Deburring of die castings is a huge project, and labor cost is high. Robot deburring puts higher requirements on consistency of die casting molds. Focus is on introduction of design and manufacture technology of die castings without flash burrs in mold design and manufacturing stage. It is mainly explained from aspects of shrinkage rate of castings, mold matching clearance, mold strength design, manufacturing accuracy, etc., and calculation formula is deduced based on data statistics. Results show that use of technical standards in design and manufacturing stage of mold can effectively realize die casting without flash burrs, and standard is still valid in maintenance stage of mold.
To remove flash burrs of die castings, enterprises need to invest a lot of manpower and material resources, and number of burr workers in enterprise is 1 to 2 times number of die casting workers. Labor cost of deburring die castings is very high. It will be very common for robots to replace manual labor in the future. This is an important way to reduce labor costs in the future. However, use of robots to deburr must ensure consistency of castings, gap of parting line is reasonable and stable, size and deformation fluctuations of castings cause robot to damage castings and tools, and deburring effect cannot reach ideal state.
Reasons for burrs of die castings are the three major elements of die casting, namely, die casting machine, die casting mold, and die casting process. In terms of die casting process, burrs may be generated if mold temperature, aluminum liquid temperature is too high, and casting pressure is too high; in terms of die casting machine, insufficient clamping force and mold plate deformation may also generate burrs; in terms of die casting mold, manufacturing accuracy of parting surface, core pin, and ejector hole of mold is not enough to generate burrs; among them, poor design accuracy and manufacturing accuracy of mold are main reasons for burrs of die castings. In Figure 1, burrs on parting surface of wheel hub are connected in large pieces, workload of burrs increases, even casting size becomes out of tolerance and thickened due to expansion force of mold, and use of raw material aluminum increases. In this case, production needs to be stopped for countermeasures. In Figure 2, there are burrs on nested end face, chamfered surface, and inner hole. Nested end faces are not processed, but burrs must be cleaned up. Butt-joint holes are completely connected and closed by aluminum sheets, which makes burr cleaning more difficult.

 

Figure 1 Wheel hub slider gap burr

 

Figure 2 Box nesting burr
In order to define thickness of burr, a conical mold gap is made on die-casting mold, as shown in Figure 3. Aluminum material is ADC12 for die-casting verification of minimum wall thickness. Forming is clear when wall thickness is ≥0.09 mm, forming is discontinuous below 0.09 mm and above 0.06 mm.
In order to define length of burr, die-casting mold is made on die-casting mold with different gaps on mold parting surface to test burr length, as shown in Figure 4. When wall thickness is 0.08 mm, length of overflow from casting body is about 4 mm; when wall thickness is 0.07 mm, length of overflow from casting body is about 2 mm; when wall thickness is 0.06 mm and below, length of overflow from casting body is less than 1 mm, and it is difficult to form. Aluminum film exists like powder, and shot blasting of die casting can completely remove it. Summarize above test results to define mold fit clearance standard of Figure 5.
Definition 1: Gap C≤0.06 mm during die casting mold production is design and manufacturing standard for molds without flash burrs

 

Figure 3 Minimum thickness test of aluminum alloy

 

Figure 4 Thin wall length test of aluminum alloy die casting

 

Figure 5 Die casting mold parting surface and core pull slider gap
This paper studies and verifies shrinkage rate, strength design, fit clearance and manufacturing accuracy of mold design. Test conditions: Toshiba 800T die casting machine, aluminum material ADC12, aluminum liquid temperature 640 mold plate, casting pressure 660 kgf/, mold material 8407 series.

Die casting is in a high temperature state when it is ejected from mold, then shrinks when it cools to room temperature. When designing mold, first complete 3D of casting and then determine shrinkage rate of casting. General data recommends 0.4~0.7. For mold designers, it is necessary to select corresponding shrinkage rate for different aluminum alloys.

Thermal expansion coefficients of metals involved in die-casting process are mold steel and die-casting materials. Common die-casting mold materials such as 8407, DIEVAR, H13, and SKD61 are all classified as 8407 series metal materials. For aluminum alloy die-casting, mold temperature range during normal production after mold preheating is between 75 and 425 ℃. Select corresponding mold thermal expansion coefficient according to mold use temperature (Figure 6). There are differences in temperature of molds used by different die-casting companies, and companies need to make statistical confirmation based on big data.

 

Figure 6 Thermal expansion coefficient of metal material 8407 mold steel
Thermal expansion coefficient of die-casting aluminum alloy and magnesium alloy is mainly related to Si content. Under same temperature, the higher Si content, the smaller thermal expansion coefficient. Figure 7 shows thermal expansion coefficients of three commonly used die-casting materials ADC12, A380, and AM50. Corresponding vertical line of grade is median value of material component requirement. Die-casting manufacturers can query according to commonly used Si content of their own company.

 

Figure 7 Thermal expansion coefficient of aluminum alloy

When selecting shrinkage rate in die casting mold design, it is necessary to comprehensively select mold material, aluminum alloy type, and mold temperature during die casting.
(1) Temperature of die casting and mold:
Shrinkage rate is change in casting size from moment die casting is ejected from mold to room temperature. It is necessary to measure actual temperature of mold and casting. When casting is ejected, temperature of mold is measured. Result of thermal imager measurement is that temperature of mold area corresponding to casting is 289.5 ℃ (mold is still cooling rapidly at the moment of casting ejection), external temperature of casting is 350 ℃, and internal temperature is about 400 ℃ (this thermal imager has a delay in measuring temperature, and temperature is about 50 ℃ lower than actual temperature), see Figure 8.

 

Figure 8 Thermal imaging temperature of mold
(2) Correspondence table of thermal expansion coefficients of mold steel materials and die casting materials at different temperatures:
Table 1 is a correspondence table of changes in thermal expansion coefficients of die castings and molds at different temperatures made based on Figures 6 and 7 and literature. Thermal expansion coefficients of three commonly used die-casting materials: ADC12, A380, and AM50.
Table 1 Correspondence table of change of thermal expansion coefficient of die casting and die temperature

In Table 1, when casting is ejected from mold, temperature inside casting is 350 ~ 400 ℃, and corresponding mold temperature is 280 ℃ ~ 300 ℃. At this time, nominal dimensions of casting made of ADC12 and 8407 series mold steel are close. After testing temperature of mold and casting, statistically analyzing size and temperature changes of mold in production, reasonable recommended values in table are determined as reference data for mold design.
Case 1: Principle of baking casting to increase temperature and then ejecting casting when die casting is difficult to eject in mold:
When casting is difficult to eject in mold, the most common method of on-site problem handling is to apply a layer of grease to joint between casting and mold, then bake casting with natural gas (Figure 9), which often ejects casting again.

 

Figure 9 Method of baking castings when castings are difficult to eject

 

Figure 10 Dimensional changes of castings (nominal size is 40 mm) and molds at different temperatures
According to Table 1, relationship curve of nominal size of casting is 40 mm (other dimensions have same slope) and mold with temperature is made in Figure 10. Size of casting and mold will increase with increase of temperature, but slope is different. Point A in figure is intersection point where expansion of casting and mold are equal. As temperature rises again, size of casting is greater than size of mold. One of reasons for difficulty of ejecting castings is that pause in production causes casting temperature to drop below point A where casting and mold sizes are equal. Shrinkage of casting makes size smaller than size of mold, resulting in greater clamping force. Baking method is used to increase temperature of casting, increase thermal expansion, and even exceed size of mold, which reduces clamping force. In this process, method of baking while trying to eject can be adopted. Many companies do not have conditions and do not need to conduct temperature tests to wait until conditions are met before ejecting. This method is fast, simple and effective. Otherwise, mold needs to be removed by mold worker, which is very time-consuming and laborious.

 

Figure 11 Process of casting and mold size changes due to temperature

Die castings are generally complex in shape and have many sizes. In design of new products, empirical formula can be directly applied. If in the case of copying mold, big data analysis is applied to see if there is any regularity in size deviation of casting in production. Under condition of die-casting process being determined, bias of size deviation is consistent, such as long-term large or small. When copying mold, this statistical result needs to be implanted to correct design, and existing mold can also be corrected. However, it should be noted that mold cannot be easily modified when big data analysis fluctuates irregularly, but size should be stabilized from process to find countermeasures.

When designing mold, it is necessary to consider mold's parting line, parting surface, contact surface of pin, matching surface of core pull slider, matching surface of concave and convex molds needs to be designed with a reasonable matching clearance, which is main reason for flash burr in mold.

Four sets of molds were selected to measure temperature of movable mold, fixed mold, concave and convex parts, core pull slider, pin and surrounding areas of mold, results in Table 2 and Table 3 were obtained.

 

Figure 12 Temperature measurement of different parts of mold
Table 2 Temperature measurement table of mold pin and nearby

Table 3 Mold concave and convex parts/slider temperature measurement table

In Table 2, temperature of pin and nearby parts is 50.2 ℃ higher. In Table 3, temperature of convex part of mold and corresponding part of core-pulling slider is 28.8 ℃ higher than that of concave part.

Mold step parting and core pull slider parting have concave and convex mold parts. Due to different heat dissipation and heat absorption, different matching of movable mold and fixed mold with moving and fixed surface of equipment in production, there are differences in heat transfer, so there will be different mold temperatures. Different temperatures correspond to different thermal expansion of mold. If matching clearance is not considered, it may cause damage and collapse of contact surface of mold, resulting in flash burrs at this part of casting.
In Figure 5, it has been defined that 1: gap C≤0.07 mm during die casting mold production is design and manufacturing standard for molds without flash and burrs. This standard is gap at this location of mold during normal production.
(1) Clearance of concave and convex parts of step parting
In daily work, it is difficult for mold designers and manufacturers to determine actual values of these two temperatures. Based on multiple tests and verifications on site, temperature difference of concave and convex parts of same mold is 28.8 ℃ (30 ℃) according to Table 3.

(2) Clearance of butt pin

 

Figure 13 Clearance of butt pin Figure 14 Clearance of core puller and pin
(3) Clearance of pin and slider
In Figure 14, pin and slider are matched. First, it is necessary to consider that top of pin and slider have same ejection angle. Pin needs to be designed to prevent rotation, and clearance of pin must also be designed. Since slider is in mold closing and opening stage, movement fluctuation of core puller and manufacturing accuracy are ±0.03 mm, clearance of concave and convex slider are added together.

 

Figure 15 Core-pulling slider hits pin with a 0.15 clearance burr
Different die-casting plants have different production processes and may have different clearances. Daily data accumulation and analysis and improvement are needed. Overall, difference is not large. Fluctuation of 0.01 to 0.03 is normal. Above recommended calculation formula is still valid.
After parting surface and clearance of pin are reasonably designed, it is completely possible to achieve burr-free die casting. There is a wrong view that die casting without burr can be achieved only when mold design and manufacturing have no clearance. Actual situation is that fit is too tight, mold collapses and deforms to produce larger burrs, and causes damage to mold.
Case 2: Pin of a new mold or a newly replaced one is normal in the first die casting, but what is reason why pin becomes bent or broken when die casting dozens of molds?
Clearance of pin is not considered when new mold is designed and manufactured. After mold is produced for dozens of shots, temperature of pin rises, which is different from temperature near mold. Thermal expansion causes length of pin to increase, resulting in pin being hit and bent or broken. In particular, pins that are matched with slider are more likely to break and bend: first, slope of top of pin is not designed, second, pin is not anti-rotation, and third, clearance of match is too small. After above designs are completed, they need to be manually confirmed during cold assembly of mold, clearance is checked with a feeler gauge after slider is pushed in, and pin clearance is reasonably adjusted during production. Pins longer than 200 mm need to review stability of product size and daily monitoring and tracking to detect abnormalities in time.

When die casting is nested, exposed part of nesting matches mold, and flash burrs caused by unreasonable clearance design are difficult to remove. In Figure 16, exposed abcg segments of nesting have burrs. Ag segment needs to be processed, burrs on casting can be left uncleaned, but bc segment is not processed. After casting is processed, bc segment still has burrs. Burrs falling off during engine assembly and use are not allowed. How to prevent generation of burrs needs to be calculated in mold design stage.

 

Figure 16 Burrs on nesting
In Figure 17 of mold design, nesting sections a to g are all exposed, h is three evenly distributed nesting positioning pin holes, and nesting section a is pressed tightly against mold.
Effective water-sealing (aluminum liquid-sealing) surfaces of nesting and mold are nesting surface a and inner diameter surface g. Water-sealing surface here is to prevent aluminum liquid from entering nesting part that needs to be exposed.

 

Figure 17 Design of matching clearance between nesting and mold (where dimensions are all nesting dimensions)
Distance between water-sealing surface of nesting in Figure 17 and section a is only 1.5 mm, which is too short. b section of R angle should also participate in water-sealing effect, otherwise bcde sections may produce burrs due to poor water-sealing effect of a section.

 

Figure 18 Design of matching clearance between nesting and mold (where dimensions are all nesting dimensions)
Table 4 Design of clearance between nesting and mold (nesting without preheating)

Combined with matching conditions of various surfaces of nesting and mold, design clearance standard table 4 of case is formed.
With increase of service life of mold, deformation and wear of surface matching nesting and mold increase, and flash burrs will also increase. The best design is to design nested mounting pins, positioning pins and other molds related to nesting as a combined inlay type, and make spare parts for timely replacement.

If strength and rigidity design of die-casting mold is unreasonable, deformation of mold during die-casting will also cause generation of flash burrs.
Definition 2: Deformation of die-casting mold δ≤0.05 mm is maximum deformation of mold to restore its original state

 

Figure 19 Strength design of die-casting mold
When mold has a core-pulling slider, mold rigidity is not enough, slider retreats during die-casting, resulting in increased burrs and aluminum running on parting surface. Figure 20.

 

Figure 20: Strength design of core-pulling slider
Force of forming insert on mold frame
When casting has a complex deep cavity, mold will be in the form of an insert. Mold insert in Figure 21 is in direct contact with mold frame. Casting pressure is completely applied to same contour area. Casting pressure is equally transmitted to mold frame. As number of molds increases, corresponding area of mold frame in Figure 19 sinks at the bottom, causing shape and size of casting to change. Therefore, it is necessary to identify dimensional impact of this area during mold development and include it in quality management category.
Many die-casting companies have a casting pressure of 70MPa~110MPa, or even greater, while yield strength of mold frame is 91.5. Each die-casting impacts mold frame once, so mold frame is crushed. Countermeasures: First, reduce casting pressure while ensuring quality; second, increase hardness of mold frame to increase yield strength; third, replace mold frame material QT50 with P20.

 

Figure 21 Inserts for complex deep cavities
Case 3: Wheel hub die casting(Figure 22). During cold die casting, slider contact surface is penetrated without burrs, but class produces burrs for about 1 hour and even blocks the entire parting surface. Calculated deformation is 0.075 mm＞0.05 mm, and slider cannot be restored to its original position under force, resulting in aluminum running and flash burrs.

 

Figure 22 Wheel hub slider surface burrs
Countermeasures: Option 1 is to reduce casting pressure from 780 kgf/ to 540/. Prerequisite is that CPK verification of tensile strength of product must be carried out. This case verifies that CPK is qualified.
Option 2 is to reduce temperature of mold frame: the first 50 molds of mold production have no flash burrs. During production process, mold frame QT50 rises from normal temperature of 20 ℃ to about 100 ℃. At this time, tensile strength decreases and deformation of mold frame increases. At this time, method of using tap water to reduce temperature of mold frame can be considered.

 

Figure 23 Wheel hub mold slider structure and size

By analyzing thermal expansion coefficient of die-casting mold steel and die-casting aluminum alloy, shrinkage rate is correctly selected according to type of die-casting aluminum alloy during mold development and design. Temperature difference between concave and convex parts of die-casting mold will lead to different thermal expansion. At the same time, fitting clearances of different parts of mold are also different. This needs to be corrected during mold design and manufacturing to improve accuracy of mold. Yield strength of mold changes with temperature of mold during use. As long as mold design is reasonable and manufacturing accuracy is improved, die casting without flash and burrs can be achieved in mold. Accuracy of casting can be guaranteed only when mold accuracy is improved. Good consistency of castings is a prerequisite for normal use of die-casting burr robots. This article only analyzes mold. Flash-free burrs of die-castings also need to be analyzed from all aspects such as equipment and technology.