Internal pressure injection molding
Internal pressure injection molding or fluid injection technology ( FIT ) is a specialized injection molding process for the production of hollow workpieces. After a conventional injection molding step or after a defined partial filling of the mold, a temporary filler ( water or inert gas, usually nitrogen ) is injected into a partially filled mold in such a way that it acts as an inner molding (core). As the melt is displaced from the center, a cavity grows on the one hand and the melt is pressed onto or into the outer mold on the other. After the melt has solidified, the fluid escapes again.
It is thus similar to the sandwich process in multi-component injection molding .
Fluid injection technique
Gas injection technology
In the gas injection technology ( GIT ) - also: Gas injection molding , internal gas pressure injection molding ( GID ) - displaces the gas and the melt takes at pressures up to 300 bar, the remaining filling. Injection can take place through the machine nozzle and thus through the sprue system or through a separate injection needle directly into the molded part in the cavity . Another variant is the complete filling of the cavity with melt and subsequent blowing out of the melt into a secondary cavity or blowing back into the screw cylinder.
Gas is always preferred when shrinkage is to be compensated for, mass accumulations cannot be avoided, duct cross-sections are very small, water cannot be removed from the component or the size of the injector is decisive.
Water injection technology
The water injection technology ( WID ), and water injection molding or short WID called, is basically equal to the internal gas pressure injection molding with the difference that is introduced instead of gas water over a so-called injector in an injection-molded component. For a long time, the technical implementation failed due to the technical and operational difficulties associated with the medium water (tightness, corrosion). Research projects at the Institute for Plastics Processing (IKV) at RWTH Aachen University, however, showed ways of realizing the process in a practical manner, so that this process has now been successfully established on the market.
Advantages that result from the use of water are the significant reduction in the cycle time (greater heat capacity of the water compared to nitrogen in the case of GIT ) and an improvement in the surface structure, which is particularly interesting for media lines.
Water automatically comes into play if the cross-sections and the channel length are too large for the gas injection technology, depending on the material, and if a smooth, closed surface is required for unreinforced plastics, e.g. B. in the sanitary sector . In general, however, in addition to the low warpage, the lower achievable residual wall thickness also plays a central role. From a business point of view, the significantly shorter cycle times and the non-incurring gas costs are in the foreground when choosing water. In the case of large quantities, this can lead to a reduction in investment costs by up to 50% (halving the production lines due to the increase in efficiency of each individual line)
Selection of the process and combined gas and water injection
Basically, these two primary processes have their specific field of application, whereby this is mainly defined by the component requirement. In the transition area of the process selection, the total costs, the expected number of units or, pragmatically, which system technology is already available, decide in individual cases.
In some applications the known standard methods of fluid injection are no longer sufficient. A growing number of special processes are then available, such as the combination of water and gas injection in one component. Areas with larger cross-sections (e.g. handles) are formed using fluid injection technology, but gas injection technology is used simultaneously to compensate for shrinkage on the ribs. Typical applications are panels with rear ribs and grip areas, door pockets, motorcycle, scooter and luggage racks.
use
By removing unnecessary material from the component core and the holding pressure from the inside acting almost without pressure loss from the inside, new designs and an otherwise unattainable quality of the components, in particular their surface, are possible. The targeted formation of cavities, which must already be taken into account when designing the molded part , enables significant material savings with comparable stiffness of the molded parts and thus results in more economical and at the same time lighter design variations compared to normal injection molding. In addition, shorter cycle times result from the faster cooling through the water or gas injection.
The possible uses of the method do not only extend to conventional thermoplastic materials. It can be used for almost all molding compounds with swelling flow behavior. This applies to most thermoplastics , a large number of thermosets, and also many elastomers .
Particularly suitable molded parts for the application of internal pressure injection molding are, for. B. long, thick-walled parts, such as handles or interior car door modules. These parts are characterized by very large wall thicknesses . It is thus possible to produce plastic parts with very large wall thicknesses without sink marks in one operation. This enables material savings of up to approx. 50%. Significantly shorter cycle times can also result from shorter cooling times due to the smaller wall thicknesses. Two-part products that were previously glued or welded together after the injection molding process can be produced in one operation.
Additional advantages:
- Reduction of the closing force (not necessarily)
- Realization of long flow paths
- Better surface compared to foamed parts
- Partly simpler tool constructions
Disadvantage:
- Additional costs
- Empirical determination of the setting parameters
- Hole at the injection point
- Strength / tightness when sealing is often insufficient
Individual evidence
- ↑ a b c d e Walter Michaeli : Introduction to plastics processing . 5th edition. Hanser Verlag, Munich / Vienna 2006, ISBN 978-3-446-40580-6 , chap. 6.3.3. Process variants , p. 129–132 ( limited preview in Google Book search).
- ↑ Note. Fluid is meant here as liquid