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Creating and Troubleshooting a  Plastic Injection Mold Process - Barrel Temperatures

Plastic Injection Molding Process Temperatures - Barrel Temperature Settings

The next things we are going to setup in our process are the injection molding process barrel temperatures.  There are a few ways to set your temperatures and this will be dependant on the type of resin you are molding with and your particular process requirements.  If you are not sure what you should be using, consult your material supplier specification for recommended heat profiles.  The common heat profiles that are used in injection molding are a forward taper, flat or humped, and reverse taper.  It would be impossible to cover all the possibilities without covering all the various types of plastic resin.  The amount of throughput will affect your overall temperature goal.  For example, in a high throughput situation, you will need to run hotter temperatures overall to compensate for reduced residence time in the barrel, while a low throughput setup will not need as much temperature as exposure time to the barrel heats will make up for the lower overall temperature. 

 

A forward taper melt temperature profile is where you use lower temperature settings in the rear and increasing in temperature as you move forward up the barrel.  A reverse taper is just the opposite, starting hotter in the rear and getting cooler in the front.  An example of where a reverse taper profile would be desirable and useful, is with a glass filled nylon.  Running a higher melt temperature in the rear will reduce breakage of the glass fibers and also reduce the wear on the injection screw and barrel by reaching the melting point sooner in the profile and thus reducing the need for excessive friction to accomplish this.  Using a flat or forward taper is more common with non abrasive or amorphous resins, as they will not pose as severe a threat to the injection screw and barrel with regard to wear and the screw will add to the melt temperature of the plastic resin through frictional means as well.  Again throughput always needs to be considered and designing the best profile for your particular plastic resin and part geometry will vary greatly.

 

You should never rely on the temperature “settings” of the machine for a reliable “actual” melt temperature.  A lot of things can happen or go wrong with your temperature control systems, and they should be treated as nothing more than guidelines or set points for reference.  Unless you have a “melt stream” thermocouple in your nozzle area with which to get this reading, actual plastic melt temperatures should be taken by use of a hand held pyrometer or thermometer with the appropriate probe attached.  Make sure you keep your probe moving while you are taking temperature readings from the actual plastic melt or purge, as plastic is also a good insulator and will quickly coat your probe if you don’t, leaving you with incorrect readings.  Another tip to get accurate readings is to heat the probe with a torch or lighter, being careful not to over heat the probe, to near the expected melt point of the plastic resin.  That way when you put the probe into the purge for a measurement, you will get a quick and more accurate reading before the plastic purge cools too much. 

 

These hand held pyrometer or thermometers and probes, are available through most plastic supply catalogs and are very similar in their accuracy and use.  It is also important that you don’t try to take actual melt temperature readings with one of the many “infrared thermometers” available on the market today.  These have become quite inexpensive and while they have a definite use and purpose, it is not for checking melt temperature of plastic.  These instruments were designed with the uses of checking motor, relay, and other electrical or hydraulic components for “hot spots” in mind.  They rely on the reflectivity of a surface to take their measurements, thus are not an accurate instrument for the purpose of melt temperature readings.  Since these can only check the surface temperature of an object, you will get very inaccurate and usually lower incorrect temperature readings from a plastic purge when they are used in this manner.

 

In establishing proper processing conditions, one the most important things to keep in mind is the melt temperature.  If your melt temperature is too high, burning, black or brown streaks and specs, & splay are just some of the likely defects.  With temperatures that are too cold, non-melted plastic pellets, poor knit or weld line quality, and poor surface finishes are some of the likely possibilities.  Take care to develop this part of your process as the part quality literally starts here and it is a very important part of your process.

 

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Written by: WM8C, August 14th, 2006.  Not for use without written permission

 

 

 

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