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Low pressure casting relies on a four-dimensional window for stable production: filling pressure, liquid rise rate, mold temperature, and holding time.
Lower and upper limits of pressure window
The filling pressure is usually taken as 0.02-0.08MPa. Below 0.02MPa, the aluminum liquid does not rise enough, and the far end is prone to insufficient pouring.
Above 0.08MPa, the filling speed is too fast, and the risk of gas entrapment and mold sticking increases. Thin walled components can be set to a higher value, while thick walled components can be set to a lower value.
The pressure also needs to match the diameter of the riser. As the diameter of the pipe increases while the pressure remains constant, the flow rate will increase, and the actual filling speed has exceeded the set value.
The liquid lifting speed determines the degree of gas entrainment
It is recommended to control the liquid level rise speed during the liquid lifting stage at 50-150mm/s. If it is too fast, turbulence will occur and the oxide film will be rolled into the casting.
The inclusions formed by turbulent flow are mostly distributed on the surface of the inner cavity of the casting, and are only exposed after machining, which belongs to implicit losses.
Segmented liquid lifting is a common practice: slow exhaust in the front section, acceleration in the middle section, and smooth filling during deceleration in the end section. Splitting into three sections can balance efficiency and quality.
The mold temperature window is more sensitive than pressure
The mold temperature for aluminum alloy casting is usually maintained at 200-350 ℃. Below 200 ℃, the risk of cold insulation significantly increases.
Above 350 ℃, both mold sticking and shrinkage become more pronounced. The lifespan of the mold coating will also be significantly shortened.
The uniformity of mold temperature is more important than absolute value. If the temperature difference on the surface of the mold cavity exceeds 15 ℃, the solidification sequence between thin and thick areas will be disrupted.
Algorithm for holding time
The holding time is often estimated based on wall thickness, ranging from 1-1.5 seconds per millimeter. When the wall thickness is 6mm, hold the pressure for 6-9 seconds.
Insufficient pressure holding, gate not solidified, aluminum liquid refluxed after pressure was removed, forming shrinkage cavities. The holding pressure is too long, the rhythm is wasted, and the gate is difficult to cut off.
Thick and large parts need to be separately extended for pressure maintenance. The unified holding time of the entire mold often sacrifices the efficiency of the thin-walled area.
How to calibrate a four-dimensional window
First, use simulation to provide the initial window, and then use T0 trial to make single factor adjustments. Change only one parameter at a time and record the defect changes.
Suggest conducting a 3x3 orthogonal experiment covering three factors: pressure, mold temperature, and holding pressure. Nine sets of experiments are sufficient to identify trends.
When delivering low-pressure casting molds, Xinfeng Mold will attach a parameter recommendation table, indicating the initial window and adjustment direction of each wheel type.
Troubleshooting of window drift
After the same mold runs for a period of time, the parameters become invalid. First check the water path for scaling. If the flow rate decreases by 20%, the mold temperature will significantly shift.
Check the liquid level of the insulation furnace again. Fluctuations in liquid level can alter the static pressure head, resulting in deviations between the actual filling pressure and the set value.
Finally, check the composition and temperature of the aluminum liquid. The fluctuation of composition will change the liquidus temperature, and the original window is no longer applicable.
Solidification and handover of parameters
After the window is confirmed, it should be written into the process card, indicating the applicable wheel type, machine and mold temperature conditions. The process card is transferred with the mold, and mass production has a basis.
Do not directly use the old card when changing the wheel type. After the changes in wall thickness and wheel rim width, the filling time and holding time need to be recalculated.
Frequently Asked Questions
Question: What is the filling pressure?
Answer: Normally 0.02-0.08MPa, adjusted according to wall thickness.
Q: What is the appropriate liquid lifting speed?
Answer: Suggest 50-150mm/s, segmented control is better.
Question: What is the mold temperature controlled at?
Answer: 200-350 ℃, with a temperature difference controlled within 15 ℃.
Question: How is the holding time calculated?
Answer: Estimated at 1-1.5 seconds per millimeter of wall thickness.
Q: What to look for when parameters suddenly fail?
Answer: First check the water flow rate, and then check the liquid level inside the furnace.
Q: Do we need to conduct an orthogonal experiment?
Answer: It is recommended to cover pressure, mold temperature, and holding pressure with a size of 3 × 3.
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The core of differential pressure casting is a differential pressure curve, consisting of four stages: establishment, filling, holding, and releasing. The slope of each stage determines the density and surface quality.
Pressure difference magnitude and equipment capability
The pressure difference for differential pressure casting is usually set at 0.05-0.15 MPa. Above 0.15MPa, the sealing burden between the mold and the tank body is significantly increased.
Below 0.05MPa, the shrinkage capacity is close to that of low-pressure casting, and the process advantage of differential pressure cannot be reflected.
Before setting, it is necessary to confirm the ability of the machine to establish pressure difference. The larger the tank volume, the higher the gas flow rate required to establish the same pressure difference.
Slope of the four segment curve
The first paragraph establishes a pressure difference, and the slope should not be too steep. Establish smoothly within 30-60 seconds to avoid severe fluctuations in the aluminum liquid level.
In the second stage of filling, the pressure difference continues to rise. The charging speed is controlled by adjusting the opening of the intake valve, and the common charging time is 5-15 seconds.
Maintain pressure in the third section and keep the pressure difference at the set value. The holding time is calculated based on wall thickness and is usually 10% -20% shorter than low-pressure casting.
The fourth stage of pressure relief should be done slowly. If the pressure is released too quickly, negative pressure may form inside the casting, and the thin wall may be deformed by suction.
Sealing is a prerequisite for the validity of a curve
Any leakage in the sealing rings of the upper and lower tanks, the interface between the mold and the tank body, or the flange of the liquid lifting pipe will cause the pressure difference to fail to establish.
During debugging, first conduct a short tank pressure test. Maintain the pressure difference for 60 seconds, and if the pressure drop exceeds 5%, search for the leakage point.
The sealing ring will age at high temperatures. It is recommended to replace every 5000 molds, or based on the change in hardness, replace when the hardness increases by 15%.
Key points of coordination on the mold side
The parting surface of the mold must be able to withstand pressure difference. If the gap between the parting surfaces exceeds 0.05mm, the aluminum liquid will penetrate under the pressure difference.
The exhaust system needs to be redesigned. Under differential pressure conditions, the back pressure of the mold cavity is higher, and the exhaust area needs to be increased by 20% -30% compared to low-pressure molds.
Xinfeng Mold will consider both the exhaust channel and the pressure difference curve on the differential pressure mold to avoid insufficient exhaust and material shortage at the filling end.
The relationship between common defects and curves
Curling often occurs when the slope of the filling section is too steep. Extending the filling time by 2-3 seconds significantly reduces the probability of gas entrapment.
Shrinkage often occurs when there is insufficient pressure difference in the pressure holding section. Increasing the pressure by 0.01-0.02MPa or extending the holding time can improve shrinkage.
The surface cold insulation is mostly related to low mold temperature and has little relationship with the curve. This type of problem should return to mold temperature and coating, rather than repeatedly adjusting the curve.
Debugging sequence
First, empty the tank and maintain pressure to confirm sealing. Close the mold again and confirm that the parting surface is in contact. Finally, test the mold with the material and adjust the curve in sections.
Adjust only one section at a time. Simultaneously changing the charging type and maintaining pressure, it is impossible to determine which parameter is working.
After determining the curve, write it into the parameter library and label the wheel type, machine, and mold temperature conditions. When replacing the machine, recalibration is required and cannot be directly applied.
Frequently Asked Questions
Question: What is the pressure difference?
Answer: Normally 0.05-0.15MPa, depending on the wall thickness and machine.
Q: How long does the filling time take?
Answer: Commonly 5-15 seconds, depending on the wheel shape and gate.
Q: Why is pressure relief slow?
Answer: If it is too fast, negative pressure will form, and thin-walled areas are prone to deformation.
Q: How long does it take to replace the sealing ring?
Answer: It is recommended to use 5000 molds or when the hardness increases by 15%.
Q: Do we need to increase the exhaust?
Answer: Yes, it increases by 20% -30% compared to low-pressure molds.
Q: Do I need to recalibrate when changing machines?
Answer: Yes, the curves for different tank volumes are not universal.
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The pouring system of gravity casting needs to be solved under three constraints: pouring time, gate position, and hot spot distribution. Any imbalance of these factors can lead to shrinkage or cold shut.
The pouring time is determined by the wall thickness
The pouring time is often estimated based on the wall thickness, with 1-2 seconds per millimeter. For parts with a wall thickness of 8mm, the pouring time is about 8-16 seconds.
The time is too short, the flow rate of aluminum liquid is high, and the risks of gas entrainment and sand flushing increase. If the time is too long, the temperature at the front end will decrease, and cold insulation is inevitable.
Thick and large parts can be appropriately extended, while thin-walled parts should be shortened. The unified time applies to parts of the same family, and cross category calculations need to be recalculated.
The position of the gate determines the temperature field
The gate should be located at the thick wall, allowing the high-temperature aluminum liquid to pass through the thick wall first and then flow towards the thin wall. This solidification sequence is consistent with the direction of shrinkage.
The gate is located at a thin wall, where the thin wall solidifies first and the thick wall loses its contraction channel, resulting in axial shrinkage and looseness.
When using multiple gates, it is important to avoid liquid flow reversal. Hedging will generate eddy currents and involve oxide films, which can easily lead to inclusions in the inner cavity of the casting.
The distribution of hot nodes should match the riser
The hot spot is the final solidification point and requires a riser to compensate for shrinkage. The volume of the riser is usually taken as 1.5-2.5 times the volume of the heat node.
The riser position deviates from the hot spot, and the contraction channel is cut off. The supplementary distance generally does not exceed 4 times the wall thickness, and if it exceeds, additional cold iron needs to be added.
The function of cold iron is to create a temperature gradient and guide the hot spot towards the riser direction. The thickness of the cold iron is often taken as 0.8-1.2 times the wall thickness.
The ratio of sprue to runner
The ratio of the cross-sectional area of the sprue to that of the runner and sprue is commonly 1:2:4 or 1:3:6. The proportion determines the flow velocity of each section.
Proportional imbalance, low flow rate in the transverse runner, and insufficient upward movement of slag. If the flow rate is too high, it will also wash away the sand mold.
Xinfeng Mold will first conduct filling simulation in the design of the gravity mold pouring system, confirm the liquid flow path, and then determine the number and position of gates.
The relationship between exhaust and pouring
The exhaust capacity of the pouring system is often overlooked. Insufficient exhaust from the mold cavity, increased back pressure, decreased filling speed, and increased risk of cold insulation.
It is recommended to configure the exhaust plug area at 1% -2% of the projected area of the cavity. Thin walled components can take higher values, while thick walled components can take lower values.
The exhaust plug should be located at the end of the liquid flow, not the front of the liquid flow. The position was wrong, and the exhaust plug was quickly blocked by aluminum liquid.
Common Misconceptions
The first misconception is to increase the gate to improve the filling capacity. If the gate is too large, it will form a new hot spot, and the position of shrinkage will only shift.
The second misconception is to ignore the pouring temperature. For every 20 ℃ decrease in temperature, the flowability significantly decreases, and the original pouring time is no longer applicable.
Misconception three is to simply copy the parameters of similar components. The wall thickness difference is 1mm, and the pouring time and gate area need to be recalculated.
Verification method for pouring system
The first piece cutting is the most direct method. Cutting along the flow path to observe the relationship between shrinkage and gate position is more useful than looking at the appearance.
Thermocouples for temperature measurement can also be used to place points at different positions in the mold cavity. If the solidification time difference exceeds 3 seconds, it indicates that the temperature gradient design needs to be adjusted.
Frequently Asked Questions
Question: How to determine the pouring time?
Answer: Estimate based on 1-2 seconds per millimeter of wall thickness.
Q: Where should the gate be located?
Answer: For thick walled areas, ensure sequential solidification.
Question: What is the volume of the riser?
Answer: Usually take 1.5-2.5 times the volume of the heat node.
Question: What is the thickness of the cold iron?
Answer: Usually take 0.8-1.2 times the wall thickness.
Q: How to match the exhaust area?
Answer: Configure according to 1% -2% of the projected area of the cavity.
Q: Can we simply copy the parameters of similar components?
Answer: No, if the wall thickness difference is 1mm, it needs to be recalculated.
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The differential pressure of mold cost is higher than low pressure, and low pressure is higher than gravity, but the success or failure is determined by the full life cycle cost.
The magnitude difference of mold unit price
Taking passenger car wheels as an example, a single set of gravity molds costs about 60000 to 150000 yuan, low-pressure molds cost 180000 to 450000 yuan, and differential pressure molds are 15% to 25% higher than low-pressure molds.
The price difference comes from the complexity of the structure. Differential pressure molds need to withstand the pressure difference of the tank body, and require higher rigidity and sealing surface accuracy of the template.
The price difference of structural component molds is more obvious. The low-pressure mold for the subframe costs 400000 to 900000 yuan, while the gravity mold of the same size may only be half of it.
Unit cost is the benchmark for comparison
The mold cost needs to be amortized over individual items. When producing 100000 pieces per year, the mold cost of 300000 yuan is divided into 3 yuan per piece; When producing 3000 pieces per year, it is 100 yuan.
In addition to the mold, the production rate of the process also needs to be calculated. The proportion of sprues and risers in gravity casting is often 35% -45%, while in low-pressure casting it is about 25% -35%.
For every 10 percentage points difference in material utilization rate, the cost of a single piece of material will differ by about 8% -12%. This part is often overlooked.
The difference between energy consumption and labor
Low pressure casting requires a continuous insulation furnace for insulation, with a single unit power often ranging from 30-60kW. The annual electricity bill is a fixed expense.
Differential pressure casting also requires an increase in compressed air or inert gas consumption. The gas consumption cost is calculated based on the tank volume and the number of cycles.
Gravity casting equipment is simple, with low energy consumption, but requires high manual labor intensity. The difficulty of recruitment has become an implicit cost in some production areas.
Quality cost cannot be solely based on scrap rate
The scrap rate of gravity casting is often higher than that of low-pressure casting, especially on thin-walled parts. For every 1 percentage point increase in scrap rate, the cost per unit increases by approximately 1.5% -2%.
Low voltage components have better density and lower subsequent testing and repair costs. For safety components, this difference may exceed the price difference of the mold.
The quality advantage of differential pressure components is most evident in high-strength wheel types. On conventional wheel types, the premium of differential pressure is difficult to recover by quality.
Type change and flexibility
Gravity molds change quickly, usually within 0.5-1.5 hours. When there are multiple varieties and small batches, this advantage will be amplified.
Low pressure mold changeover takes 1.5-3 hours and requires takeover and machine adjustment. Frequent changes will eat up the beat advantage.
The change of differential pressure mold is the slowest, and the reset of the tank seal requires additional time. Suitable for large quantities of small varieties.
The pricing logic of Xinfeng Mold
For the same round, the quotations for the three processes will separately list the mold cost, trial mold cost, and recommended batch range, making it easier for customers to calculate according to the project.
This kind of listing is more useful than giving only a total price. The purchaser can use this to determine which process is more cost-effective in their own batch.
Unified caliber for quotation comparison
Before comparing prices, unify the specification sheet and clearly state the materials, coatings, and number of mold tests. The caliber varies, and the price difference is incomparable.
Converted based on the cost per unit. The mold cost divided by the batch size, plus materials and energy consumption, is the number that can be used for decision-making.
Frequently Asked Questions
Q: What is the price difference among the three types of molds?
Answer: Differential pressure is 15% -25% higher than low pressure, and low pressure is about 1 time higher than gravity.
Question: Should we only look at the mold price?
Answer: No, it should be calculated based on individual amortization and quality cost.
Q: What is the yield rate of gravity casting?
Answer: It is usually 55% -65%, with a high proportion of sprues and risers.
Q: Does low-pressure casting consume high energy?
Answer: Insulated furnaces typically have a capacity of 30-60kW and need to be included in the cost per unit.
Q: Which one to choose in small batches?
Answer: Gravity molds change quickly and are more suitable for small batches of multiple varieties.
Q: Can the premium of differential pressure be recovered?
Answer: High strength wheel types are acceptable, while conventional wheel types are more difficult.
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Simulation can detect 70% -85% of the shrinkage position in advance, but it cannot replace the trial mold. It can only reduce the trial mold cycle from four rounds to two rounds.
What can simulation solve
The air entrainment, cold insulation, and insufficient pouring during the filling process result in a high degree of agreement between simulation results and actual measurements, usually above 80%.
The accuracy of predicting the location of shrinkage during the solidification process is about 70% -85%. The position of the hot spot is basically accurate, but the quantitative deviation of the shrinkage volume is relatively large.
Simulation can also calculate the temperature field of the mold, which can be used for waterway layout. The value of this aspect in mold design is often higher than defect prediction.
What simulation cannot solve
It is difficult to accurately model the factors of sand core disintegration, coating detachment, and aluminum liquid gas content through simulation.
The mechanical deformation, ejection synchronization, and sealing failure of the mold are structural issues that require finite element analysis rather than casting simulation.
The surface roughness and mold sticking tendency of castings rely on experience and process testing, and the reference values provided by simulation are limited.
The setting of boundary conditions determines the credibility
The interface heat transfer coefficient is the most sensitive parameter. The value deviation is 20%, and the solidification time may deviate by more than 30%.
Suggest using the measured temperature curve to calculate the heat transfer coefficient. Simulations without calibration can only show trends, not absolute values.
The setting of pouring temperature should be consistent with the site. The simulation uses 700 ℃ while the on-site use uses 680 ℃, and the conclusion cannot be matched.
The position of simulation in the development process
The correct sequence is: structural review, simulation, mold design, T0 trial mold. Simulation can only be modified before mold design.
After the mold drawings are produced and simulated, if any problems are found, only minor repairs can be made. Major changes such as gate positioning come at a high cost.
Xinfeng Mold uses simulation as part of the design input for wheel hub and structural component mold projects, rather than as a post validation tool.
How to read simulation results
Look at trends rather than numbers. The position of the shrinkage area is more instructive than the shrinkage rate.
Comparing schemes is more useful than looking at results alone. The difference in filling time and hot spot position between the two gating schemes can directly support decision-making.
The simulation report should include boundary conditions, material parameters, and grid size. Without these three items, the results cannot be reproduced.
Grid and computational costs
The grid size is often taken as 1/3-1/2 of the wall thickness. The mesh is too coarse, and the flow details at the thin walls are lost.
The filling and solidification calculation of a set of wheel hub molds usually takes 4-12 hours. Complex structural components may exceed 24 hours.
When computing resources are insufficient, filling analysis can be performed first, followed by local solidification analysis. Step by step calculation can shorten the cycle.
Configuration of simulation team
Simulation is not something that can be used just by buying a set of software. Parameter calibration requires people who understand both the process and simulation, and such people are even rarer than software.
Small factories can choose to outsource simulation. The simulation cost of a set of wheel hub molds is often between 5000-15000 yuan, which is more cost-effective than maintaining a dedicated team.
Frequently Asked Questions
Question: What is the simulation accuracy?
Answer: Shrinkage position is 70% -85%, and air entrapment is over 80%.
Q: Can simulation replace trial molding?
Answer: No, we can only reduce the trial rounds.
Q: What is the most sensitive parameter?
Answer: The interface heat transfer coefficient is greatly affected by a deviation of 20%.
Question: At which stage should simulation be placed?
Answer: The plan can only be changed before the mold design.
Question: What is the size of the grid?
Answer: Usually take 1/3-1/2 of the wall thickness.
Question: How long does it take to calculate once?
Answer: Wheel hub parts take 4-12 hours, and complex parts take even longer.
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H13 or 8407 for the mold cavity and ductile iron for the mold frame are common combinations, and material mismatch can result in a trade-off between lifespan and cost.
Two mainstream choices for cavity materials
H13 is the most commonly used hot work die steel in China, with a hardness of 45-52HRC, a relatively controllable price, and a short supply cycle.
8407 has higher purity, good isotropy, and better thermal cracking resistance than ordinary H13. The price is usually 30% -50% higher.
The difference in lifespan between the two on conventional wheel types is about 20% -30%. The advantages of 8407 are more obvious for high-end wheel types or thin-walled parts.
The difference between electric slag remelting and ordinary forgings
The content of inclusions in ordinary forgings is relatively high, and the transverse performance is lower than that in the longitudinal direction. The mold is prone to cracking along inclusions under repeated thermal cycles.
The sulfur and phosphorus content of electroslag remelted steel is lower, and its isotropy is improved. Under the same conditions, the lifespan can be increased by 20% -30%.
The price difference is about 25% -40%. Whether it is worth it depends on the target lifespan and replacement cost of the mold.
Mold frame materials are often underestimated
The mold frame is often made of ductile iron QT500-7 or medium carbon steel welded parts. Ductile iron has good shock absorption and is suitable for large tonnage machines.
The cost of welding formwork is low, but the rigidity uniformity is poor. After long-term use, the area near the weld seam is prone to deformation.
The price of the overall casting mold frame is 8% -12% higher, but it has good rigidity and is suitable for large-sized wheel hub molds.
Matching of inlay materials
Wheel rim inserts wear out quickly and can be treated with H13 nitriding. The impact of the spoke inserts is small, and ordinary H13 is sufficient.
There should be a difference in material hardness between the insert and the mold frame. The hardness of the two is similar, and they will bite into each other after long-term use, making disassembly difficult.
In the material scheme of the wheel hub mold, Xinfeng Mold will list the materials and heat treatment requirements by part, rather than uniformly labeling the entire mold.
Matching of materials and processes
The differential pressure mold can withstand higher differential pressure loads, and the template material needs to be selected from a higher strength grade. Ordinary low-pressure molds can be moderately relaxed.
The cavity of water-cooled molds needs to withstand greater temperature gradients, and the resistance to thermal cracking is more important than wear resistance.
The wear of gravity molds mainly comes from demolding, and the value of surface treatment is higher than that of upgrading the substrate material.
Balance between cost and lifespan
The material cost of a set of molds accounts for 30% -45% of the total price. The lifespan improvement brought about by material upgrades needs to be converted based on the number of modules.
The lifespan has increased from 30000 to 40000 cycles, resulting in a 25% increase in material costs and a 6% decrease in single cycle costs.
But if the product lifecycle is only 20000 cycles, material upgrades are a waste. The selection of materials should first depend on the actual output of the product.
Boundary of material substitution
Replacing 8407 with regular H13 can reduce the quotation by about 20%, but the risk of cracking in thin-walled wheel profiles increases. Substitution needs to have boundaries.
Before replacing, check the product output first. For projects with a lifecycle of only 10000 cycles, the impact of material degradation is controllable; It is not recommended to use more than 50000 molds.
Frequently Asked Questions
Question: What is the difference between H13 and 8407?
Answer: The price of 8407 is 30% -50% higher, and the lifespan is 20% -30% longer.
Q: What material is used for the mold frame?
Answer: QT500-7 ductile iron or medium carbon steel welded parts are commonly used.
Question: Is electric slag steel worth buying?
Answer: When the target lifespan is high, the price difference is 25% -40%.
Q: Should the hardness of the inlay be different from that of the mold frame?
Answer: Yes, the hardness is close and it will bite, making disassembly difficult.
Q: How much does material cost in the mold?
Answer: Usually 30% -45% is the main cost item.
Q: Do we need to upgrade materials for small product quantities?
Answer: No need, selecting materials based on actual production is more economical.
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