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Purging is Critical on Reconfigured IM
May 2008
| THE DETAILS |
| Part: |
truck dash lens |
| Mold: |
two-cavity cold runner |
| Injection unit: |
80-mm 24:1 barrier screw with spiral mixhead and ball check screw tip. |
| Barrel temp: |
290°F |
| Material to be purged: |
30-melt PP with blue concentrate |
Each of the many Cincinnati Milacron (Batavia, OH; www.milacron.com) all-electric Powerline NT injection molding machines currently molding parts comes standard with the company's patented two-stage injection system — the Sidewinder, as it's known. Molders using a Powerline like having separate control of melt quality in the first stage and injection rates, pressures, and shot repeatability in the plunger-axis second stage.
Every rule has to have an exception, and sure enough, an unnamed medical molder already using Powerline machines wanted another one — but hold the Sidewinder, they said. The customer is not always right, but in this case single-stage injection made sense.
Being as intimately familiar with reciprocating screw technology as they are, Milacron's engineers promptly configured a Sidewinder-less 550-ton Powerline with a single-stage injection unit that met customer specs. But as Milacron's director of technology Mark Elsass told IMM, "No machine goes to a customer without full testing under production conditions."
The molder listed material/color changeover, normally not much of a concern with the two-stage unit, as a critical production factor. Therefore, Milacron, as part of its machine verification, ran a purging test with a couple grades of Dyna-Purge from Shuman Plastics (Buffalo, NY; www.dynapurge.com): Dyna-Purge M for high-temperature resins, and Dyna-Purge K for lower-temp grades. (See box above for details.)
The Powerline ran 20 cycles of good blue parts before purging. Using 6.5 lbs of purging compound followed by 1 lb of virgin PP they cleaned the machine such that the first part in clear PP was good. Both Dyna-Purge grades worked.
Shuman Plastics did not participate actively in this test. Milacron ordinarily uses Dyna-Purge compounds and ran the test with stock on hand. All parties were happy with the result, including the one that counts most—the molder.
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