For previous article, please refer to Die-casting design specifications—structural design self-inspection.
This series mainly introduces specifications related to die-casting design, including an introduction to basic knowledge, material selection, structural design specifications, introduction to die-casting molds and materials, and how to self-check during structural design. Next, we will introduce them to you one by one. This article mainly introduces die-casting. Causes and solutions of structural defects of parts are as follows;

Flow marks and patterns (stripes) are traces left by molten alloy that first enters mold cavity, forming an extremely thin and uneven metal layer that is compensated by subsequent molten alloy.

 

Problem causes:
(1) Metal temperature is too low:
(2) Punch speed is too slow:
(3) Nitrogen pressure of gas storage bottle is too low:
(4) Die-casting mold temperature is too low:
(5) Poor exhaust:
(6) Excessive paint accumulation:
(7) Punch or pressure chamber wear:
(8) Gate is unreasonable and causes splashing into cavity:
(9) Insufficient injection specific pressure
Solution:
(1) Ensure correct temperature of molten metal and check temperature control device:
(2) Determine correct injection speed and make it constant:
(3) Check gas storage bottle pressure gauge and oil supply indicator, add nitrogen if necessary;
(4) Ensure correct mold temperature:
(5) Add or modify vents and overflow grooves:
(6) Appropriate amount and concentration of paint:
(7) Replace if necessary:
(8) Improve gate design:
(9) Increase injection pressure

Cold slit (cold joint): A cold slit (cold joint) is a gap that occurs when lower-temperature metal flows meet each other but do not fuse.

 

Problem causes:
(1) Two metal streams are connected to each other, but they are not completely fused and there is no inclusion between them. Bonding force between two metal streams is very weak.
(2) Pouring temperature or die-casting mold temperature is low
(3) Improper alloy selection and poor fluidity
(4) Sprue position is incorrect or flow path is too long
(5) Low filling speed
(6) Injection ratio is lower than pressure
Solution:
(1) Properly increase pouring temperature and mold temperature
(2) Increase injection pressure and shorten filling time
(3) Increase injection speed and increase cross-sectional area of inner gate
(4) Improve exhaust and filling conditions
(5) Correctly select alloys to improve alloy fluidity

Under-casting: Under-casting means that molten metal does not fill mold cavity, and incomplete filling appears on die casting.

 

Problem causes:
(1) Reasons for poor liquidity:
a. Alloy liquid absorbs gas, oxidizes inclusions, and contains high iron content, which makes it poor in quality and reduces fluidity;
b. Low pouring temperature or low mold temperature
(2) Poor filling conditions:
a. Specific pressure is too low
b. Too much gas is involved, back pressure of mold cavity becomes high, and mold filling is blocked.
(3) Poor operation, excessive paint spraying, paint accumulation, and gas failure to evaporate
Solution:
(1) Improve quality of alloy liquid
(2) Increase pouring temperature or mold temperature
(3) Increase specific pressure and filling speed
(4) Improve diversion method of molten metal in pouring system, add overflow grooves and exhaust grooves in under-cast parts.
(5) Check whether die-casting machine capacity is sufficient

Air holes and pinholes: Air holes and pinholes are holes (mostly round or elliptical) with a regular shape, a smooth surface, and a dark gray surface formed by gas involved in die casting.

 

Problem causes:
(1) Metal pouring temperature is too high: (2) Smelting process is improper or metal is not purified overnight: (3) Charge is not clean: (4) Fullness of pressure chamber is small: (5) Gas is trapped during filling, exhaust hole is blocked, exhaust is not smooth and overflow tank is insufficient: (6) Poor runner design: (7) Too much paint on die-casting mold
Solution:
(1) Maintain correct pouring temperature: (2) Improve smelting and purification process: (3) Dry and purify charge: (4) Increase fullness of pressure chamber: (5) Increase fullness of pressure chamber, increase thickness of inner gate, reduce punch speed, and improve overflow system: (6) Improved runner design: (7) Reduce paint.

Shrinkage cavities and porosity: Shrinkage cavities and porosity are holes with irregular shapes and rough surfaces caused by insufficient internal compensation during condensation process of die castings.

Problem causes:
(1) Insufficient specific injection pressure during solidification shrinkage of casting: (2) Poor structure of casting, with hot spots and uneven wall thickness: (3) Insufficient capacity of overflow tank or overflow is too thin: (4) Residual cake is too thin: (5) Punch of vertical die-casting machine returns too quickly: (6) Metal pouring temperature is too high
Solution:
(1) Increase injection pressure; (2) Improve the structure and eliminate hot spots: (3) Increase capacity of overflow tank or thicken overflow opening: (4) Thicken remaining material cake: (5) Ensure a certain pressure holding time: (6) Control pouring temperature and lower it as much as possible.

Bubbles (bubbles): Bubbles (bubbles) are bubbles formed by expansion of accumulated gas under the surface of die casting.

 

Problem causes:
(1) Too much gas trapped in metal: (2) Temperature of molten metal is too high: (3) Temperature of die-casting mold is not high: (4) There is too much die-casting paint: (5) Injection system is unreasonable and exhaust is not smooth: (6) Opening mold too early,
Solution:
(1) Add overflow grooves and vents to defective parts, reduce punch speed: (2) Ensure correct temperature: (3) Control temperature of die-casting mold; (4) There is less paint and no uniformity: (5) Modify pouring System: (6) Extend pressure holding time and mold retention time.

There are small particles or lumps in casting matrix that are harder than metal matrix, making processing difficult and causing serious tool wear. After processing, castings often show hard points of different brightness.
Problem causes:
(1) Charge is not clean: (2) Alloy is not purified enough or slag is not removed: (3) Slag and oxides are brought in when taking alloy liquid: (4) Die-casting mold is not clean: (5) Graphite inclusions in paint too much
Solution:
(1) Ensure that charge is clean: (2) Purify alloy and select a flux that is easy to remove slag: (3) Prevent slag and gas from mixing into spoon: (4) Pay attention to cleaning die-casting mold: (5) Graphite coating must be mixed evenly and pure

Mucous membrane is metal adhesion to the surface of die-casting mold
Problem causes:
(1) Temperature of molten metal is too high: (2) Temperature of die-casting mold is too high or too low: (3) Iron content in (aluminum alloy) is too low: (4) Improper use of release agent: (5) There are hot spots in die-casting mold; (6) Temperature of die-casting mold or molten metal is too high; (7) Casting or runner is not solidified.
Solution:
(1) Maintain correct pouring temperature: (2) Maintain correct die-casting mold temperature: (3) Increase iron content to 10%: (4) Use release agent correctly: (5) Whether cooling water is smooth or increase cooling speed: ( 6) Ensure correct mold temperature and molten metal temperature: (7) Increase die-casting cooling speed.

Depression: A depression is a concave part on the smooth surface of a die-casting part, and its surface is in a natural cooling state.

 

Problem causes:
(1) Wall thickness of castings is too different, and sinks mostly occur in thick walls.
(2) Mold is partially overheated and overheated part solidifies slowly.
(3) Low shot ratio
(4) Due to air holding, cavity gas cannot be discharged and is compressed between cavity surface and metal liquid interface.
Solution:
(1) Wall thickness of casting should be designed to be as uniform as possible
(2) Mold local cooling adjustment
(3) Increase injection pressure
(4) Improve cavity exhaust conditions

Cracks: Cracks are filamentous gaps formed by destruction or disconnection of alloy matrix on die castings. There are two types: cold cracks and hot cracks. Cold cracks are not oxidized, and hot cracks are oxidized.

 

Problem causes:
(1) Iron content in alloy is too high or the silicon content is too low
(2) Content of harmful impurities in alloy is too high, which reduces plasticity of alloy.
(3) Aluminum-silicon alloy: Aluminum-silicon-copper alloy contains too much zinc or copper; aluminum-magnesium alloy contains too much magnesium
(4) Temperature of mold, especially core, is too low
(5) There are drastic changes in casting wall, shrinkage is blocked, and stress is formed at sharp corners.
(6) Mold retention time is too long and stress is high
(7) Uneven force during ejection
Solution:
(1) Correctly control alloy composition. In some cases, pure aluminum ingots can be added to alloy to reduce magnesium content in alloy; or aluminum-silicon master alloy can be added to alloy to increase silicon content.
(2) Change casting structure, increase rounded corners, increase draft angle, and reduce wall thickness difference
(3) Change or increase the ejection position so that ejection stress is equal to 8
(4) Shorten mold opening and core pulling time 5. Increase mold temperature and keep mold temperature stable

Flash: Flash is a flake of metal that appears on the edge of a die casting.

 

Problem causes:
(1) Insufficient clamping force
(2) Injection speed is too high, resulting in a pressure impact peak that is too high
(3) Debris on the parting surface has not been cleaned up
(4) Insufficient mold strength causes deformation
(5) Insert and slider are worn and parting surface is not flush
Solution:
(1) Check mold clamping force and pressure increase, and adjust die-casting process parameters
(2) Clean cavity and parting surface
(3) Trim mold
(4) It is best to use a closed injection end time control system to achieve flash-free die casting.

Deformation is the overall deformation of die casting geometry that does not match design requirements.
Problem causes:
(1) Poor structural design of castings, causing uneven shrinkage
(2) Mold is opened too early and casting is not rigid enough.
(3) Ejector rod is improperly set and force is uneven during ejection.
(4) Improper gate removal method
(5) Due to rough surface of mold causing large local resistance, deformation during ejection
Solution:
(1) Improve casting structure
(2) Adjust mold opening time
(3) Reasonably set position and quantity of ejector rods
(4) Choose appropriate gate removal method
(5) Strengthen surface polishing of mold cavity to reduce demoulding resistance

Scratches (strain marks, mold sticking scars): Along demoulding direction, due to metal adhesion, mold manufacturing slope is too small, causing strain marks on the surface of casting, and in severe cases, it becomes a scratched surface.
Problem causes:
(1) There is damage to cavity surface
(2) Slope of mold exit direction is too small or inverted.
(3) Deflection during ejection
(4) Pouring temperature is too high or too low, and mold temperature is too high, causing alloy liquid to adhere.
(5) Release agent is not effective when used
(6) Iron content of aluminum alloy components is lower than
(7) Cooling time is too long or too short
Solution:
(1) Repair mold surface damage, correct slope, and improve smoothness
(2) Adjust ejector rod to balance ejection force
(3) Replace release agent
(4) Adjust iron content of alloy
(5) Control appropriate pouring temperature and mold temperature.
(6). Modify inner gate to avoid direct penetration into core wall or special treatment on core surface.