Plate Process Optimization
Process sheet for injection molding:
For the plate, it took a bit of experimentation with the injection molding machine to determine the appropriate process parameters. We started out with a shot size of 20mm, but soon determined that the mold wasn’t filling completely, and there was a short shot every time we injection molded. Bumping the shot size up to 22 mm seemed to produce the best results, with no visible defects. We also experimented with the injection pressure, eventually settling on 500 psi, which produced a plate that had no warping.
Waffle Process Optimization
Process planning sheet for waffle:
As with the plate, a lot of experimentation was necessary before we arrived at proper parameters for making the waffle. Initially, our shot size was much too large (32mm) and we had difficulties with the packing phase. This, in combination with inadequate holding pressure, led to undesirable flash. To alleviate this problem, we reduced holding pressure, increased packing time, and reduced shot size. After minor adjustments of these parameters and increasing the injection speed from 50% to 70%, our waffles looked very edible.
Summary of Overall Process Optimization
Once we had the basic process parameter settings down for the plate, our biggest problem was trying to minimize the sharpness of the edge of the plate. We originally thought that the sharp edge was caused by a bit of flash, so we tweaked a bunch of the process parameters to try to eliminate this feature. Unfortunately, none of the changes in process parameters made a difference in reducing the sharpness of the edge of the plate, and we determined that the issue was primarily caused by the location of the parting line on the piece — it would be extremely difficult to completely eliminate the sharp edge, given the design at that time.
With this in mind, we re-machined the core mold, this time extending the curvature of of the plate so that there would be a smoother edge. We then ran the molds on the injection molding machine, using the our original process parameters. Since the change we made to the plate was not drastic, the original process parameters worked well, and those became our final process parameters to use with the re-machined molds.
After injection molding a few plates and waffles, we realized that the snap fit was too loose. The waffle component did not shrink as much as we thought it would. We re-machined the core mold of the waffle to get a better snap fit with an overlap of 0.01 inch, whereas before, there was no overlap in between the ID of the waffle and OD of the plate. With the re-machined core mold, the snap fit was tight; we were not able to get the waffle off the plate.
Since our waffle component has uneven thickness, during the process optimization stage, we wanted the part to have the least warping without introduction of defects such as flash or short-shot. When we played around with parameters, we encountered problems balancing pressure and injection speed. The mold was filling very slowly and occasionally had cases of short-shot. When we increased the pressure, flash occurred. To address this, we opened up the runner to allow more area for the plastic to flow in. After doing this, our parameters were easier to optimize. We found an appropriate balance of injection speed and pressure (70%, 500 psi) that addressed both short-shot and flash issues. The shot size of 27mm was optimal for the packing time of 6 seconds without causing flash. Ultimately we were able to achieve consistent results with these parameters.
During our production run, we encountered another problem when trying to achieve the perfect golden-brown waffle color. We discovered that mixing two colors was much more consistent than mixing three colors. When we mixed cream, brown, and yellow, our color ranged from light brown and cream to olive green and dirty yellow. We decided to stick with just the cream and brown plastic pellets.
When the only setting we could change was temperature, we found that 625oF worked well with our part. When the display was fixed, we tried to optimize time-wise so production could go just as smoothly but in a shorter amount of time. First, we decreased the heat time from 30 seconds to 22 seconds, and the form and open delay times from 15 seconds to 10 seconds. With those settings, even though production was faster, the base of the butter was not clearly defined enough to snap together with the waffle and plate. Next, we increased the heat time to 25 seconds, but the base was still not well defined enough. We increased the temperature until 640oF, at which point the base of the butter was well defined enough to fit with the rest of our parts, and the top of the butter that’s exposed in the yoyo was still round enough to give it a soft buttery look.
Final thermoforming settings
Not pictured: butters 11-120