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Plastic Injection Molding Machine Processing
Plastic Injection Settings, Speeds, and Pressures | Basic Injection Control settingsInjection Settings
Hold Pressure Phase The hold pressure phase of the process is meant to maintain the custom injection mold process pressure inside of the cavity until the gate or gates freeze off which then will be released, as pressure no longer need be applied after that point. Once gate freeze off has been attained, no further pressure can be added or removed from the plastic inside the injection mold cavity, as the gates basically act as a cork after that point of the process. Once gate freeze has occurred, and the hold pressure has been released from the cavity, the cooling portion of the cycle may begin.
Hold pressure on most newer machines is also provided in segments with individual time and pressure settings for each. There is typically only one hold speed for the entire hold pressure phase. This again is usually a very low speed of less than 30%, as we are not trying so much to move plastic at this point of the injection molding process as we are maintaining that pressure in the mold cavity until the gates have frozen preventing any further pressure loss. If your particular machine is not equipped with a pack pressure option, you can sometimes use the first one or two segments of your hold pressure phase as “pack pressure”. This again will depend on the abilities of your individual machine and controller capability. Most machines built in the last 10 – 15 years have at least 5 and 10 segments of hold pressure.
The machine settings controlling these functions are: Hold Speed – this is usually on one speed setting that covers all of the hold segments. Hold Pressure Profile – each segment of hold will have an independent pressure setting and can be ramped up or down for each segment change. Hold Time - each segment of hold will have an independent time setting and can be changed up or down for each segment change. Use as few of these segments as necessary to provide a good molded product. Many molders have a tendency to take the approach that if there are 10 segments injection hold pressure, that we should use 10 segments of injection hold pressure. In reality, most injection molding situations will require less than 2 pressures segments and rarely more than 5. Some exceptions to that might be when you are using one or two segments as pack. Hold pressure also will most commonly remain the same over the profile or reduce to lower pressures in each phase when using multiple segments.
Screw Rotate and Back Pressure Phase The screw rotate and back pressure portion of the process occur during the cooling phase of the injection molding portion of the cycle. Back pressure is applied as a resistance pressure to the screw as it rotates, while developing the shot for the next machine cycle. The purpose of the back pressure is to aide in providing a consistent density of the shot size as the screw rotates and conveys plastic resin towards the front of the screw. This is very important for the injection molding process “shot to shot” consistency that is required by today’s high precision injection molding process requirements. On most machines, back pressure is usually limited somewhere around the 25% mark of total system pressure. Back pressure in most cases is a very important function of the molding process, but it also contributes the wear of the screw and barrel components, thus keeping this as low as possible and still be able to produce a consistent and homogenous melt stream is very desirable. Depending on the resin you are processing, this could be very low in the range of 50 PSI or upwards of 300 PSI or more. Back pressure also contributes “mechanical heat” to the melt stream and the higher the back pressure is set, the more mechanical heat that will be added to the injection molding process through it’s use.
The machine settings controlling these functions are: Screw Speed – This setting controls the RPM’s that the screw turns at up to the machine rated speed. This can be a few hundred RPM’s on a smaller machine, but typically will be less than 150 RPM’s on larger machines without special options. This setting also contributes to the time that it take to get the screw from it’s forward position during the injection molding process and the full shot size position. The ideal condition is to get the screw back to its full shot size position just prior to the opening of the mold at the end of the cycle, to prevent excessive idle residence time for the plastic in the barrel. Back Pressure Profile– This function on most newer machines, is also segmented with individual pressure and position settings for each segment.
Written by: WM8C, August 8th, 2006. Not for use without written permission
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