Wobbly Salt'n'Pepper Shaker
INTRODUCTION
The purpose of this project was to explore the manufacturing process of an original product idea through all phases of prototyping to DVT to EVT to mass production. Inspired by wobble cups and roly poly toys, our team designed a set of salt and pepper shakers that will wobble but not get tipped over. It is also designed to have a removable screw-on cap for unlimited refilling.
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The criteria to proceed with this project include:
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Comprised of three or more parts
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Two or more mfg. processes and materials
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Dimensional fit between the two parts
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Requires at least one tool or fixture to make or assemble
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Be small enough to keep part for 10+ units in a single locker
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One purchased part
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Requires a secondary process (sanding, painting, polishing etc.)
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<$10 per unit to produce
DOCUMENTATION
One major key to success for this project was highly organized and consistently updated documentation. In this case, Google Drive was our chosen platform for quick and accurate file edits and collaboration.
While sharing every single document and process will prove extremely comprehensive, this page will only focus on some of the key components of this project. Please use the menu on the right for quicker navigation through the various sections.
KEY CHALLENGES
#1: Wobble ratio
The following equation is used to calculate the wobble ratio:
(Mass of body + cap + nut + rod + salt) / (Mass of base)
Through the concept design phase, we found that the ideal wobble ratio that allows the object to wobble without tipping is equal to or below 0.25.
#2: Food safe material
Various food safe materials are researched for the shakers to avoid consumption hazards by consumers
#3: Dimensional tolerance and fit
Design elements must be explored to reduce the risk of the cap falling off when pouring seasoning.
#4: Fatigue of tooling*
How will the tooling and machinery wear over time?
#5: Product quality*
Will the operators read the SOP properly? Will the final product meet our standards as well as those of our customers?
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*Unsolved challenges for the project due to lack of time for testing
DESIGN & DEVELOPMENT
Part 1: Shape
The product was designed to fit in the palm of the average hand size and consist of at least 3 parts: base, body and cap. Between the original and final iteration, a few shape modifications were made to achieve the ideal wobble ratio that allows successful dynamic wobble-ness without tipping.
Original Design
- Wobble ratio was above required 0.25
- Center of mass was higher causing instability
- Base shape was completely semi-circular
Final Design
- Wobble ratio at 0.227 when filled with salt
- Center of mass was lowered for more stability (0, 0, -0.24in.)
- Base shape was flattened to semi-spheroid for more contact area and stability
Part 2: Assembly
When considering the security of the cap during use (pouring seasoning), the parts involved in the assembly was modified.
Original Design
- Cap relies on press fit with a rubber gasket that contacts the inner rim of the body
- Body is secured to base with epoxy glue
Final Design
- Cap is fastened and secured by an acorn nut and nylon rod
- Body redesigned to have appropriate wall thickness for injection molding
- Body mold designed with collapsible core to facilitate extraction
- Body is still attached to base with epoxy glue
Part 3: Materials
Material selections were based on the wobble ratio and mass manufacturing feasibility
Cap:Machined aluminum
Body: Cast food safe resin
Base: Cast concrete
Acorn nut + rod: nylon
Cap: Injection molded PP CoPolymer
Body: Injection molded PET
Base: Die cast pwter
Original Design
- Body material is food-safe but too dense
- Base material was too light for desired wobble ratio (density 2.4 g/cm^3)
Final Design
- Body material is lighter and food-safe
- Base material was denser and preferred to meet wobble ratio (density 7.3 g/cm^3)
Bill of Materials (BOM)
Using aPriori software, the following costs were found for a set of 2 shakers:
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Part costs:
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Cap = $2.54 ($1.27 / part)
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Body = $4.12 ($2.06 / part)
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Base = $5.62 ($2.81 / part)
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Tooling costs:
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Injection molding (Cap) = $9,610.31​
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Injection molding (Body) = $12,416.74
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Pewter casting (Base) = $21,718.92
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Purchase costs (via Alibaba):
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Rod = ~$0.012 / part​
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Acorn nut = ~$0.003 / part
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Final COGS for one boxed product (2 shakers) = $23.23
Cash Flow Analysis
The following assumptions make up our hypothetical cash flow:
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Assume 1 unit = 2 shakers
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No. of sellable units to make on day 0 = 1,000 (2,000 shakers)​
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Pre-orders 2 months prior
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Sales growth set and assumed at 5%
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Final price set to $35 (markup 51%)
- No long lead times
- Verification testing costs
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EVT - 10 samples - 6 x COGS - 5 mo. prior to MP*
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DVT - 15 samples - 3 x COGS - 3 mo. prior to MP*
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PVT - 25 samples - 1.5 x COGS - 1 mo. prior to MP*
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Breakeven = 18 months
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Max cash outlay = $256,185.12
*MP = mass production
Breakeven point
Manufacturing: Assembly Line
Using Flexsim, the following assembly line was simulated and used to estimate our process plan (assembly and packaging time)
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Assembly Time = 18.5 min/part
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Assembly Labor = 8.5 min/part
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Packaging Time = 3.5 min/package
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Note: 1 package = 2 parts​
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No. of packages / 40-hr work week = 524
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Quality check done on 10% of parts
Manufacturing: SOPs
Two hypothetical SOPs were created: one for a drop test (quality test), and one for part assembly.
Product Specifications
A hypothetical specifications document was created as shown below.
Quality Test Plans and DFMEAs
The following documents illustrate our hypothetical quality test plans and design failure modes and effects analysis:
Packaging
The packaging was designed to contain 2 shakers as one product. The graphics and printed information were designed using Adobe Illustrator to provide a clear visual of the product and an easy-to-follow set of instructions of how to use the product.
Materials:
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Main: white corrugated cardboard (UPrinting)
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Insert: recyclable 2-circle die cut insert (EcoEnclose)
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Other: silica gel packet inside each shaker (Alibaba)
Cost Estimate: $1.40/product