For the processing of thermoplastics the predried granulate is melted in the plasticizing unit of an injection molding machine and then conveyed by the screw to the molding via a spreader system.
This spreader system is also called a gating system. The primary requirement for the gating system is extremely careful transport of the melt to the molding. On the basis of
* the geometry of the molding,
* processing-related reasons and
* economic aspects
the gating system can be divided into the following areas:
* hot runner,
* cold runner,
* sprue and
* Optimal component quality
* no sequential faults
* gentle processing of the plastic melt taking into account process and component requirements Gating system of a center console
Structure of a gating system
A number of demands are placed on a gating system both by the processor and by the raw material supplier. To meet the constant cost pressure, processors attach importance to smooth and optimally automated production. Furthermore, they require a large processing window, but for reasons of economic efficiency wish to realize a short cycle and be faced with little sprue waste. From a rheological point of view, however, low shear, temperature increase and pressure loss are optimal conditions for trouble-free production. To meet these, in part, conflicting requirements, the gating system is divided into different sections.
* In the hot runner the melt is maintained at the proper temperature by means of electric heating. In this way even larger distances can be bridged over in the mold without pressure losses. The hot runner ends in a hot nozzle, which represents the boundary line between the heated and unheated zone.
* The cold runner is the link between the hot runner and sprue section and bridges the section that can no longer be reached with the hot runner. A large temperature gradient is produced between the cold mold wall and the melt in the unheated cold runner. Therefore, the melt in the cold runner is exposed to greater shear and accordingly is subjected to a greater load. Short cold runners make it possible to keep pressure loss and material load within acceptable limits.
* The sprue is the transition area between cold runner and molding. Varying sprue designs are used here, depending on the design and demands on the part.
* The gate point is the transition point from the sprue to the molding. Often rheological necessities and demands on surface quality clash at this point. Two aspects must be kept in mind:
1) A small gate is less visible and can be removed from the mold better.
2) In a gate whose size is too small the melt is exposed to an extremely high shear that may lead to surface problems, such as streaking and collapse.
Design of a gating system
The gating system guides the melt prepared in the injection molding cylinder to the molding and has direct and indirect effects on the quality of the molding. Against this background, the article designer and the tool and die maker should take these aspects into account right from the design phase in order to guarantee a trouble-free processing process and constant component quality.
In the design of a gating system the focus is on the component. Factors that influence the design of the gating system include:
* targeted surface quality,
* mechanical requirements,
* specified thermoplastic,
* complexity of the geometry and
* economic efficiency.
Runners and sprues that are dimensioned too small lead to excessive load on the melt due to high shear in the gating system. In extreme cases this may lead to chemical degradation, resulting in a drastic reduction in the mechanical capacity and streaking in the gate section or over the entire molding. To ensure optimal processing, the following points are coordinated step by step:
* gate location,
* gating technology,
* gate type and
* dimensioning of the gating system.
What used to be determined in individual steps and frequently through initial matching is now possible with the help of constantly optimized software for simulation of the molding and of the filling process prior to die making.
When determining the gate location, the following must be given consideration:
* the flow path,
* the filling volume and
* the position of the joint lines.
The first step should be to check the filling situation on the basis of the filling pressure curves. The latter provide material-specific information on the fluidity in comparison to the pressure needs as a function of the wall thickness. With regard to the location of the gate points, the respective flowability of the material should not be exhausted so as to avoid unnecessarily restricting the processing window. If the molding is filled in several gates, it must be kept in mind with respect to the joint that
* a joint line is created that may be visible and represents
* a mechanical weak point.
Therefore, it is often necessary to select the location of the gates such that the joint line is not in optically critical exposed sections and areas subjected to mechanical load. Furthermore, the location of the sprue may depend on specific component requirements and production processes. If, for example, IMD (in-mold decoration) is used for a molding, the flow over the film has a decisive influence on the gate location.
After determination of the sprue location the next step is to define the gating technology. Depending on component size and requirement, a number of methods are available, from three-plate to cascade technology. Selection of the technology to be applied depends on the targeted result. The mold construction time and the costs are the primary factors in the making of prototype molds. Therefore, a simply designed cold runner system is usually used here, possibly in a three-plate mold. If runnerless injection molding is required, the appropriate hot nozzles must be selected to support such a process.
Type of gating system
The transition area between the cold runner and the molding is designated as the sprue. The point where the melt flows into the molding is called the gate. The selection of sprue and gate depends on the requirements to be met by the gate point and the filling characteristics.
* Tunnel gating system The tunnel gating system meets the requirements of automatic production and is therefore applied very often. In connection with the design, however, adequate cross-sections must be ensured in order to avoid high pressure losses and application of unnecessary shear to the melt, depending on the length of the tunnel.
* Film gating system The film gating system is especially well suited for optical components, such as headlight lenses. The arriving melt is placed across the width of the film gate via a pre-distributor and an even flow into the molding is achieved through skillful dimensioning of the pre-distributor. This type of gating enables low-stress filling, but requires mechanical removal of the sprue.
* Tunnel gating system with auxiliary pivot or bent tunnel gating system The tunnel gating system with auxiliary pivot or bent tunnel gating system makes it possible to place the gate point on the concealed internal side of the component in such a way that the gate is not visible. The molding is injected by means of a pivot that is connected to the molding. The pivot is held by an ejector pin, it remains on the molding and must be removed later. There are a number of different variants that have been derived from the basic versions according to the conditions and requirements.
Dimensioning of the gating system for plastic mold
During the filling process the melt is pressed through the gating system. Because of the specific melt viscosity, filling volume and injection speed, a varying amount of pressure is required to fill the molding. Substantial pressure losses may occur in the gating system depending on the length and diameter distribution of the gating system. The diameters in the individual runners should be selected such that the processing conditions of the melt are not exceeded. If these limits are exceeded, there is a risk of a melt degradation that has a direct influence on the mechanical characteristics, e.g. impact value.
Post time: 02-11-2017