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'Common Practice' is Not Always the Best Solution

A frank conversation with a 2nd generation molder



"Give it more pressure!"


That was the first thing they told me as I began my career in the plastic injection molding industry. I later learned that my problem was not simply an incomplete part, but a faulty process, a faulty mentality. I was trying to cover over a symptom without actually solving the root cause of the problem.


I have seen this time and time again throughout my career. Guys chasing alarms all day and night. Making adjustments to different variables at the same time and walking away without confirming their "fix" actually did not cause a bigger problem. Is it laziness? Perhaps. Could it be complacency? Maybe. Or could it be that we're missing something here? We might need to look at the bigger issue. Hundreds of new molders year after year receiving the same bad advice: "More pressure!".


Take short shots for example.


An incomplete part is obviously a bad part. It is no good to anyone. Depending on the length of the cycle time, you might have an angry operator scrambling to find the "good" parts among the shorts by the time your process guy arrives. As a company, you may be able to grind the bad parts and reuse them on another job, but this requires time, labor, and other costs related to re-processing material. But worse yet, imagine that somehow some of these bad parts make it to the customer. Now everyone looks bad!


Lets examine a common problem and potential solutions. Keep in mind this is only a starting point. A good processor will take the time to find and fix the root cause.


SHORT SHOTS


Problem: A situation where one or more cavities fail to properly fill. This results in an incomplete part.


What Causes Shorts: Shorts can be caused by a wide variety of factors. Flow is important. Perhaps the material being used is too viscous causing the flow to be inadequate. The material begins to solidify before reaching the desired fill point, resulting in an incomplete part. Insufficient venting or degassing may also cause shorts. During cavity fill, the air or gas ahead of the flow path may not escape properly so any space being occupied by gas or air cannot be occupied by the plastic material resulting in a short shot.


What can you do: Increase flow, decrease viscosity. "More pressure!" is not always the answer in this case. More pressure means more wear and tear on equipment. More pressure means more money spent long-term. More pressure may actually cause other problems. If the material and part allow it, increase melt temperature instead.


Check the basics first. Do you have material in the hopper and barrel? Is your cushion established and is it being made? Are you "bottoming out" the screw? Analyze the situation. Find the root cause. After observing the situation you may find that it may not be a machine-specific parameter but another secondary process that has changed.


Finding the root cause and addressing that issue will be better for everyone. "Common practice" may not always be the best way to go.

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Vico Plastics Inc. 608-884-9096

410 Haugen Rd. Edgerton, WI 53534