Why is the exhaust groove design of LPDC mold unreasonable? Why is the casting prone to gas entrapment?
Low pressure casting relies on low-pressure gentle filling, and the design of the exhaust groove directly determines the porosity defect rate of the casting, which is the core link of process design.
1、 The influence of exhaust groove size parameters on the filling exhaust effect
The conventional depth of the exhaust groove for aluminum alloy LPDC molds is 0.1-0.2mm, and the width is 8-15mm. Within this parameter range, the gas exhaust efficiency inside the mold cavity is the highest. Industry test data shows that the porosity defect rate can be controlled within 3% under the appropriate parameters. If the depth of the exhaust groove exceeds 0.25mm, the aluminum alloy melt is prone to material migration and edge flying, which increases the subsequent cleaning time. The daily cleaning time for a single set of molds will increase by more than 25 minutes, affecting the production line rhythm. When the depth of the exhaust groove is less than 0.08mm, the gas discharge resistance increases, and the gas wrapped around the filling front cannot escape. The probability of porosity defects in the thick walled position of the casting increases to 27%. For symmetrical castings of wheel hubs, the total opening area of the exhaust groove generally accounts for 1.2% -2% of the parting surface area, while for complex castings of shells, the proportion of exhaust area needs to be increased to around 2.5%. Xinfeng Machinery, in the mold design stage, combined with filling simulation, centrally arranged exhaust structures at the final filling convergence position.
2、 Applicable scenarios of different forms of exhaust structures
The open exhaust system with parting surface is easy to process and cost-effective, suitable for wheel hubs and simple disc castings. 70% of standardized LPDC molds on the market adopt this exhaust solution, which is easy to maintain. The embedded block exhaust structure is suitable for deep cavity and thin-walled shells, with multiple exhaust channels and less aluminum blockage. The processing cost is about 30% higher than that of the proportional exhaust, and it is mostly used for new energy motor shell orders. Exhaust plug exhaust is suitable for locally isolated thick spots, with limited single point exhaust capacity. It is generally used as an auxiliary structure and cannot be used as the main exhaust solution alone. The frequency of exhaust plug blockage is about every 2000 cycles and needs to be cleaned. Negative pressure exhaust is a high-end supporting solution that requires an external negative pressure device, which can further reduce the porosity rate by half. The equipment investment is high, and only a few high-end European casting production lines are equipped with it.
3、 High frequency demands from overseas customers regarding exhaust structure
Southeast Asian customers tend to prefer simple split exhaust, prioritize controlling mold costs, and rarely add exhaust plugs. About 81% of inquiries do not accept the markup caused by complex embedded exhaust. Tier 1 North American automotive customers require a simulated gas flow animation for filling, which needs to prove that the exhaust layout can avoid gas pockets. This is a mandatory item for technical review, and if the review fails, the project cannot be approved. European customers will require the exhaust structure to have a quick disassembly and maintenance design, which is convenient for the foundry to clean independently and reduce downtime for maintenance. Customers have high requirements for utilization rate. Middle Eastern engineering machinery casting customers typically require an increase in the number of exhaust slots due to thicker casting walls and higher total gas volume, and do not accept simplified exhaust solutions.
4、 Maintenance and life decay law of exhaust groove
The accumulation of aluminum shavings and oxide scales will gradually reduce the effective cross-section of the exhaust groove. After continuous production of 1200-1800 molds, the exhaust efficiency decreases by about 40%, and the porosity defect rate gradually rebounds. Using hard alloy inserts as exhaust slots improves wear resistance and reduces the risk of scratching the slots during cleaning. The effective service life is extended by 60%, making it suitable for long-term uninterrupted production lines. When cleaning the exhaust duct, using a hard scraper can easily cause deformation of the groove. After the groove is deformed, the airflow becomes turbulent. Even if it is cleaned thoroughly, intermittent porosity defects may still occur, which many overseas foundries tend to overlook. Excessive spraying of die-casting release agent can cause residue to deposit in the exhaust channel. It is recommended to perform a basic blow every 8 hours to reduce the probability of blockage.
5、 Google's long tail traffic layout ideas
Exhaust defects belong to the pain point category of search terms, and most users have already encountered production problems. They have a strong willingness to convert commercial inquiries and have a higher conversion rate than general terms. Product+Problem Category: LPDC mold porosity from poor venting、low pressure casting mold vent design rules， Suitable for popular science blog posts on process faults. Tutorial class: How to set vent slot depth for aluminum LPDC, undertaking research traffic for designers and process engineers. Comparative category: vent insert vs parting line vent for LPDC die, suitable for structural comparison content, undertaking traffic during the decision-making period. The content includes a schematic diagram of defective samples with air holes and a simulated airflow diagram for filling. When overseas engineers encounter defects, they will prioritize searching for solutions to the problem.
6、 Common misconceptions in exhaust design
Misconception 1: The more exhaust slots there are, the better. Too many exhaust slots can damage the sealing of the parting surface, easily causing large areas of burrs and increasing subsequent polishing costs. Misconception 2: When the exhaust groove is opened to the front position of the filling mold, the melt seals the exhaust port in advance, trapping the gas inside the mold cavity and exacerbating the porosity defect. Misconception 3: LPDC exhaust can directly copy the exhaust parameters of high-pressure die-casting. HPDC has a fast filling speed, and the exhaust groove parameters are significantly different from those of low-pressure casting, resulting in a skyrocketing defect rate when directly applied.
FAQ
Q: What is the conventional depth range of LPDC exhaust groove? A: Aluminum alloy is commonly used with a thickness of 0.1-0.2mm, which can be fine tuned according to the casting wall thickness and filling speed. Q: How many times do exhaust plugs usually need to be cleaned? A: Under normal operating conditions, cleaning should be done once every 1500-2000 cycles to prevent aluminum slag blockage. Q: What types of castings are suitable for negative pressure exhaust? A: Thin walled, structurally complex, and prone to gas accumulation new energy shell castings have outstanding effects. Q: What are the main causes of exhaust duct blockage? A: The main reasons are the accumulation of aluminum oxide scale, release agent residue, and splashing aluminum shavings. Q: Is the large flying edge caused by the deep exhaust groove? A: Most likely, it is due to the excessive depth of the exhaust groove or the large gap between the parting surfaces. Q: What is the appropriate proportion of exhaust area for LPDC mold of wheel hub? A: It is recommended that the total exhaust area of the parting surface account for 1.2% -2% of the parting surface. Q: What are the drawbacks of embedding exhaust blocks? A: The processing cost is higher, and it is necessary to reserve positioning structures, which increases the cost of molds. Q: Can simulation software predict the position of the airbag? A: Sure, the filling simulation can identify the gas convergence zone in advance and guide the exhaust layout.
