What Is Design for Manufacturing? Part 1 of 4

Design for Manufacturing (DFM) is the process of designing parts, components or products for ease of manufacturing to make a better product at a lower cost. This is done by simplifying, optimizing and refining the product design. The acronym DFMA (Design for Manufacturing and Assembly) is sometimes used interchangeably with DFM.

The five principles examined during a DFM are process (in this case, injection

molding), design, material, environment and compliance/testing. Ideally, DFM must occur early in the design process, well before tooling begins. Properly executed DFM also needs to include the stakeholders — the engineers, designers, contract manufacturer, mold builder and material supplier. This “cross-functional” DFM intends to challenge the design — to look at the design at all levels — component, subsystem, system and holistic levels — to ensure the design is optimized and does not have unnecessary cost embedded in it.

Chart 1 is a visual representation of the effect of an early DFM. As the design progresses through the product life cycle, changes become more expensive and more difficult to implement. Early DFM enables design changes to be executed quickly, at the least expensive location.

Pulling stakeholders together early in the design process is easier if you’re developing a new product. However, even if you’re dealing with an established product, challenging the original design is a necessary element of a thorough DFM. Too often, mistakes in a design are repeated by replicating a previous design.

Key Elements for a Well-Executed DFM Review

Review every aspect of your design by examining the original drawings, disassembling the product and analyzing competitive and similar products. Look into how lead users, such as those in the medical and automotive fields, approach similar challenges. Determine if someone else has addressed this problem in a different way and consider whether there is an opportunity to improve your solution.

Here are the key focus areas to help guide the team during the DFM review.

Considerations

Design

• Maintain a constant wall thickness to ensure uniform and efficient part cooling.
• Provide appropriate draft angles; typically, a draft of 1 to 2 degrees is acceptable.
• Apply a draft of 1 degree for every 0.001" of texture depth on textured side walls.
• As a general rule, ribs should be approximately 60% of the nominal wall thickness.
• Ensure smooth transitions between thick and thin features.
• Avoid excessively thin wall thickness, as this can increase injection pressure.
• Eliminate undercuts or features that require side action. All features should align with the mold opening whenever possible.
• Specify the loosest tolerances that will still produce a quality product. It’s important to review the design with the manufacturer to ensure it adheres to sound manufacturing principles.

An effective DFM process streamlines design while meeting cost, material, specification, and scheduling requirements. The result? A manufacturable design ready for production.

Material

• Mechanical Properties: What is the required strength of the material?
• Optical Properties: Should the material be reflective or transparent?
• Thermal Properties: How heat-resistant does the material need to be?
• Color: What color should the part be?
• Electrical Properties: Does the material need to function as a dielectric, meaning it acts as an insulator rather than a conductor?
• Flammability: How flame- or burn-resistant must the material be?

Environmental

• Your part or product must be designed to withstand the environment it will encounter.
• All the form in the world won’t matter if the part cannot function properly under its normal operating conditions.

Compliance/Testing

• All products must meet safety and quality standards.
• These may include industry standards, third-party standards and internal company-specific standards.

Questions

Which direction will the tool pull?

Here’s how the process works: A tool, or mold, consists of two halves. Hot plastic resin is injected into the mold and then quickly cooled. After cooling, the two halves are pulled apart to reveal the finished part. If any feature of your part moves in a direction that is different from the pull of the mold, it can complicate the tooling process, resulting in higher costs for the tool.

Are there any undercuts or features that might become trapped?

Undercuts are protrusions or recesses in a design that can prevent the mold from easily separating from the part. These features can become caught in the tool, potentially causing damage. If an undercut is essential to the design, a slide can be used to facilitate the mold's release. However, this option will increase the cost of the tooling. It is often better to eliminate the undercut by modifying the design instead.

How consistent are the wall thicknesses?

It’s been said that about 70% of the manufacturing costs of a product — materials, processing and assembly — are determined by design decisions. If that’s the case, then you want to make sure you are adhering to the best design practices possible.

The thicker areas of plastic parts are designed for added strength. However, thickness also influences the cooling time of the part — the longer the cooling time, the greater the risk of sink marks, which create weak spots in the part. Also, longer cycle times increase production costs because more press time is required to mold the part.

To address these issues, engineers will often reduce the thickness in certain areas and reinforce them with ribs. That said, walls that are too thin can easily break, which is not desirable either. Typically, wall thicknesses range from 3 to 5 mm. Engineers also pay attention to the transitions between thin and thick walls, ensuring these transitions are gradual to maintain structural integrity.

Does the design require draft angles?

Straight sides or walls can cause parts to stick to the mold, making removal difficult. Draft angles are slight tapers on the walls or sides of the mold that help with the proper ejection of the part. The larger the draft angle, the easier it is to remove the part from the mold.

Additional questions to assess mold design:

• Where will this part be gated, and would it be beneficial to use simulation for this assessment? 
• What type of gate will be used for this part? 
• How will this part be ejected from the mold? 
• Which half of the mold will the part remain on when the mold is opened? 
• Does the part design result in "thin" or "weak" steel conditions within the mold?

Risk Assessment

Any skilled engineer understands the importance of examining the tolerances specified in the part’s drawings. Tolerance refers to the allowable variation in a specific dimension, and it’s common for manufacturers to receive drawings from customers that contain unreasonably tight tolerances, which can complicate the request for quotation (RFQ) process. To lower tooling costs, facilitate manufacturing, reduce defects and increase yields, it’s advisable to loosen tolerances.

Chart 2 shows the drop in yield and the rise in cost as the tolerance increases. The bell curve shows measurements on a particular dimension, including the Upper Spec Limit (USL) and Lower Spec Limit (LSL), which is based on the tolerances. The tighter the tolerance, the narrower the bell curve must be for the dimensions to be in spec.

This chart highlights the trade-off between tolerance, cost and yield. As tolerances tighten (higher Cpk values), manufacturing costs rise while yield improves. Finding the right balance is key to optimizing production efficiency and profitability. Source | Virtual.Mold.Design

All manufacturing processes have limits, defined by the Upper Specification Limit (USL) and the Lower Specification Limit (LSL). If you are unsure about these limits, consult with your contract manufacturer or the relevant trade organization. There is extensive data available on common manufacturing processes that can help guide you in setting reasonable specifications.

Tolerances should be determined by the manufacturing process, the materials used and the sensitivity of the features to variations. Effectively managing tolerances is a crucial aspect of good DFM. It’s important to justify the tolerance values indicated on the drawings.

An effective DFM process ideally simplifies the design while meeting the customer’s requirements for cost, specifications, materials and scheduling. In other words, the design is considered manufacturable and ready to advance to the next stage of production.

Part 2 of this series in MoldMaking Technology’s August issue will address flow simulation as an essential tool for understanding the behavior of molten plastic within the mold cavity and how proper simulations can guide decisions on gating, runner layouts and cooling designs.