Shannon Wetzel, Senior Editor
(Click here to see the story as it appears in the Nov./Dec. issue of Metal Casting Design & Purchasing.)
The permanent mold and diecasting processes share a common ground. While the molds used in sand molding, investment casting and lost foam casting are expendable, metal diecasting and permanent molds are used repeatedly. The material and manufacture of the metal molds make initial tooling costs more expensive than the other processes, but if the volume of production is high enough, this cost difference can be overcome. Permanent mold and some types of diecasting also offer superior mechanical properties because the metal mold acts as a chill.
Despite the similarities, each process may better suit a particular application, depending on the property requirements, casting size, production rate and design complexity.
Diecasting part size ranges from a few ounces to more than 100 lbs., but most parts fall on the lighter side of the range. Diecasting’s minimum size is smaller than most other casting methods, so the process usually is associated with small parts with thin sections.
The demand for larger, more complex die castings with improved quality and lower cost has led to the development of high precision equipment and the extension of casting technologies to larger pieces with heavier wall thicknesses. Still, unusually large parts cannot be diecast.
Diecast parts trend toward the less complex, partly because the metal cores must be designed to be pulled straight out of the casting. This limits the shapes of the cores and passageways of the casting.
Diecast parts also have strong dimensional accuracy and excellent surface finishes. Aluminum alloys can be diecast to tolerances of
+/-0.004 sq. in. and feature finishes as fine as 50 RMS. Walls can be cast as thin as 0.04 in.
In the diecasting process, also called high pressure diecasting, metal molds, or dies, are preheated and coated with a die release agent prior to each shot of metal. Premeasured amounts of molten metal then are metered into a shot sleeve and forced into the die under extreme pressure (usually from 10,000 to 15,000 psi).
Rapid filling of the mold and solidification under pressure can produce a dense, fine-grained and refined surface structure with excellent properties, including fatigue strength. But the typical injection speeds of the metal into the mold do not allow enough time for air to escape the die cavity. If turbulence occurs as the metal flows through the shape of the casting, porosity results. The use of a vacuum during die filling (vacuum diecasting), larger ingates with slower shot velocities (squeeze casting) or semi-solid metalcasting (in which metal somewhere between the liquid and solid phase is injected into the die) can overcome these problems and produce parts that can be heat treated and welded.
In designing for a die casting, thick sections may be less strong than thinner areas, because they can breed shrink porosity as the outer layer solidifies before the interior metal.
Dies have a relatively long wear life and can be used for up to 100,000 shots, depending on the application, so when large quantities are required, diecast parts cost less in the end, despite the high start-up costs. However, because the molds used in diecasting must be stronger than those used in permanent molding, they can be more costly, and the number of castings required to justify the use of diecasting is higher than permanent mold. For high volume jobs, the diecasting process, which is highly automated, often produces parts with the lowest per-unit price. Production runs above 10,000 pieces are connected with this method most often, but rapid tooling technology advances have made shorter runs—between 500 and 2,000 pieces—more economical while also significantly reducing lead times to one to four weeks (Fig. 1).
Because of the shot chamber method of introducing metal into the mold, metal loss in diecasting is usually low.
Casting size for permanent mold ranges from less than a pound to more than several hundred pounds. Surface finish varies between 150 to 400 RMS, basic linear tolerances are about +/-0.01 sq. in. and minimum wall thickness is 0.1 in.
In the permanent mold process, molten metal is poured into a mold made in two halves either directly by gravity, by using low pressure, or by tilt-pour, where the metal is poured into a cup attached to the mold that is then tilted from a horizontal to a vertical position.
Like diecasting, the metal mold aids in quicker solidification of the casting material, which results in highly desirable fine-grained structures that have high strength and soundness. While diecasting can produce castings with closer dimensional limits and thinner sections, permanent mold casting can produce parts with higher soundness (Fig. 2).
Porosity that often occurs in diecasting lowers the mechanical properties of the part and may cause blistering during thermal treatment. Permanent mold castings typically contain lower levels of entrapped gas, resulting in superior pressure tightness and soundness.
Permanent mold casting generally is used in high production volumes that will compensate for the high tooling costs, although these costs are generally not as high as with diecasting.
You will get efficient and thoughtful service from Yuhui.
The wear life of a permanent mold can range from 10,000 to 120,000 castings. A general number of castings needed to be produced annually for permanent mold to be economical is 3,000, although this varies by metalcasting facility and casting size. Permanent mold jobs with production runs as few as 100 a year are possible.
When designing for permanent mold castings, be aware that the process should not be expected to cast key ways, exterior screws or threaded designs or holes. Because all casting features must be machined into the metal mold, the permanent mold process cannot produce the complexity capable with sand molds. However, permanent molding can be paired with sand cores for semi-permanent molding, and this method allows metalcasters to achieve higher complexity in the design (Fig. 3). The use of metal cores is more economical, but when a casting has cavities that do not allow a core to be pulled straight out, an expendable sand core often will do the trick. Too many sand cores in a semi-permanent mold casting can result in the deterioration of its strength advantages, so highly complex castings may be better cast in a full sand mold process.
Each casting process has characteristics that are beneficial for different applications. Here are a few guidelines when considering diecasting and permanent mold:
Rules of thumb provide a base understanding of the strengths and weaknesses of diecasting and permanent mold, but if doubts remain on a suitable choice, contact a metalcaster from each process. Ultimately, metalcasters will have the best knowledge of a process’ capabilities. Plus, potential suppliers will have the best knowledge of their own process capabilities. Often, they also will be able to show you additional design measures to achieve your goals in cost-effective ways.
Design plays a crucial role in the die casting process, directly impacting the quality, cost-effectiveness, and efficiency of the final product. To achieve high-quality die cast parts that meet performance requirements, manufacturers must consider various design factors throughout the production cycle. In this article, we will provide advanced design tips for die casting that can help optimize your manufacturing process in various industries. From material selection to tooling design optimization and surface finish considerations, these tips will enable you to produce cost-effective components with superior quality and reliability.
The best result can come from the die casting process only when you follow certain design tips. Here are some of the major tips that one must follow.
When designing for die casting, it is essential to carefully consider material selection based on specific properties required for the application. Strength, durability, corrosion resistance, and thermal conductivity are key factors to evaluate when choosing materials for die cast parts.
Aluminum alloys are commonly used in die casting due to their lightweight nature and excellent mechanical properties. However, other materials such as zinc alloys or magnesium can also be suitable depending on the application requirements. It is important to balance material performance with cost and availability considerations.
The geometry of a part has a significant impact on its manufacturability and performance in die casting. Designers need to consider factors like shape complexity, thin walls, deep pockets, sharp corners, or undercuts when developing part geometry. Special tooling or cooling techniques may be required to address these challenges effectively.
For example, thin walls can lead to porosity or insufficient filling of molten metal. Conformal cooling channels can improve heat dissipation during solidification. Deep pockets may cause shrinkage defects or slower solidification. Incorporating core slides or complex tooling can help achieve uniform filling and efficient ejection.
The tools must withstand high temperatures, pressures, and stresses during the casting cycle while ensuring efficient filling, solidification, and ejection of parts. Factors that impact tooling design optimization are as follows:
The surface finish of die-cast parts is essential for both aesthetic appearance and functional performance. Depending on the application requirements, different surface treatments such as shot blasting, polishing, anodizing, plating or painting may be necessary. Surface treatments offer several benefits, like enhanced appearance, corrosion resistance, and functional performance.
Treatments like polishing or painting can improve the visual appeal of die-cast parts, making them more appealing to end-users. On the other hand, shot blasting can improve surface roughness. Anodizing or plating can provide an additional protective layer against corrosion.
By maximizing efficiency in part design, manufacturers can reduce production steps and costs while maintaining quality standards. The tips to maximize efficiency through design are as follows:
To ensure optimal design for die casting, manufacturers must engage in iterative processes based on feedback and testing results. This involves making design changes informed by simulation software predictions and actual manufacturing tests. Simulation software enables designers to predict potential defects before production begins. By analyzing factors like filling patterns, solidification rates, air entrapment risks or shrinkage effects during cooling, simulations help optimize die casting process parameters.
Design is a critical aspect of the die-casting process, impacting the quality, cost-effectiveness, and efficiency of the final product. By selecting materials carefully, optimizing part geometry and tooling design, selecting the right surface finishes, maximizing efficiency through design, and fine-tuning through iteration and testing, manufacturers across industries can produce quality components that meet performance requirements. The advanced design tips provided in this article ensure manufacturers not only meet their customer’s expectations but also maximize their manufacturing capabilities.
Optimizing cooling channels helps to achieve uniform solidification and minimize cycle time. Factors like channel placement, sizing, and shape optimization using simulation software or conformal cooling techniques help improve heat transfer efficiency and reduce part defects.
When choosing a surface treatment for your die-cast part, consider factors like desired appearance, corrosion resistance requirements, and functional performance needs (e.g., lubricity).
If you want to learn more, please visit our website Die Casting Mold.