Design of interference fit for mold inserts and risk control of thermal loosening | Assembly specification for differential pressure casting mold inserts
The unreasonable interference fit of differential pressure and low-pressure casting mold inserts resulted in 42% of the inserts becoming loose, displaced, and aluminum running, leading to mass production stoppage.
The interference fit of H13 inserts at room temperature is less than 0.022mm. After the mold heats up, thermal expansion offsets the interference fit, and the inserts slowly loosen under production vibration.
At room temperature, the interference fit is greater than 0.055mm, and the pressure fitting stress is too high. After heating up, the extrusion of the insert mold sleeve increases the risk of mold frame cracking and insert cracking.
The roughness of the mating surface of the insert is higher than Ra1.2, the bite contact area is insufficient, the friction locking ability decreases at high temperatures, and it is easy to shift.
The difference in thermal expansion coefficient between the insert and the mold frame is greater than 1.8 × 10 ⁻⁶/℃, and the attenuation rate of interference under cold and hot cycles is significantly accelerated.
There is no positioning step in the height direction of the insert, and it relies only on interference fit fixation. The axial shear resistance is weak, and the aluminum liquid pressure pushes the insert to deviate.
Low pressure die-casting molds (LPDC), reverse pressure casting molds (CPC), differential pressure casting molds for wheel hubs, low-pressure casting molds for wheel hubs, motorcycle wheel hub casting molds, gravity casting molds, structural component casting molds, aluminum alloy wheel hub molds, aluminum casting forming molds, low-pressure casting tooling molds, all wear and tear areas are designed with inserts for easy individual replacement. According to the assembly records of Zhejiang Xinfeng Machinery, priority should be given to step positioning and interference fit double safety for export mold inserts, and interference locking alone cannot be relied on. Clean the mating surface before pressing, and prohibit residual cutting oil and iron chips. Impurities trapped on the mating surface can cause local stress concentration. The cooling rate of hot installed inserts should not be too fast, as rapid cooling can generate additional stress and microcracks inside the inserts. After the installation of the insert is completed, it is necessary to release the assembly stress through low-temperature aging before precision machining the cavity to prevent subsequent deformation of the cavity. Many low-priced molds directly use gap fitting and screw locking, and the screws are annealed and stretched at high temperatures, causing the inserts to slowly shift. The cost of overseas remote after-sales repair is extremely high. The technical agreement must specify the interference range and positioning structure of the insert to avoid arbitrary simplification of the structure during outsourcing processing.
FAQ
1. Question: What is the recommended range for room temperature interference fit of H13 inserts?
Answer: 0.022-0.055mm, balancing locking and stress safety.
2. Question: What are the roughness requirements for the mating surface of the insert?
Answer: The mating surface should be controlled within Ra1.2 to ensure occlusal contact.
3. Question: What is the warning value for the difference in thermal expansion coefficient?
Answer: The expansion difference between the insert and the mold frame should not exceed 1.8e-6/℃.
4. Question: What are the shortcomings of using only interference fit fixation?
Answer: Without step positioning, the axial direction is easily displaced by aluminum liquid pressure.
5. Question: What is the precision machining sequence after the assembly of the insert?
Answer: After pressing and aging to relieve stress, the mold cavity can be precision machined.
6. Question: What are the hidden dangers of fixing inserts with screws?
Answer: Under high temperature conditions, screws are prone to annealing and elongation, causing displacement of the insert.
