Q. What are the prototyping options with a design intended for die cast production?
A. Beyond 3D computer product modeling, there is a wide range of prototyping options for eventual die casting. They include prototype parts produced by conventional machining from stock of the same alloy family; by CNC machining directly from CAD files; by one of several of the new rapid prototyping (RP) techniques now including Fused Deposition Modeling (FDM); and by creating an RP or conventionally modeled master for plaster molded or investment cast prototypes. Gravity cast prototypes are sometimes used. Since only production from an actual die casting die can yield a part with precise die cast characteristics, a single-cavity prototype die is the most thorough approach to critical product performance evaluation, although the alternative requiring the greatest prototype investment and lead time. Today's most widely accepted prototyping for metal parts, CNC machining, and newest RP prototyping process, FDM, are discussed below. The limitations and prerequisites for using various prototyping processes for functional evaluations prior to die casting die design are discussed in Technical Bulletin (No. 022), "Prototyping for Die Casting," (Resource Center Engrg. Bulletin section), and in the NADCA Product Design for Die Casting Manual, described in the Resource Center Reference Manual section.
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Q. What are the advantages of using CNC machined prototypes for a design intended for die casting?
A. Developments in 3D-CAD, CAM and CNC programming have made the machining alternative increasingly desirable. Parts can be CNC machined from wrought or sheet stock, working directly from customer CAD files, depending on the type and accuracy of the files. After transfer to a CAM program interfacing with specialized CNC workstations, total machined prototype lead times can often approximate rapid prototyping production scheduling. Validation of form and fit is assured and virtually all functional tests can be performed, with the prototype capable of withstanding the most extreme handling. Many multiple prototypes can be produced in the same time frame at reasonable additional cost. CNC machining can produce parts in metal alloys of the type to be die cast, to near identical part weights and to the specified die casting tolerances with precise details. For Al 380 die castings, A1 6061-T6 aluminum plate is generally used for CNC prototypes. For Mg AZ91D die castings, AZ31 Mg plate is recommended. Zamak 3 stock is available to prototype Zamak No. 3 zinc die castings. Planned post-castng coatings and finishes can be applied to machined "hogouts" to closely approximate the appearance of the proposed die casting.
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Q. What are the advantages of using Fused Deposition Modeling prototypes for a design intended for die casting?
A. The rapid prototyping process called Fused Deposition Modeling (FDM) enables the production of far stronger RP prototypes in durable ABS plastic, directly from STL design files. FDM parts are built and bonded, extruded layer by layer, from 3-D computer data. An FDM prototype can be geometrically complex and produced to tolerances of ±0.005 in. (±.127mm). Because of the strength of the ABS plastic part, it can be evaluated rigorously for form and fit and used in many functional tests. Most RP methods often have difficulty reproducing very tight toleranced sections, such as in sections containing ribs, bosses and holes; in these cases precision CNC machining can be performed on the strong FDM ABS part to the required critical specifications. FDM prototypes are being generated by CWM on every new die casting project to expedite production and shorten total lead times: FDM models help assure that die designs result in first-piece success and aid in the simultaneous construction of die cast tooling, trim dies, machining fixtures, finishing masks, and any required subassembly gauges or fixtures. FDM parts can also be used as masters for investment casting in an alloy approximating the properties of the planned die casting.
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Q. What exactly is a prototype die and when should it be used?
A. Prototype dies are usually requested by a customer to produce a small number of castings under precise die cast production conditions where thorough functional product testing is critical before committing to full production dies. Lesser grades of die steel can be used in prototype dies. Where full functional testing is not required, a range of lower cost prototyping options, with lower lead times, are available.
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Q. In planning for the design and construction of a new die casting die, what are the general considerations that should be addressed?
A. When planning for die design and construction, the die casting engineer will address items such as the quality of the die cavity steel to be used and its heat treatment; any cored holes and moving die components required to produce special features in the proposed part; the specific casting alloy to be used; short- and long-term projected part volume; estimated cast part weight; the desired as-cast finish for the part; and any cast-in identification symbols or graphics required. The economics of lower-cost unit dies vs. single and multiple-cavity dies should be discussed; required post-casting machining to tighter specifications than can be cast in place; and the details of the first-piece approval process required. A special "New Die" Tooling Checklist is available from your CWM Sales/Engineering Representative or from the CWM Sales Dept. It is also provided in the NADCA Product Standards Manual (See description & Discount Order Form), and is also available for instant download in the Resource Center Engr. Bulletin section.
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Q. What is the expected die casting die life for a
new die casting die?
A. The life of a die casting die in terms of parts which can be die cast under production conditions will vary with the alloy being used, the specific product features required to be cast and the specified tolerances to be maintained on those features. The required as-cast component surface finish, and the final specifications for the cast surface after post-casting finishing operations have an important impact on die life. The relative expected die life for parts made in the commonly used die casting alloys, however, can be estimated as follows: If conventional high-technology production of average and larger sized components results in die life of a 1x volume of parts of a given product design in Al 380 and Al 360 alloy, Mg AZ91D alloy can be expected to yield a volume of 3X to 5X parts. The same part produced in Zn No. 3 alloy can be expected to provide a virtually unlimited die life, i.e., a life normally equal to the lifetime project volume for the part. This long die life for Zinc is matched as well in the Miniature Zinc and ZA-8 4-slide die casting process.
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Q. Why does a set of die casting dies require the die investment it does?
A. A die casting die or mold is a closed vessel into which molten metal is injected at a high rate of speed, under high pressure and temperature, then rapidly cooled until the solidified part is sufficiently rigid to permit ejection from the mold. To survive these operational extremes and stresses, the die casting die must be built from highest-quality tool steel, heat-treated to the required hardness and structure, with dimensions of the die and cavity machined to exacting specifications for tolerances and part surface finish, to net-shape or near-net-shape production. It is this die quality and precision die construction that makes today's advanced die casting possible.
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Q. Does the product designer need to be concerned with the location of the ejector pins built into the die casting die?
A. Ejector pins are used to push the casting out of the die after the metal shot has been made and the casting solidified. The die caster will always attempt to locate ejector pins in a nonfunctional area of the casting, such as in an overflow, on a boss, in the bottom of a deep pocket. The size, location and number of ejector pins required is important for successful part production. Each ejector pin will leave a slight impression on the cast surface, so they are not placed against the cosmetic surface side of the part. It is vital that the product design engineer point out all such cosmetic part surfaces, and understand where all ejector pin impressions will occur. Guidelines on ejector pin locations are discussed in the NADCA Product Standards Manual (See description & Discount Order Form).
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Q. Is a trim die always necessary in preparing for die cast production?
A. Since the trim die is the tool that trims the runner, overflows, and flash from the die cast part, it is an essential requirement with all conventional die casting production. Depending on the shape of the casting, the trim die may be a simple open-and-close trim die or it may include as many slides as the die casting die itself. Trim dies can require as much attention to detail in design as the die casting dies.
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Q. How important is the exact location of the parting line on the casting, where the die casting die halves meet?
A. Location of the die casting's parting line, on which the die caster must have the last word for successful production, affects part aesthetics, integrity, mechanical properties, dimensions and the simplicity with which the casting can be trimmed. For cosmetic surfaces, the small ridges of flash remaining where the two die halves join must be removed or the parting line relocated. To maintain mechanical properties and dimensions in critical areas, the parting line may need to be relocated. An inappropriate parting line location can also result in more complicated and costly trimming operations.
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Q. What is a unit die and what are its advantages and disadvantages?
A. A unit die is a lower cost production tool that has a standardized main die frame and replaceable die cavity units. These replaceable units are designed to be readily removed from the main die frame without the labor intensive removal of the standard frame from the die casting machine. However, unit dies limit the use of core slides, with their advantage of enabling production of complex as-cast features in a part, because of the configuration needed for interchangeable unit die inserts and the resulting limited space available.
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