Screw Speed and Back Pressure

The advantage of screw machines is that work done by the screw heats and plasticises the mix, under conditions which can be carefully regulated by jacket temperature, screw speed and back pressure. A natural rubber mix leaves the extruder section of the machine 20-30 °C higher than the jacket control temperature. 

In most cases, more energy is introduced into the material by friction than by the hot cylinder wall. Ultimately, this energy must be produced by the drive motor. Therefore, the amount of energy introduced is especially dependent on the screw speed and back pressure setting. The screw speed influences the time required to prepare the material. 

Melt temperature too low Lengthen cycle Increase temperature gradually Increase screw speed and back pressure... 

Asaclean s purging procedure consists of cleaning of the barrel vent hole, cleaning of the screw, cleaning of the nozzle and cleaning of the hot runner mold. Their purging compounds do not work by a chemical reaction. No soak time is required for an effective purge. The maximum allowable screw speed and back pressure are used for their materials. 

Computer controlled injection-moulding machines store the optimum process parameters for each mould/polymer combination. However, a number of machine settings affect each polymer variable the melt temperature prior to injection is influenced by the screw speed, the back pressure on the screw, and the barrel and nozzle heater temperatures. There is no simple way of measuring the melt temperature at the nozzle any thermocouple projecting into the melt would be sheared off by the high flow stresses. Therefore, there is open-loop control on the melt temperature, with the value fluctuating both during and between cycles. 

Time, temperature, and pressure are the primary factors influencing the surface flnish of the molded part. Time factors include rate of injection, duration of ram pressure, time of cooling, cross-linking time, and rotation of the screw (RPM). Pressure factors are applied pressure (high or low), back pressure on the extruder screw, and pressure loss before the charge enters the cavity, which can be caused by a variety of restrictions in the mold cavity. Temperature factors are mold (cavity and core), barrel, and nozzle temperatures, as well as the melt temperature because of back pressure, screw speed, and frictional heat. 

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. 

Bubbles High moisture content Trapped air Check each step of drying process Check dried resins exposure to air Force air out of feed vent Increase screw speed and/or back pressure... 

As the screw rotates at fixed speed, the back pressure affects the screw torque and hence the work done and so the melt temperature the melt temperature increases if the back pressure is increased. 

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. 

Rated capacity usually 60 to 80%. If the shot capacity is too low e.g. less than 30%, then degradation can occur reduce risk by using low screw speeds, barrel temperatures and back pressures. 

Low screw speeds and minimum back pressure are recommended. Back pressure need only be high enough to compact the molten material in front of the screw as the screw rotates and re-... 

The control system is there to ensure repeatability during moulding operation. It monitors both the hydraulic system and the process parameters such as temperature, injection speed, screw retraction speed and injection and back pressure. The ability to control the process has a direct impact on final part quality, part to part consistency and economy. The nature of the control system may vary from a simple relay switch to a complex microprocessor system with closed-loop control. Some of the components of the machine control system will now be introduced. 

The higher the screw speed and the higher the back pressure the higher the melt temperature will be (at the same cylinder temperature)... 

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