Home » 13 Die Casting Defects and How to Avoid Them
13 Die Casting Defects and How to Avoid Them
Do you kown what the most common die casting defects are? What cause the defects? And how to prevent these defects? Look through this article, and you will get the answers and solutions you need. There are 13 types of very common die casting defects listed as below:
Gas porosities appear in the body of the die casting parts, round or oval, with smooth surface and bright white or bright yellow color. The machined surface can be identified by visual inspection after machining, and the non-machined surface needs to be identified by X-ray detection.
A：Released hydrogen from molten metal alloy
The higher the smelting temperature, the higher the solubility of hydrogen in molten metal alloy. With the cooling and solidification of the die casting, the solubility of hydrogen decreases and is released from the molten metal to form the gas porosities. Sources of hydrogen:
Moisture in the air enters the molten metal alloy and decomposes into hydrogen;
Moisture, greases on metal alloy ingot enter the molten metal alloy and decomposes into hydrogen;
Moisture on smelting tools enters the molten metal alloy and decomposes into hydrogen;
B：Involved gases during molten metal alloy filling
In die casting process, the molten metal alloy is filled into dies with high pressure and high speed, if the alloy liquid can’t flow orderly and stably, it is easy to cause turbulences and gets gases involved. Turbulences are caused by:
Unreasonable designing of venting launders or overflow launders;
Blind areas in gating system;
Unreasonable die casting parameters and injection speed;
C：Gases from mold release agents
Mold release agents decompose and produce gases when get heated by molten metal alloy. Or too much use of mold release agents causes gas volatilization.
Make sure metal alloy ingots dry and clean;
Control smelting temperature in case of overheating, and do degassing treatment to molten metal alloy;
Chose reasonable die casting parameters, especially the injection speed;
In order to molten alloy flows stably and gases discharge easily, make sure sufficient length of sprue and runner (>50mm);
Use mold flow analysis software, set up venting launders and overflow launders at the positons that may form gas porosities;
Chose high quality mold release agent and control spraying quantity;
Die Casting Defect 2: Drags and Soldering
Drags are parallel with the dies opening direction, with a certain depth, and appears as strip-shaped scratches on the surfaces of die casting parts. Soldering is caused by the abnormal sticking of metal alloy and die cavity, and appears as some areas of die casting parts surfaces are with excess material or missing material. Both of the defects can be identified by visual inspection.
Damages on the surfaces of die cavity;
Insufficient hardness of die cavity, or low roughness of cavity surfaces;
Insufficient draft angles, even under-cut;
Inclined ejection of die casting part since unreasonable ejection mechanism;
Overheated molten metal alloy temperature, or overheated dies temperature;
Poor quality mold release agent;
Insufficient Fe element of metal alloy;
Repair the damages of die cavity surfaces, and improve the roughness;
Make sure the hardness of dies(HRC45-48), and adjust the draft angles;
Optimize the ejection mechanism to ensure balanced ejection;
Choose high quality mold release agents;
Control molten alloy temperature and die temperature;
Adjust Fe element content;
Adjust the direction of ingate to avoid molten alloy impacts the die cores;
Die Casting Defect 3: Blisters
Blisters appear on the surfaces of die casting parts as different size of bulges. These bulges are caused by gas expansion underneath the surfaces. Blisters can be identified by visual inspection.
Involved gases during the injection process;
Poor performance of venting caused by unreasonable design of gating system;
No degassing treatment of molten alloy, or overheated smelting temperature;
Overheated die temperature, insufficient pressure-holding time, insufficient solidification time, all of which result in less strength of die casting parts and cause the gas expansion in the casting.
Too much use of mold release agent, or too much use of lubricant on the piston;
Insufficient blowing time after spraying of mold release agent.
Adjust die casting processing parameters and injection speed;
Optimize the gating system, add venting launders and overflow launders;
Lower die temperature of where the defects may occur, so as to reduce the pressure effect of gas;
Adjust smelting process;
Prolong the pressure-holding time, and blowing time after spray;
Optimize the using quantity of mold release agent and lubricant;
Die Casting Defect 4: Cracks
Cracks appear on the surfaces of die casting as linear or irregular patterns, which have the tendency of extension with the external force. Crack includes cold crack (the material is not oxidized at the crack) and hot crack (the material is oxidized at the crack). This defect can be identified by visual inspection.
Too high Fe element content or too low Si element content in molten alloy;
Excessive harmful elements that lead to the decrease of plasticity of die casting parts;
Too high Zn element content in Al-Si alloy, or too high Mg element content in Al-Mg alloy;
Too low die temperature;
Non-uniform wall thickness results in abnormal shrinkage of die casting;
Too much stayed time of die casting in dies results in the adding of internal stress;
Unbalanced ejection of the die casting;
Ensure correct alloy elements content during smelting;
Optimize the structure of die casting part, and avoid non-uniform wall thickness;
Adjust or add ejection positions to ensure balanced ejection of die casting parts;
Shorten opening time or core-pulling time properly;
Improve die temperature properly (the working temperature of dies between 180° and 280°);
Die Casting Defect 5: Deformation
The geometric shape of the die-casting part does not match the drawing, and the overall deformation or local deformation occurs. Deformation can be identified by gage inspection or visual inspection.
Premature mold opening results in insufficient rigidity of the casting;
Unreasonable ejection mechanism design leads to unbalanced ejection of castings;
Drags occurs when ejection of casting;
Improper gate removal method;
Optimize casting structural design;
Adjust mold opening time;
Reasonably set the position and quantity of ejection pins;
Eliminate unfavorable factors of demoulding;
Choose reasonable gate removal method;
Die Casting Defect 6: Flow Marks
On the surface of the die-casting part, there are stripes in the same direction as the flow of the metal liquid, and there are clearly visible non-directional lines that are different from the color of the casting base. These stripes and lines have no tendency to extend. Flow Marks can be identified by visual inspection.
The liquid metal that first enters the cavity forms a thin and incomplete metal layer, and the incomplete parts are filled with the liquid metal that enters later, thereby leaving traces.
Die temperature is too low;
The cross-sectional area of the ingate is too small and improperly positioned, causing molten metal to splash during filling;
Insufficient filling pressure on the molten metal;
Too much mold release agent and lubricant;
Increase die temperature;
Adjust the position and cross-sectional area of the ingate;
Adjust the injection speed and pressure of molten metal in the runner;
Choose suitable mold release agent and lubricant, adjust the amount as well;
Die Casting Defect 7: Cold Flow
On the surface of die-casting parts, there are obvious, irregular, and depressed linear lines. These lines are small and narrow, and the edges of the junction are smooth, which may extend if with external forces. Cold flow can be identified by visual inspection.
Two streams of metal liquid butt together, not fully fused;
Moten metal temperature or die temperature is too low;
Fluidity of metal alloy is poor;
The design of gating system is unreasonable, resulting in long flow distance of molten metal.
Injection speed or injection pressure is too low;
Metal liquid does not flow smoothly in the die cavity;
Properly increase pouring temperature and die temperature;
Increase injection pressure, and shorten filling time;
Increase injection speed, as well as the cross-sectional area of the ingate;
Improve venting condition of die cavity;
Choose proper metal alloy to improve the fluidity;
Ensure metal liquid flowing smoothly in the die cavity;
Die Casting Defect 8: Turtle Cracks/Network Cracks
There are net-like, hair-like protrusions or depressions on the surface of the die-casting. With the increase of die-casting times, these protrusions or depressions expand and extends continuously. Turtle cracks can be identified by visual inspection.
Turtle cracks occurs on the surface of die cavity;
Improper material of die casting mold, or improper heat treatment process;
The temperature difference of the die-casting mold changes drastically in a very short time;
Filling temperature is too high;
Preheating of die casting mold is insufficient or uneven;
The die cavity is rough;
Choose proper material of die casting mold and heat treatment process;
The filling temperature should not be too high, especially for high melting point alloys. Under the premise of not affecting the quality of die castings, adopt the lower filling temperature as possible;
Preheating of die casting mold must be sufficient and even;
After reaching a certain service life, the die must be annealed to eliminate internal stress;
The surface of the gating system and the die cavity is regularly polished to ensure good roughness;
Choose proper cooling method to ensure thermal balance of die cavity;
Die Casting Defect 9: Sinks
In thick-walled areas, there are depressions on the surface of die casting parts. This defect can be identified by visual inspection.
The uneven wall thickness of the casting results in uneven shrinkage during solidification.
The die is partially overheated, and the casting solidifies slowly in the overheated area;
The injection pressure is too low;
Due to the poor venting performance of die cavity, the gas is compressed between the cavity surface and the metal liquid surface;
Short pressure-holding time results in poor feeding effect;
Optimize casting structural design to avoid uneven wall thickness;
Avoid partial overheating of die cavity;
Increase injection pressure;
Improve venting performance of die cavity;
Prolong the pressure-holding time;
Die Casting Defect 10: Short Filling
Some material is missing in partial area of the casting surface. This defect can be identified by visual inspection.
Poor fluidity of molten metal alloy caused by:
Metal liquid involves gases, inclusions, or contains too much Fe element;
Filling temperature is too low, or die temperature is too low;
Poor filling condition caused by:
Injection pressure is too low;
Too much gases involving results in high back pressure in the die cavity, and high back pressure hinders the filling process eventually;
Using too much mold release agent or lubricant leads to too much gas release in the die cavity;
Choose proper metal alloy and improve the quality;
Increase filling temperature or die temperature;
Increase injection pressure and speed;
Improve the design of gating system and the flow diversion of metal liquid, add overflow launders and venting launders in the short filling areas;
Choose proper mold release agent and lubricant, and control the using amount;
Die Casting Defect 11: Flashes
On the edge of the parting surface of the die-casting part, there are thin and irregular-shaped metal sheets. Flashes can be identified by visual inspection.
Clamping force of die is insufficient;
Injection speed is too high;
The garbage on the parting surface is not cleaned up
Deformation occurs on the die since insufficient strength of die material;
The inserts or sliders of die are wore out that results in metal liquid escapes from the gaps;
The hinges of die casting machine are wore out or deformed;
The filling temperature is too high;
Adjust die casting processing parameters, and ensure sufficient clamping force;
Clean up parting surface and cavity of the die;
Ensure regular maintenance of dies and die casting machines;
Die Casting Defect 12: Inclusions
Irregular-shaped holes are on the surface or inside of the casting, with inclusions inside. Inclusions on the surface of casting can be identified by visual inspection, while inclusions inside the casting must be identified by UT or X-Ray.
Purity of furnace charge is low;
Metal alloy liquid is not purified properly, or slags are not removed thoroughly;
Metal alloy liquid is polluted by garbage when ladling out;
Die cavity is not cleaned up;
Too much graphite inclusion in the mold release agent;
Ensure the purity of furnace charge;
Purify metal liquid properly, and thoroughly remove slags;
Clean up the ladles, and prevent involving slags;
Clean up die cavity;
Graphite must be stirred and mixed evenly, if mold release agent contains graphite;
Die Casting Defect 13: Interlayers
Interlayers are sandwich-like layered defects on the surface of die casting parts, and are usually seen on the parting line. This defect can be identified by visual inspection.
There are excessive material or flashes sticking on the edges of sliders, inserts or parting surface. The excessive material or flashes are not fused by the molten metal alloy when filling.
Clean up the die cavity and slider every shot;
Tips to solve die casting defects
Since every defect may be caused by various factors, to solve defects in die casting production, you are strongly recommended to follow ‘easy things first’ principle as below:
Clean up parting surface, die cavity, ejection pins—improve the quality of mold release agent, improve spraying process—increase clamping force, increase molten metal amount. All of these simple actions may solve big problems.
Adjust die casting processing parameter, injection speed, injection pressure, injection time, pressure-holding time, die opening time, filling temperature, die temperature.
Change metal alloy ingots to high-quality ones—change the ratio of new material to recycled material—improve smelting process.
Repair the die—mending gating system—add ingates—add overflow launders—add venting launders.
To better understand the tips, here is a demo to refer to.
Flash defect may be caused by:
Die casting machine problem as insufficient clamping force;
Processing problem as over-high injection speed;
Die problem as deformation, garbage on parting surface, wore-out slider, insufficient die rigidity, etc.
A smart engineer is supposed to solve this defect by following the sequence: clean up parting surface increase clamping force—adjust processing parameters—repair the die. Check the result after every step, and the defect may be eliminated the first step.