Plastics Technology

APR 2017

Plastics Technology - Dedicated to improving Plastics Processing.

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It's simple, right? Just unscrew the nozzle tip and screw on the new one. Nope. For those of us who have done this a few times, it does not always go this smoothly. Molding involves hundreds of details … and I do mean hundreds. This one, the nozzle tip, may seem minor. It may be small, but it can cause significant trouble, downtime, lost production, and significant profit loss. How can such a small part cause so much trouble? The accompanying illustration shows the three most common types: general- purpose, reverse-taper, and free-flow. These screw into the nozzle body. They come in different lengths, but like the nozzle body, do your best to keep them as short as possible as they often add a significant pressure drop, which makes filling and packing the parts more diffi- cult. The general-purpose tip is the most common at most molding shops. However, I consider this a "no-purpose" tip, to be discarded. Why? There are a number of reasons: 1. The G-P tip has a long land, which creates a larger pressure drop. 2. It provides a dead space that is difficult to purge clean. Molders can waste 50 to 100 shots changing colors or materials. 3. Research by well-known molding expert Steve Brammer (The Humble Nozzle) show it can produce higher viscosities and temperatures. (Brammer's blog is available at short.ptonline.com/HumblNozzl.) Add these up and you are talking thousands of dollars lost per day in each plant. Small part, small price—but it needs attention. My personal preference is the free-flow type, because it resolves each of the above three problems. Whatever your pref- erence, I would avoid mixing nozzles/nozzle bodies—these also have significant issues and are really Band-Aids meant to compensate for poor screw design. After you pick among the styles, you have to figure out how to control the tempera- ture. You do not want the nozzle body or the nozzle tip to change the temperature of the melt coming out of the end cap of the barrel. Instead, you want the tip to maintain that temperature, since a hotter or colder ring of plastic around the flow channel will show up as a haze, blush, or halo in your part. Controlling nozzle-body and tip temperature is easier said than done, as there is no easy way to measure. I have seen as much as 200° F (110° C) variation from setpoint in a nozzle body and tip. In addition, there can be a problem in cold runners, where you want the tip at melt temperature but it conducts heat to the sprue bushing of a cold mold, so plastic freezes in the nozzle-tip orifice. You will have to experiment to find what setpoint provides the correct temper- ature. (For more details on temperature control of the nozzle body, see my column in the Feb. '13 issue.) Now, here is the procedure for replacing nozzle tips: Please Note: No one procedure is correct for all applications. Never mind ISO 9000; you have a brain—use it. This procedure must be modified for your own situation or conditions. You are responsible for your own safety. How to Pick, Remove, and Replace a Nozzle Tip This small component can cause big problems. Here's my procedure for picking and replacing yours. Free Flow The author's preference among these options is the free-flow tip, as it has a short land, is easier to purge, and does not produce higher melt temperatures and viscosities. Different Styles of Nozzle Tips Dead Space General Purpose Reverse Taper By John Bozzelli I would avoid mixing nozzles/nozzle bodies; these have significant issues and are really Band-Aids meant to compensate for poor screw design. 28 APRIL 2017 Plastics Technology PTonline.com K now How INJECTION MOLDING

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