Mold Manufacturing!

One of the more challenging components of the Yolette is the set of red and black inserts that line the interior of the yo-yo face. We considered molding individual square pieces and inserting them over the ring piece during the assembly process, but decided that we wanted a cleaner approach that would involve fewer component parts. As such, we decided to design the feature to be manufactured using a double-shot process.  

The red and black pieces will be manufactured using the same mold. They are designed such that each red and black piece can clip together to form an alternating colour pattern. To assemble the two pieces, one colour will be flipped, offset by 22.5 degrees, and placed on top of the other so as to produce a symmetrical part. This set of two injection molded parts will then be inserted into the mold for the faceplate (aligned using shallow guide wells), which will be injection molded around it.

The red colour ring insert:

The combined red and black insert:

These parts are rather thin, and our molds were therefore designed very carefully. The wells in each side of the mold are 0.05in deep to form a 0.1in thick part. The runner that connects all the pieces is 0.025in thick. Two ejector pin holes are located in two opposite wells.

The cavity mold:

The core mold: 

Mold Dimension Justification

The mold dimensions are determined from a scaling factor and the desired dimensions of the final part. The specific mold dimensions are determined via two steps. For most components, the molds were measured through extrapolating several empirical measurements of like-parts. Beginning with the assumption that the thicknesses in the parts are designed to be as closely uniform as possible and as thin as possible to minimize the effects of dishing and similar phenomenon, similar thickness and geometry parts were measured in the lab. Several sets of molds and a quantity of their manufactured parts were provided as examples. Taking a set of measurements and approximate general values, the shrinkage values were determined to be approximately 1-2%. In the case of the body, retaining ring and roulette wheel second-shot, the average of the measured values for similar geometry and thickness parts was very close to 2%. This gave us a universal scaling factor of 1.02 for all dimensions.

However, because the yoyo has a double-shot part, which requires a well sealed set of shutoff surfaces created by the inserted part, simply measuring a variety of similar example parts is not sufficient. In order to determine a more precise shrinkage allowance, we must use true measurement methods. This begins with the color ring insert. In order for the double-shot to work well, the insert needs to be well clamped so that both faces seal against the mold, creating a shutoff surface without flash incursions. In order to do this, the part must be compressed slightly in order to have the correct response when the injection pressure is applied (i.e. primarily it doesn’t begin to leak around the edges).

Therefore, despite the estimated shrinkage value of just under 2% from measuring similar-thickness parts (the part is very thin and low volume, so therefore unlikely to shrink as much as the larger and thicker parts), a scaling factor of 1.02 was again selected in order to create a very slightly oversized part. This allows for it to be better compressed in the faceplate mold for the double shot. Furthermore, because there are still inherent inaccuracies in this estimation process, and because of the importance of the fit for this part manufacturing process, a more accurate value for the shrinkage allowance, and therefore, dimension scaling factor for the faceplate molds, needs to be determined. Under the advice of Mr. Dave Dow, we will manufacture an appropriate number of color ring inserts and accurately measure them to determine their true dimensions and variance. This will allow the dimensions of the faceplate mold to be precisely set, resulting in maximum quality of the roulette wheel face.

Mold Manufacturing Process

The mold manufacturing process is a two-staged series of machining processes. First, all single-shot molds are machined on the CNC Lathe and CNC Mill. These molds include the color ring insert core and cavity, body core and cavity, retaining ring core and cavity and thermoform die. The color ring insert molds are particularly important because, although calculations have been performed with what are believed to be reasonable and accurate approximations and assumptions, the exact behavior of the shrinking and other manufacturing parameters are not known. The second stage is the machining of the faceplate molds (core and cavity) once an appropriate number of the color ring inserts have been manufactured and measured to allow for an optimization of the design.

Because the inner faceplate part is a double-shot process in which grey plastic is injection-molded over two color rings (black and red) inserted into the mold, it is necessary to design the part specifically to the real dimensions of the color ring parts. Therefore, this mold will be made after a suitable quantity of the color ring inserts have been made and measured, to determine the true dimensions and variation of the parts to optimize the mold design. These optimizations include small alignment divots to center and hold the inserts in the mold and creating the correct tolerance gap to allow the part to be slightly compressed by the clamping force, allowing a good seal to prevent grey flash from leaking onto the faces of the colored spaces and an overall well-manufactured part that will fit appropriately onto the body.

Going into a bit more detail on the machining of the molds, most require a two-step process for each mold-half. First, any radially symmetric features are machined on the CNC Lathe due to the significantly decreased cycle time and optimized tool paths. Next, the mold is moved to the CNC Mill, where small or nonradially symmetric features are added. For all molds except for two, they begin on the lathe, where in addition to the design features, facing cuts are added to smooth the surface to provide an optimum parting-line seal to prevent flash or other losses of pressure.

For the color ring inserts, the majority of the features can only be machined on a mill. They therefore begin on the mill, with the surface-quality face cuts being done on the mill with a large 2-inch diameter shell mill. The tools are switched to a ½” endmill, which performs the majority of stock removal, either cutting away the face of the core, resulting in the raised shutoff surface, or cutting out the center pocket where the internal circular runner will be formed upon the closing of the molds. The majority of the manufacturing time occurs in milling the pockets for each red (or black) colored “square” on the roulette wheel face with a 1/16” endmill. Finally, on the cavity side, 2 runners are cut in the mold with a 1/8” ball endmill. 2 runners are made in order to allow for a larger cross-sectional surface area for the plastic to flow through to avoid flow restrictions between the external runners and internal circular runner. Pictures of this mold are above, showing the lack of “turnable” features, necessitating all machining to be done on a mill. Note the small pockets which consume the majority of the cycle time. To reference the complete detailed machining process plans for all molds, see the attached PDF. Note, the process plan for the faceplate mold has not been finalized, as described above, in order to accurately measure and optimize the mold design for a double-shot process.

Manufacturing time

We performed thorough examinations of the estimated manufacturing time for each of our yo-yo components, as can be seen in this chart. Specifically for the manufacturing of the mold for the colour ring insert, we came up with the following estimates:

Colour Ring Insert Mold	Cavity	Lathe	0 sec
		Mill	37 min 41 sec
	Core	Lathe	0 sec
		Mill	40 min 40 sec

These estimates were made using the machining time estimates from our Mastercam machining profile.

We also made estimates of the fabrication time of the part:

	Step:	Percentage of Total Time	Total Time
Colour Ring Insert	Close Mold	3	0.98
	Injection	11	3.58
	Pack and Hold	29	9.4
	Part Cooling	50	16.28
	Open and Eject	7	2.28
	Total:		32.55

Cooling time was calculated using the equation:    where  is the characteristic thickness of the part and α is the thermal diffusivity. Here, the characteristic thickness was taken to be the average of the biggest and smallest thickness values in the part. For the material used in this manufacturing process, polypropylene, α = 9.6x10-6 m2/s.

Once the cooling time was found, the other times were found using their relative percentage weights (percentages of total manufacturing time were taken from those given in lecture).

Scheduling

While we have estimates for shrinkage, we plan on shooting the colour ring parts and measuring the average shrinkage before fully dimensioning the mold for the disk. Since we are manufacturing double shot molds, it is crucial that the respective dimensions match up. As such, our schedule has slightly changed: we are waiting on injection molding time in the lab before our final disk mold can be completed. Furthermore, the injection molding process for the disk plate will take longer than initially estimated, since the double shot process requires a human operator to place a colour ring part into the disk mold before every injection. This will add an estimated 15-20 seconds for each part on average, which corresponds to approximately 160% increase in manufacturing time for this part.

The manufactured molds!

Upon running our mill program, we were left with sections of very thin metal along the intersection between the wells and the centre, and also between the wells and the runners. We have since filed these off to allow plastic to flow through the entire mold. Furthermore, the ejector pin holes were reamed out another 0.001" to allow for a slip fit for the ejector pins.

We're keen to start some injection!