[Abstract] This article mainly discusses relationship between gate and "adaptability" of mold from area of gate and angle of guide through examples, providing ideas for future molding design and solution of similar problems in diecast productions.
So-called mold adaptability refers to smoothness of producing qualified die casting part for same die on different die casting machine, operated by different personnel, and under a wide range of process parameters. In actual production process of diecast, there are many factors that affect quality of diecast, such as: injection pressure, injection speed, fast-press starting position, furnace insulation temperature, mold temperature, mold structure, etc. Which of these factors have a greater impact on the quality of diecast?
A common view among die-casting production enterprises is that influence of mold on quality of die casting part accounts for 70%, and in-gate parameters play a significant role in "adaptability" of mold. Here are a few examples to explain influence of in-gate in mold on "adaptability" of mold and how to deal with it.

Under certain injection conditions, when area of in-gate is too large, filling speed will be too low, metal will solidify prematurely, and even cause insufficient filling; while area of in-gate is too small, injection will be intensified, heat loss will be increased, eddy currents will generate and involve too much gas, intensify erosion of mold, leading to early scrap of mold.
Size of cross-sectional area of in-gate is often determined empirically during design and drawing process. Cross-sectional area of in-gate is calculated based solely on empirical formula, which cuts close relationship between cross-sectional area of in-gate and filling speed, filling time. There is a risk that cross-sectional area of in-gate, filling speed and filling time cannot be effectively matched. It is not clear to designer what range of design results can be modified. Cross-sectional area of in-gates obtained from different personal experiences are very different, and in practice, phenomenon of bad "adaptability" of mold will appear;
In practice, mismatch between cross-sectional area of in-gate in mold and die casting is not uncommon. When gap between such mismatches is not large, its performance is not obvious. In the use of mold, operator often feels that mold is "no easy to use"; when gap is large, it will be apparent --- die casting part can not be formed, reject rate is high, and quality is unstable.
Following is a real production example; this is a barrel-shaped shell part with an average wall thickness of about 4.5mm; alloy used is: ADC12, weight of die casting part (including net weight and slag package) is 4700 grams, and cross-sectional area of in-gate is 460 mm2; see attached picture 1:

 

Equipment used: 800T die-casting machine, diameter of injection head Φ110
Main process parameters are:
Casting temperature 650 ℃ mold temperature 230 ℃
Quick shot stroke 202 mm Quick shot handwheel opening 7 turns
During production process, it was found that die casting part were unfilling and surface quality was poor; reject rate was as high as 50% or more;
It can be seen from Figure 1 that this is a relatively simple diecast, design of gating system is basically reasonable; under normal circumstances, above die casting process can produce qualified products. In view of quality problems that occurred, we adjusted production process again in accordance with principle of easy first and then difficult. However, it was basically invalid!
To this end, we draw P-Q2 drawing according to parameters of mold and die-casting machine; as shown in Figure 2, we find from drawing:

 

Main reason for abnormal production is caused by inconsistency between cross-sectional area of in-gate and injection system. We know that for larger products, when gate area is small, filling time will be too long, cavity cannot be completely filled, a large area of cold partitions on product surface and a large number of cold blocks are mixed, and overall strength is severely reduced; this requires that actual filling time that injection molding system can achieve is shorter than filling time required for diecast. Calculation of the longest filling time required for diecast can be found in following formula:
T = K * X * 1000 * (TI-TF + S * Z) / (TF-TD)
Among them:
T is the longest filling time required for diecast, unit is ms
K is a coefficient, which is related to mold material used. Value of common mold steel H13 is 0.0346.
X is average wall thickness of diecast, unit is mm
TI is temperature of metal liquid, unit ℃
TF is minimum flow temperature of metal liquid, unit ℃
S is target solid percentage, unit %
Z is solid coefficient, unit %
TD is mold temperature, unit ℃
Filling time calculated according to above formula is 88.8ms. This is an empirical calculation value that is related to process parameters and wall thickness of diecast, has nothing to do with cross-sectional area of in-gate; it can be known from calculation of P-Q2 that when gate area is 460 mm2, minimum filling time of die casting system composed of die casting mold and equipment is 86.3ms; obviously, this value is almost same as the longest filling time required for die casting. Under this condition, during production process, high process parameters are required, slight fluctuations in process will cause various defects on the surface of die casting; if such a mold is produced on a higher-performance die-casting machine, it is also possible to smoothly produce qualified die casting part, but it is difficult to produce normally on existing die casting machines.
According to calculations, when in-gate area reaches 700 mm2, minimum filling time that system can reach is 64.9ms. This value has a larger adjustment space compared with the longest filling time required for diecast; It leaves ample room for adjustment of die casting process, and such a mold can basically be adapted to be produced on various diecast equipment with different properties.
With trial production of modified mold, operators generally report that mold is "useful" and reject rate has dropped to about 3%.

Although shape of in-gate of die casting part has a great relationship with shape of die casting part, actual design is very different; however, referring to some molding design manuals and some successful empirical data, it is generally possible to design a more reasonable injecting part. For design of various gates, please refer to relevant materials, this article will not repeat them. Following is a brief description of import of local details of gate design on "adaptability" of mold.

Figure 3 on the right is a schematic diagram of a common die casting mold runner system. In the use of mold, it is found that guiding angle of in-gate (that is, angle between edges of two ends of in-gate, as shown in Figure 3) plays a vital role in filling of die casting part. Improper guiding angle will cause "adaptability" of mold to be greatly reduced. Following uses a common part on motorcycles as an example to explain it;

 

Alloy used in this product is: ADC12, weight of die casting (including net weight and slag package) is 705 grams, average wall thickness is 4.3mm, and gate area in a single cavity is 145 mm2;

 

Equipment: 280T die casting machine, diameter of injection head is Φ60, and mold structure is  two cavities.
Main process parameters are:
Holding temperature 630 ℃ Mold temperature 220 ℃
Quick firing stroke: 95 mm  Quick pressure shooter wheel opening 4.5 laps
During production process, it was found that product has serious oil stains and skinning ( so-called skinning refers to local delamination of die casting part) in parts shown in Figure 5; different operators have different production rates, which is a typical phenomenon of poor mold adaptability. Skilled operators can basically produce normally, while those who are less skilled can hardly produce normally.

It can be seen from runner part of die casting system that fan angle is very small, and guiding angle of in-gate is almost zero (see Figure 5). After metal liquid enters cavity, it quickly impacts on opposite side of cavity, closing two slag packages on both sides, causing air, mold release agent and gas. in local area shown in Fig. 5 to be unable to be discharged normally, resulting in severe skinning and oil contamination in die casting.

 

In terms of human factors, it mainly lies in spraying control of release agent. Because diecast productions uses manual spraying of release agent, everyone's experience is different, spraying difference is large; if spraying agent is sprayed too much, waste products are produced, too few sprays, and mold sticking is prone to occur; due to poor mold venting, slight spraying differences will cause product scrapping; only individual skilled operators can carry out production; requirements on spraying amount of release agent have reached a nearly harsh level;
According to above analysis, solution to problem is to improve exhaust of defective parts; it is well known that adding slag packages and exhaust at defective parts is the best way; however, due to limitation of mold structure, there is no place to add slag packages and exhaust slots on the mold. In the end, it was decided to still use gating system and improve exhaust of area shown in Figure 5 by changing guiding angle of in-gate, so that alloy liquid can better achieve sequential filling, delay closure of exhaust duct, improve filling of mold. Figure 6 is a modified schematic diagram. Guiding angle of in-gate is increased at the position shown at A. Through change of guiding angle of in-gate, filling quality of mold is improved, "adaptability" of mold is improved. Replacement of different die casting machines and different operators can ensure smooth production of die castings, shift output is greatly increased, and rejection rate is almost reduced to zero;

 

In the production of shell parts, a problem that often occurs is cracking of die casting at in-gate, which causes product to be scrapped. Occurrence of this problem is mainly manifested in die casting part produced by molds whose in-gates are opened above movable mold. Crack occurrence site is shown in Figure 7. Operators often find this mold "not easy to use".

 

If we carefully observe cracked part of die casting, we will find that all cracks extend along leading edge of in-gate to inside of die casting; main reason for its formation is large internal stress caused by local sharp corners at in-gate. Removal of gate is mostly performed by knocking. When operator removes gate, due to local force at the gate, die casting is cracked.
To deal with this kind of cracking problem, a simple treatment can be done to mold, and local sharp corners shown in Figure 7 can be eliminated. After processing mold, similar problems did not occur in die casting.

Through analysis of above examples, we can see that design of gate of die casting mold has a great impact on "adaptability" of die casting mold. A reasonable parameter of gate is to produce high-quality products and ensure mold's "adaptability" At the same time, improper processing of local details of gate is often an important factor for poor adaptability of mold; a reasonable combination of design of in-gate and optimization of local details is an effective way to improve mold's "adaptability", and it is also a fundamental guarantee to improving product quality and reducing rejection rate.