Injection molding screw rotation speed

For each material a target melt temperature will be given. For a given machine, the actual injection cylinder settings to achieve this melt temperature, will depend on, for example, the screw rotational speed, the back pressure, the shot size and the molding cycle. 

Most of the compounds were extrusion compounded in a conical, partially intermeshing, counter rotating twin screw extruder (Haake Reomix TW-lOO). The extruder speed was set at 50 rpm and the barrel temperature profile was set to produce a melt temperature of 260°C at the die. Samples were injection molded in a 31.8 MT Battenfeld press with a 59 cc shot size. Where noted, samples were compounded in a 60 cc Brabender internal mixer and compression molded. 

The modified screw was placed back into the injection-molding press and evaluated for performance. The barrel temperatures were maintained at 245, 255, 260, and 260 °C for the feed zone through the last barrel zone, respectively. This temperature setting was lower than that used for the original screw. The screw was rotated at a speed of 235 rpm, and the back pressure was set so that the pressure at the tip was 10 MPa. The 0.244 kg part and runner system was plasticated in 4.2 s for a specific rate of 0.89 kg/(h-rpm). All parts produced were completely free of the splay defect. The modifications were able to eliminate the bubbles and the unmelted material. 

This section covers melt temperature and profile, screw rotation, injection speed and pressure, mold temperature and back pressure, cycle management, clean-out procedure, and shutdown and startup procedures. 

Even though the CRD-NRV is designed for injection molding machines the same principle can be used in conventional extruders. In this case there is no axial motion of the slide ring however, there will still be a difference in rotational speed between the slide ring and the rest of the screw. The velocity difference will enhance the mixing action. 

It is important to minimize the rotation speed of the screw. High speeds can be used, in conjunction with the appropriate backpressure, to mold thin and/or long parts. Injection pressure should also be as low as possible. Dimensional stability improves when pressure is lowered because of a reduction in the residual stresses in the molded part. Injection pressure may have to be increased if an improvement in the weld line or a reduction in sink marks is desired. Part design and equipment capability should always be considered in the selection of injection pressure. Backpressure should be kept at the lowest possible value. Increasing it can sometimes help increase the stock temperature. 

Before blending, the light fraction films were ground to pieces of a few mm and the heavy fraction to particles of about 100 im size. The blends were prepared in a Krauss-Maffei double screw extruder at T = 180°C and a rotation speed of 20 rpm. The extrudates were injection molded with a Negri and Bossi nb 55 machine, at T = 220°C in a cold mold and specimens for mechanical tests were produced. 

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