The exjection process (also: extrusion injection molding) is one of the special injection molding processes. The process is a combination of extrusion and injection molding and is used to manufacture long, thin-walled and structured components, also from viscous thermoplastics, on relatively small injection molding machines with low clamping forces.
The basic idea of Exjection is to move the mold cavity , which is normally fixed during injection molding, through the mold structure in relation to the injection point and thereby decouple the component length from the achievable flow path. This makes it possible to manufacture parts and components from the usual, highly viscous molding compounds without restricting the length of the part.
The injection process
After the mold has been closed on the injection molding machine, the injection process begins, analogous to conventional injection molding, with the injection into the cavity from the fully dosed cylinder. The slide of the injection mold is in the starting position. The entire component geometry is molded into the slide, with the mold cavity being open over the entire length of the gate. A corresponding part of the mold on the fixed nozzle side of the mold partially seals the mold cavity and acts as an extrusion tool with a calibration unit .
The melt is injected into the cavity via a nozzle and a heated nozzle insert, fills the end area and flows away from the gate into the open cavity. A filling pressure of several 100 bar builds up at the nozzle. Even before the melt leaves the corresponding, partially variothermically heated, area of the cavity and steps outside, the slide begins to move in the opposite direction to the injection direction.
The subsequent injection process is primarily pressure-controlled, with the flow front speed and slide movement being harmonized. This means that the gate rushes behind the flow front and a uniform pressure profile with low shear rates is created based on the swelling flow and the viscosity of the melt. This state is quasi-stationary and therefore, with a constant cross-section, independent of the subsequent length of the component to be produced. The filling process with constant process parameters avoids weld lines and results in an even pigment and filler distribution in the component.
The slide speed and the injection profile are controlled via the component length using default values and speed profiles. The controlled variable is the profile cross-section and the component thickness, which determine the cooling rate and thus the slide speed depending on the spray material and the process temperatures. In the filled cross-section, the filling pressure, which also acts as holding pressure, is built up with its maximum at the gate. Depending on the viscosity of the melt, the filling speed and the component wall thickness, filling pressures, and thus also holding pressures, can be selected in the range from about 50 bar to 200 bar . A good structural impression and the avoidance of sink marks are the direct consequence of this condition. The pressure is effective in the mold cavity until the melt has solidified over the entire wall thickness. Each volume element in the component is subject to a time-dependent holding pressure curve. After solidification, the extruded surface leaves the calibration zone of the nozzle insert and steps outside.
Shortly before the gate reaches the end of the cavity, the speed of the slide is reduced and it is brought to a standstill. The end area of the component is filled, shaped and solidified using a conventional injection cycle with injection, pressing and cooling. After the plasticizing unit has been dosed , the mold can be opened. The return movement of the slide to the starting position is still used for cooling, then the component can be demolded, ejected and removed. The total cycle time for the extrusion process corresponds to that of an injection molding process with cascade control with filling from one end of the component.
Realizable component geometries
The extrusion process is primarily intended for the production of profile-like components and strips. Characteristic for extrusion components is a large ratio of component length to the dimensions of the component cross-section. Typical product examples for Exjection are, for example, light band covers, cable ducts, high-performance cable ties , decorative and decorative strips , toothed racks or slide strips.
From the material side, Exjection is suitable for processing all molding compounds that can be processed on conventional injection molding machines. In the thermoplastics sector, this range extends from standard plastics (PS, ABS, SAN, PP) to engineering plastics (including ionomer, POM, PBT, PC, PMMA, PA6, PA66) to thermoplastic elastomers and thermoplastic polyurethanes . As a result of the gentle process sequence, Exjection is particularly suitable for processing high-performance plastics such as polyetherimide (type: Ultem; manufacturer: Sabic Innovative Plastics), polyphenylene sulfone (PPSU, type: Radel R; manufacturer: Solvay ) or PEEK (type: Victrex PEEK; manufacturer : Victrex).
Combination with special injection molding processes
Special injection molding processes such as in-mold labeling, hybrid technology and multi-component technology can be implemented in combination with exjection. The basic process conditions (low process pressure, gentle filling of the component) with Exjection can offer corresponding advantages for the combination of Exjection with a special process. For example, the low level of shear rate in connection with the non-stationary injection point in the production of in-mold decorated components allows the use of sensitive decorations such as real wood veneers .
Developer of the injection process
In 2005 IB Steiner , an Austrian engineering office with a focus on component and process development in the field of plastics, based in Spielberg in Styria, applied for a patent as an in-house development. Since registration, IB STEINER has developed the process in close cooperation with another Austrian company, Hybrid Composite Products , until it is ready for the market. Other project partners involved in the development of the process are the companies Arburg and Engel Austria , both global manufacturers of injection molding machines, as well as other companies from the plastics industry such as the supplier of standard parts Hasco , the company Oerlikon Balzers as a specialist in the field of surface coating, and Böhler Edelstahl as Supplier of tool steels as well as the mold makers CAD-Plast and Dema Engineering.
Available machine platforms
Currently, two manufacturers offer corresponding injection molding machines with hardware platforms for the injection process. These are two large European suppliers of injection molding machines, the Austrian company Engel Austria and the German machine manufacturer Arburg . Both manufacturers offer series-ready solutions for the implementation of Exjection production cells, using both fully electric and hydraulic injection molding machines as the basis.
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- The cavity is cut - Exjection: ready for series production. In: Plastverarbeiter. 9/2009, pp. 76-78.
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