The development of new products and the renewal of old molds are often encountered in the production of zinc alloy die casting. Generally speaking, the internal quality and external quality of zinc alloy die castings depend on die-casting production process parameters such as liquid metal temperature, die-casting pressure, speed and speed switching point. But in most cases, it is also closely related to the design and manufacture of zinc alloy die casting mold. The factors affecting the zinc alloy die casting mold include the number of cavities, the design of gating system, the type of gate, and the structure of zinc alloy die casting itself. In order to avoid the defects of zinc alloy die casting parts caused by the design of zinc alloy die casting mold, and check and correct some problems in the production of zinc alloy die casting mold, it is necessary to test the completed zinc alloy die casting mold.
The main purpose of mold test is to find out the optimized process parameters and the modification scheme of zinc alloy die casting mold. It is required that even if the material, machine setting or environment and other factors change, the zinc alloy die casting mold can still ensure the stable and uninterrupted mass production of qualified products, and maximize the “adaptability” of zinc alloy die casting mold, Not just to get a few good test samples.
1.1. Find out the problems existing in zinc alloy die casting mold
The zinc alloy die casting molds produced by the zinc alloy die-casting factory all hope to be a zinc alloy die casting mold with good “adaptability” (i.e. wide molding parameters or process conditions). However, due to poor consideration in design or inadequate processing in the manufacturing process, some problems often occur, such as out of tolerance in size, unsmooth local transition, inappropriate structure, etc., resulting in unqualified zinc alloy die castings or impossible trial production of zinc alloy die casting molds. The test mold is the simplest, direct and effective way to find out the problem of zinc alloy die-casting mold.
1.2. Find out the best forming conditions
In the process of mold test, the mold test personnel can find out a set of the most suitable process data, which can be used as the default value of the set machine parameters during mass production, and adjust the best process conditions for mass production of zinc alloy die-casting mold according to this set of data.
1.3. Verify CAE aided design and improve follow-up work efficiency
CAE analysis software is the result of computer simulation based on the specification of real machine, the performance of real alloy and real external environment. The designers of zinc alloy die casting mold make corresponding amendments to the design of zinc alloy die casting mold according to the simulation; During the real machine mold test, the analysis data can be directly input as the preset value, so that the mold test personnel can test and analyze whether there are deficiencies in the design of zinc alloy die casting mold. The defects of the test module can be compared with the simulation results to provide a basis for the subsequent process parameter setting.
2.1. Determine reasonable mold test steps
One of the purposes of mold trial is to determine reasonable mold trial steps and make appropriate records for the smooth production in the future, which is of great significance to the formal production in the future.
2.2. Strictly inspect the zinc alloy die casting mold
According to the design drawings of zinc alloy die casting mold, check the relevant dimensions (especially the size of gate socket interface) in detail; Find out the action sequence of each moving part of the die (core pulling and sliding block); Confirm that the model, injection position, oil pipe interface, pressure chamber diameter and interface size of the die casting machine to be used are consistent with the actual requirements of the mold. Special attention should be paid to that the die-casting machine used in the mold test must be consistent with the model of the die-casting machine used in the mass production of the mold, otherwise, the zinc alloy die-casting mold may not be used.
2.3. Check the clearance between the pressure chamber and the gate sleeve and between them and the punch
The interface size between zinc alloy die casting mold and die casting machine is matched, and the matching clearance is very important; Its good fit clearance is the basis to ensure the quality of zinc alloy die castings.
2.4. Fixing of zinc alloy die casting mold
Before lifting the mold, the thread accuracy and effective thread depth of the lifting ring hole must be checked to prevent the sliding buckle of the lifting ring when lifting the mold. When the die is fixed, the U-shaped groove corresponding to the T-shaped groove of the die-casting machine is opened on the mold base, which not only facilitates the installation of the zinc alloy die casting mold, but also ensures the reliable installation.
3.1. Setting principle of process parameters
When setting process parameters for mold test, as long as the zinc alloy die-casting parts can be qualified and meet the technical requirements of customers, the parameters such as liquid zinc temperature, injection speed and pressure should be as low as possible to protect the zinc alloy die-casting mold and machine.
3.2. Basic requirements for mold test
Try to use the same alloy, die casting mold temperature, pouring temperature and cooling time as mass production in the future. These factors will greatly affect the correct determination of the size of zinc alloy die castings. It is also important to select the appropriate die casting coating.
3.3. 7 groups of main process parameters
Practical experience shows that the main parameters that have an important impact on the quality of zinc alloy die castings are as follows:
① Pouring temperature and quantity of molten metal (thickness of residual cake).
② Temperature of zinc alloy die casting mold.
③ Slow injection speed.
④ Fast injection speed and injection pressure (fast injection accumulator charging pressure).
⑤ Fast injection switching position (fast injection stroke).
⑥ Boost trigger pressure and boost specific pressure.
⑦ Pressure holding and cooling time.
If the pouring temperature is too high, the shrinkage of zinc alloy is large, which makes zinc alloy die castings easy to produce cracks, coarse grains and sticking patterns; When the pouring temperature is too low, zinc alloy die castings are prone to cold shut, surface cracks, insufficient pouring and other defects. Generally, whether the metal pouring amount is appropriate is judged from the thickness of the remaining material cake. The thickness of the material cake is too thin, which is not conducive to the feeding of the casting, and the thickness of the material cake is too thick, which will bring serious potential safety hazards.
Improper slow injection speed will make the air in the pressure chamber wrapped in the alloy liquid, resulting in the increase of pores in zinc alloy die castings. After the design of zinc alloy die is completed, the fast injection stroke can be calculated, which is generally not different from the actual situation. The determination of the fast injection conversion position can be completed as follows: the position at the end of the injection retreats by a certain distance (the calculated fast injection stroke); A small amount of pre and post adjustment is made according to the production of zinc alloy die castings.
In the process of testing, when adjusting parameters, only one parameter is adjusted at a time, so as to distinguish the effect of single parameter change on the quality of zinc alloy die castings.
3.4. Check the first mock exam, then adjust the process parameters. After making the first die castings in the trial mold, the following inspections shall be carried out.
① Residual cake thickness.
② Boost specific pressure.
③ Mold filling speed.
④ Ejection length of zinc alloy die castings.
⑤ Quality of zinc alloy die casting, gate and slag ladle.
The purpose of the above inspection is to prevent the finished product from filling, shrinkage, flash, mucous membrane and even damage to the zinc alloy die casting mold. According to the inspection results, the process parameters are further adjusted to achieve the best die casting process conditions.
In the process of mold test, the test mold produced after the process parameters are stable requires the process personnel to conduct on-site inspection according to the requirements of customers. The main inspection contents include:
① Machining allowance measurement of main parts
② The burr and flash distribution of zinc alloy die castings, especially the castings with sliding block. Although the water running at the parting surface of the sliding block has little effect on the size of the product, it will bring serious hidden dangers to the subsequent die production
③ Dissect zinc alloy die castings and check internal pores, shrinkage cavities, etc.
④ The appearance of zinc alloy die castings is strained and cracked. The mold problems that can be judged intuitively on site need to be handled before trial production, so as to provide customers with good test molds as much as possible.
Mold test is an actual inspection process of mold use. After mold test, samples, inspection records and process parameter records in the process of mold test shall be properly kept (including various pressures, pouring temperature, mold temperature, time, etc.); These records are the basis for the analysis and modification of zinc alloy die casting mold and the formulation of mass production process in the future; At the same time, it also accumulates practical experience for the die design of similar zinc alloy die castings in the future, especially provides the basis for real parameter setting for CAE filling simulation analysis in the future.
It is a patient and meticulous work to solve various problems in the mold test. We should have a detailed understanding of the structure of zinc alloy die-casting parts, die-casting process and the structure of zinc alloy die-casting molds. We must not modify the zinc alloy die-casting molds blindly to avoid unnecessary losses.
Finally, it should be emphasized that the purpose and focus of mold test is to optimize the zinc alloy die casting mold and process to meet the requirements of mass production, rather than just testing several zinc alloy die-casting samples.