Boston Skyline Top Plate Initial Injection Mold Result

These past 3 weeks we've updated our YoYo design to make the manufacturing easier.  For the top plate of ourYoYo - since we are insert molding a wood ring and a circular metal skyline- we had to come up with an interesting and fairly complex design. Insert molding is especially challenging because special care needs to be taken to ensure that the insert parts are properly constrained and positioned. Before we get into details here is an image of the part and the mold to get you acquainted with the objective.

Figure 1: Initial Injection mold of Boston Skyline

Top Plate

Figure 2: Cavity of the Insert Mold of Boston

Skyline

Figure 3: Core of the Insert Mold of Boston

In order to constrain the wood and the metal - the first part of the mold to notice are the 4 arced curves each with a step down/lower curve and a bump in the core of the mold. These arcs are meant to position the metal insert radially and vertically. The bump on each of the arcs exists to constrain the orientation of the metal such that it doesn't rotate in the mold. The other important part of the mold for the metal are the cylindrical magnets that have been press fit into the mold. These magnets are 3/8" in diameter and are positioned to rest on the different buildings of the Boston skyline.  Image 4 below shows a good example of how of magnets are positioned relative to the buildings in the Boston plate.

Figure 4: Back of the Boston Top Plate. Notice the

holes from the magnets on each of the buildings.

With the metal insert fully positioned and constrained in the mold, we designed the wooden ring to rest on the cavity side of the mold and be positioned overlap a small portion of the outer diameter of the metal plate. Through testing with the injection mold machine, we found that the wood would compress significantly from the high injection pressure of the process. This resulted in a fair bit of flash on the top face of the wood ring. In order to solve this issue we modified the mold to fully compress the wood ring before the injection molding took place. Our final design consideration for constraining the wood was to cut a small circular retraining groove in the mold that overlapped a small portion of the outer diameter of the wood . This produced the effect of allowing a circular plastic ring on top of the wood to constrain it vertically after decompression once it left the mold. Now we'll discuss more about the process parameters that we settled on for the injection molding of the insert plates. For this discussion please note that the Boston and Cambridge insert plates represent the left and right plates of the YoYo respectively and had the same injection molding parameters.

Machining the Molds

The cavity half of the mold used the lathe first. It began by being faced off with the facing tool to ensure flatness and avoid any potential flash due to the mold templates we received having defects. The cavity of this piece was only meant to have a space to hold the wooden insert and a ring around and underneath the wood to allow the plastic to move underneath the wood to hold it in place. As such, the next step is to have the lathe dig out these two concentric rings with the grooving trepan. The mold was then finished on the lathe by having the turning trepan finish the corners between the rings and the center and the boring trepan finish the corners between the rings and the outside of the mold. The cavity mold was then transferred to the mill so that the runner path could be cut using the 1/8” ball nosed endmill.

           The core half of the mold was faced on the mill using the 2” face mill because it did not require any work to be done on the lathe. The core mold needed to hold the steel insert which included having a housing for it so that it wouldn’t slide or move as well as magnet holes to hold it down. The core mold also needed to include the ejector pin holes, sections at the faced off level to hold the wood insert into the cavity and space under the steel for the plastic that would hold the entire piece together. The first step in the milling process was to drill all the ejector pin and magnet hole leads. This was done by creating leads with the 3/16” center drill at a depth of 0.1” then drilling through with the 0.12” standard drill using pecks. The deepest section of the piece at 0.09” where the plastic would reach was milled next; first with a 3/8” flat endmill to clear most of the area, then remachined with a 3/32” flat endmill to reach into the ring around the supports for the wood and to finish the corners. The seating for the steel insert was the last thing done on the mill, creating a circular contour into the supports for the wood with some divets to hold the steel piece more securely in place. The core mold then had all the ejector pin holes drilled then reamed with a 1/8” bit and ream and all the magnet holes were drilled and reamed with a 0.18” bit and ream.

Step	Operation	Machine	Tool	Justification
1	Rough	Lathe	T0101	Facing cut using the facing/turning tool
2	Groove	Lathe	T0909	Peck operation to fit custom shape pocket geometry with the grooving trepan
3	Finish	Lathe	T0707	Clean the inner corners from step 2 with the turning trepan
4	Finish	Lathe	T0808	Clean the outer corners from step 2 with the boring trepan
5	Contour	Mill	#9	Cut path for runner along the line closest between the cavity and the sprue with the 1/8” ball-nosed endmill

Step	Operation	Machine	Tool	Justification
1	Facing	Mill	#12	Face cut with the 2” face mill
2	Drill	Mill	#13	Center drill the holes for the ejector pins and magnets with the 3/16” center drill
3	Peck Drill	Mill	#17	Make thru lead holes for the ejector pin and magnet holes with the 0.12” standard drill
4	Pocket	Mill	#6	Clear the bulk of the material to a 0.09” depth with the 3/8” flat endmill
5	Pocket (Remachining)	Mill	#2	Clean the previous cut and cut around the ring and in corners to a 0.09” depth with the 3/32” flat endmill
6	Contour	Mill	#2	Cut the fit shape of the metal insert into the piece at a 0.0625” depth using the 3/32” flat endmill
7	Drill	Drill Press	1/8” Bit	Drill through the holes for the ejector pins
8	Drill	Drill Press	0.18” Bit	Drill through the holes for the magnets
9	Ream	Drill Press	1/8” Ream	Ream the holes for the ejector pins
`10	Ream	Drill Press	0.18” Ream	Ream the holes for the magnets

Injection Molding the Cambridge Side

For the injection molded insert piece of Cambridge we kept many of the standard parameters.  The only parameters we altered considerably were the shot size, injection speed, injection pressure, and cool time.

Before we could start working with the BOY Injection Molding Machine we needed to set-up our molds in the larger configuration that would be placed into the injection molding machine.  A major component of this step was the ejector pin length and the associated total shim thickness.  We needed to select ejector pins that were long enough to reach through the larger configuration when it was compressed (there are four springs on the four corners of the larger configuration) to press onto the injection molded part once it was formed and pop it out of the mold.  We knew the width of the configuration and measured the section of the mold the ejector pins needed to travel through and the sum of these two measurements became our minimum measurement.  We selected the ejector pins that were longer than our minimum measurement and then selected shims to attach to the configuration to make up for the difference between the ejector pin length and the minimum measurement which was the length we wanted.  There was also a standard ¼ “ ejector pin that was placed into the configuration.  

For detailed parameters for each part please see Table 1.  We started out with part number 1 with what we thought would be reasonable parameters with common settings for shot size, injection speed, injection pressure, and cool time.  Our first piece served as the foundation on which we would make other changes.  The first part came out with a frozen gate and flash (Figure 6). For the second part we thought that reducing the shot size would help to prevent flash but when we ran part 2 we experienced a short shot (Figure 5).  The plastic did not even make it all the way around the outer most ring of the design and was nowhere close to filling in the center cavity.   The third piece was us going back to our original parameters from the first part to see if we obtained different results.  We still saw flash and a frozen gate.  For the next few tries we reduced the shot size to enough just to fill the mold to help prevent any extra material from being forced around the wooden ring and resulting in flash.  We also increased the injection speed thinking that if we filled the mold more quickly perhaps we could avoid the frozen gate problem.  We also increased our injection pressure in the hopes that this could also help us prevent the frozen gate.  Both increasing the injection pressure and increasing the injection speed also were done to try and make sure we had plastic filling in the mold completely (examples in Figure 7).  In addition we reduced our cooling time since we know that most shrinkage happens while the piece is cooling in the mold we altered the cooling time to see if we could get slight variations in the size of our part to experiment with the snap fit to the body piece to see what cooling time was ideal.   

One curious thing we encountered was that the location of the flask relative to the mold was the same for multiple runs.  It turns out that the molds we were using were not perfectly parallel.  The core mold of the Cambridge side insert molding piece was not level.  To solve this issue we faced off the back of the core mold so that when it is placed in the configuration it is parallel to the cavity.  This helped us quite a bit in reducing the flash.  

Another avenue we used to reduce the flash was to face off the cavity side of this mold too.  Our wooden ring is held by the cavity mold and we had aimed for the wood to be pinched between the two halfs of the mold so that plastic could not sneak around the wooden ring and form patches of flash.  But we soon realized that our wood was a bit thinner than expected so we needed to reduce the depth of the shape the ring fits into in the cavity by facing off the front of the cavity.  

Figure 5: Two views of Part 2, a short shot.  The opaque plastic did not reach all areas of the part.  In the image on the right you can see the gate on the left edge of the image and on the right edge you can see that the plastic did not even make it all the way around the outside ring.  It is also very clear that there was not enough material to fill up the space in the mold because there are large areas in the center of the piece that that plastic has not reached.  

Figure 6: These two images are examples of flash.  It is evident that the plastic overflowed at a few locations around the wooden ring.

Figure 7: These two images are of frozen gates or potential short shots.  The metal piece is held up in the mold by magnets supporting each of the main shapes.  Due to the magnets we expect that a mold completely filled with plastic would leave circles void of plastic behind each of the main shapes.  In part number 5 you can see a few complete circles but it is also clear in other shapes in part 5 and in the piece on the right that the plastic did not completely fill the mold.

Figure 8: This is an image of very minimal flash (there is a small amount in the upper left section of the ring) but this piece shows what a completely filled piece with minimal flash is meant to look like.  

Figure 9: This is an image of the form we used for our different parameters.

List 1: A Particular Run’s Parameters

*The items in bold and italics are the parameters that we changed.  The other parameters remained constant for all the pieces.  This list is not all-inclusive of all preset parameters but it does include all the parameters that we optimized*  

Injection speed: 80%

Injection pressure: 300 psi

Holding Time (Packing Time) 60

Holding Pressure 300 psi

Shot Size: 15mm

Screw Drawback Speed: 70%

Back Pressure 300psi

Decompression Stroke: 6mm

Cooling Time: 35 sec

⅛” Ejector Pin Lengths: 5.57”

Total Shim Thickness: 0.018”

Quantity: 6

¼” Ejector Pin Number: 1

Mold Open Position: 120mm

Set-up value, mold safety position: 5mm

Set-ip value, mold safety pressure: 2.5kN

Clamping Pressure: 125kN

Ejector Forward Position 80mm

Set-up value, nozzle contact pressure (force) 50 kN

Table 1: BOY Altered Parameters for Cambridge Insert Molded Part

Part Number	Shot Size	Injection Speed	Injection Pressure	Cool Time	Notes
1	15mm	80%	300psi	35	Flash; frozen gate
2	12mm	80%	300psi	35	Short shot; frozen gate
3	15mm	80%	300psi	35	Frozen gate; flash
4	15mm	95%	300psi	35	Frozen gate; flash
5	14mm	100%	320psi	35	Frozen gate; flash
6	14mm	100%	350psi	15	Frozen gate; flash
7	14mm	100%	350psi	15	Frozen gate; flash

Key takeaways from Initial Manufacturing

Outer Part (Base)

We learned that with a part this large, to avoid dishing, we need to allow a significant amount of cooling time (at least 30 seconds). When we tried to lower the shrinking time, it did not shrink as much as we anticipated and was too large to snap fit with the insert mold parts. To remedy this, we will have to remachine part of the core mold for this part to make the inner diameter smaller for a tighter fit. We will have to settle for a long cooling time which will slow down production.

Thermoformed Stand

Our thermoformed part is the stand of our Yo-Yo which has our initials on one side, “2.008 2017” on the opposite side and waves (depicting the Charles River) on the two remaining sides.

Figure 10: Images of the thermoformed stand for the YoYo

During the process of optimizing the thermoforming, we learned that small overhangs (letters and numbers) are possible and can come out in great detail as long as there are sufficient air holes under each overhang. Additionally, we learned that heating time is crucial and that the plastic sheet forms folds on sharp corners if it is too hot and hence too soft. During the process of thermoforming multiple times, the thermoforming machine heats up. Hence, a heating time that might have worked in the beginning, will cause overheating after a couple of runs and has to be reduced.

The significant changes we made to get ready for production were to reduce the heating time from 35 to 20 seconds to prevent overheating and folding at the corners, to redrill the air holes under the letter overhangs for improved detail and to use 0.03” thick thermoplastic sheets which are sturdy enough for a firm base as well as thin enough to show all desired detail.

Boston and Cambridge Insert Plate

   To wrap up the blog post - the Boston and Cambridge insert plates are both complex parts that are very similar with parameters that were the same. The key takeaways from the initial injection molding of those parts were how much a couple of thousandths of an inch could change the quality of the part. This is what we experienced with the wood ring compressing by a small amount inside the mold and what we experienced with the unevenness of the molds producing flash. The parameter settings were discussed in detail above for the Cambridge and Boston parts - again mostly we dealt with flash and frozen gates. Ultimately though we were successful and it was fun! We’re looking forward to the large production run!