Injection Molding: 10 Key Considerations for Designing High-Quality Molded Parts (I)

There are thousands of designers who design injection molded parts, but there is an elite group of people in this large community who can design parts for injection molders. Injection-molded product design evolves through multiple development stages before all parts are finally documented and released to the molder for production. The final step of the development process is the most critical because no further design changes or corrections can be made without significantly increasing costs or project delays.
 

 
Unfortunately, plastic part design errors are only discovered after the project team has examined and evaluated the first article part. Even with today's sophisticated mold flow simulations, 3DCAD interference checks, rapid prototyping, and numerous other development tools, no one can predict every potential problem with an injection molded part. However, there is a very simple, low-cost way to minimize potential problems and ensure a perfect part. It's called working with your molder, and that's the focus of this article.
 
It doesn't matter how you think you designed your injection molded part correctly - you should establish a close relationship with your preferred molder as early in the design process as possible. Each molder has its own tooling preferences and molded part technologies that can have a significant impact on part design. These subjective preferences can influence any of the following key design-related parameters that affect injection molded parts.
 
1. material selection and consequences
2. tight tolerances
3. sink marks
4. Steel safety zones
5. Door position
6. Closing angle
7. Draw angle direction
8. Textures and drafts
9. Scheduling critical start-up phases
10. Secondary operations and fixtures
 
It is often difficult for designers/engineers to develop this relationship early in the design process, as mold selection is often delayed until the design is complete and a formal quote is released by the purchasing department. In addition, many molders will not provide any input until they are assured that the project will be awarded to them. This stalemate prevents designers from following these recommendations, often resulting in unacceptable delays or cost overruns due to tooling complexity or long cycle times. In the long run, these policies are not cost-effective because they significantly reduce the efficiency of product development. However, there are simple solutions that can resolve this paradox.
 
The first solution typically used by large companies is to generate a shortlist of preferred suppliers based on extensive analysis by internal experts on staff. Because of their mutually beneficial business arrangements, engineers typically have access to this limited group of three or four preferred molders and toolmakers throughout the development process. Smaller companies can select one or two viable molders early in the process by establishing an honest business relationship. This informal handshake agreement requires both parties to be in agreement on the estimated costs and terms of eventual business with each other. While there are no guarantees, alliances can be developed as molders and designers share their knowledge throughout the design development process.
 
It should be noted that designing quality injection molded parts requires the designer to understand all the basic design parameters associated with injection molding and to be a skilled person in the field. The molder/designer partnership is not an internship program-it should optimize the final design-to-production switch with little or no variation. If successfully completed, the final production part is usually cost-effectively molded to specification for the following reasons
 

1. Material selection and consequences

Materials are usually specified early in the design process and should be mutually agreed upon. Sometimes, the molder can purchase a large quantity of a specific resin at a significant discount. These discounts can be passed on to the customer. For example, if the designer can specify an ABS grade to match the ABS grade purchased in bulk by the molder, tens of thousands of dollars can be saved. Designers may find that certain high-performance resins may not be suitable for the molder due to viscosity, high glass content, or crystallinity. Resins may be selected for specific physical or chemical resistance properties but may be very difficult to mold or hold to specified tolerances. Molders should be consistent with the specified resin and overall part requirements as they need to actually mold the part.
 

2. Tight tolerances

While designers should always provide wide tolerances when possible, many times tight tolerances must be maintained to ensure fit, function, or appearance. These images illustrate the design details of a group of injection molded parts that need to meet reasonable but tight tolerances for aesthetic and functional requirements. The molder is included in the design review to insert his comments and commitments to maintain the specifications.
 

 
One of the biggest challenges facing any designer designing injection molded parts is to provide sufficient clearance in the design to accommodate tolerance variations. Tolerance variation depends on several variables, including material, process control, and tool design. The range of acceptable tolerances in a design varies from molder to molder. The designer must discuss reasonable critical tolerance specifications with the molder and consider possible mold modification options if necessary. This may require intentionally designing certain design features with additional clearance that can be tightened later by removing the steel from the mold. No one wants to add steel by welding to compensate for interference problems. The moldmaker may offer some recommendations for maintaining tight tolerance control, including post-machining
 

3. Sink marks

The experienced designer is always faced with the challenge of avoiding sink marks on injection molded parts. While the recommended maximum wall thickness at the bottom of a rib or tab should be less than 60% of the vertical face wall, some molders prefer 50% or less. It should be noted that this is a guideline and does not guarantee that the part will be acceptable to the QC department.

 
Avoiding dents on the cosmetic surface is always a challenge during the design development of injection molded parts. If ribs or tabs are added on the opposite side, molders are always reluctant to guarantee that the cosmetic surface will be free of any dents. The challenge is compounded when the ribs and tabs include a draft. This ribbed detail is a good example to illustrate this point. Working closely with your molder may lead to simple solutions such as minimizing draft depth, rib height or adding other features to eliminate gutters.
 
Cosmetic surface defects depend on gate location, tool quality, nominal wall thickness, material, additives, surface finish, color, and perspective. Production problems can be avoided by clearly establishing acceptable surface quality with the molder prior to making any of these decisions. A reputable molder will provide honest expectations and a backup plan before production begins. A molder may recommend eliminating all features inside the part, while others may recommend special coring techniques.
 

4. Steel Safety Zone

When we design injection molded parts, we are often faced with details that require tight tolerances, such as snap fits, alignment features, or interlocking parts. It is easy to perfectly align and match these features in CAD, but it is not easy to reproduce them during production. Details that molders cannot confidently replicate are often designed to be "steel safe". For the benefit of those unfamiliar with the term, steel safe means that the design features have enough clearance so that the toolmaker can easily machine the steel. The tooling is used to tighten the gap after the initial test of the molding. Most moldmakers prefer these precautions to avoid welding material back into the mold and then machining it.
 
Welding always affects the quality of the process, is expensive, and delays production start-up. Working closely with the molder or toolmaker early in the design process minimizes design revisions and allows you both to agree on the critical dimensions that should make the steel safe and the amount of clearance to be included in the design. Often, these collaborative, well-planned decisions have little to no impact on the tooling budget and minimal impact on the product release. Conversely, some molders expect parts to be designed exactly as expected and do not want to add clearance. This is why it is important to communicate closely with the molder you choose.
 

5. Gate Position 

Ideally, the gate location should be specified by the designer, molder, and toolmaker. Gate position is critical to almost every attribute of an injection molded part. It affects appearance, warpage, tolerances, surface finish, wall thickness, molding, and physical properties, just to name a few.
 
Some designers use mold flow simulation to determine gate design and location. If the molder agrees with their recommendations, I think that's fine. I disagree with designers who insist that their gate recommendations must remain uncompromised. In either case, working closely with the molder throughout the design cycle will ensure that the gate will not adversely affect part performance, appearance, or fit. The molder is also willing to advise the designer on the type of gate and features that may have to be added to the part geometry based on the gate design. The molder will also make tradeoffs between different types of gates, including fan gates, edge gates, or gate gates.