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| Al^sy' Evaluation of Different Injection Molding Screw Designs for Better Color Mixing | |||||||||||||||||||||||||||||||||||||||||||||||||
| every pellet knows our name™ | |||||||||||||||||||||||||||||||||||||||||||||||||
| Walter S. Smith, Timothy W. Womer, Robert A. Sickles Xaloy® Corporation, New Castle, PA | |||||||||||||||||||||||||||||||||||||||||||||||||
| Abstract Improved color mixing for injection molding can be improved by different variables. Screw speed, melt f temperature, backpressure, barrel temperature profiles, screw design, dispersion discs, etc. can all influence color dispersion. This study will focus on the screw design and its affect on color mixing and its affect on other process parameters. | |||||||||||||||||||||||||||||||||||||||||||||||||
| Resins Four resins were used for this study | |||||||||||||||||||||||||||||||||||||||||||||||||
| • PolyOne LDPE • PolyOne HDPE • PolyOne ABS • Profax 6523 PP | |||||||||||||||||||||||||||||||||||||||||||||||||
| 12 g/10min MFR 6 g/10min MFR 2 g/10min MFR 4 g/10min MFR | |||||||||||||||||||||||||||||||||||||||||||||||||
| Introduction Screw design is an important factor in achieving proper color mixing and melt homogeneity in injection molding processes. Screw design can affect melt temperature, recovery time and color dispersion. By using the proper screw for a particular application, scrap rates and cycle times can be reduced through decreasing recovery times, lowering melt temperatures and improving color dispersion. Equipment The Injection Molding Machine used for the study is a Toshiba Model ISS 85. The machine has a clamping force of 82.6 tons and a screw diameter of 36 mm. The injection capacity is 3.63 oz PS, with a maximum screw speed of 150 RPM. The machine is equipped with 4 heater zones and feed throat cooling. A low pressure dryer and a thermolater were used as auxiliary equipment for this project. There were four screws used in this project. Screw number 1 is a variation of a general purpose screw, designed for higher output. Screw number 2 is a General Purpose screw with a compression ratio of 2.83:1. Screw number 3 is a General Purpose Screw with a 2.83:1 compression ratio which is the same as General purpose screw number 2, but this screw also has 2 turns of a dispersive mixing section on the end of the screw. Screw number 4 is a patented barrier screw with a new mixing section designed for lower melt temperatures and higher outputs. The barrel on the injection molding machine is drilled every 2 L/D, and a thermocouple was inserted into each hole in order to measure the temperature gradient along the length of the barrel. The mold used for this study is a 2 cavity box mold which uses 70 % of the maximum stroke of the machine. | Experimental Procedure Each of the four resins was tested on each screw. For each test, the machine was heated up to a set temperature and allowed to heat soak for 1 hour. After 1 hour of heat-up time, the machine was allowed to run cycles for 30 minutes to stabilize. After the initial 30 minute stabilizing period, data was collected for another 60 minutes on the NETDAQ. After this 60 minute period, melt temperature was taken with a hand held melt probe. The data recorded on the NETDAQ for this study consisted of: 1. screw recovery time 2. 11 temperature readings along the length of the barrel. 3. hydraulic system pressure to measure injection pressure, hold pressure and backpressure During this study each screw design was tested with each material. The exact same process was used for each screw on the same material. The process was altered somewhat when changing between materials. During the tests, the screw speed was changed to 3 different settings to record the screw recovery time at each interval. The screw recovery time was tested at 100 RPM, 125 RPM and 150 RPM. All data was recorded on the NETDAQ every .02 seconds. The data was then extracted from the NETDAQ using a spreadsheet program. The screw recovery time, 11 temperature readings and hydraulic pressures were then graphed in excel. After all testing was completed; the Polypropylene material was mixed with a red color concentrate at 1 % mixture. This mixture was processed on all 4 of our test screws. This light mixture made the parts translucent. The parts could then be held up to a light source to evaluate any color swirls or non-dispersion of color pigments. | ||||||||||||||||||||||||||||||||||||||||||||||||
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| A Fluke Data Acquisition System was used to acquire data from the process. It will be referred to as the NETDAQ. | |||||||||||||||||||||||||||||||||||||||||||||||||
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