Abstract: Through analysis of an example of a failed die-casting mold design, key elements that should be paid attention to in die-casting runner design process are explained. When imitating runner system of other successfully designed die-casting molds, it is necessary to proceed from reality and combine structural characteristics of die-casting parts to design, change and improve it. Especially when copying molds, it is still a guideline to keep elements that are temporarily incomprehensible unchanged.

Die-casting production is inseparable from die-casting molds, quality of molds directly affects normal production of die-casting parts and even economic benefits of enterprise. A well-designed die-casting mold can maximize production efficiency of equipment, effectively reduce energy costs of die-casting production, and reduce scrap rate of die-casting parts. Experience shows that a well-designed die-casting mold mainly depends on success of design of its runner system, which is also the key to a die-casting mold.
Normal die-casting production first depends on correct position setting and shape and size design of runner system. When producing die castings with heavy weight, widely varying wall thicknesses, complex shapes, and strict requirements on tissue density, surface quality, etc., design parameters of runner need to be carefully designed. In any case, impact of improper runner system settings on die casting product quality cannot be compensated by improving other die casting parameters.
Before designing die casting gating system, it is necessary to fully analyze structure of casting and various customer requirements, and make sure that these requirements are basically met. General process of designing gating system is:
(1) Select location of gate.
(2) Consider flow direction of introduced molten metal.
(3) Divide number of strands of sprue.
(4) Design shape and size of sprue.
(5) Determine cross-sectional area of gate.
Although design of gating system is simply divided into above steps, small changes in each factor will cause major changes in design of gating system. Except that cross-sectional area of gate can be better confirmed through p-Q2 diagram, consideration of several other factors still depends on personal experience and knowledge of mold designer. For die-casting parts, there are very few cases where ideal gate position setting can be fully satisfied. It is often carried out according to principle of "ensuring main requirements and taking into account secondary links". In actual die-casting mold design, most of structural design of runner system is based on experience. Therefore, imitating and improving gating system of past successful mold designs has become an indispensable design method.
However, in process of imitating old die-casting sprue system, if it cannot be used flexibly, cases of imitation failure in actual design are still common. Following is a brief discussion based on a real case, so that we can get some inspiration from it and draw lessons from it in future mold design to improve success rate of mold design.

(1) Imitation of steering gear housing replica mold gating system. Figure 1 shows design of casting pouring system for a certain automobile steering gear housing. This is an early design structure and uses an annular main runner for filling. However, due to thicker wall thickness at intersection of core and presence of a large shrinkage cavity in the center, customer requested rectification. In order to solve this quality problem, after simulating filling of die-casting gating system, we changed die-casting sprue system to structure shown in Figure 2. Without affecting appearance of product, a small auxiliary runner is added to thick part of casting. After production practice, internal shrinkage of thick parts of casting has been greatly improved, and product quality fully meets customer's needs. Since then, design of pouring system of product has basically not changed. More than ten sets of this die-casting mold have been designed and produced. It is basically a stereotyped sprue design method. Structures of these die-casting molds have basically never been changed too much when they are copied. Only small-scale optimization designs have been carried out on local sharp corners and insufficient strength of die-casting mold to improve quality problems discovered during production.
Later, due to various reasons, a new cooperating manufacturer was replaced to make replica die-casting molds. Newly designed imitation pouring system of die-casting mold is shown in Figure 3. However, following quality problems occurred in actual die-casting production:
One is poor product formation.
Second, thick parts of product have serious internal shrinkage and porosity.
Third, internal quality decreases after machining.
Fourth, shift output decreased, mainly due to increase in spraying time, otherwise local mold sticking and cracking would occur.
Judging from castings produced by trial mold (see Figure 4), originally designed auxiliary runner completely became main runner during filling process of molten metal, while original main runner became auxiliary runner, which also affected normal filling of die-casting mold. Since this product is a very mature casting, there are mature die-casting processes that can be used for reference. From perspective of changes, in order to solve these quality problems, it is necessary to find reasons from die-casting mold design. Due to changes in die-casting mold manufacturers, there are still certain differences in design concepts of die-casting designers from each manufacturer. Although they are copying a set of die-casting molds, designers will more or less integrate their own design concepts into new die-casting mold design during design process,

 

Figure 1 Early pouring and drainage system design

 

Figure 2 Improved pouring and drainage system

 

Figure 3 Imitation designed pouring and drainage system

 

Figure 4 Castings for trial mold
In habitual design thinking, main channel opened on manifold cone is vertically upward, and there are not many diagonally pointing in the direction of ingate. Therefore, when an ingate is opened on manifold cone, a vertically upward main channel will naturally be opened on new die-casting mold according to designer's experience. As we all know, for die-casting runners, every turn of runner will bring about a large energy loss. At the same time, due to inertia of molten metal filling, material flow is filled first in linear direction. Only when front filling resistance increases, flow rate will change.
Newly designed main runner has a 90° turn, while auxiliary runner is exactly in straight direction. Therefore, during filling process of molten metal, mold is filled first by auxiliary runner and then by main runner. Judging from actual production samples, the entire filling process is basically completed by auxiliary runner, and main runner fails to play a filling role.
(2) Design change of overflow system basically imitated original design plan in design of overflow system. However, when core-pulling mouth slag bag on left side of product was redesigned, new design may have changed due to differences in design habits. New design has made small changes, that is, changing original annular slag bag into two independent small slag bags, as shown in Figure 5.
This small change did not seem to have any impact on the forming of casting, and it failed to attract attention of relevant personnel during actual production of casting. It was not until a complaint from a customer that problems caused by this design change were exposed. It turned out that during machining process, customer discovered that some products had fallen into slag bag, which affected customer's processing. This is a problem that has not occurred in the past. Size of original slag collecting bags is larger than aperture of steering gear housing. Design change of slag collecting bags makes one of slag collecting bags smaller in size. After outer size is smaller than hole diameter of steering gear housing, slag bag will fall into hole inside product and get stuck in hole during subsequent production process.
(3) Rectification of die-casting mold. According to production situation of die-casting mold, corresponding rectification and treatment must be carried out. Die-casting mold should be modified according to original die-casting mold design plan to make design of runner system more reasonable and smooth, facilitating filling of molten metal, solving problems of changing die-casting mold for small batch production, and enables product quality to meet customer needs.
Problem of small slag collecting bags falling into product holes can only be solved by connecting two slag collecting bags and increasing their overall dimensions.

 

Figure 5 Comparison of design changes of slag bag

Through analysis of above examples, we can understand that key factor that truly determines success of a mold design lies in details of die-casting design. Almost everyone knows general principles, but in actual applications, success or failure depends on one mind. Everyone's knowledge level, understanding of gating system design determine success or failure of a mold manufacturing.