Riser system production application experience

I. Overview
Among casting defects, volume defects (macroscopic shrinkage, microscopic shrinkage, sink marks) are common causes of scrapping. Castings with volume defects can only be repaired into usable castings by special repair measures (such as infiltration) under special circumstances. Most of these castings must be scrapped.
The manufacture of castings with low defect rates requires a certain degree of technical effort at all stages of the casting process. This technical work involves distinguishing between large amounts of raw materials, energy and processes and is reflected in financial data.
The use of a riser system has a good inhibitory effect on volume defects. By using external or internal cooling elements and heat-generating risers in the casting or on the surface, or specific castings (controllable directional solidification), it is usually possible to optimize the function of the riser system. . The use of risers means increased expenditures in the manufacture of castings (see Figure 1).

Figure 1 Knight statue with gate, riser and sprue
The quotient of the feeding system mass m S and the casting mass m T contained in the formula for calculating the casting yield η G can be used as an index of the amount of technical shrinkage required for the casting pressurization. The yield is defined as the quotient of the casting mass m T (excluding the casting quality of the gating system and the feeding system) and the casting mass m G (including the casting quality of the gating system and the feeding system). The pouring mass m G includes the casting mass m T , the gating system mass m G i and the feeding system mass m S , and the relationship is as follows:

Therefore, the casting yield as a cost/yield represents the technical efficiency factor of the casting and feeding technology in casting production, which decreases with the quality of the feeding system and the quality of the casting system.
Therefore, the quality of the riser system must be kept as low as possible (see Figure 2) without affecting the actual effect of the riser, ie compensating for volume changes during solidification of the molten metal.

(a) Ordinary riser (b) Fever riser (c) Mini riser
Figure 2 Comparison of ordinary risers, hot risers and micro risers (ASK Chemicals Feeding System)
In the example shown in Fig. 2, the casting material is ductile iron, the weight of the cube casting is 28.7kg, the weight of the ordinary riser is 23.0kg, the contact area with the casting is 153.8cm 2 ; the weight of the molten riser is 8.4kg. The contact area with the casting is 73.5 cm 2 ; the weight of the molten iron of the micro riser is only 1.3 kg, and the contact area with the casting is 19.6 cm 2 .
Second, the riser system should meet the requirements
All metals are subjected to thermal expansion and contraction, and the relationship between volume change and temperature is shown in Fig. 3.

Figure 3 Relationship between volume change and temperature
The volume change depends on the nature of the alloy. The shrinkage values ​​(guide values) of commonly used casting materials are shown in the attached table.
Casting material shrinkage value

When manufacturing a casting, it is necessary to compensate for the volume change that occurs when the molten metal is cooled or solidified. Gray cast iron and ductile iron also exhibit volume expansion during the solidification process (expansion phase during graphite precipitation). The volume change that occurs during solidification is divided into liquid shrinkage, solidification shrinkage, and solid state shrinkage. Only solidification shrinkage is related to the casting and feeding techniques, as this volume change must be compensated using a feeding technique. Solid shrinkage is important for model construction because the model must create more than this percentage.

Figure 4 Improved geometry to improve the efficiency of the feeding (ASK Chemical Addition System)

Figure 5 "heating" the riser's heating material to further increase the suction power (ASK Chemicals Supplementation System)
In order to ensure sufficient feeding, the riser system must meet the following basic requirements:
First, the riser solidification time must be longer than the solidification time of the casting that needs to be replenished.
Second, the solidification time of the riser neck (the connecting pipe between the riser and the casting) must not be shorter than the solidification time of the casting.
Third, in the process of shrinking the casting, that is, from liquid to solid, the riser must store enough molten metal to compensate for the volumetric shrinkage of the casting.
Fourth, the efficiency of the normal riser must be expanded to the maximum. However, for technical safety reasons, the optimum riser size and design must limit the shrinkage hole depth in the riser to no more than 80% of the riser height. Figures 4 and 5 illustrate the effect of optimizing the shape of the riser and the heat generating material on the suction power to improve the efficiency of the feed. The miniature risers that are often used today exhibit a very long burning time, that is, the material required for feeding can be minimized, or the volume of the suction is maximized, resulting in a significant increase in the yield (see Figure 6).

Figure 6 The heating curve of the micro riser
As can be seen from Figure 6, the micro riser has a significantly longer burn time (ASK Chemical Filling System) than the conventional combination with a cylindrical riser cap. The efficiency of the feeder with a cover is about 30%, and the efficiency of the micro-portion is about 70%.
(1) During the shrinkage of the casting, only the molten metal in the riser maintains continuous contact with the air to ensure the function of the riser. If the riser wall solidifies, the riser forms a closed solidified hard skin prematurely, and the contact between the molten metal and the air in the riser is blocked, and the displacement caused by the atmospheric pressure disappears. If the riser is a clear riser, it can be kept in contact with the air by adding a heat insulating substance or a heat generating substance. However, the riser area of ​​the dark riser must use a porous sand core insert or a sand edge extending into the riser cavity. (William's Wedge). William's sand cores are currently used in a range of designs, such as William's Point or William's Rod on naturally occurring risers, or William's sand core made of silica sand or a heat-generating material on a riser model. Form or insert a riser cavity.
(2) must be ensured during the solidification process
There is a continuous flow of molten metal from the riser into the casting. The intersection of the riser neck at the junction of the casting and the riser and its performance are critical to this. It is not allowed to form a partially isolated melting zone or a late solidified melting zone, which may result in the metal liquid not being supplied normally. Directional solidification is a prerequisite for the rise of the feeder.
Third, the riser system application guide
The availability of different types of risers depends not only on the casting structure (shape and size) but also on a range of technical conditions. As a result, some general application requirements and guidelines can be derived. These factors should be taken into account when designing and arranging the riser system.
(1) The riser should be optimally designed according to the shape and size of the casting to ensure the economic feasibility of the casting.
(2) The riser should be easy to form.
(3) The riser shall not impede the shrinkage of the casting.
(4) The riser should be easy to remove.
(5) For castings that cannot or cannot be machined due to their structure, the riser should be placed as far as possible on the surface.
(6) It is preferred to use a cylindrical dark riser with atmospheric pressure effect, because this riser has the following advantages: regardless of the casting conditions, since the volume of the riser metal liquid is fixed, the feeding effect is stable; good shrinkage effect Thanks to the shrinkage cavity in the riser, because the shrinkage hole is generally formed in the upper part of the casting; the dark riser is easy to form, especially in machine modeling; the possible pressure application in the dark riser reduces the microscopic shrinkage.
(7) It is recommended to use a riser when manufacturing large castings because the metal can be refilled into the riser or the surface of the riser is covered with a hot substance.
(8) When feeding multiple castings at the same time (group riser), and when the top riser cannot be used due to the highly structured surface of the casting, the side riser should be used. The dark side riser should be at least 25 mm beyond the upper edge of the casting.
(9) Use a riser with easy-to-cut pieces to reduce the amount of work required to remove the riser. Easy-to-cut risers with easy-to-cut pieces have a distinct advantage in steel castings that cannot be cut with oxidized fuel.
Fourth, further development
Mini riser
The productivity of a foundry company is highly dependent on the cleaning work required to complete the casting. In this respect, the mini riser has a distinct advantage over the normal and hot riser (see Figure 7).

Figure 7 Higher productivity and yield when using mini risers compared to conventional risers and hot risers
As can be seen from Figure 7, the use of micro risers has higher productivity and yield compared to conventional risers and hot risers.
Spring pin
In order to prevent the direct contact between the strong exothermic material and the casting, the spring pin is increasingly favored in the industry, and it also reduces the contact area of ​​the riser. In the spring pin technology, the riser subtly uses the downward movement of the mold to apply pressure to the sand layer between the riser and the casting to achieve optimum molding and precise mold contours, even in areas around the riser. influences.

Figure 8 Spring pin principle
Figure 8 shows the use of spring pin technology in the BKS heating micro-riser, which uses a cut-edge edge riser (ASK Chemicals Feeding System).
3. Easy to cut pieces
The coated sand easy to cut piece simplifies the removal of the riser. The riser breaks near the casting, reducing cleaning work. However, this method requires a large contact area. Further developments in modern forming systems have revealed another weakness: coated sand shards cannot withstand ever-increasing pressures, which can lead to casting defects. Coated sand easily cut pieces can also cause wear on the model and also on the surface of the casting (see Figure 9).


(a) Large contact area (b) Imprinted on the surface of the casting Figure 9 Formed riser with easy-to-cut piece
To develop a corresponding heat generation to form a lateral contraction, the development of an innovative spring pin with an internal spring is an indispensable step (see Figure 10). Thanks to the spring pin technology, the heat-generating material of the easy-to-cut piece no longer needs to be in direct contact with the casting. Instead, the downward pressure is applied to the molding sand under the riser. At the same time, the easy-to-cut sheet forms the required lateral shrinkage, making the removal simple.

Figure 10: The fracture zone with a specific fracture edge
Five, heating affects the riser coagulation
As with riser insulation, the size of the effective riser can be significantly reduced by heating the riser. This technical measure can be preferred when casting copper or iron based alloys. The heat released by the hot mixture prolongs the time the riser solidifies. This allows the riser size to be reduced, thereby increasing the casting yield. Extensive studies of the effects of the heat-generating mixture on the size of the desired riser have shown that the delayed action of the exothermic reaction of these exothermic materials can significantly reduce the size of the riser.
To heat the riser, a heat-generating powder (aluminum heat mixture) is typically used to cover the riser, while a riser sleeve and blank consisting of a heat-generating mixture is used to cover the riser and the selected casting.
The heat-generating mixture contains combustibles (Al, Mg), oxygenates (Fe 2 O 3 , NaNO 3 , KNO 3 , Mn 2 O 3 , SiO 2 ), fillers that reduce the reaction rate (silica sand, refractory clay, perlite, Concrete) and binder (water glass, resin). Specific components have a special role in the reaction. For example, nitrates make it easier to ignite the mixture and accelerate combustion. Fluoride reduces the ignition point and enhances the aluminothermic reaction. Due to the shortage of raw materials in recent years, foundries often use high-quality auxiliary materials in the production process to prevent possible fluctuations. Here, modern riser systems have become an important factor in ensuring reliability and efficiency in the production process by optimizing performance. There is no doubt that prior art risers contain no carcinogenic fibers and only emit minimal organic matter, thanks to the use of inorganic binders, which reduces gas pollution in the foundry and reduces the rate of porosity defects. It also protects the health of employees.
The residual sintering of the riser material after burning can make the riser residue easy to separate and prevent the molding sand cycle from being contaminated.
In the mid-1990s, ASK Chemicals was the first manufacturer to produce a completely fluorine-free riser. This has made an important contribution to improving the quality of circulating sand in the foundry industry and preventing surface defects. At the same time, it reduces the pollution caused by the treatment of old sand and helps to reduce the processing costs. As such, the latest developments in riser technology have made a lasting contribution to ensuring the competitive advantage of the foundry industry.

About the author: Dipl.-Ing.StefanFischer, European Product Manager, ASK Chemicals Filling Systems GmbH; Dipl.-Ing.UdoSkerdi, General Manager, ASK Chemicals Filling Systems GmbH; Dr.mont.StephanHasse, Foundry Technology And General Manager of Engineering Co., Ltd.

Note: This article belongs to the Metal Processing Magazine, and is not allowed to be reproduced without permission!

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