RULE PROCEDURE OF CERTAIN PARAMETERS IN CONNECTION WITH THE PRODUCTION OF PLASTIC ARTICLES.

The present invention relates to methods for manufacturing of plastic articles, and in particular to such a method giving rise to plastic articles having desired wall thickness, weight per meter and/or weight per piece.

The invention is primarily intended to be used in connection with the forming of plastic articles by extrusion, such as blown film and foil extrusion, cable extrusion plastic coating, pipe and rod extrusion, but the invention may also, after modification of the discharge section, be adapted to processes for blowing bottles and injection moulding.

By the invention it has been possible to manufacture plastic articles having precise dimensions without use of instruments for measuring the thickness or for measuring some other dimension, or complicated and expensive weighing equipment for batch weight as referred to in e.g. DE-OS-1 964 386.

An essential problem in connection with the manufacturing of plastic articles is how to be able to maintain an even quality of the articles, and then especially with regard to the wall thickness. The thickness variations occuring in connection with e.g. film and foil manufacturing, result in that nominal correct thickness vary above as well as under said thickness. The result thereof is that one either accepts a product the thickness of which is incorrect, or one has to manufacture a product being thicker than the nominal one in order to be able to guarantee desired thickness. In the first case the quality is faulty and in the second case the plastic consumption is unsatisfactory high. The latter case also gives rise not only to increased costs for the consumed plastic but also for heating, extrusion energy, transportation and so on.

To overcome the above problems, thickness measuring instruments have been used within the plastic industry, but these instruments have caused problems due to often used additives such as pigments and the like, due to variations of the temperature of the sensed article through the thickness thereof, and due to the fact that plastic temperature variations have appeared caused by conditions depending upon seasonal variations and/or product and manufacturing conditions. Accordingly, the result is that it has been necessary to identify a great number of calibration constants which the operator then has had to take into consideration for the continued production. The result thereof is that on-line measurment of the thickness generally is an unpractical and unreliable method which cannot be used successfully for manual or automatical control of the manufacturing process.

Another attempt to overcome said deficiencies is described in DE-OS-1 964 386. This attempt includes the development of a weighing method for the batch weight in accordance with which the material supplied to the extruder or corresponding machine is weighed, batch by batch or continuously, and for a set amount of weighed input material the manufactured length or time of manufacture is determined. Thereby, it is possible to calculate weight per meter or production per time unit. The weight per length or per time unit is then determined by means of a level switch mounted in the lower portion of the extruder funnel or in the pipe between the extruder funnel and the extruder. The closer to the extruder and the less area at the place of measuring the better the accurancy of the weighing. However, this leads to a very small reserve amount of raw material being maintained before the extruder, which in connection with problems in the delivery of raw material or in the weighing equipment for the input material gives rise to production stop and deficiencies in the manufactured product. Production stop means loss of time and reduced profitablity and unnecessary deficiencies in the manufactured article and involve extra trouble for taking care of these faulty articles and increased costs.

Another attempt to remedy the known disadvantages is disclosed in DE-OS-2 700 003. Therein a system is described for the manufacture of extruded profiles, comprising a metering pump provided between an extruder (screw feeder) and the extrusion die. The rotation speed of the pump is controlled by a counter fed with set values, and which in addition controls a stripping control device, in dependence of the rotational speed of the metering pump. An actual value sensor is provided between the metering pump and the extruder for controlling the pressure and/or the rotational speed of the extruder. The temperature of the mass as it leaves the extruder is kept substantially constant.

By this known solution several changes occur contemporaneously, i.a. such that the amount of manufactured product per time unit varies. This leads to i.a. cooling problems which in turn may lead to variations in the quality of the end product.

In the conventional technique in use today for manufacturing plastic articles, an extruder is used which comprises a rotary screw arranged within a cylinder, said screw being responsible for the processing and homogenization of the plastic material, as well as for the discharge thereof to a tool or to a mould for forming the final product. In connection with extrusion, this takes place continuously while injection moulding and in some cases blow moulding make use of a discontinuous discharging, preferably carried out by an axial movement of the screw, in which case the screw operates in a way similar to a piston pump.

The flights of the screw do not work with complete sealing against the surrounding cylinder wall but a slight flow backwards of the material takes place in the gap between the wall and the flights of the screw, which gives rise to the pumping efficiency being effected on one hand by the viscosity and pressure of the melt, and on the other hand depending upon the degree of wear of the screw. These facts give rise to variations in the discharged amount per time unit, in turn resulting in increased scrap level due to product variations and to bad raw material efficiency.

The disadvantages of the prior art methods are overcome with the method according to the present invention, as defined in claim 1

Also in this case, the plastification section may comprise a screw rotating within a cylinder. However, as the plastification section does not need to be used to build up pressure for the subsequent extrusion, this section may be optimized in order to only carry out the processing of the plastic, i.e. the plastification and the homogenization. This leads to a better efficiency with relation to this section and thereby one also obtains energy savings. Also other types of plastification and homogenization principles may be used such as kneading and/or melting during heat supply and stirring.

The discharging section is based upon a volumetric metering device, such as a gear pump or a piston pump. The piston pump may be provided with a device making an alteration of the displacement thereof possible, e.g. by changing the stroke length and/or diameter thereof.

The change in density of the plastic due to effect from pressure and temperature is in accordance with the invention used for control of some parameters in connection with the manufacturing of plastic articles. The density of the raw material is stated in the material specification and may be used as initial value and be used in combination with measured pressure and temperature for calculation of the density at the place of measuring. The density of the additives are normally very close to the density of the plastic and further, constitute so relatively small amounts that the additives do not affect the density of the plastic itself in any degree that would effect the final result.

By measuring pressure and temperature on the suction side of the discharge pump in connection with extrusion, it is possible to determine the production in weight per time unit, preferably grams per minute, and together with measurment of the length, to determine the weight per meter and in connection with the injection moulding and blow moulding weight per manufactured unit. The pressure and temperature measurment on the suction side of the pump constitute in this connection two parameters from which it is possible to determine the density of the plastic mass at the discharge from the pump and which, due to the fact that the volume discharged from the pump is known by determining either its r.p.m. or the total number of revolutions per discharge or the discharge of the piston pump per cycle, can give the basis for determining the production.

The density may also be calculated by controlling the process to constant pressure and temperature on the suction side of the discharge pump. In this case, the density may be set as the calculated value of the density at the set points given for pressure and temperature.

By the above steps brought into practice, the objective is reached to arrive at acurate control of desired dimensions of the final product without obtaining unreliable safety in the production due to too small reserve of material before the extruder, which might lead to immediate production stop if there are break downs in the material delivery or problems in the input weighing system.