Mass-Manufactured Nautical Yo-Yos

February 2017   Bianchini

My team of 5 people designed a nautical-themed yo-yo, which we designed for mass manufacturing and of which we fabricated 50. We learned how to design for manufacture, to use injection molding and thermoforming machines, and to use MasterCAM to program CNC mills and lathes. I specifically did the design, tooling manufacturing, and production run of the ship steering wheel featured on both sides of the yo-yo.

2.008 design and manufacturing 2

2.008 is the second Manufacturing + Design class for MIT mechanical engineers, and it focuses on mass manufacturing techniques and processes.  The ongoing semester project was to manufacture 50+ yo-yos of custom design, using manufacturing techniques such as injection molding and thermoforming.

This video was one of our final deliverables at the end of our semester, and it shows my team's progress through the design and manufacturing.

Design

The yo-yo design in SolidWorks.

We made a nautical-themed yo-yo, each side featuring a ship steering wheel, compass arrows, and a textured panel designed to look like waves.  Each yo-yo features the following parts:

Part Quantity

Manufacturing Method

Body 2 Injection Molded
Steering Wheel 2 Injection Molded
Outer Ring 2 Injection Molded
Wave Pattern 2 Thermoformed
Hex Nuts 2 Off-the-shelf
Brass Nail 2 Off-the-shelf
Set Screw 1 Off-the-shelf
Axle Spacer 1 Off-the-shelf
String 1 Off-the-shelf

Table 1:  Yo-yo components, quantities, and manufacturing methods.

In addition to each of the components, we kept track of critical dimensions and tolerances, a tactic that is crucial in large scale production.

Critical Dimension

Target Value

Tolerances

Measurement Method

Overall yo-yo diameter

2.52"

+/-0.015"

Calipers

Overall yo-yo width

1.485"

+/-0.010"

Calipers

String gap

0.1"

+/-0.005"

Calipers

Body rim for snap fit (OD)

2.2176”

+0.005”

-0.000”

Micrometers

Snapring rim for snap fit (ID)

2.2076”

+0.000”

-0.005”

Micrometers

Waves thermoformed piece OD

1.85”

+/-0.010"

Calipers

Waves thermoformed piece ID

0.5”

+/-0.010"

Calipers

Steering wheel center hole diameter

0.09”

+0.005”

-0.000”

Calipers

Steering wheel max OD

1.785”

+/-0.010”

Calipers

Steering wheel max thickness

0.166

+/-0.015”

Calipers

Total mass

0.07kg

+/-.01kg

Scale

Max rotational speed

197 rad/s

+/-10 rad/s

Logger pro

Table 2:  Critical dimensions for the yo-yo assembly/components.

Design for manufacturing

For injection-molded parts, common defects include shrinkage, flash, and short-shot.  Shrinkage is the most difficult to alleviate, and it can be helped by maintaining a consistent thickness of the part throughout its shape.  This led us to redesign one of our parts in order to combat some future problems down the road.

Compare the thickness through the red line from the original design (bottom) to the designed-for-manufacturing iteration (top).  Injection molded parts with thick sections typically can show shrinkage marks because the outer edges harden before the molten inside, which then shrinks when it freezes.

Mold design for injection molding

Our yo-yo had three unique injection molded parts.  I designed and manufactured the steering wheel, so I will go more into detail on that part.  Because of the 3D-machined features, the part led itself to a symmetric split plane through the center of the part.  Save for the central hole, all other features of the part were reflected symmetrically in either mold half (called the core and cavity molds).  The central hole became a steel pin that was press-fit into the aluminum mold after its machining.

The mold halves for the steering wheel.  They were CNC milled using operations made in MasterCAM software.

Mold manufacturing process plans

In large scale manufacturing, process plans are essential to predict time and money costs associated with manufacturing.  We made a process plan for each of our manufactured tooling (e.g. molds for injection molding, molds for thermoforming).

Part

Task

Time Estimate

Description

Body

Setup injection molding machine

30 mins

Put core and cavity mold in machine, learn how to operate controls

 

Calibrate pressure and other parameters

30 mins

Make test pieces and adjust controls to make optimal parts

 

Injection mold 100 bodies

2 hours

Put hardware in mold, close mold, inject plastic, let cool, release mold, repeat

Body Molds

Setup core and cavity on mill jig and run drill pattern on ProtoTrak for ejector pin holes.  Then run gate mill program

30 mins

Load program onto mill, set z-zero, put in center drill, center drill holes, change to drill bit, drill holes, change to end mill, mill gates.  

 

Turn core on lathe

10 mins setup

4 mins run (according to mastercam)

Put rectangular stock in chuck, load program, verify program, run.

 

Turn cavity on lathe

10 mins setup

3 mins run (according to mastercam)

Put rectangular stock in chuck, load program, verify program, run.

Snap Ring

Set up injection molding machine

30 mins

Put core and cavity mold in machine, learn how to operate controls

 

Calibrate pressure and other parameters

30 mins

Make test pieces and adjust controls to make optimal parts

 

Injection mold 100 snap rings

1.5 hrs

Put hardware in mold, close mold, inject plastic, let cool, release mold, repeat

Snap Ring Molds

Set up mill with rectangular stock

10 min

Put in stock, load program, run

 

Machine cavity on mill

50 minutes

Set z-zero, face stock with 2” face mill, machine pocket with ½” end mill, machine around islands with 3/32” end mill

 

Set up lathe with rectangular stock

10 min

Put in stock, load program, set zero

 

Turn core on lathe

3.5 mins

Run core turning

 

Set up mill with turned core

10 mins

Put in stock, load program, set zero

 

Drill ejector pin holes on mill

7 mins

Machine series of 6 ejector pin holes, first with center drill then with pecking drill

Wheel

Setup injection molding machine

30 mins

Put core and cavity mold in machine, learn how to operate controls

 

Calibrate pressure and other parameters

30 mins

Make test pieces and adjust controls to make optimal parts

 

Injection mold 50 wheels

2 hours

Put hardware in mold, close mold, inject plastic, let cool, release mold, repeat

Wheel Molds

Setup core and cavity on mill jig and run the programs.

Core:  1 hour 15 minutes


Cavity:  5 hours 10 minutes

Core:  Load program onto mill, set z-zero, put on 2” facing bit, face mold, put in center drill, center drill holes, change to drill bit, drill holes, change to ½” end mill, mill top, change to 3/32” flat end mill, rough cut pockets, change to 1/16” flat end mill, finish cut pockets


Cavity:  Load program 1 onto mill, set z-zero, put on 2” facing bit, face mold, mill

Waves

Set up thermoforming machine with wave mold

30 minutes

Set up heating elements, insert plastic and position mold.

 

Thermoform 100 wave pieces

2 hours

Heat plastic sheets, vacuum press mold, cool, remove sheet, repeat

 

Punch out exterior and interior circles

2 hours

Individually cut out the outer diameter and interior through hole of the cooled sheets

Wave Mold

Set up 3D model in printing software, print

4 hours

Construct the mold using additive manufacturing methods. Cure if necessary.

 

Total Time:

19 hours +

45 min

 

Table 3:  Time estimates for manufacturing each of our custom parts, including their tooling.

Our timing assumptions for the machining of the molds came from the development of MasterCAM files for each part and determining the run time based off MasterCAM’s estimates. We took our injection molding estimates based on discussed standard setup times, giving ourselves some slack to calibrate the machine, and then standard injection and cooling times--making sure to account for larger cooling times for thicker parts.

Injection molding process optimization

Injection molding machines carefully control temperature and pressure with respect to time, and these profiles greatly vary the outcome of the parts.  Before doing the production run, we made practice parts as we optimized the temperature/pressure profiles to achieve minimal defects and maximal quality.  Those parameters for the body piece, as an example, are as follows:

~~~Injection Hold~~~
Injection Hold Pressure Profile:  P7-P16 = 500, 300, 300, 300, 300, 300, 300, 300, 300, 300
Injection Hold Time:  Z2 = 8.00s
Cooling Time:  Z4 = 10.00s
Set Screw Feed Stroke (shot size):  C1 = 2.20in

~~~Injection Boost~~~
Injection Speed Profile:  V12-V21 = 3.0, 3.5, 4.0, 3.0, 2.0, 1.0, 0.8, 0.4, 0.2, 0.1
Injection Boost Pressure:  P6 = 1700psi

~~~Screw Feeding~~~
Screw Feed Delay Time:  Z3 = 3.0s

~~~Ejector~~~
Ejector Counter:  2
⅛” Ejector Pins Length:  5.57in, Quantity:  6
Total Shim Thickness:  0.00in
¼” Ejector Pin Number:  #2

We arrived at these values with a couple of things in mind that we wanted to improve:  dishing and rate.  There also was a consistent visible weld line/air bubble across all our test parts, however this spot was going to be covered after assembling with the snap ring.  Thus, we decided to not worry about this flaw.

To fix dishing, we considered changing the pressure profile so that it would pack in more material while cooling, hopefully filling in some of the dished areas.  This resulted in our pressure profile holding 300 psi for the duration of the holding time.

To improve the rate, we considered decreasing the cooling time, at the potential expense of dimensional accuracy since the part shrinks more when it cools for longer outside of the mold.  Thus, we decreased the cooling time and checked the dimensions to ensure the variations were still within specifications.  This resulted in our decreasing the cooling time from 20 seconds to 10 seconds, which still allowed us to keep our parts within our design specifications.

We began with generic numbers based on our knowledgable shop staff.  We tweaked the numbers as we went by tracking the defects in each of the practice parts like below.