Defects in die castings:

Die casting is a process that can produce geometrically complex metal parts. lots of problems will occur when you don’t pay attention to it during production.

A high level of technical expertise and attention to detail is required to prevent small errors from causing huge losses to the company in the mass production of new parts.

Preventing this from happening is entirely about quality control of high capabilities.

In here, Five-Star tooling discusses some of the die casting defects that may occur during a die casting process, as well as ways to address and avoid them.

The shortcomings we will discuss include:

One, Flow marks

Another name: stripe

Features: the surface of the casting is in accordance with the direction of the flow of liquid metal, and the local felt and smooth lines are felt by hand. This defect has no direction and can be removed by polishing.

Causes

1. Two strands of metal flow out of sync, filling the cavity and leaving traces.
2. Mold temperature is low, such as zinc alloy mold temperature is lower than 150 degrees, aluminum alloy mold temperature below 180 degrees, are easy to produce such defects.
3. Filling speed is too high.
4. Too much paint

Elimination measures

1. Adjust the sectional area or location of the gate.
2. Adjust the mold temperature, increase overflow tank.
3. Adjust the filling speed properly to change the flow pattern of the cavity filled with metal liquid
4. The coating is thin and uniform

Two, cold separation

Another name: cold junction (butt joint)

Features: the lower temperature of the metal flow to each other, but not fusion, and the gap appears irregular linear, there are two kinds of penetrating and non-penetration, under the influence of external force, there is a trend of development

Causes

1. Pouring temperature of the liquid is low or die temperature is low.
2. Alloy composition does not meet the standard, poor liquidity
3. The liquid metal split filling, poor fusion
4. Gate unreasonable, the process is too long
5.Low filling speed or bad exhaust
6.Lower than the pressure

Elimination measures

1. Appropriately increase pouring temperature and mold temperature
2. Change alloy composition, improve flowability
3. Improve the pouring system, improve the filling conditions
4. Improve discharge conditions and increase the discharge
5. Improve the injection speed, improve exhaust conditions
6. Increase the specific pressure

Three, abrade

Other names: tension, stretch mark, mould sticking mark

Features: along the direction of the release, due to metal adhesion, the mold manufacturing slop is too small, resulting in the surface of the casting strain marks, serious when it becomes pulled to surface.

Causes

1. The casting angle of the core and mold wall is too small or inverted
2. Core, wall pressure scars
3. Alloy adhesion mold
4. Casting out deviation or core axis deviation
5. The surface of the wall is rough.
6. Paint often can not be sprayed
7. The content of iron is aluminum alloy is less than 0.6%

Elimination measures

1. Adjust the mold to ensure the manufacturing slope
2. Light indentation
3. Reasonable design of the gating system to avoid metal flow around the core, wall, and appropriately reduce the filling speed
4. Fix the die structure
5. Polished surface
6. Paint dosage is thin and even, can not leak spraying paint
7.Appropriate increase in iron content to 0.6~0.8%.

Four, sag

Other names: depression, depression, shortness of breath, edge collapse

Feature: a concave part on a smooth surface of a casting, whose surface is in a state of natural cooling

Causes

1. The casting structure design is unreasonable, there are local thick parts, produce hot knot.
2. Alloy shrinkage is big
3. The gate cross-sectional area is too small
4.Lower than
5. Mold temperature is too high

Elimination measures

1. To improve the casting structure, so that the wall thickness is slightly uniform, thickness difference between the larger connections should gradually ease the transition, remove the heated festival
2. Choose the alloy with small shrinkage
3. Set the gating system properly and increase the sectional area of the gate appropriately
4. Increase the injection force
5. Adjust the mold heat balance condition, adopt the temperature control device and cooling, etc.

Five, bubbles

Another name: bubbling

Features: skin of casting, formed gas bulging bubble

Causes

1. Mold temperature is too high
2. The filling speed is too high, the metal flow into the gas too much
3. Coating gas, an excessive amount of unburned, before pouring, the volatile gas enclosed in the surface layer of the casting
4. The exhaust is not smooth
5. Premature opening
6. Alloy smelting temperature is too high

Elimination measures

1. Cooling die to work temperature
2. Reduce the injection speed, avoid eddy gas inclusion
3. The small air quantity amount of coating, thin and uniform, then die out
4. Cleaning and adding an overflow tank and exhaust duct.
5. Adjust the die leaving time
6. Trimming smelting process

Six, stoma

Other names: air hole, air

Characteristic: the gas formed by the gas in the die-casting part has a regular shape and relatively smooth surface

Causes

The main cause is the inclusion of gas
1. The choice of gate location and improper flow guide, resulting in the metal liquid into the cavity, resulting in a positive impact and vortex
2. Sprue shape design is not good
3. The pressure chamber is not full enough
4. The gate speed is too high, resulting in turbulence
5. Poor ventilation
6. Mold cavity location is too deep
7. Too much paint, not burned before filling
8. The burden is not clean, poor refining
9. Machining allowance is too big

Elimination measures

1. The gate position and the flow guiding shape which is favorable to the cavity gas exclusion are selected to prevent the metal liquid from blocking the discharge system on the parting surface first. The cross-sectional area of the nozzle
2. Sprue gate should be possible within the large cross-sectional area
3. Increase the pressure chamber fullness, as much as possible the use of smaller pressure chamber, and quantitative casting
4. In order to meet the molding conditions, increase the gate thickness to reduce the filling speed
5. In the last part of the cavity filled open the overflow tank and exhaust, and should avoid overflow and exhaust by a liquid metal seal
6. The deep cavity at the vent plug, use the mosaic form to increase the exhaust
7. Paint dosage is thin and even, burn out after filling, use the small amount of paint
8. The burden must be handled clean and dry, strictly abide by the smelting process
9. Adjust the injection speed, the transition point of the slow injection velocity and the fast injection velocity
10. Reduce pouring temperature and increase specific pressure

Seven, shrink hole

Another name: retraction

Features: in the process of condensation, due to inadequate internal compensation caused by irregular shape, the rough surface of the hole

Causes

1. Alloy pouring temperature is too high.
2. The wall thickness of the casting structure is uneven, resulting in a hot knot.
3. The pressure is too low.
4. Overflow tank capacity is not enough, overflow is too thin.
5. The pressure chamber is too small, the remaining material (material cake) is too thin, and finally, the tonic can not play a role.
6. The gate is smaller.
7. The local temperature of the mold is on the high side.

Elimination measures

1. Comply with alloy smelting standard, alloy liquid overheating time is too long, reduce pouring temperature.
2. Improve the casting structure, eliminate metal accumulation parts, uniform wall thickness, slow transition.
3. Appropriately increase the specific pressure.
4. Increase the overflow tank capacity, thickening overflow.
5. Increase the pressure chamber fullness, using quantitative casting.
6. Appropriately improve the gating system to transfer the pressure well.

Eight, pattern

Features: smooth stripes, casting presents on the surface visible to the naked eye, but with the hand feel, color is different from the base metal lines, the removal of 0# can slightly wipe cloth.

Causes

1. Filling too fast.
2. Paint dosage is too much.
3. The mold temperature is low.

Elimination measures

1. As much as possible to reduce the injection speed.
2. Coating dosage is thin and even.
3. Improve the mold temperature.

Nine, crack

Features: the alloy matrix on the casting is broken or broken to form a slit with a fine wire. There are two kinds of penetrating and non-penetrating, which have a trend of development.

Cracks can be divided into two kinds of cold cracks and hot cracks. The main differences are: cold cracks, cracks in the casting, the metal has not been oxidized, hot cracks, cracks in the casting, the metal is oxidized.

Causes

1. The casting structure is unreasonable, the shrinkage is blocked and the casting fillet is too small.
2. The core pulling and ejection device skew in the work, uneven force.
3. Mold temperature is low.
4. Opening and core pulling time is too late.
5. The use of alloy improper or harmful impurities is too high so that the plasticity of the alloy decreased. Zinc alloy, lead, tin, cadmium, iron, and high aluminum alloy: zinc, copper and iron, copper alloy high silicon high magnesium alloy: aluminum, silicon, high iron

Elimination measures

1. Improved casting structure, reduce the wall thickness difference, increasing cast fillet.
2. Fix the die structure.
3. Improve the mold working temperature.
4. Shorten the mold opening and core pulling time.
5. Strictly control harmful impurities, adjust alloy composition, comply with alloy smelting specification or re-select alloy grade.

Ten, less casting

Other names: insufficient irrigation, unclear outline, incomplete corner.

Features: the metal liquid is not filled with cavities, and the casting is filled with incomplete parts.

Causes

1. Alloy flow caused by:
a. Metal liquid gas content is high, serious oxidation, resulting in decreased liquidity.
b.The alloy pouring temperature and die temperature are too low.
c.The gate speed is too low.
d.The nitrogen pressure in the accumulator is not enough.
e.The chamber is low in fullness.
f.The design of the casting wall is too thin or poor in thickness.

2. The poor casting system causes:
a.The gate position, the diversion way and the number of the inside gate are not properly chosen.
b.The gate cross-section is too small.
3. Poor exhaust conditions caused by:
a.Poor ventilation.
b.Excessive paint, no drying, burnout.
c.The mold temperature is too high, the gas pressure in the cavity is higher, it is difficult to discharge.

Elimination measures

1.Improve the fluidity of alloy:
a.Adopt the correct smelting process, excluding gas and non-metallic inclusions.
b.Increase the pouring temperature and die temperature of the alloy.
c.Increasing the injection velocity.
d.Adding nitrogen to raise the effective pressure.
e.Adopting quantitative pouring.
f.Improve the casting structure and adjust the wall thickness properly.

2. Improve the gating system:
a.The correct choice of gate location and diversion way, for non-good shape castings and large castings using multi-gate, is beneficial.
b.Increase the cross-sectional area of the gate or increase the injection speed.

3. Improve the exhaust conditions:
a.Add overflow tank and exhaust duct, a deep cavity can be equipped with breather plug.
b.The use of paint, thin and uniform, dry burn after die.
c.Reduce die temperature to working temperature.

Eleven, prints

Other names: putting a mark, insert block or moving block splicing mark.

Features: casting surface due to the mold cavity insert, knock and push rod, movable block parts such as convex and concave mosaic left traces of.

Causes

1. Uneven adjustment of tappet or end wear.
2. The mold cavity, the slider splicing part and its active parts with poor coordination.
3. The push rod area is too small.

Elimination measures

1. Adjust the push rod to the correct position.
2. Fastening inserts or other movable parts to eliminate undue irregularities.
3. Increase the area or increase the number of push rods.

Twelve, mesh burr

Other names: mesh marks, mesh patterns, cracks, burrs.

Characteristic: a mesh raised mark on a casting surface resulting from thermal fatigue on a mold cavity surface and a metal spike.

Causes

1. The mold cavity surface cracking caused by traces of the most concentrated heat conduction near the gate area, friction resistance, withstand the strongest erosion of molten metal, alternating hot and cold the drama, the easiest to produce hot crack, crack formation.
2. Die material improper or heat treatment process is incorrect.
3. Mold temperature difference.
4. Alloy liquid pouring temperature is too high, mold preheating is not enough.
5. Mold cavity surface roughness Ra is too big.
6. Excessive metal flow and positive erosion wall.

Elimination measures

1. The correct selection of mold materials and reasonable heat-treatment process.
2. The die must be preheated to the working temperature range before die casting.
3. As much as possible to reduce the pouring temperature of the alloy.
4. Improve the mold cavity surface quality, reduce the Ra value.
5. The block shall be annealed periodically to eliminate stress.
6. Design the gating system correctly, and use the smaller injection speed as much as possible to meet the well-formed condition.

Thirteen, colored spots

Other names: spot, black spot.

Characteristic: the spot on a casting surface that is different from base metal, usually formed by coating carbide.

Cause

1. Paint impurities or excessive use.
2. Paint containing too much graphite.

Elimination measures

1. The use of paint should be thin and uniform, can not pile up, use compressed air to blow.
2. Reduce the graphite content in the coating or choose the graphite free water base coating.

Fourteen,the surface not smooth

Features: a small particle or lump of hardness higher than the metal matrix exists in the matrix of the coating, which makes the machining difficult and the tool wear seriously. After the machining, the castings often show the hard points with different brightness.

Cause

The alloy mixed or precipitation than the base metal hard metal or non-metallic materials, such as AL203 and free silicon, etc..
1. Alumina(AL203).
(1)Aluminum alloy is not refined.
(2)The mixture is mixed with oxide.

2. Complex compounds composed of aluminum, iron, manganese, and silicon, mainly formed by MnAL3 in the molten pool at a relatively cool point. Then, MnAL3is used as the core to precipitate Fe and silicon to react to form compounds.

3. Free silicon mixed material
(1)Aluminum-silicon alloy with high silicon content.
(2)Free silicon exists in A1Si alloy casting in semi-liquid.

Elimination measures

1. Smelting to reduce unnecessary agitation and overheating, to maintain the purity of alloy liquid, aluminum alloy liquid in the furnace for long-term heat preservation, should be periodically refined to gas.
2. The aluminum alloy contains titanium, manganese, iron and other components, should not make segregation and keep clean, with dry refining agent refining, but in the aluminum alloy containing magnesium, should pay attention to compensation.
3. An aluminum alloy containing copper, iron content for a long time, should reduce the silicon content to below 10.5%, appropriately raise the pouring temperature, so that the silicon precipitation.

Fifteen, brittleness

Features: casting metal grain is too large or small, easy to break or fracture of the casting.

Causes

1. Excessive overheating of liquid alloy or holding time too long.
2. The intense cold, fine crystallization.
3. Aluminum alloy impurities, zine, iron, and other content are too much.
4. The content of copper in aluminum alloy is beyond the specified range.

Elimination measure

1. The alloy should not overheat, avoid alloy long time insulation.
2. Increase the mold temperature, reduce the pouring temperature.
3. Strictly control the chemical composition of the alloy.
4. Keep the coating of the crucible intact and well.

Sixteen, leakage

Features: the die casting is leaking, leaking or seeping through the test.

Causes

1. Lack of pressure
2. The design of the pouring system is unreasonable or the casting structure is unreasonable.
3. Alloy selection is improper.
4. Poor ventilation.

Elimination measures

1. Increase the specific pressure.
2. Improve the pouring system and exhaust system.
3. The use of good alloy.
4. Try to avoid processing.
5. The casting is impregnated.

Seventeen, the chemical composition does not meet the requirements

Features: by chemical analysis, the alloy elements of castings do not meet the requirements or too many impurities.

Causes

1. The ingredients are incorrect.
2. Raw materials and scrap without analysis shall be put into use.

Elimination measure

1. The burden should be chemically analyzed before it can be used.
2. The burden should be strictly managed, the old and new materials should be matched in a certain proportion.
3. Strictly abide by the smelting process.
4. Smelting tools should brush paint.

Eighteen, mechanical performance does not meet the requirements.

Features: the mechanical strength and elongation of casting alloy are lower than the required standard.

Causes

1. The chemical composition of the alloy is not up to standard.
2. The casting has porosity, shrinkage hole, slag inclusion and so on.
3. The sample treatment method is not equal.
4. The casting structure is unreasonable, limiting the casting to the standard.
5.Improper smelting process.

Elimination measure

1. Ingredients melting to strictly control the chemical composition and impurities content.
2. Strictly abide by the smelting process.
3. According to the requirement of the sample, in the production process should be regularly cast on the process of testing.
4. Temperature, as far as possible to eliminate alloy oxide formation of various factors.

Common defects of die castings and their causes
Common defects, influencing factors, deformation, shrinkage hole, porosity, crack, cold separation, slag inclusion, mold sticking marks, forming factors.

The relationship between B class factors and die casting
The defect causes metal pressure possible causes (pressure) injection rod speed pressure 1-2 speed transfer point pressure chamber with the solidification time temperature of metal injection quantity.