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A special screw configuration was developed specifically for long fiber applications:
| - |  | First the matrix polymer is melted |
| - | | Then the melt is intensively but gently mixed and homogenized |
| - | | And finally after the continuous fiber glass strands are drawn in as rovings by the rota-tion of the screws, the rovings are impregnat-ed with polymer, cut to length and dispersed. |
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The diameter ratio Do / Di determines the free volume of a closely intermeshing, co-rotating twin screw, at a given shaft center distance.
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| There is an upstream gravimetric feeder for the matrix polymer and, if necessary, gravimet-ric feeders for additives and pigments as well. The components in the formulation are fed in a continuous stream, and this ensures uniform formulation at all times. |
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The rotation of the screws in the ZSK draws in the continuous strands. The screws are specially designed to be compatible with the matrix viscosity and roving characteristics.
They produce a steady pulling force, good impregnation and disper-sion of the individual filaments and good fiber length distribution in the melt which is ready for insertion into the press. Each roving is monitored individually to ensure that a miss-ing or dropped roving is detected immediately and a signal sent to the machine controller.
For quality control purposes, the shut-down limit can be set at the controller. Rov-ing spools can be changed without stopping the machine. Every roving feed station has an active and a passive slot.
Following compounding, the melt is continu-ously extruded though a die. The sequence to form of a completed blank for the batch press process is technically complex. First, the melt is cut to length at the die outlet. The flat ex-truded material is fed into a warming unit and separated into blanks. The system buffers the blanks, and a forced-air convection oven keeps them warm until the last blank has been extruded and cut. The set of blanks is then placed onto the staging conveyor. Immediately prior to the press operation, an operator or a robot with needle-type grippers picks up the blanks and places them into the press mold.
| The advantages of height-profiled extrudates |  | |
The shape and the position of the extrusion in the mold has a major influence on the press force required for the mold fill as well as on twisting and thickness tolerance characteristics.
The current state-of-the-art method involves placing the blanks into the tool in two or three layers. This method was and still is used during production of glass mat reinforced thermoplastics. To automate the process, very complex and heavy needle-type grippers with up-and-down stroke cylinders are needed to carry out the stacking operations.
Given these difficulties, an obvious solution was to add a servo motor to the dies, which already had a manual height adjustment. The motor can be used to change extrusion height during the extrusion process. Careful attention has to be paid to the interaction between the extrusion, the cutting unit and the discharge conveyor.
The extraction speed must be adapted to the extrusion speed. A change in height is always accompanied by a change in speed, and corrections have to be made to compensate for the displaced material or filling in the die.
Height-profiled extrusions have a number of other benefits:
| - | | Up to 40% reduction in press force (depending on glass content and part geometry). |
| - | | Less part distortion due to reduced fiber orientation |
| - | | More uniform part thickness |
| - | | Needle-type grippers can be simplified, because it is usually possible to use grippers that do not have a stacking function |
Normally, only grippers are used to handle height-profiled extrusions, and this significantly reduces the investment in automation equipment. To some extent, existing handling devices can be used for larger or heavier parts, because the grippers are lighter.
| Fast commissioning | | |
Existing lines are often converted from GMT to D-LFT. To minimize down time on existing presses, the entire D-LFT system features a modular, plug-in design. The systems are completely assembled in the factory and operated using the process material. This can reduce commissioning time, including acceptance runs, to two weeks.
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