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A Basic Custom Injection Molding Machine Process
Plastic Injection Machine Controller | Basic Plastic Machine Control settings
High pressure | 1st Stage | Boost - Depending on the area in which you live, you will hear a number of different terms used for this phase of the custom injection molding cycle. No matter what it is referred to as in your plant, it remains the same thing and serves the same purpose, and that is to inject melted plastic into your injection mold. This in many ways is the most important part of the basic injection molding cycle, as it has a great bearing on the initial cosmetics and size of your finished plastic injection molded product. You can control the hydraulic pressure and speed with which you perform this phase, and on most newer machines the “fill” or “injection phase” can be broken down into 10 segments or more to aide in precise control of this part of your injection molding process. We will get deeper into the profile settings, what they are, and what you can do with them in the troubleshooting segment of this series. Under normal plastic processing conditions we want to fill the part as close to the 95 -99% full cavity range as we safely can, allowing for the natural resistance of the plastic to absorb the remaining inertia of the screw. When we reach the injection transfer or “cut-off” point setting, we are doing exactly what it sounds like and that is transferring from fill or 1st stage pressure, to pack/hold or 2nd stage pressure. The machine settings controlling these functions are:
Fill Pressure – the pressure setting for fill or 1st stage. In most processes today, this would be set to the maximum of the machines ability which is usually between 2000 and 2500 psi. The reason for setting this to maximum, is so that you do not “pressure limit” this part of the process.
Fill Speed – The speed control for the fill portion process. We want to control the fill speed of our process and not the fill pressure.
Fill Speed Profile – these are the fill segments we spoke of earlier. These profiles can have as few as 2 steps or many as 15 or more. 5 to 10 steps are usually more than sufficient to handle the processing conditions for most processes.
Transfer, transition, or cut-off position – The point of transfer from 1st to 2nd stage injection. This can be done on most newer machines by position (most common), hydraulic pressure, time (least accurate and repeatable), and cavity pressure.
Pack Pressure Phase
Pack or second stage pressure is used just as it implies. When used properly it’s purpose is to pack out the injection molded part. Assuming you followed the 95 to 99% fill rule, you will use the pack pressure to fill out the remainder of your plastic part, until reaching the desired cavity pressure for you process. Typically, there are only one or two segments for this function but I’ve seen up to five, and it will have independent time and pressure settings for each. Pack pressure if used correctly, will finish filling and “pack your plastic into the cavity. Often the 1st step of this pressure will be “equal too or slightly greater than” your transfer pressure so as to prevent the screw from bouncing back at transfer. Again, if you follow the 95 to 99% fill rule on the 1st stage filling process, you can even use a lower pressure pack in many cases without creating a screw bounce situation. This bounce is caused by two things, the first is overfilling the cavity in the 1st stage of fill and second, packing with too low of a pressure when the first condition exists. If the inertia of the screw is fully expended before the cavity is fully filled as should be, the likelihood of this happening is low.
Normal pack setting options, if your machine is so equipped are:
Pack time – Typically, you will have one or two segments of time available if your machine has the pack option installed. This time should coincide with the amount of time to finish filling and pack the cavity to it’s peak pressure for the desired dimensions.
Pack Pressure – settings for the pressure of each segment.
Pack Speed – this is not an option on all equipment but very useful if you have it, especially if you are using your pack to finish filling and packing the part. This will be a low speed option and on some machines it’s even limited to 50% or less.
Written by: WM8C, August 8th, 2006. Not for use without written permission
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