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Creating and Troubleshooting a Plastic Injection Molding Process
Now that we have covered all the basic plastic injection molding machine components and their functions, as well as the simplest of custom plastic injection molding process techniques, we can begin to take a closer look at the injection molding process itself and also common problems and troubleshooting methods to employ when trying to solve them. We had earlier discussed the main elements of the custom injection molding process in brevity, and now we can take that knowledge and review them more in depth. This will include how to best use this knowledge to design some robust injection molding processes for our custom injection molded products. Below is the basic molding process components and small graphic to explain them.
Cycle Time –Main Components of the Injection Molding Cycle
Total Injection Molding Cycle Time The amount of time it takes to perform all of the steps in creating an injection molded part. This includes part removal and all aspects of time from “mold open to mold open”.
Mold Close The amount of time it takes to move the clamp from it’s full open position to a it’s fully closed position.
Fill Time The amount of time it takes to fill the cavity with plastic and reach the transfer or cut off point.
Pack/Hold Time The amount of time to complete the pack and hold phases on the injection process
Cooling Time /Screw Rotate Time The amount of time it takes to cool the part before removing it from the mold.
Mold Open Time The amount of time it takes to open the mold, remove the part, and begin the new cycle.
Mold Cooling – Chillers: Once the machine and mold have been set up and the hopper has been loaded with the plastic resin we will be using, we are now ready to begin entering the beginning parameters, auxiliary equipment, and machine settings with which we will produce our plastic part. The first things we will want to set are the mold cooling temperatures. This is typically done with one of two things, a chiller which just as it sounds, supplies chilled water, or a mold temperature controller that supplies warm to hot water to the mold.
With a chiller, the temperature of this water will typically be between 60 and 33 degrees. You can also use a chiller to supply water to a mold that is below freezing or 32 degrees F, (0 deg. C) but and additive such as glycol will need to be used in order to prevent the chilled water from freezing. This is not a common process temperature and is usually reserved for extreme molding conditions. Chilled water above the freezing point up to about 60 degrees however is very common. The biggest disadvantage to using chilled water is condensation, that tends to build up on the mold cooling lines as well as the mold surfaces themselves during humid periods. Once the dew point rises above the temperature being used, this will occur. Depending the part of the country or world you live in, this can be many months of the year. You can improve the condensation issue on the water lines themselves by using insulated hoses but there isn’t a lot that can be done for the mold itself. The biggest risk involved with chilled water is rusted molds and mold surfaces on humid days.
I have seen some highly polished mirror surfaces turned to a rusty disaster within a fairly short time simply because someone forgot to turn the water temperature back up to at least 70 degrees or so while the mold sat idle. However, a huge advantage is to be had in many cases by using chilled water by reducing both the gate freeze off and cooling times required to mold a given plastic part, therefore providing in many cases a substantial savings in over all cycle time, which translates to dollars on the bottom line of any custom injection molding operation.
Mold Temperature Controllers, also known as thermolators, mold heaters, or sterlco’s (and I’m sure a few more) depending where you live, are the most commonly used item in most injection molding processes for mold cooling. Even though is sounds a bit like a misnomer, the water even though it’s being heated, is still cooler than the temperature of the plastic being injected into the mold and therefore is still cooling the plastic. Common mold temperature controller settings are between 90 and 220 degrees F (32 – 104 C) depending on the resin and surface quality required. There is no real risk of condensation at these temperatures but care and caution in handling of the lines is in order. The most common injury to people using these units are burns. Always replace any hoses or fittings that show even a small amount of wear or are damaged in any way before the unexpected happens and your taking someone to the hospital for treatment. You also should always cool the water temperature down to at least 90 degrees (32 C) before disconnecting any lines from the mold or mold temperature control unit. In some rare cases with exotic engineering resins, hot oil units may be used for mold cooling but we will not discuss these units in any detail here.
Once you have your mold temperature set to the desired point, we can move on to the next temperature settings which will be the barrel temperatures.
Written by: WM8C, August 9th, 2006. Not for use without written permission
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