Permanent Mold Life and What Shortens It
A permanent mold is tooling, and it is replaced or repaired on a schedule that its life sets. That life can be as few as 100 pours or as many as 250,000, and the published guidance puts the difference down to a short list of operating variables plus the way the mold and its cores are designed.
How long a permanent mold lasts
Mold life can vary from as few as 100 pours to as many as 250,000 pours or more, depending on the variables set out below. A mold for an aluminum piston can be expected to produce 250,000 castings before it requires repair. After 250,000 more castings the repaired mold needs a major overhaul, and with repeated repairs and overhauls it can produce as many as 3.5 million castings before it is discarded.
A piston mold has a relatively simple design, and it runs much longer than a mold that needs elaborate internal coring and external inserts, such as a cylinder head mold. Magnesium alloys are also kinder to a mold than aluminum alloys of similar size and shape, because molten magnesium does not attack ferrous metal molds. How large that advantage is depends heavily on how well the mold coating holds up.
Molds are often made from cast iron, because casting the mold close to the finished shape cuts machining cost and because cast iron resists attack by molten aluminum better than steel does. Steel is weldable and easier to repair, so steel molds are often used for high-production castings. In the casting of gray iron, mold life is expected to be short compared with the casting of similar shapes from aluminum alloys.
The variables that shorten mold life
The published list of major variables runs from the metal being poured through the way the mold is run and stored. Several of them are operating decisions rather than material decisions, and they sit above the material choice in importance.
| Variable | Effect on mold life |
|---|---|
| Pouring temperature | The hotter the casting metal is poured, the hotter the mold is operated, which leads to rapid weakening of the mold metal. |
| Weight of casting | Mold life decreases as casting weight increases. |
| Casting shape | Mold walls must dissipate more heat from castings with thick sections than from those with thin sections. A large variation in section thickness sets up a temperature differential across the mold, and mold life falls as that differential grows. |
| Cooling methods | Water cooling is more effective than air cooling, but it substantially decreases mold life. |
| Heating cycles | A continuous run, with the mold held at a uniform temperature, gives maximum mold life. Repeated heating and cooling over a wide range shortens it. |
| Preheating the mold | Preheating to operating temperature with a gas flame or electric heaters greatly increases mold life. Thermal shock is one of the principal causes of mold failure. |
| Mold coating | The coating protects the mold from erosion and soldering by keeping the metal off the mold surface, and it increases mold life. |
| Mold material | The material chosen from the published selection sets the ceiling on mold life. |
| Storage | Improper storage leads to rusting and pitting of the mold surfaces, which reduces mold life. |
| Cleaning | Abrasive blasting, dipping in caustic solution and wire brushing are all used. Caustic is hazardous to the operator, and blasting and brushing cause excessive mold wear if they are not carefully controlled. Glass beads are the safest blast material. |
| Gating | A poor gating system reduces mold life greatly by causing excessive turbulence and washout at the gate areas. |
| Method of mold operation | The same materials are used on automatic and hand-operated equipment, but tooling on hand-operated equipment has the shorter life because of the abuse it takes. Tools on automatic equipment can last up to twice as long. |
| End use of the casting | Where the structural function counts for more than appearance, a mold can be run for more pourings before it is discarded. |
Major variables that affect the life of permanent molds, reproduced from the mold life section of Permanent Mold Casting, in ASM Handbook, Volume 15, Casting.
Two entries in that list are worth pulling out. Water cooling is more effective than air cooling but it substantially decreases mold life, and preheating the mold to its operating temperature greatly increases it because thermal shock is one of the principal causes of mold failure. A continuous run at uniform temperature is the best case, and repeated cycling over a wide temperature range is the worst.
How mold design shortens life
Mold design has a marked effect on life on top of the operating variables. Variation in mold wall thickness causes excessive stress during heating and cooling, which leads to premature failure by cracking, and abrupt changes in thickness without generous fillets do the same. Small fillets and radii reduce life because checking, cracking and ultimate failure often start at those points.
Draft is part of the same picture. Less draft is required on external mold surfaces than on internal ones because of casting shrinkage, and a 5 degree draft is desirable. Lower draft angles decrease the number of castings that can be made between mold repairs.
| Mold surface | Draft |
|---|---|
| Internal mold surfaces | 3 degrees |
| External mold surfaces | 2 degrees |
| Desirable practice | 5 degrees |
Draft angles for permanent molds, reproduced from the mold design section of Permanent Mold Casting, in ASM Handbook, Volume 15, Casting. Less draft is needed on external surfaces than on internal ones because of casting shrinkage, and lower draft angles decrease the number of castings that can be made between mold repairs.
Projections in the mold cavity also shorten life. They become extremely hot, which raises the risk of extrusion, deformation and mutilation when the casting is removed. Inserts can sometimes be used to replace worn or broken projections and extend the life of the rest of the mold.
Heat checking and mold erosion
Mold erosion from metal impingement and cavitation, both of which come from improper gating design, contributes to rapid weakening of the mold metal and to heat checking. These design errors show up as rapid dimensional variation during a long run. Mechanical abrasion from insufficient draft or from a poorly designed ejection system has the same effect.
Heat checking is a surface crack network produced by the temperature cycles the mold face sees. It is the failure mode that copper alloy castings add to the picture, because their higher pouring temperatures put the mold cavity under more severe cycling. The surface of the casting can be no better than the surface of the mold cavity, so heat checks and other imperfections in the mold are reproduced on the castings.
What holds a mold together in service
A mold coating is applied to mold and core surfaces to act as a barrier between the molten metal and the mold while a skin of solidified metal forms. It is used to prevent premature freezing, to control the rate and direction of solidification, to minimize thermal shock to the mold material, to prevent soldering of molten metal to the mold, and to vent air trapped in the cavity.
Preheating is the other routine measure. Molds are preheated to their approximate operating temperature before the operation starts, which cuts the number of unacceptable castings made while the operating temperature is being established. Preheating the mold face in an oven is the best method because the thermal gradients are smaller, though it is usually impractical for larger molds.
Permanent mold life, printable PDF
The mold life range, the variables that shorten it and the draft angles that go with it, in one PDF with our contact details.
Grades and stock
The tool steels that appear in permanent mold tooling are hot work grades. The details of each are on the H11 tool steel page, which is the general-purpose chromium grade used for zinc molds and for aluminum and magnesium cores, and on the H13 tool steel page.
The material selection that sets the ceiling on mold life is set out on the permanent mold and core materials page, which lists the mold and core materials by casting alloy and pour count.
Heat checking is a damage mode with its own signature at each scale of examination, and the way it is separated from wear and overload is set out on the thermal fatigue resistant tool steels page. For the analysis route when a mold has failed, the tool and die failure analysis page covers how the failure is classified.
Permanent mold life, printable PDF
The mold life range, the variables that shorten it and the draft angles that go with it, in one PDF with our contact details.
Before you use these values
This page is a reference summary of published practice and it is not an Aobo Steel specification. Mold life figures are typical published values for the alloys and processes described, and a different casting alloy, mold design or operating discipline will move them. Final decisions on mold replacement and repair are made against the casting drawing and the production schedule.
Source: ASM Handbook, Volume 15, Casting (9th Edition Metals Handbook), ASM International, 1988.