Thankfully, shooting two parts in one mold was successful. We had doubts as the overall surface area of our parts was quite large.
We optimized to reduce shrinkage around the critical dimensions, those being the press fits for the bearing and ballast nuts. The nut press fit was very tolerant of changes in shot temperature and cooling time. The most extreme values deviated by no more than three thousandths of an inch from one another.
The bearing press fit dimensions were more variable. The OD of the bearings we're using is .315", we aimed for an interference fit of ten thousandths. Regardless of variations in parameters, the dimensions stayed very consistent. However, we found that the cavity shrank into the shape of an ellipse. The major diameter averaged to about 0.311" while the minor averaged to 0.307".
This made for 0.009" and 0.013" of interference respectively. The fit was slightly snug, but went easily with the help of an arbor press.
Later, we increased cooling time to 30 seconds in an attempt to further refine dimensional accuracy. During our trial run with clear plastic, this did not pose any issues. However, when we added black dye to the plastic, the results were catastrophic. Bowties wedged themselves in the core and punctured or cracked when the ejector pins were fired. Although the critical dimensions were preserved, the parts were not. We observed that this was provoked by long cooling times. We solved the problem by reducing the cooling time from 30 to 20 seconds, and by using copious amounts of mold release spray on the core.
We also had to tweak the feed stroke to maximize packing while preventing flash. We did this by trial and error, increasing the value by 0.05 cubic inches from our initial value of 1.6 to our final value of 1.85. We noticed flash began to appear at around 1.95 cubic inches.
The injection hold pressure profile did not require changing. We played with the values to see their effect on the part, and found that it had none in the range to which we changed them. They seemed uncorrelated.
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| One of the damaged bow ties. |
The remainder of the parameters were unchanged.
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| A full production run, mostly successfully |
Process Parameters Sheet:
| Injection Hold | ||||
| Injection Hold Pressure Profile P7-P16 | ||||
| 653 | 600 | 600 | 600 | 600 |
| 600 | 400 | 376 | 350 | 350 |
| Injection Hold Time | Z2 = | 8s | ||
| Cooling time | Z4 = | 20s | ||
| Set Screw Feed Stroke | C1 = | 1.85 cubic in. | ||
| Injection Boost | ||||
| Injection speed profile: V12-V21 | ||||
| 6.3 | 6.3 | 6.3 | 6.3 | 6.3 |
| 6.3 | 6.3 | 6.3 | 6.3 | 6.3 |
| Injection Boost Pressure | P6 = | 1100 psf | ||
| Screw Feeding | ||||
| Screw Feed Delay Time: | Z3 = | 20s | ||
| Ejector | ||||
| Ejector Counter | AZ = | 2 | ||
| Ejector Pin Lengths: | 4x 5.690 in. | 6x 5.570 in. | 1x Sprue | |
| Total Shim Thickness: | 0.062 in. |
