Most will agree that the process of mechanical design review that exists in organizations today is far from being fully efficient. The lack of a formal set of specific requirements at each stage of design often results in time consuming and chaotic reviews, which in turn leads to fewer actionable suggestions to be incorporated by the designer. Moreover, scenarios where the review process is hijacked with an intent of discussing trivial details (which otherwise can be discussed off line) is considered usual. Adding to the list of inefficiency, resource simulation tools for melt flow simulation and structural analysis are often employed on fundamentally flawed or premature designs, resulting in wastage of precious resources.
A planned approach and a well thought-through set of deliverables at each stage can ensure a focused, efficient and fast review process, making it a true Poka Yoke tool which would prevent design deficiencies from being carried to the next phase of design.
Having considered these, let us take a look at a systematic and comprehensive approach towards design stages and milestones.
This is part one of a two-part series. The first part will describe the stages and the second part will address the tooling and prototyping requirements from preliminary models such as wax and stereolithography to hard tools capable of producing over a million parts.
This discussion will be restricted to molded plastic parts – which in most instances are complex for a typical product.
Pre-Concept and On-going Dialog
Objective
- To define basic product requirements (cost, environmental ruggedness, service life, etc.)
- To determine the number of sub-components
- To set preliminary form, size, weight and cost target
- UL and other agency requirements
Activities:
The following are initiated:
Materials
- Firm up UL requirements for the materials (UL 94 V-0, V-1, etc.)
- Discuss possible materials with the appropriate engineer(s) including the UL ratings
- Minimum thickness based on UL rating
- Final materials short listed
- Custom color development initiated (if required)
- New material approval initiated (procurement and central configuration control group)
Design
- Review experiences and lessons learned from previous similar designs
Concept Review
Objective
- ID surface defined
- Number of parts defined
- Preliminary structural calculations such as snaps performed
- Material identified
- High levels areas of DFM (Design for Manufacturability) identified
Activities:
Materials
- Final materials identified
- Custom color matching started (if required)
Design
- Minimum wall determined based on UL requirements
- Uniform wall thickness planned (no more than 25% amorphous and 15% variation for semi crystalline materials)
- Ribs and bosses no more than 50% of the wall at the base planned
- Cored holes length no more than 3 times the width or diameter at the base planned
- Secondary operations identified
Tooling
- Texture selected
- Draft per texture planned
Analysis
- Flow Simulation (Mold Flow, etc.)
- Preliminary gate location, number and type
- DFMPro1 run to confirm preliminary design
- Processing limitations and long term properties identified and possibly addressed
Detailed Design Review
Objective
To finalize a design that is ready for an engineering verification (EV) build and comprises of the following:
- Issues highlighted in Concept Review resolved
- Part geometry ready for tooling
- Specific areas of concern identified
- Materials finalized
- Design fallbacks for identified risky areas
- Full DFMPro performed along with reasons for exceptions
Activities:
Materials
- Material finalized
- Color match approved
Design
All of the areas from the previous reviews, plus:
- Nominal wall established
- Nominal wall meets minimum UL requirements
- Nominal wall meets flow requirements
- Wall thickness transition length > 3 times step height
- Final gate locations confirmed
- Thick to thin transitions confirmed
- Knit lines not in critical areas
- Preliminary check for hesitation in flow. See figure 1 below:
Figure 1. When a thin section is surrounded by a thick wall, the melt flows faster in the thick area causing the melt in the thin area to slow down and “hesitate”. This will leave serious visual and mechanical deficiencies in the thin area
- Draft included (including special requirements for textured areas)
- Design features adjusted for secondary operations (decoration or printing, ultrasonic welding, heat staking)
- Stress and strain calculations performed and verified
Tooling
- Checked for trapped steel conditions
- Side action avoided where possible
- Side actions identified
- Minimum 5 degrees for bypass shut off*
(*Two pieces of steel coming to together to create an undercut such as the bump on a snap as shown below. The draft allows prevention of galling between the mating pieces of steel. See figure 2).
Figure 2 The 2 green surfaces on the left and right of the snap fit need drafts since the two halves of the mold will slide by each other here too.
Analysis
- Flow Simulation
- Flow length
- Filling pressure
- Quality of fill (hesitation, short shot, shear, etc.)
- Venting (identification of critical locations)
- Warpage (preliminary estimate of warpage)
- Cooling (preliminary identification of critical areas)
- Cooling (propose or review cooling)
- DFMPro report reviewed and exceptions approved
- Design Engineers run DFMPro analysis at intermittent stages of design process to aim for a good design meeting performance and manufacturability criteria
- Reports are submitted for review along with any exceptions needing approval
- Structural
- Processing weaknesses and long term properties analyzes
- Stress and strain analyzes performed
Final Design Review
Objective
- Review of action items generated and modifications needed based on verification testing
- Structural integrity confirmed by simulation
- Areas of weakness redesigned
- Material performance confirmed
Activities:
Materials
- Material performance verified (toughness, chemical resistance, visual – if parts already tooled)
Design
- All of the areas from the previous reviews reconfirmed
Tooling
- Reviewed with the tooling engineer
- Final gates and runners design initiated
- Tooling material/type selected based on the resin, finish and tool life required
Analysis
- As needed based on design changes from the Detailed Design Review
Release for Production Tooling
Objective
- Comprehensive transition of design to tooling including inspection and quality control documents
Activities:
Materials
- Meeting held between molder, resin supplier and engineering to ensure timely delivery of colored material and to agree on any specific requirements such as MFR (Melt Flow Rate) range
Design
- Ensure fidelity of data transmitted
- Ensure that latest revision of data is transmitted
- Any special information such as zero draft, texture and texture areas, etc. are fully documented and understood
- Inspection fixtures discussed and documented
- Inspection documents created and understood
Tooling
- Meetings held between tooling engineers (internal and external), simulation engineer and design engineer leading to optimum tooling design and cost
Analysis
- As needed based on design changes from the Detailed Design Review
To be continued in part two of this blog article….
- DFMPro is a design for manufacturing assistant for design engineers tightly integrated with popular CAD systems. It helps design engineers quickly review their designs for ease of manufacturing and assembly prior to taking the designs forward for design reviews or manufacturing. Thus, it helps the organization avoid rework, improve product quality and reduce the time to market. To know more, visit dfmpro.com