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Moving moulds part 2 | moulding masterclass


Figure 1: Injection unit and screw combinations for 2,100kn Sumitomo SHI Demag Systec 210/580 model Injection unit Screw diameter L/D ratio


430


mm 35 20


g


40 20


45 20


600 840 1450


40 45 50 45 50 60 50 60 70 20 20 20 20 20 20 20 20 20


Specifi c injection pressure bar 2644 2025 1600 2423 1914 1550 2402 1946 1351 2426 1905 1400 Maximum shot weight (PS)


153 210 266 210 294 383 326 402 579 482 695 946


machine and achieve optimum performance capability and output at the standard cycle in the minimum amount of time. As both designated machines in this case had the same clamping unit specifi cation and shot capacity, then it tends to be taken for granted that they are identical from a process setting and moulding performance viewpoint. Taking a closer look at the information given in Figure


2, however, it can be seen that there are subtle differ- ences between the injection unit attributes of the two 2,100kN machines. These differences are not defi ciencies but are characteristics of the injection units and will have a signifi cant effect on the selection of process parameters and the relevant outcome when transferring either the box or lid mould tools from Machine B to Machine A. In this scenario, we will be considering the box component and related mould tool to be installed into each moulding machine for the same component quality standard, output and performance effi ciency to be achieved. In comparative terms both Machine A and Machine B


possess the same clamping force capability, screw diameter and geometry, shot volume, screw stroke, hydraulic pump pressure (in this case 180 bar), and general construction. It can be seen from Figure 2 that with the same maximum pump pressure, the maximum available specifi c injection pressure is 398 bar lower for Machine A than Machine B, assuming a maximum pump pressure of 180 bar hydraulic. This means that Machine A will require a higher hydraulic pressure of 1.196 bar compared to Machine B for each 1 bar incremental change. Such a small difference may seem insignifi cant but when the box mould tool is sited in Machine A the transfer of identical hydraulic pressure settings will not produce the same component as produced from Machine B. Based upon the process trials when commissioning


the box mould tool using Machine B, it was found that an optimum fi ll time of 0.58s was required to obtain both the visual and structural requirements of the box moulding. At this fi lling time an injection pressure of 158 bar hydraulic pressure (2,127 bar specifi c) was recorded. When converting this fi lling time into a volumetric injection rate, a value of 179cm3


/s, while the injection rate for Machine A is


Figure 2: Injection unit specifi cations for Machine A and Machine B shows performance differences despite similar screw diameters, L/D ratio, and shot capacities


Injection unit classifi cation Screw diameter Screw geometry L/D ratio


Specifi c injection pressure mm


Machine A Machine B 430 40


600 40


Standard Standard 20


20 bar


Maximum cylinder head volume cm3 Maximum shot weight (PS) Maximum injection rate Plasticising rate (PS) Maximum screw stroke Nozzle sealing force


g cm3 /s g/s Number of heating zones Screw cylinder heating capacity kW 224cm3/s. Such differences will require a different


linear speed to be set to achieve the same injection time for both machines, highlighting the importance of recording both the injection time as well as the linear injection speed. A more important feature regarding the 0.58s fi ll


time is related to the injection pressure needed to achieve box mouldings of consistent quality when the mould tool is sited in Machine A. The derived injection pressure value of 2,127 bar used in Machine B cannot be achieved from Machine A, where the maximum specifi c pressure value derived is only 2,025 bar at a corresponding 180 bar hydraulic pump pressure. This lack of injection pressure will prevent the mould


cavities being consistently fi lled on a cyclic basis, causing component quality and productivity issues. In moulding terms the cavities will be fi lled using Pressure Control rather than the more preferred technique of Speed Control (reference Article Nine). Even though Machine A can deliver the required volumetric injection volume in cm3


/s of molten


PP is obtained. Upon reviewing the data given in Figure 2, the maximum injection rate for Machine B is 187cm3


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2025 231 210 224 52


mm 184 kN


80 4


11.1


2423 231 210 187 32


184 80 4


11.1 Source: Sumitomo SHI Demag


/s to suitably fi ll the


box mouldings in 0.58s, the inherent resistance in both the hot runner system and impressions of the mould tool necessitates a fi lling pressure of 2,127 bar. In an attempt to reduce this resistance pressure with the objective of achieving a value of 2,025 bar or below,


January/February 2014 | INJECTION WORLD 27


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