic surface imperfections are dependent on tool quality, gate location, 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. The best solution is that provide honest expectations and backup plans well before production starts.

3. Important tolerances

Although designers should always provide generous tolerances whenever possible, there are many times tight tolerances must be maintained for fit, function or appearance. 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.

4. 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. Some designers use mold flow simulations to dictate gate design and location. That’s great if the molder agrees with their recommendations. The designers shouldn't 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.

5. Steel safe areas

When designing injection parts, usually need to face 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 design, enabling both parties 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.

6. Shut-off angles

Shutoff angle and bypass 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. 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 wasting 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 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

The 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. And 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.

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 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.