Plastics Technology

AUG 2018

Plastics Technology - Dedicated to improving Plastics Processing.

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When processing via "Scientific Molding," injecting plastic into the mold or cavity is separated into two stages: filling (or "first stage") and pack-and-hold ("second stage"). During the last few weeks I've been getting lots of emails and have had several discussions focusing on just how full the part should be at the end of first stage. And since we call for a greater than 90% full part at the end of first stage, does this mean by weight or by volume? Good questions. Where do we find the answers? To start let's define the terms, since there are host of different names, labels, or expressions for first and second stages. Terms such as fill, pack, hold, high-pressure, low- pressure, V-to-P, etc. have been used. Do they all mean the same thing or are there differences? Having numerous terms that are not clearly defined drives me to near insanity. It makes communications diffi- cult, breeds misunderstanding, and confusion, and makes training especially difficult. So, to be clear on what I mean by first and second stage, see Fig. 1, which is an Injection Pressure (plastic pressure) vs. Time graph depicting first and second stage. This graph is typical for most but not all injection molding processes. (It would not apply, for example, to micro-molding, where the parts are too small to separate first stage from second stage.) In short, first stage fills the part to greater than 90% full, and second stage packs out the part with the remaining amount of plastic needed to finish filling the part, remove sinks, and provide the appropriate part dimensions. Next let's cover the procedure to make a first-stage-only part. The goal is to make a short shot that is greater than 90% full. Seems simple enough; after all, the instructions to make a first-stage-only part is to take off second stage. Problem is, there are three different ways to take off second stage: 1. Take the second-stage timer to zero. 2. Take second-stage pressure to a very low value. I suggest 5 to 50 psi (0.5-3.5 bar) and leave at least 0.50 sec or longer on the second-stage timer. 3. Take second-stage timer to zero and reduce pressure to a low value. Try each one … my bet is you will not pro- duce the same first-stage-only part. So, which one is best? I suggest the second approach, and let me explain why. There is the issue of momentum or over-travel on most molding machines. That is, the screw does not stop at the set cutoff position. It is critical that the processor sees, under- stands and deals with this over-travel in developing first stage. Also, it is critical that the processor ensures that this first-stage- only part is done under conditions where injection or first stage is not pressure limited. Many go with 10% higher set or available Fill Based on Volume, Not Weight Having numerous terms that are not clearly defined drives me to near insanity. Most parts should be 90-99% full after first-stage—by volume not weight. Here's why that's important and how to make a first-stage-only part. Get more insights on Injection Molding from our expert authors: short.ptonline.com/moldingKH Learn more at PTonline.com KNOW HOW INJECTION MOLDING Time, sec 3000 2000 1000 20,000 16,000 12,000 8000 4000 0 First Stage FIG 1 First and Second Stage 0 1 2 3 4 5 6 7 8 9 10 As shown here, first stage fills the part to greater than 90% full, and second stage packs out the part with the remaining amount of plastic needed to finish filling the part, remove sinks, and provide the appropriate part dimensions. Plastic Pressure, psi Second Stage Plastic Pressure in the Nozzle 22 AUGUST 2018 Plastics Technology PTonline.com K now How By John Bozzelli INJECTION MOLDING

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