Loss of casting runner
Anyone who knows about die casting will know that the runner or remaining material is a part of the casting. Although there is no profit value, it cannot be avoided in the production process. This part of the cost is generally only calculated as a fixed ratio of the cost of the casting. At the same time, in view of the recyclability of zinc alloys, the most common local treatment method is real-time throwing back to the machine furnace for remelting. Due to the need to control quality problems, the use of central melting furnaces to recycle runners or waste products is gradually accepted by the industry (Figure 1). As for the slag, larger die-casting plants may recycle them on their own, and generally sell the remaining materials back to the raw material suppliers for new materials. The local recycling price of zinc materials is generally 50 to 70% of that of new materials. Without good environmental conditions, the slag handling can easily cause air pollution.
Take a 160-ton hot chamber die-casting machine as an example. Each time it produces at least 150 grams of runners (excluding overflow wells), assuming three shifts, the production cycle is 20 seconds, the machine utilization rate is 80%, and the annual output is The mouth runner can reach 190 tons. Another example: using an 80-ton machine to produce 100 grams of runners each time, the same assumption is made but the production cycle is changed to 12 seconds, and the annual output of runners exceeds 210 tons.
It can be seen that the runner design affects the importance of cost.
Various recycling methods
Among the recycling methods, directly throwing the runner back into the furnace is the simplest and cost-saving method. The flow channel just produced by reflow melting does not need to be preheated, and the storage space is reduced, but it is difficult to control the quality of the molten material, including more slag, the furnace temperature is difficult to control, and the alloy composition cannot be known; more importantly, it relies on Operating staff’s craftsmanship, such as the proportion of new materials, and observing the changes in boiler water, while the staff puts overflow wells and flashes into the furnace, not only will make the situation worse, this method of directly melting the waste also hides it. The high defect rate, instability of mold design and die-casting parameters have prevented managers from making effective improvements. This method is not suitable for the production of castings with high surface quality requirements, and it is difficult to accurately calculate the cost of runner loss.
Central furnace recovery nozzles and defective products have become popular in die-casting plants with large output. Its benefits are very obvious, that is, centralized processing of recycled materials can improve furnace efficiency and control alloy quality. If the molten metal is directly fed into the furnace from the central furnace, the die casting machine material temperature can be kept stable and there is less slag. If equipped with automatic feeding control, the change of the liquid level can be minimized. Currently popular central melting furnaces are divided into several categories: cast iron crucible furnaces with larger capacity, stainless steel crucible furnaces, and continuous melting type non-crucible furnaces. Zinc liquid transportation is also divided into several categories: there are overhead crane type liquid material transportation, ground trolley type (trackless or tracked) holding furnace (with feeding device) transportation and holding trough type gravity conveying device, which connects the furnace and the central furnace Connected. Its disadvantage is that the investment is relatively large, it is only suitable for a single alloy (small crucible furnace is not discussed here), and the workshop occupies a large area, so small die-casting plants (less than five machines) are not suitable, and the old workshops are difficult to transform and cooperate. Therefore, it is generally only re-planned when a new plant is built. The use of a small crucible furnace to remelt the sprue material, due to the lack of economies of scale, will cost more than the central furnace, so it is not used as a reference for calculation.
Calculation of melting cost
Just use the central melting furnace to calculate the melting cost of the runner as a reference. Take a company with five 80-ton or 160-ton die-casting machines as an example. Assuming that the investment of the equipment is 500,000, it is divided into ten years. About 1,000 tons of runner recycling materials are processed every year (the actual situation should be melted in proportion to the new materials, and it is purely convenient to calculate the cost of remelting).
The melting cost per kilogram of gate material is $0.93. According to the calculation of the above five machines, the annual production of 1000 tons of runner nozzles involves nearly 10 million yuan. If the recycling of defective products is included, this figure is even more alarming (such as The average casting weight is 100 grams and the defective product rate is 5%, the cycle is 12 seconds, and the annual recovery of defective products is about 53 tons). Although the larger the processing quantity, the lower the melting cost, but environmental protection and strict quality control costs are not calculated here. It can be seen that the cost of gate remelting is quite alarming, and the die casting plant must minimize the cost. Therefore, how to reduce the gate weight is an important key to controlling costs.
- Land occupation rent 20.000 HKD
- Equipment investment apportionment of HKD 50.000
- Interest cost HKD 5.000
- Maintenance and repair HKD 25.000
- Fuel oil fee (100 liters of oil residue per ton·US$2/liter) HKD 200,000
- Electricity fee (1 USD/kWh) 30.000 HKD
- Wages (including operating workers, management personnel, quality control personnel) 100.000 Hong Kong dollars
- Metal loss 5% (USD 10/kg) HKD 500,000
- Total: HKD 930.000
Calculation method of apportioning runner cost
The cost of refining the nozzle must be included in the production cost of the casting. The most common method is to multiply the material used by a fixed percentage. For example, if the raw material price is $10/kg, and the nozzle melting cost is 3% of the weight of the casting, $10.3 will be used when calculating the material price of the casting. Although this method is simple, it may cause deviations in the cost calculation and hide the true cost of water outlet recycling. Now you can use the following example for comparison:
- The net weight of casting A is 400 grams, and the weight of the nozzle runner is 100 grams.
- The net weight of casting B is 400 grams, and the weight of the nozzle runner is 250 grams.
If calculated with a fixed percentage:
- The cost of casting A and casting B should be the same ($10.3 x 0.4) = $4.12.
If calculated by actual recovery cost:
- Casting A should be ($10 x 0.4 + $0.93 x 0.1) = $4.093
- Casting B should be ($10 x 0.4 + $0.93 x 0.25) = $4.233
This difference may seem small, but with 20 seconds as the production cycle, the machine utilization rate is 80% and the production is in three shifts, each machine produces 1,261,440 times per year. The difference is as follows:
- Runner nozzle cost Casting A Casting B
- Difference Fixed ratio method HKD 5.197.132 HKD 5.197.132 HKD 0
- Actual cost method HK$5.163.074 HK$5.339.675 HK$176.601
- Difference HK$34.058 HK$142.543
If the fixed ratio method is used, the cost of casting A and B are the same, but in fact the cost of casting B is higher. It can be seen from this case that using the fixed ratio method to calculate casting B not only underestimates the production cost, but also indirectly encourages designers not to reduce the weight of the nozzle runner as the goal, and should promote the application of the actual cost method (see the table below).
To reduce the weight of the gate, it is more common to use a short gate (short nozzle) design and reduce the thickness of the template. It uses a longer machine nozzle (generally 20mm longer than normal), combined with a deep cavity gate mold design to reduce the weight of the gate. The following is a new hot chamber die casting runner design.
Hot chamber die casting runner design
Die-casting runner is the path of molten metal flowing from the nozzle into the mold cavity. It is composed of the branches of the sprue and the runner. Due to the need to attach the castings and facilitate demoulding, the sprue must have an inclination. At the same time, the splitter block on the movable template can reduce the thickness of the sprue; adding a cooling water channel in the splitter block facilitates the balance of mold heat, shortens the cooling time, and pulls out the casting and ejects it. A study conducted by the Australian CSIRO in the early 1970s found that, with acceptable errors, the zinc alloy liquid can be summarized as follows in the case of die casting:
Liquid behaves as incompressible fluid
Comply with general principles of fluid mechanics
The high Reynold number indicates that the flow process is turbulent.
According to the above research results, the ideal flow state of molten metal should be:
1. The runner section is circular
Since the circumference/area ratio is the lowest, the surface resistance of the circular cross-section pipe is the lowest, so the pressure loss is also the lowest. Compared with the same trapezoidal cross-sectional area, the periphery is reduced by more than 20%.
2. The flow pipe is straight
Bent pipes will generate bias flow, mix bubbles into the melt, and cause pressure loss. Especially when the bending radius/pipe diameter ratio is less than 1, the pressure loss increases rapidly.
3. The flow channel profile gradually shrinks in terms of liquid flow
The rapid change of the pipe profile, whether it becomes larger or smaller, will cause high pressure loss and eddy currents. The best solution is to gradually reduce the profile to compensate for the resistance loss caused by the pipe surface.
Link to this article：The Design And Application Of Hot Runner For Zinc Die Casting
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