Zinc pressure die Casting Processes
Pressure die casting is a process in which molten metal is injected at controlled high velocity and pressure into the cavity of a mould (die) which is usually made of high quality tool steel. Less expensive die materials may be used when production quantities are relatively low.
Fill times may be as brief as a few milliseconds for the smallest components and as long as half a second for the largest. The extremely short time required to fill the die minimizes the tendency for the metal to solidify prematurely and enables it to flow through and fill very thin sections. The foundry maintains the temperature of the die approximately 150°C (300°F) below the solidification temperature of the metal, which causes it to solidify much more rapidly than with other casting processes. The rapid solidification develops properties in the casting that cannot be developed by other processes.
High injection pressure, typically 14 to 69 MPa (2 to 10 ksi), is maintained during filling and solidification, promoting complete cavity fill, faithful reproduction of intricate details, excellent surface quality, and excellent dimensional precision. After ejection from the die, few machining operations are necessary; in many cases high precision components are produced to net shape by zinc die casting. The short cycle time associated with zinc die casting and the capacity to make several castings in each shot make the process advantageous for high production volumes.
The pressure die casting process for many years was primarily an art rather than a science. Techniques for conveying molten metal to the die cavity, controlling solidification, and ejecting the casting were developed on an empirical basis, with the industry practicing the techniques that appeared to work best. In the late 1960ʹs, basic research was initiated by ILZRO in concert with the zinc die casting industry to better understand the fundamentals of thermodynamics, heat transfer and fluid flow that govern the die casting process. The outcomes of this work have been quantified and transferred to the die casters in a variety of design packages, elevating die casting from an art to an applied science.
Many die casters are applying this science to produce castings with wall thick‐ nesses, surface finishes and dimensional tolerances that were unthinkable a few years ago. The casting shown in Figure 1 is an example. Cored holes, hubs and other features are being produced with zero draft and very close tolerances, eliminating costly finish machining operations. The specific capability is often proprietary, and it varies with the individual die caster.
One of the most important product capabilities is reduced wall thickness. A die casting with a 0.5 mm (0.020 in.) wall thickness is shown in Figure 2. Previously, wall thicknesses for structural features were often specified according to the minimum that could be cast, rather than the mechanical and structural require‐ ments. Decorative components required sufficient wall thicknesses to bury porosity in order to ensure a quality surface.
Pressure die castings can now be designed with more emphasis on structural criteria and less restraint from process limitations. Many pressure die casters use shot control systems that measure, control and monitor the flow of metal into the die. Vacuum systems, when used with shot control systems, virtually eliminate porosity and promote filling of intricate die features. Monitoring of the pressure die General Fill time and solidification Injection pressure and cycle time Practice and science Product qualities
Figure 1. The holes in this zinc housing are die‐cast to tolerances of 0.025 mm (0.001) and less
Designers can realize the benefits of high‐technology die casting by consulting with zinc die caster who employs advanced technology from the early stages of design. Interaction can substantially reduce product cost by eliminating many finish machining operations and reducing metal content.
The cycle of research, technology transfer and application is an ongoing process that is keeping the zinc die casting industry competitive with alternative processes. Product designers, who work closely and early with the die caster, will maximize material utilization and process benefits.
Figure 2. Some of the walls in this zinc electronic component are die‐cast as thin as 0.50 mm (0.019 in)
Pressure Die‐Casting Processes
There are two basic pressure die‐casting processes: hot‐chamber and cold‐chamber. The hot‐chamber process is used for zinc Alloys 3, 5, 7 and ZA‐8. The cold‐chamber process is used for ZA‐12 and 27 alloys. The Hot‐Chamber Shot Cycle The basic components of a hot‐chamber die‐casting machine and die are illustrated in Figures 3 and 4. The process is called hot‐chamber because the plunger and cylinder are submerged in the molten metal in the holding furnace. The energy to inject the metal into the die cavity is supplied by a hydraulic pump and stored in an accumulator, from which it is released as needed to produce plunger movement. The operating sequence for the hot chamber cycle is illustrated in Figure 5. The die is closed to begin the cycle. Next the plunger is driven downward, forcing molten metal out of the cylinder, through the gooseneck, nozzle, runners and gates into the die cavity, filling the die and maintaining pressure on the metal as it solidifies. Two basic procedures Basic machine components Operating sequenceAfter solidification, the die is opened while the plunger returns to the starting position, refilling the shot cylinder. Then the casting is ejected and the cycle repeated. A die lubricant, or mould‐release compound, is sprayed onto the die periodically to facilitate release of the casting. Figure 3. Cross sections of a hot chamber die casting machine. Figure 4. Cross section of a die used in a hot‐chamber die‐casting machine.
The last area of the die cavity to fill generally has the coolest metal which may cause casting defects. Current casting technology applied to the metal feed system and good thermal design of the dies can prevent this type of defect. In some cases it is necessary to locate overflow cavities, or overflows, at the die parting surface in these areas. The overflows receive the cooler metal and allow the higher tempera‐ ture metal following it to fill the die cavity. Overflows are used only when necessary and are carefully sized because they constitute additional extraneous metal for recycling.
Figure 5. Operating sequence for the hot‐chamber die‐casting cycle.
The Cold‐Chamber Shot Cycle
The cold‐chamber process is employed for ZA‐12 and 27 because the casting temperatures of those alloys are in the range at which zinc‐aluminium alloys would attack the steel in the gooseneck and piston. It is also the process used for casting aluminium. The sequence for the cold‐chamber cycle, illustrated in Figure 6, is essentially the same as for hot‐chamber. Instead of submerging the cylinder, the molten metal is transferred, usually by hand or automatic ladle, into a horizontal shot cylinder. The system does not employ a sprue; instead, the piston stops before reaching the end of the cylinder at a distance of approximately one third the diameter of the cylinder, forming the characteristic ʺbiscuitʺ. The shot cycle is otherwise identical to the hot‐chamber process. Figure 6. Operating sequence for the cold‐chamber die‐casting cycle. Operating sequence
Cycle Characteristics The submerged shot cylinder gives the hot‐chamber process six advantages: − The shot cylinder fills automatically while the die is opening, reducing the cycle time. − Fewer steps in the cycle make it easier to automate the process. − Less cooling than when molten metal is transferred to the shot cylinder of the cold‐chamber machine reduces a metal temperature variable. − Injection pressures are usually lower, placing less load on the machine clamping system. − Better fluidity enables thinner walls to be cast and promotes sounder cast‐ ings. − The molten metal is less subject to oxidation from atmospheric exposure. There are four significant characteristics common to both cycles. − The casting, as normally ejected from the die, is attached to metal that so‐ lidified in the sprue, runners, and overflows. The casting(s) with the extra‐ neous metal, sometimes called the shot, is transferred to a trim die, where the casting is separated. The metal that is removed can be immediately re‐ cycled. − Metal injection normally terminates with a sharp increase to full pressure. This promotes complete filling of the die cavity, feeds voids caused by shrinkage during solidification, and promotes uniform metal density. Ex‐ cessive pressure can increases the load on the damping system of the die‐ casting machine to the point where metal is forced between the die mem‐ bers causing flashing. This effect, which is sometimes referred to as ʺdie blowʺ or ʺflashingʺ is well controlled by die casters using the latest technol‐ ogy. Cold chamber machines can be operated with a pressure intensifica‐ tion system which multiplies the applied pressure and so increases the effectiveness of feeding shrinkage. Because the outside skin has already so‐ lidified when intensification is applied, the improvement is achieved with‐ out forcing metal between the die members. − Maximum metal pressure multiplied by the projected area of the entire shot imposes a force on the machine structure that dictates the clamping force required. Die‐casting machines are rated by the number of tons of locking force they can develop without distortion during long‐term operation. Lar‐ ger machines require a greater investment and run slower than smaller ma‐ chines. The size of the machine thus affects the cost of the product. − The maximum metal pressure can cause the die halves to separate slightly, adding a variation in casting dimensions across the parting surface, giving the aforementioned ʺdie blowʺ. This effect can be controlled to reduce di‐ mensional tolerances by many die‐casters who employ high‐technology die‐casting practices. Six advantages Four common characteristics
Figure 7. ʺShotʺ of metal with sprue, runners and overflows prior to trimming.
Die Casting Dies
The typical hot‐chamber die, illustrated in Figure 8, is constructed in two sections: the cover or fixed half and the ejector or moving half, which meet at the parting surface. The shape of this surface and the direction that the die members move relative to the casting are affected by the design of the casting. The cover half is mounted on the front or stationary platen of the machine. The sprue, which directs the molten metal toward the die cavity, is in this half, and it is aligned with the nozzle of the casting machine. The ejector half contains the ejector mechanism and, in most cases, the fixed cores and the runners, which convey the metal from the sprue to the die cavity.
The cold‐chamber die is arranged in essentially the same manner except that the biscuit replaces the sprue. Figure 8. Typical hot‐chamber die casting‐die Die sections: moving and fixed
The die cavity, which forms the casting, is machined into both halves of the die block or into inserts that are installed in the die blocks. Guide bars or leaders pins extend from one die half and enter holes in the other half as the die closes to ensure alignment. The die and the casting are usually designed so that the casting remains in the ejector half when the die opens, allowing the casting to move free from the cover half. The casting is then pushed off the ejector half by ejector pins that come through holes in the die and are actuated by the ejector plate, which in turn is powered by the machine hydraulic system. The location of ejector pins is often critical to both the process and the design, and should therefore be clarified in the early stages of design.
Dies may contain fixed and/or movable cores in either half to produce complex shapes. Fixed cores are anchored in the die. Consequently the casting must be designed to permit their movement parallel to the direction of the die opening. Movable cores (see Figures 5 and 6 above), which are locked in place, when the die closes, are actuated by cam pins or hydraulic cylinders. They may be incorporated into either half, but the best location from the die casterʹs viewpoint is in the die parting. Movable cores add to die fabrication and maintenance costs, and may increase cycle time. However, they can be employed to advantage when they allow features to be cast that would otherwise require finish machining operations. Relative motion between die members requires clearance, which tends to increase as the members wear. As the clearance increases, molten metal may be forced between the members, leaving thin fins on the casting, called flash, which must often be removed. The designer should visualize the die construction required to produce the casting to predict the occurrence of flash and provide for its removal where necessary.
The simplest die configuration contains one cavity (Figure 9a). Dies may contain more than one cavity so that several castings are made in each cycle, which is advantageous when production volumes are high. A die with several identical cavities is called a multiple‐cavity die (Figure 9b). A die with several cavities of different shapes is called a combination or family die (Figure 9c). Combination dies are restricted to castings that are compatible both in size and in required production quantities. They are frequently used to make several or all of the components in an assembly.
Figure 9. Schematic illustrations of four die configurations. A Single cavity die ; B Multiple cavity die ; C Combination die ; D Unit die Die cavity Fixed and movable cores Clearance between die members Cavity layout
Unit dies, which are particularly appropriate for lower production volumes, consist of a die holder into which a number of standard size blocks are fitted, each with one or more cavities (Figure 9d). The cavities may be identical or different, provided that they are compatible in size. Unit dies facilitate quick changeover and provide a high degree of production flexibility. However, they contribute an additional tolerance in the die alignment (ejector side to cover side stack up), and they tend to increase the lengths of runners.
Die Fill
Much of the recent research and development work designed to improve die casting technology has focused on proper filling of the die with molten metal. However, the flow of molten metal is subject to some turbulence, which tends to entrain air, forming porosity in the casting. When porosity occurs it tends to accumulate away from surfaces in the core, particularly in thick sections.
Die castings solidify from the surface inward. (See above ʺCharacteristics of Zinc Die Castingsʺ section). The surfaces exhibit the best mechanical properties and are normally smooth with no visible defects. Metal removal operations that cut deep enough to penetrate the surface skin expose the metal beneath which could contain porosity. It is essential that machining not be performed, or at least held to very light cuts on castings that must be pressure tight or require a smooth surface. Otherwise impregnation may be required.
The metal delivery and distribution system (referred to as the metal feed system), which is crucial to proper cavity fill and minimal porosity, should be designed with smooth transitions and gradual changes in the direction of metal flow to minimize air entrapment and porosity. It should also be positioned relative to the cavity to give good filling pattern. The technology for sizing the system and proportioning its various features has been developed through ILZRO research and transferred to the die casters.
Some die casters are also utilizing advanced vacuum systems which, when used with good die design and process control, virtually eliminate porosity from zinc die castings. These systems involve modifications in the die gate and runner system design and the use of automatic forced evacuation of air entrapped in the die cavity. The vacuum systems produce sound sections, even in complex castings. Porosity Casting surface layer Die filling system characteristics
Economics of Die‐Casting
The high production rates and high precision achieved by zinc die‐casting give the process distinct economic advantages. The total manufactured cost per component is determined by the costs of metal, production equipment and labour, the die‐ casting die, and finishing operations (the cost optimization module in the ʺDesign Guideʺ enables an estimate of the casting cost to be made).
Metal Cost
The cost of die‐casting alloys is subject to fluctuation, similar to any commodity. Where desirable or required, the purchaser of die‐castings can enter into a contract that fixes alloy prices for a specified time.
The real metal cost is determined by the listed price of the alloy, the cost to melt, and the cost of reprocessing both the metal trimmed from the casting shot and casting rejects. Die casters are systematically reducing metal processing costs by reducing rejects and minimizing the amount of metal trimmed from the casting. Equipment Cost A die‐casting machine represents a substantial capital investment, especially when it is equipped with high technology process controls. The cost ascribed to the die‐ casting machine for producing a casting is a significant part of the cost of produc‐ tion, and it is influenced by the production rate. The production rate is determined by interdependent factors such as machine size, cycle time, casting weight, the number of castings produced per cycle, and the reject rate.
Although these factors are primarily the concern of the die‐caster, they may be affected by the design of the product. For instance, a casting may have a relatively massive, thick‐walled feature which requires the longest time to solidify and thus governs solidification time. Where possible, such features should be redesigned to reduce the mass of metal or increase the area available for heat transfer to reduce solidification time and consequently cycle time (refer to the ʺDesign Rulesʺ module in the ʺDesign Guideʺ).
The cost of operating a die‐casting machine, per component cast, is frequently a trade‐off of machine size, die complexity (e.g. core slides), number of pieces cast per cycle, and scrap rate. These factors vary among zinc die‐casters, so that there may be some variations in the prices quoted by equally competent die‐casters. Die Cost Zinc alloys are cast at lower temperatures than other die‐casting alloys and exhibit less tendency to attack the die steel. Therefore zinc alloys offer greater die life and lower maintenance costs than other die‐casting alloys. Alloy + melting + trimmed metal and rejected parts Machine + cycle time Design casting to minimize cycle time Operating costs Relatively low casting temperature
Zinc die‐casting dies are made from specially developed alloy steels. They incorporate one or more die cavities, which are as intricate in configuration as the castings they produce. Dies may be further complicated by components such as core slides. The high initial cost of die casting dies is justified when production rates are high enough to keep the amortized cost per casting at a reasonable level. The cost may also be justified in lower volume production, when casting a complex component to very close tolerances eliminates costly secondary operations.
Normal service subjects zinc die‐casting dies to severe operating conditions. Although the dies are made from high quality, engineered tool steel, the injection of molten metal and the subsequent rapid cooling induces thermal shock and cyclic thermal stresses which cause deterioration and ultimately failure of the die steel. The rate of deterioration is a function of the quality of the die steel and quality of heat treatment of the die, the total amount of heat transferred from the molten metal to the die and the associated temperature increase.
The high‐speed flow of molten metal through the die can cause die steel erosion, called die wash. The amount of erosion on one shot is infinitesimal, but the accumulation over many shots must be recognized. The most severe erosion occurs at locations of high metal flow velocity, sudden change in the direction of metal flow, and at irregular die features and die sections, where these locations cannot be provided with sufficient cooling. Often, these factors can be mitigated in the design of the product and the die through early die‐caster consultation.
The most obvious visual effects of die degeneration on the casting are loss of sharp definition of the more intricate details, deterioration of surface quality, and thickening of some sections. As the die degenerates, it must be removed from production periodically and repaired; ultimately, the die cavities, and eventually the entire die, may need to be replaced.
Costs of Finishing Operations
Five types of finishing operations are commonly performed after die‐casting and prior to machining operations or surface treatment: trimming, vibratory finishing, polishing, sanding and grinding.
Trimming: Die‐castings are ejected from the dies attached to their sprue or biscuit, runners, overflows, and flash. They are normally separated in trim dies that shear off the unwanted features, which may be remelted and recast. In some cases, particularly with small and mini‐ size castings, degating dies separate the casting during ejection, eliminating the need for a separate trimming operation. Trimming is a metal shearing operation; therefore the casting must have sufficient strength and rigidity to withstand deformation in the trimming operation. The extraneous metal in the casting shot is utilized to support the casting in the trim die as much as possible, but the casting may be required to provide additional features for support. It is therefore advantageous to consult with the die‐caster early in the design process to ensure that the casting can be trimmed without complication.
It is sometimes necessary to trim the entire parting line. Therefore, trim die cost is minimized when the parting surface is kept in a single plane. Further economies can be realized by eliminating or avoiding irregular features on the parting line. However, the cost of complex, multistage trim dies can be justified when costly secondary operations are eliminated, lowering final piece‐part costs. Thermal shock Die steel erosion Five types of finishing operations
Vibratory Finishing: Vibratory finishing is often employed when flash is not accessible to trim dies, such as the flash that occurs at the interface of moving die members (for example, core slides). The castings are placed in a container with abrasive media and subjected to high‐rate oscillations. The abrasive media preferentially attacks protruding features such as flash.
Polishing: Parting lines usually appear on die‐castings as small ridges, wherever two die members meet. In the case of decorative components, these lines are usually removed to enhance appearance. Flash height and thickness depend upon the condition of the die and the control of process variables. In certain applications, advanced casting processing can greatly minimize or eliminate flash.
The visible lines that remain on the casting where flash has been removed must be eliminated in most decorative components. It is also necessary in many applications to polish the surface of the casting by buffing prior to decorative finishing, particularly bright chrome plating. Buffing is performed with large, soft wheels that move across the surface of the casting at high speed in the presence of a very fine abrasive (buffing compound).
Sanding and Grinding: Belt sanding and grinding, which are often hand opera‐ tions, are occasionally employed to remove a heavy flash line. However, hand operations incur the risk of removing too much metal in critical areas, leading to field failures. The product designer must identify critical structural areas to ensure that the casting will not be weakened by grinding and belt sanding.
Characteristics of Zinc Die‐Castings
Zinc alloys solidify and cool very rapidly in die‐casting, giving the metal a fine grain structure with good mechanical properties. Heat is transferred from the casting through the surface of the die cavity into the die where it has been tradi‐ tionally removed by cooling water flowing through cooling channels in the die. Some die‐casters now use closed‐loop heat exchangers with oil, which improves die temperature control, allows for localized heating where required, eliminates scale build‐up in coolant passages, prolongs die life, and enhances casting quality. Zinc die‐castings solidify from the surface to the centre, generating two distinct zones in each wall section, as shown in Figure 10. The skin, which has finer grain structure, is typically 0.4 mm to 0.5 mm (0.015 to 0.020 in) thick. The skin is usually free of porosity, which tends to occur in the core of thick sections. The finer grain structure and absence of porosity give the skin superior mechanical properties. Since skin thickness is relatively constant, and not a function of total wall thickness, thin walls are stronger per unit of wall thickness than are thick walls. This important point is not widely recognized by designers.
Figure 10. Schematic of a section through a zinc die‐casting showing the skin and core zones. Rapid cooling and die temperature control Solidification
The parting line that is formed at the interface of the ejector and the cover dies is significant in product design because in‐gates and overflow gates (orifices that conduct metal into and out of the die cavity) are usually located in the parting surface and appear as thickened areas of the parting line. Gate location may affect the mechanical properties and/or appearance of the casting. Consequently, the product designer should work with the die‐caster to determine mutually satisfac‐ tory gate locations.
When the extraneous metal is trimmed from the casting, the exposed surface is interior or core metal, which may have some porosity and appear pitted. If this area is subjected to cyclic tensile stresses, the porosity may act as stress raisers, leading to premature fatigue failure. Porosity may also cause an unsatisfactory appearance. It is sometimes necessary to redesign the casting, moving the parting line to an area of lower stress. Figure 11 shows a cross section of a zinc die‐cast handle that was subjected to cyclic bending, which developed tensile stress on the top surfaces and compression on the bottom. Surface porosity in the gates at the parting line created stress raisers near the area of maximum stress. The handle was redesigned to locate the parting line closer to the neutral axis in an area of lower stress. The revised parting line offered the disadvantage of placing the flash in a more noticeable
location, requiring additional buffing.
igure 11. Sketch showing on of d e
Preventing Die‐Casting Defects
Fcross secti a zinc door handle, original design and redesigne to reduc stress on the parting line.
Casting defects are undesirable in critical structural areas, particularly those subject
Cracks
cases, die‐casting cycle parameters, such as die temperature, direction of to fatigue. Minor defects on decorative surfaces can become very obvious after surface treatment. Defects are generally eliminated by the die caster through control of the casting process. However, the product designer should be conversant with casting defects, particularly those that are affected by design practice.
In most
metal flow, and lubrication techniques, are adjusted to eliminate cracks. However, the design of the casting may contribute to the formation of cracks, and the die‐ caster may request minor product changes to remedy the problem. Parting line and trimming Porosity in trimmed part of the casting
Restrained shrinkage cracks (sometimes referred to as ʺhot tearingʺ) may occur when a long expanse of cast metal is terminated by a restraining rib. The metal contracts exceeding the hot strength and forming a crack at the junction of the restraining rib. This type of defect can often be eliminated by adding more ribs to distribute the shrinkage forces or by decreasing the holding time to eject the casting before shrinkage has reached its normal value.
Thick‐walled frame members intersecting at right angles may be susceptible to corner cracks, resulting from restraint and relatively slow freezing characteristics. Adequate radii at the intersections will reduce the stresses. It may also be necessary for the die‐caster to increase the metal injection pressure.
Visible cracks (also known as ʺcold shutsʺ) may appear at the junction of a hot stream of metal and a returning cooler stream. The die‐caster may be able to correct this problem by redirecting the filling pattern.
Shrinks
Ragged, irregular porosity in the form of shrinks often results when the die is overheated locally. The spot can be cooled or the casting cycle time increased. In cases where the cycle time cannot be increased, it may be necessary to enlarge the gate and/or runners to feed the troublesome spot. In other cases, it has been found necessary to add flat or vertical ribs to the cast wall to improve the feeding characteristics.
Shrinkage defects are more likely in thick sections, and the best remedy is to redesign the component with metal saving cores. Shrinkage defects may also occur when the casting pressure is too low; the solution is usually to increase the pressure on the metal. Smooth Internal Porosity Smooth internal porosity in the form of bubbles occurs when gas is trapped, rather than expelled from the die. When they are located close to the casting surface, the pores can cause blisters if the die casting is subsequently reheated. The die‐caster may change venting, gating, fill conditions or lubrication to reduce porosity problems. Proprietary vacuum systems, combined with proper process control, can virtually eliminate gas porosity.
Gross (Shrinkage) Porosity
Shrinkage pores are irregular in size compared with gas pores. They tend to occur in heavy sections when the metal pulls away from itself on solidification. Shrinkage pores may affect the integrity of the die‐casting, especially when they are in the vicinity of certain machined features, such as tapped threads. They do not contain gas, and are not a source of blisters.
Shrinkage pores are usually controlled by varying die‐casting parameters. In some cases it is necessary to redesign the casting, eliminating the thick walls. Holes for tapped threads should be cored, so that the threads are formed in metal that is free from shrinkage pores.
Segregation
Alloy constituents may segregate in ZA 12 and 27 during melting and solidification. Proper metal‐handling techniques prevent segregation during melting, and the solidification process in die‐casting is too rapid to permit segregation.
Surface Blisters
Surface blisters occur following ejection or reheating when porosity near the surface is severe. One remedy is to increase the time that the casting is held in the die, allowing the walls to cool and thereby increase in strength. A more satisfactory remedy is reducing or eliminating porosity as discussed earlier.
Galling or Drag
Drag is usually caused by undercuts in the die, which can be polished out as required. In some cases, draft is inadequate and the die cavity must be reworked. In other cases, the ejector plate advances unevenly, cocking the casting and causing drags; repair of the ejector plate will then be required. Mild drag, which causes only a burnishing of the surface, is tolerable in non‐ visible areas, such as holes that are cast with very low or zero draft.
Warped Castings
Castings can warp during ejection when one portion of a casting sticks in the die due to a local undercut or insufficient draft. The undercuts must be polished out, or the die cavity reworked to provide the required draft. Draft requirements should be reviewed with the die‐caster as the component is being designed. Warping may also be caused by an overheated die, in which case local cooling will be added or cycle time increased.
Unusual features sometimes shrink unequally, causing warpage. Ribs are some‐ times added to distribute the shrinkage. In some cases the shrinkage can be compensated by local cooling in the die.
Heat Checking
Heat checking in the die, caused by thermal fatigue, produces a pattern of raised fins on the surface of the casting. The life of the die can be prolonged by polishing the area at the first sign of failure. The appearance of raised fins will be delayed by proper preheating of the dies, operation at proper temperatures, and fog lubrication of the die faces at frequent intervals.
