CO2 was injected into the autoclave to impregnate the plastic granules and processed using conventional injection molding equipment.
Foamed PC parts have been reduced by 16% in weight and the surface has been smoother due to the gas counter-pressure (Source: Kunststoff-Institut) Trends in electrification of automobiles The current trend toward lighter-weight products has become a current trend.Now Kunststoff -Institut Lüdenscheid and Linde Gas have developed a new physical foaming process that eliminates the need for costly reworking or machining of injection molding equipment that has the flexibility to appeal to small batch production and mass production companies. If combined with the gas back pressure method, you can also give the product a beautiful appearance of the surface.
Numerous OEMs are aggressively tapping the potential of lightweighting as they assemble vehicles, and plastic parts are also being put on the line of sight as regulations push forward to reduce emissions of hazardous substances and a series of discussions around electric vehicles, Foaming technology is an effective way to reduce the weight of injection molded parts, and in the automotive industry where weight loss is the primary concern, other industries are also interested in foaming technology because it reduces the amount of material used, which means Cost reduction.
Foam injection molding is also technologically advantageous because it significantly reduces the viscosity and hence the melt flow properties, as the filling pressure of the foamed material becomes smaller, to a certain extent, no definite dwell time is required, Reduce injection molding machine clamping force and the stiffness of the mold to switch to a smaller injection molding machine, lighter injection mold means that can bring significant savings.Originally need to use steel compact injection molding machine to be able to process large parts , If the use of foam injection, may only need aluminum mold can be.
Improve dimensional accuracy by foaming
Foaming techniques can have a positive effect on the quality of the part, and many plastic anisotropy shrinks as one of the major causes of component distortion and dimensional accuracy.As foaming reduces shrinkage because bubbles can create a complex internal component Of the pressure retention, thereby reducing the difference between the contraction, thereby improving the dimensional accuracy of the components.
For a variety of reasons, there are a variety of foaming technologies available on the market today where we make a distinction between chemical and physical foaming processes.
Comparison of chemical and physical foaming process
In the chemical foaming process, the blowing agent is mixed with plastic particles, usually masterbatches, in a weight ratio of 0.5 to 2% The advantages of chemical foaming include easy handling and processing with existing conventional injection molding machines, Compared with the foaming process, the foaming pressure is low and the foaming of the thin-walled part is limited, and the product decomposition caused by the chemical reaction may cause residue on the mold.
The mold or injection molding machine should be equipped with a nozzle or end closing system to prevent the synthetic foams from escaping from the plasticizing unit. The position of the screw should also be controlled to ensure repeatable processing. The bubble causes an uncontrolled back pressure on the screw after the metering work has been completed. Both physical requirements must be met when performing physical foaming.
In addition to the above requirements for injection molding machines, the known physical foaming method requires modification of the injection unit and the screw, with the addition of peripheral equipment to control the supply of gas, depending on the process, which is why it is necessary The reason for the special plasticizing unit to adapt to different processes lies in it.
For the physical foaming technologies currently available on the market, foaming agents are mixed directly with plastic granules or melts in an injection molding machine.The most famous representative technique is the ProFoam process (Arburg / IKV) The foaming agent in the interior enriches the plastic particles and exerts pressure on the rear sealed screw; and the MuCell process, which adds the blowing agent to the blowing agent in the reformed plasticizing unit.
Since there is no chemical reaction, the product does not deform and no build-up on the mold Another advantage of physical foaming is the high foaming pressure, so thin-walled parts can achieve good foaming.
A Simple New Physical Foaming Process
The new physical foaming process, developed jointly by Kunststoff-Institut Lüdenscheid and Linde Gas, combines the advantages of simple chemical foaming and efficient physical foaming.
Figure 1 The new foaming process: Unlike other physical foaming technologies, the plastic particles are impregnated in the foaming agent before they are processed in the machine (source: Kunststoff-Institut) added before the injection molding process One step: The dried material is immersed in carbon dioxide (CO2) as a propellant in the pressure vessel (Figure 1) The amount of gas entrained by the particles varies depending on the type of plastic material, temperature, wetting time, and pressure According to current knowledge, polycarbonate consumes the most gas (Figure 2).
Figure 2 Increases in weight by 100g at two different impregnation times and the effect of increased pressure is evident (Source: Kunststoff-Institut) With the introduction of CO2-containing PET beverage bottles, it is recognized that CO2 will rapidly Diffusion into and through plastic particles, but joint research by Kunststoff-Institut Lüdenscheid and Linde Gas showed that CO2 remains in the plastic particles for a considerable amount of time after loading and pressure have been released, even under open storage conditions, Sometimes for 2-3 hours.
Figure 3 Continuous weighing of foamed parts made of different plastics (reference: compact parts) Each point corresponds to one part Confirmed that after several hours, the impregnated plastic showed reproducibility. Source: Kunststoff-Institut) So far, many materials have been studied so that one can reproduce the foaming result in hours, and the part begins to rekindle only as the gas gradually escapes from the plastic particles Even more exciting is the fact that, like chemical foaming, people are also conducting physical foaming studies on a variety of materials on conventional injection molding machines.
Figure 4 Material impregnated gas supply. Multiple injection molding equipment (source: Linde gas) can be supplied from the same autoclave. Similar to central drying, an autoclave can also be used with several injection molding machines (Figure 4) Materials. In addition to further research on materials, Kunststoff-Institut Lüden scheid and Linde Gas also collaborate with peripheral technology partners on the concept of industrialization and are expected to launch products by 2018.
Reverse the gas pressure to improve the surface appearance of the part
A disadvantage of all foaming processes is that bubbles suddenly break at the melt front, causing cracks to form on the surface of the molded part (Figure 5). As with the prevention of significant flow marks, the use of a variable mold temperature process is a suitable method , That is, the mold wall temperature of the injection and hold-up phase is slightly increased so that the melt in which the bubble ruptures is compressed again before the melt is solidified in the cavity wall.
Figure 5 Streaks (top) when bubbles reach the surface Gas Backpressure (bottom) Prevents bubbles from breaking at the melt front (Source: Kunststoff Institut)
The resulting surface is comparable to solid molded parts, and in many cases the flow marks and dents are better. The costs associated with molds, peripherals and energy consumption depend on the material, the geometry of the part , Mold concept and processing cycle.
Another method is the gas counter-pressure process, but this method has so far been used very little, mainly to prevent the bubble in the melt front rupture (Figure 5). It will slightly reduce the level of foaming, but even with the traditional model Temperature control, you can still get a solid surface (title map).
Table 1 The degree of foaming achieved by the different plastic materials fluctuates between 10% and 60% (PC) (Source: Kunststoff-Institut) It sounds simple, but not so easy to achieve from a sealing and technological point of view Even if the mold is properly sealed, the gas must be controlled to fill the cavity, so the gas (nitrogen) pressure must not only be repeatable but must increase rapidly and drop again quickly and in a controlled manner.
Figure 6 Quick response to the pressure control module (Linde) is required to keep the reverse pressure at the end of the flow line consistent by releasing the gas as it enters.
Here a pressure control module similar to the internal gas pressure technology can be used (Figure 6). Once closed, the control unit of the pressure control module receives a signal and increases the gas pressure level in the cavity before injection begins To prevent the melt from further increasing pressure during flow, creating a fill problem, the pressure control module's control valve must respond quickly and keep the back pressure at the end of the flow channel constant by releasing the gas Here, in addition to the pressure control module, Whether the location of the air passage is correct or not is crucial.
in conclusion
Today's industrial companies need to conserve resources, and in the automotive industry this goal can only be achieved by reducing the weight of vehicles, so the demand for foamed plastic parts is expected to increase in the future.
The process developed by Kunststoff-Institut Lüden scheid in collaboration with Linde Gases can bridge the gap between chemical and physical foaming processes and help more polymer manufacturers foam existing foams into physical foams. According to the authors New possibilities, such as the production of 3D-style seals, and the further development of new pressure control modules, can push the gas backpressure process to become an alternative to future temperature-mode processes.