Zinc die castings can be prototyped using several techniques, depending on specific properties of interest. One prototyping method is to machine the shape from a solid piece of zinc alloy. This method is especially useful when a limited number of parts with relatively simple features are required. It should be noted that mechanical properties of machined zinc components will be inferior to those of a like shape die cast part.
Zinc-alloy die castings can be prototyped as gravity castings either in the designated alloy or the ZA alloys, depending on the specific property of interest. For appearance or conceptual studies, plaster-molded castings in No. 3 alloy can be used, but their mechanical properties will be inferior to those of die castings. If tensile and yield strengths are the major factors in prototyping zinc-alloy die castings, ZA-12 gravity castings can be used. However, their ductility and impact strength will be lower than die cast Zamak alloys, while wear resistance will be greater.
When only a few prototypes are needed, plaster casting is also a good choice. For a large number of prototypes, either sand or graphite permanent-mold casting can be considered.
Types of Dies
Conventional miniature die casting machines use single- or multiple-cavity, two-part dies. Miniature Casting Corporation’s miniature, four-slide dies produce parts up to 1.5 in. in any direction, or under 1 oz. in weight.
The term “parting line” refers to the seam on the casting created by the parting plane of the two die halves. In many instances, a natural parting line will be established either by the shape of the part or by a previous fabrication method. Given a drawing for a new part, MCC will first select a parting line that will yield the flattest die surface. This selection depends on the following factors:
- The best gating location to achieve complete filling of the die cavity consistent with strength and surface-finish requirements.
- The simplest die with a minimum number of features formed by separate, movable members.
- Easy removal of the casting from the die on completion of the casting cycle.
- Maximum utilization of the casting machine’s locking force. (Generally, the plane of the casting’s largest projected area should coincide with or be parallel to the parting plane). This is most crucial when large slides are required.
- Most advantageous use of the principal die movement in coring.
- The desirability of positioning of close-tolerance elements in the same halves of the die.
- The surface-finish requirements may indicate that a particular parting line is objectionable.
Die casting is the most accurate of the casting processes. The reason for this is that the zinc alloy is molded in steel dies machined to very tight dimensional tolerances.
Zinc alloy can be cast to the tightest tolerances due to its low melting temperature and its ability to quickly fill a die before it freezes.
The tightest die casting tolerances are held for features formed in the same die part. In precision casting, tolerance for linear dimensions is ±0.002 in. for the first inch. In certain circumstances MCC can hold tolerances to ±0.001 in. Individual applications should be reviewed with the Sales Engineer.
Parting line tolerance for dimensions perpendicular to the parting plane is in addition to linear dimension tolerances. Typically, this tolerance is +0.003/-0.000 inch.
Moving-die component tolerance is added to linear dimension tolerance for total dimensional tolerance.
Size of the Die Casting
A casting’s maximum size depends on the size of the die casting machine available. In the case of Miniature Casting Corporation, maximum size is approximately 1.5 in. square. There is no limit on minimum size; in fact, zinc die castings weighing 1 gram (1/28 ounce) or less are in regular production. Small components may be produced in multiple-cavity or combination dies to contain costs.
MCC recommends parts be produced on a single-cavity die when there are exacting tolerances on part-to-part variation.
Zinc die castings should be designed for the thinnest practical wall thickness consistent with strength and stiffness requirements. Thin-wall sections provide the following function/economic benefits:
- Cost savings: Metal saved through thinner sections cuts the cost of the casting. The die casting machine cycle time is reduced and production rate increased, providing further cost savings.
- Weight savings: Since thinner sections can be cast in zinc, it is often practical to produce a lighter product in zinc than in metals that have a lower density. Wall sections as thin as 0.020 in. to 0.025 in. (0.50mm to 0.64mm) are practical for many small zinc die castings. Wall sections as thin as 0.015 in. (0.37mm) may be cast in zinc in certain situations. Section thickness can be retained where necessary for strength or impact resistance, or supported and strengthened by ribs.
- In general, thin sections of a zinc die casting are tougher than thick sections, with higher impact and fatigue strengths. These improved properties result from the superior grain structure of thin sections. The uniform fine grain structure of the surface layer, normally present as the “skin” of a zinc die casting, occupies the full thickness of thin sections up to approximately 0.040 in. (1mm). The thin section consists entirely of a fine dendritic structure of alpha phase and a uniform eutectic network. The thicker 0.080 in. (2mm) section has only a thin skin of uniform structure below which is a coarser dendritic formation of zinc-rich solid solution and less uniformly dispersed eutectic.
Variation in Section Thickness
Sectional thickness should be as uniform as possible. Where variations in section thickness are necessary, the transition should be gradual. Thin sections cool more rapidly than thick ones, and in extreme cases unequal contraction of non-uniform sections may cause distortion or cracking of the casting. Careful design will minimize the effects of distortion.
Draft is preferred on all surfaces of a casting perpendicular to the parting line of a die and on all surfaces parallel to the line of pull of movable members. The amount of the draft angle varies with the depth of draw and depends on whether the surface is an outside or an inside surface.
When molten metal solidifies, it shrinks away from the walls of the cavity or onto steel protruding into the cavity. Therefore, minimum precision draft for inside walls is generally recommended at 0.75° per side, with outside walls requiring half as much draft. The extent to which internal surfaces require draft is more involved. It depends on the method used to strip the casting off the internal surface-either ejector pins (internal surfaces rigidly fixed in the ejector die), or stationary surfaces (in the case of a movable member). In the cover die, it is difficult to provide a stripper mechanism and a minimum of one-degree taper is desirable on internal surfaces.
The miniature four-slide process is more flexible with draft requirements than traditional two-part dies, in many cases not requiring draft angles at all. For best economy, the designer should provide for the maximum taper that the part’s performance will permit. Consult MCC for specific casting draft angle requirements.
Ejector Pin Marks
Once the casting cycle ends, the part must be removed from the die. Typically, this is accomplished by pins fastened to a pair of ejector plates that mechanically or hydraulically passed through the ejector die. While ejector pins may take any shape that can be machined, circular pins are most common.
Ejector pins are most effective and create the least distortion if they can be placed under a vertical wall (vertical in the sense that a major dimensional length or width is parallel to the axis of the pin). Since the wall thickness in a die casting is generally thinner than the diameter of the ejector pin, a boss on the casting may be required to clear the pin in its travel.
Ejector pins will leave small marks on the surface of castings. The die should be designed where possible with ejector pins positioned so that these marks do not appear on significant surfaces of finished castings. Measurements over ejector pin locations may vary. Therefore, it is best to place pin locations in non-critical areas.
Threads, Gears and Gear Teeth
External threads may be cast in zinc. The most common practice is to machine the female thread in the separate die halves; in which case a seam appears across the thread parallel to the thread axis. This is not objectionable with most thread classes.
Variations in metal shrinkage can create an error in the pitch, but if the thread is not excessively long, the total error will be slight. Threads to 32-per-inch are commonly cast in zinc.
Internal gears are as easily die cast as external gears, whereas internal gears made by other methods are much more expensive, especially if the gear is small and the teeth must run the entire depth of the hole.
In addition to strength, there is another factor to consider when using die cast gears. When one gear is cast, the mating gear should also be cast. Die cast gears generally cannot be expected to run smoothly with a gear that has been machined.
Design Points to Remember
- Final selection of the parting line should be the responsibility of the caster. A number of factors will influence this decision including: ejection, configuration coring, finish specifications, gating, type of die and flash removal.
- Cores and slides often save much more than they cost to incorporate.
In addition, cores often allow high volume production of complex parts that are ready for assembly.
- Wall stock should be as thin as possible, consistent with strength and finish requirements.
- The total quantity of castings as well as volume per production run may have an important influence on part cost. This is especially true if machining and finishing operations are keyed into the production cycle. The longer the production run, the lower these costs will be for each individual casting.
- Small bosses or studs can be formed integrally, simplifying assembly of the finished product. The automotive industry often does this with letters, escutcheons and decorative trim. Assembly can then be completed by simply inserting the projections into pre-punched or pre-drilled holes, securing the part with steel spring clips or other inexpensive means.
- Irregular or eccentric gears or cams, difficult to machine from solid blanks, can be die cast as easily as parts with uniform profiles.
- Interchangeable die sections can produce castings with the same external shape and size but with different length, cavity or hole requirements. This greatly reduces the tooling for similar parts, especially when one of the parts has low production requirements.
- Elimination of machining is one of the greatest benefits of the die casting process and every effort should be made to take advantage of the precision of miniature die casting.
- External threads can be cast either by positioning the part so that half of the thread circumference is formed in the cover die, the other half in the ejector die; or by using side slides to accomplish the same purpose. Loose die pieces or threaded inserts can also be employed.
- The surface-finish specifications should only be as good as is necessary for appearance and functional purposes. Finer surface finish requirements decrease die production and increase maintenance costs.
- High injection pressures and the use of process control equipment and techniques produce die castings with little internal porosity.
- Closest tolerances can be held between elements located in the same die component. Closer tolerances than those listed in the Die Casting Standards, published by the North American Die Casting Association, often can be held, but such customer requirements should be discussed with MCC before the job is begun. Tolerances and limits are subject to variation, depending upon such features as the size and shape of the casting, the die construction and the casting pressures employed.
- Bi-metal assemblies can be produced by using a precast component of one alloy as an insert or interlocking part of another casting.
- Fillets and radii should be as generous as possible to aid metal flow and avoid stress concentrations.
- Large, flat sections are more difficult to cast without surface imperfections than contoured or ribbed members. If the exterior must be flat, some degree of surface imperfection must be tolerated and more liberal allowances may be necessary, particularly regarding flatness.
- If the only difference between the right and left parts of an assembly is the position of a flange or other integral member, a single die may possibly be used for both pieces. Parts could be formed into the desired shape after casting by a simple bending or machining operation to create the necessary difference.
- Design’s effect on all secondary operations (including machining and finishing) should be considered before finalizing the design, so that the die casting process will provide the maximum benefit.
MCC, as a full-service casting facility, provides designers and engineers assistance in obtaining the proper coating finish in order to achieve the desired functional or cosmetic results. When selecting a top-coat system, the designer must fully understand the properties of the casting and coating as well as the service conditions the product will be expected to withstand. Many engineering applications do not require surface coating of zinc castings. Their excellent casting and material properties often enable them to be utilized as-cast. Coatings and finishes are specified to enhance the properties of castings by providing a decorative finish, increasing corrosion resistance, and improving engineering properties.
The coating systems available for zinc castings offer a wide spectrum and are most often classified into three categories: chemical, metallic and organic finishes.
These finishes are usually based on proprietary chemical solutions applied by an immersion or spray process. The chemical solution reacts with the surface of the casting to form a complex conversion coating system. No current is involved in the formation of these coatings, which are usually based on zinc chromate or zinc phosphate formation. Two of the more popular solutions for chemical blackening are based on chlorate or molybdate salts.
Chromate Coatings. The primary purpose of these chemical conversion coatings is low-cost corrosion protection. In addition, with the colors now available, these finishes can be aesthetically pleasing. This coating process involves a controlled oxidation reduction reaction.
Phosphate Coatings. Phosphate conversion coatings are used primarily as a precoat for organic finishes. Metal surfaces do not provide a good base for paint films, since these surfaces remain conductive. The underlying casting will corrode when the organic surface is broken or when the atmosphere diffuses through organic coatings.
The majority of metallic coatings on zinc castings are electrodeposited, while a small percentage are electroless deposited or vapor deposited. Metallic coatings are favored for their appearance, corrosion resistance, wear resistance and electrical properties.
Electroplating. In the electroplating process, the metal to be plated onto the casting is introduced into solution by dissolution of a metallic salt or by metal dissolution as an anode. The zinc parts to be plated become the cathode. The most common methods of electroplating are rack electroplating for large parts and barrel electroplating for smaller parts.
Cu/Ni/Cr System. This system is used extensively in both indoor and outdoor applications, combining a decorative appearance with corrosion, wear and tarnish resistance. Bright-acid copper over the copper strike is generally used to provide excellent leveling and brightness prior to the nickel plate. This can eliminate the need for polishing or buffing before subsequent plating.
Founded in 1962, Miniature Casting Corporation (MCC) offers customers a progressive and highly-motivated workforce dedicated to the manufacture of miniature four-slide zinc die castings. Experience and technical expertise enable MCC to produce tight-tolerance, complex components in a cost-effective and timely fashion. An integral component of MCC’s customer responsiveness is its complete in-house four-slide die cast machine design and build capability.
MCC dovetails comprehensive quality programs with cost-effective and time-sensitive operational efforts, keeping pace with the requirements of today’s higher technology marketplace. Quality assurance is enhanced by workforce cross-training and a focus on process documentation and review.
MCC’s operational philosophy is to work with the customer’s individual production and schedule requirements, minimizing lead times without compromising product cost. Customer service programs will continue to be developed, enhancing response times for quotes and tooling or product information follow-up.
Consider miniature zinc die casting as the alternative fabrication process for design flexibility and cost reduction. Then consider Miniature Casting Corporation, the company with the desire, resources and personnel to make your project a success.