DE102011054278B4 - Method for producing multilayer articles - Google Patents

Abstract

A method for producing multilayer articles from a plastic melt in a Spritzgiessgießwerkzeug, in particular of preforms with a barrier layer, wherein the plastic melt is introduced with a non-changing injection volume of each layer in the injection mold and- determines a change in a flow profile and thus a height and / or thickness of at least one layer, in particular a presence of the barrier layer, determined in the manufacture of the object and possibly monitored and / or regulated / will.

Description

The invention relates to a method for producing multilayer articles from a plastic melt, in particular in an injection mold, in particular of preforms with a barrier layer.

State of the art

Basically, in the WO 2009/040077 A1 a method for monitoring, documenting and / or regulating an injection molding machine with an injection mold into which a melt is introduced, described, wherein a viscosity of the melt is determined directly in the injection mold. The determination of the viscosity is effected by the respective quotients of shear stress and shear rate on the basis of pressure differences, the geometry of the cavity and the flow velocity of the melt. For example, cavity pressure sensors or tool wall temperature sensors could be used for the determination.

From the WO 89/11959 A1 For example, a method is known in which hollow articles can be made from different melts. The melts are injected successively into a cavity and allowed to solidify one after the other.

From the EP 0 034 158 B1 For example, a method of manufacturing a multi-ply article is known. The melt is entered in different layers in a cavity. After entering a relative thickness of the layer is determined by determining the relative flow rate of the individual plastics based on an offset of the respective ram, with which the plastic is pressed into the cavity for each position.

WO 2007/140 447 A1 shows a method for controlling the supply of polymer in a sequential injection molding process. The individual layers are introduced successively into a cavity of an injection molding tool. In this case, a first material is supplied to a plurality of cavities. Independently, for each cavity, the property is determined which is indicative of the volume or flow of the material. Thereafter, the supply of the first material is stopped and a second material is simultaneously supplied to the cavities.

Also the WO 2005/018909 A1 shows a sequential injection process, wherein a first material and a second material supplied to an injection device and there is a melt output, which comprises the first and the second material is prepared. This is then fed to a mold comprising a warm and a cold half. The temperatures of the various zones of the warm half are controlled by temperature sensors to control characteristics of the melt, such as the thickness of the core material. The material is then fed to the cavities.

Object of the invention

The object of the invention is to provide a way of monitoring the presence and / or the thickness and / or the state of a single layer.

Solution of the task

To achieve the object, the features of claim 1.

In the production of, in particular, PET bottles, which are used by the millions in the beverage industry, so-called preforms are usually produced by injection molding, which are then blow-molded into the final bottle. Already during injection molding of the preform, a so-called barrier layer of a second, likewise transparent material is often injected in the center of the melt (similar to coextrusion). This layer is intended to prevent the carbon dioxide escaping from the bottle at an early stage and the product is no longer salable. This is problematic, for example, for very long transport routes. Described is the production of such preforms, for example in the EP 1 980 493 A2 ,

From a technical point of view, the difficulty is to recognize whether and when the barrier layer is actually present even to the bottleneck, i.e., to the area of the screw connection, as this is not visually discernible due to the transparency of the layers. This barrier layer can also be made of PET, but also of other materials such as PE, PP, etc. Neither past attempts with special pressure sensors, nor the use of temperature sensors could be used for clear detection of this barrier layer. Today, this can only be detected with downstream systems, which are very costly.

In general, however, it is within the scope of the invention to detect not only one or more barrier layers of PET preforms, but of all multilayer articles which are produced by injection molding or extrusion (or a combination of both). These include, for example, food containers for margarine or ketchup bottles. Depending on the application, the barrier layer can serve to prevent, for example, oxygen, carbon dioxide or light from penetrating into the vessels.

Although it is already an immense advantage to only detect the parts with a barrier layer that is too low or too filled, and then sort out the bad parts (thus monitoring the process), the process should also advantageously be regulated and changed.

Incidentally, it is not only about detecting the presence of a barrier layer, for example, digitally (barrier layer there or not there), but also a creeping process change of a layer, for example due to contamination, to be gradually detected.

Incidentally, the manufacturing process should also be monitored and / or regulated with respect to the condition of one or more barrier layers.

Preferably, the viscosity, in particular the shear rate, of at least the position to be monitored and controlled in the injection molding tool is determined. The present invention now offers a comparatively simple solution via the measurement, monitoring and control of the viscosity and here in particular the shear rate, but especially not exclusively in the area of the screw connection. The viscosity or shear rate values are determined by detecting the melt front. This is done either with the help of pressure or temperature sensors. However, it is also conceivable to use so-called digital / binary switches (e.g., optical fibers) to detect the melt front.

The basic idea is that the flow profile basically changes when, instead of two different melts, only one melt flows through this area. Since the injection volume of each individual component or each individual layer does not change during this process, the shear rate of the individual layers must inevitably change. At a lower shear rate the barrier layer is missing, at a higher, "optimized" shear rate it is present. Preferably, of course, the shear rate of each layer is monitored so that a very accurate distinction is made between the actual plastic layer and the barrier layer.

In order to determine only the shear rate, it would be conceivable to use any combination of pressure and temperature sensors, as above all the time difference is determined, which requires the melt to flow through the flow path between the two sensors. The simplest and most cost-effective variant would be the arrangement of two temperature sensors, since they are both small and cheaper than pressure sensors.

Instead of monitoring or regulating one or more barrier layers based on a viscosity / shear rate measurement between two sensors, the filling time of the melt in the cavity can alternatively also be determined and used as a basis.

By the regulation, e.g. the height of the barrier layers reached (in the preform, for example, to the bottleneck) or the thickness of the barrier layers are changed.

In the context of the present invention, the sensors are now to be combined. This means that the sensor has a sensor housing in which there are at least two sensors. This can be any combination of pressure and temperature sensors, depending on what is desirable. In this way, a viscosity sensor or a shear rate sensor is realized, which can be arranged in a very simple manner in the mold wall of a cavity. At the same time, the distance of the two sensors from each other is determined and determined. It does not need to be determined retrospectively or postponed.

Claims (4)

Method for producing multilayer articles from a plastic melt in an injection molding tool, in particular preforms with a barrier layer, wherein - The plastic melt is introduced with a non-changing injection volume of each individual layer in the injection mold and - Determines a change in a flow profile and thus determines a height and / or thickness of at least one layer, in particular a presence of the barrier layer in the manufacture of the article and possibly monitored and / or regulated / will. Method according to Claim 1 , characterized in that the viscosity, in particular the shear rate, determined by the layers in the injection mold and / or the filling time of the melt from the layers in the cavity and / or monitored and / or regulated. Method according to Claim 1 or 2 , characterized in that the melt front is detected by two spaced apart in a housing arranged sensors. Method according to at least one of Claims 1 to 3 , characterized in that the Presence of a barrier layer in the neck area of preforms is determined and / or monitored and / or regulated.