Our “holder” part is the core of the entire yo-yo. Each half of the yo-yo uses a holder, each
snap-fit into its half of the body, held to each other by a nut and screw. It’s very important that the holder fit very
precisely, otherwise the whole thing falls apart (or doesn’t come together in
the first place)!
The basic design of the holder hasn’t changed since the last
blog post, but we ran into some problems with specific dimensions. It turns out that due to differing amounts of
shrinkage between the holder and body pieces, the holder couldn’t fit. A bit of tweaking was required.
Figure 1: The core mold for the
holding part. The center pin-like feature is accomplished by inserting the nut,
screw, sleeve combo.
Figure 2: The cavity mold for the
holding part. It doesn't show the runner or the 1/4 in centered hole.
We measured several identical holders, front body and back
body pieces to get an idea of the shrinkage we were dealing with. While the thickness was satisfactory, the
outer diameter wasn’t what we wanted. We found the shrinkage was very
consistent, which is a good sign. The
results are tabulated below.
Measured diameter of
Holder Part (in)
|
Measured
diameter of Holder Cavity Mold (in)
|
Ratio of
part to mold (in)
|
Shrinkage
|
1.789
|
1.809
|
0.989
|
1.063%
|
Now, our holder part had to fit both in to the front body and back body parts. We decided to leave the back body part
unchanged and modify the holder and front body parts to work. We found the inner diameter of the Back to be 1.727 in. In order to get a
snug snap fit, the holder part needs to be ten thousandths of an inch wider in
diameter than the hole it is fitting into, so our target Holder outer diameter is 1.737
in. With a shrinkage of 1.063%, this
corresponds to a Holder Mold Cavity
diameter of 1.755 in.
Target
outer Holder Part diameter (in)
|
Shrinkage
|
New
Holder Cavity diameter (in)
|
1.737
|
1.063%
|
1.755
|
Table 2: Measurements for new Holder Cavity Mold
Our process plan for the molds for the Holder Part is below.
Step
|
Operation
|
Machine
|
Tool
|
Justification
|
1
|
Drill/Cbore
|
Mill
|
1/2"dia.
2fl End Mill
|
Used to
create a hole for the screw, nut, and sleeve combo
|
2
|
Drill/Cbore
|
Mill
|
# 2
Center Drill
|
Used to
create center holes to drive the #31 drill for the injector pins
|
3
|
Drill/Cbore
|
Mill
|
#31 Drill
|
Used to
bore the injector pin holes all the way through
|
Manufacturing Plan
1)
Holding
Part – Lathe Core
2)
Holding
Part – Lathe Cavity
3)
Holding
Part – Mill Core
4)
Holding
Part – Mill Cavity
Process Plan
Holding Part –
Lathe Core
Step
|
Operation
|
Machine
|
Tool
|
Justification
|
1
|
Lathe
Rough
|
Lathe
|
T1010
|
Used to
remove large bulk of volume
|
2
|
Lathe
Finish
|
Lathe
|
T1010
|
Used to
clean the face up to a smooth finish
|
3
|
Lathe
Rough
|
Lathe
|
T0808
|
Used to
clean out the corners
|
Holding Part –
Lathe Cavity
Step
|
Operation
|
Machine
|
Tool
|
Justification
|
1
|
Lathe
Rough
|
Lathe
|
T0101
|
Used to remove
large bulk of volume
|
2
|
Lathe
Finish
|
Lathe
|
T0101
|
Used to
clean the face up to a smooth finish
|
3
|
Lathe
Rough
|
Lathe
|
T0707
|
Used to
clean out the corners
|
Holding Part
–Mill Core
Step
|
Operation
|
Machine
|
Tool
|
Justification
|
1
|
Drill/Cbore
|
Mill
|
1/2"dia.
2fl End Mill
|
Used to
create a hole for the screw, nut, and sleeve combo
|
2
|
Drill/Cbore
|
Mill
|
# 2
Center Drill
|
Used to
create center holes to drive the #31 drill for the injector pins
|
3
|
Drill/Cbore
|
Mill
|
#31 Drill
|
Used to
bore the injector pin holes all the way through
|
Holding Part –
Mill Cavity
Step
|
Operation
|
Machine
|
Tool
|
Justification
|
1
|
Drill/Cbore
|
Mill
|
1/8"dia.
2fl Ball End Mill
|
Used to
create runner
|
2
|
Drill/Cbore
|
Mill
|
# 2
Center Drill
|
Used to
create center hole for the screw, nut, sleeve combo
|
In addition to the holder part, we made some changes to the
rest of the yo-yo, detailed below in summary.
Figure 3: The core mold for the front body piece.
Front body: Technically, the front body refers to several
parts that snap together to form Mike’s face.
For purposes of this post, I’m referring to the inner part, which
fastens directly to the holder part. We
found that it was slightly too wide to fit our brand new holder, so we shaved
down the core mold inner diameter at the top from 1.781 in. to 1.731 in.
Figure 4: The core mold for the back piece.
Back: While we kept the diameter of the back piece
the same in order to fit our newly designed holder, we found that it was too
shallow to snap grab on to the holder.
We increased the depth of the back part, increasing the height of the
core mold by 0.015 in.
Optimization
parameters:
To
optimize our process, we began with a base set of parameters suggested by Dave
and decreased the time of each parameter until we were satisfied; just before
quality faltered. We have optimized parameters for injection molding of
the teeth and the back, as well as the parameters for the thermoforming of the
eye. The process sheets can be found below.
INJECTION MOLDING PARAMETERS – Back
The base set of
parameters worked mostly well except for a small amount of short shot in one
end, which was fixed by increasing injection volume of plastic. On top of
the short shot, the edges of the part kept getting burned so injection speed
was slowed down. Due to this, cooling time was reduced to 20 seconds, and
feed stroke to 2.3 inches. We found that reducing cooling time to around
8 seconds undesirably shrinks the part because the mold is not holding it in
place while it cools. A 20 second cooling time allows for the part to lower
temperature while adhering to the mold that prevents shrinkage.
INJECTION
HOLD
INJECTION HOLD PRESSURE
PROFILE: P7 – P16
|
|||||
700
|
700
|
676
|
650
|
625
|
|
600
|
580
|
550
|
500
|
450
|
|
INJECTION HOLD TIME
|
Z2 = 8.0s
|
||||
COOLING TIME
|
Z4 = 20.0s
|
||||
SET SCREW FEED STROKE
|
C1 = 2.3in
|
||||
INJECTION
BOOST
INJECTION SPEED
PROFILE: V12 – V21
|
|||||
1.0
|
1.1
|
1.3
|
1.6
|
2.0
|
|
2.4
|
2.0
|
1.0
|
0.7
|
0.5
|
|
INJECTION BOOST
PRESSURE
|
P6 =
|
||||
INTRUSION
TIME: if needed
|
ZIN =
|
INTRUSION
SPEED: if needed
|
V23 =
|
SCREW
FEEDING
SCREW FEED DELAY TIME:
|
Z3 = 12.0s
|
EJECTOR
EJECTOR COUNTER:
|
AZ = 2
|
EJECTOR PIN
LENGTH: 5.647
TOTAL SHIM
THICKNESS: 0.005
Run time for 105
parts= 18585 seconds
INJECTION MOLDING PARAMETERS – Teethfill
Unfortunately,
the base parameters suggested by Dave resulted in short shots as the part was
very intricate and the plastic cooled before the part could completely form. In
order to fix this the injection speeds were increased so as to make sure the
mold is filled before the plastic cools. We settled on a cooling time of
10.0 seconds, and a feed stroke of 0.6 inches.
INJECTION
HOLD
INJECTION HOLD PRESSURE
PROFILE: P7 – P16
|
|||||
700
|
700
|
676
|
650
|
625
|
|
600
|
500
|
550
|
500
|
450
|
|
INJECTION HOLD TIME
|
Z2 = 8.0s
|
||||
COOLING TIME
|
Z4 = 10.0s
|
||||
SET SCREW FEED STROKE
|
C1 = 0.6in
|
||||
INJECTION
BOOST
INJECTION SPEED
PROFILE: V12 – V21
|
|||||
2.0
|
3.0
|
4.0
|
5.0
|
5.0
|
|
4.0
|
3.5
|
3.0
|
1.0
|
0.5
|
|
INJECTION BOOST
PRESSURE
|
P6 =
|
||||
INTRUSION
TIME: if needed
|
ZIN =
|
INTRUSION
SPEED: if needed
|
V23 =
|
SCREW
FEEDING
SCREW FEED DELAY TIME:
|
Z3 = 5.0s
|
EJECTOR
EJECTOR COUNTER:
|
AZ = 2
|
EJECTOR PIN
LENGTH: 5.057
TOTAL SHIM
THICKNESS: 0
THERMOFORM PARAMETERS – Eye
We began
thermoforming with a base set of parameters and reduced times for each step
individually just before quality was compromised.
*First numbers are
base parameters. Second numbers are optimized parameters. Some
intermediate steps are not shown.
HEAT
TIME: 35s -> 30 s
Reducing heat time to 30 seconds allowed the material to deform
appropriately. Anything less than 30 seconds did not heat up the sheet enough
to be molded properly.
FORM
TIME: 22.5s -> 14 s
1st PLATEN
OPEN DELAY TIME: 17.5 -> 14 s
Reducing this open delay time for more than 14 seconds causes
the piece to warp as it is ejected from the mold. At 14 seconds, warping
does not occur.
COOLING
FAN DELAY: 2.5s -> 1 s
COOLING
FAN TIME: 15s -> 8 s
AIR
EJECT/PLATEN HOLD TIME: 0.5 s
MAIN
VACUUM DELAY: 0
MAIN
VACUUM TIME: 120 -> 100
TOP OVEN
TEMP: 650˚F -> 655˚F
BOTTOM
OVEN TEMP: 650˚F -> 655˚F
Increasing oven temperature reduced heating time.
MATERIAL
USED: Clear plastic
TIME: 6
minutes 40 seconds for 7 parts
TIME FOR
110 PARTS: 6285.4 seconds
