Working in a die-casting factory will encounter many technical problems; for examples defective castings; pouring runner design; casting Porosity; etc. So how do these problems occur? Today I have a collection of possible causes of Porosity in die-casting parts; and what we should do to prevent it; therefore hoping to help die-casters in their daily work! also improve the die casting products and die casting mold better and better.
What is Porosity in die casting parts?
As you know; There have small bumps on the surface of die-casting parts; which are found after die-casting; are exposed after polishing or CNC Machine processing, or appear after Coating or electroplating; therefore it will affect parts of the appearance and parts of life; in actual production, we have to reduce the porosity of die-casting parts as much as possible, and make our parts with better appearance and stronger. however how do porosity come out? and What is the root cause of the porosity coming out?
Causes of Porosity in Die Casting
The design of the mold and cast parts
The purity of the metal or alloy being used
Pressure and shot speed of the machines
Shrinkage of the material wall thickness
Too much lubricant in the die
Sharp corners in the mold
Low metal temperatures
Air trapped in the metal
The most common way to check for porosity is an X-ray of the material; using computerized tomography or by cutting and polishing a section and then analyzing it under a microscope.
How to prevent Porosity in die casting parts or Reduce them?
The key to controlling the Porosity is to reduce the amount of air mixed into the casting material and die casting process; lets check one by one.
Die Casting Parts Design Factors
Check the wall thickness of die casting parts design is even or not. generally it should be not thicker than 8mm.
Die Casting Die Design Factors
The factors of equipment; mould and tooling mainly involve mold quality and equipment performance.
Is the die-casting mold design reasonable? Will it cause Porosity?
factor from die casting molds:
a. The gate position and the shape of the diversion are improper, causing the molten metal to enter the cavity and produce frontal impact and vortex.
b. Check design of the sprue shape?
c. Is the speed of the inner gate too high, causing flow dissipation?
d. Venting enough or not?
e. Is the position of the mold cavity too deep?
f. Is the machining allowance too large? It penetrates the dense layer of the surface; exposing the subcutaneous porosity? The machining allowance of die castings should be smaller; generally around 0.5mm; which can reduce the weight of the castings and reduce the amount of cutting processing; Reduce costs and avoid exposure of subcutaneous porosity; The margin should not be greater than 0.5mm; so that the processed surface can hardly see porosity; because it is protected by a hard layer.
Human Factors
Is the release agent sprayed too much? Because the release agent has a large amount of air; when the amount is too much; it is not burned out before pouring; so that the volatile air is wrapped on the surface of the casting; Therefore, under the same conditions; some workers will produce more porosity during operation; Choose a release agent with a small amount of air, and the dosage is thin and uniform, and the mold is closed after burning.
The slag and venting duct are cleaned frequently.
Is the mold opening too early? Has the mold been preheated? The temperature of each part should be slowly and uniformly, so that the surface temperature of the cavity and core is 150C ~ 200C.
Is there isolation of products produced when the mold temperature is low at the beginning?
If there is no pre-heating device, whether to use aluminum alloy material, quickly push it into the cavity to preheat or use other methods to heat it?
Do you take clean aluminum liquid and inject the oxide layer into the press chamber?
When pouring, whether to put the spoon close to the injection port of the pressure chamber to avoid flying waves, oxidation or entrapment in air to cool down, etc.
As soon as the molten metal is poured into the pressure chamber, does the injection begin, and does the temperature decrease?
For cooling and mold opening, do you choose the mold opening time according to different products?
Is there any reason that I am afraid that the molten aluminum will fly out and dare not use the normal die-casting pressure? Do I dare not try to increase the specific pressure appropriately?
Does the operator strictly follow the die casting process?
12. Is there any quantitative pouring? How to determine the pouring amount?
Equipment, tooling factors
Is the venting hole blocked and the air cannot be released?
Is there too much lubricant on the punch? This is also one of the sources of Porosity.
Does the gate position and diversion shape first seal the overflow system on the parting surface?
Is the position of the inner gate unreasonable? After passing through the inner gate, the metal immediately hits the shaped wall and generates eddy currents, and the air is drawn into the metal flow?
The position of the venting slot is incorrect, causing poor venting conditions?
Is the area of the venting slot large enough? Is it blocked, and is the location at the last filling place? Is the exhaust part of the mold cleaned frequently? Avoid losing the exhaust effect due to blockage of the release agent.
Is the mold temperature too low?
Is the runner turning smooth enough? Increase the inner gate appropriately?
Is there a vent plug in the deep cavity, or use die insert to increase the vent?
Is there any part that is difficult to vent due to unreasonable die-casting mold design?
Is the total cross-sectional area of the overflow less than 60% of the total cross-sectional area of the inner gate, and the slag discharge effect is poor?
Is it possible to increase the thickness of the inner gate to reduce the filling speed under good molding conditions?
Is the gate speed too high, the pouring movement is dramatic, and the metal flow is seriously involved in gas?
Is the cross-sectional area of the inner gate too small and serious jetting?
Whether to fill in order to facilitate the release of cavity air. Is there sufficient length for sprue and runner?
Material factors
Have you paid attention to the composition control of the supplier’s raw materials? What is the iron content? (Required below 0.7)
Is the purity of aluminum guaranteed?
Is the recycled material used too much and the slag removal action is not done well?
Do you add too much waste slag bag in the molten aluminum during the production process, and pour it along with the oxide scale during pouring?
Do you control the proportion of recycled materials? How to implement it?
Can recycled material be added to the molten aluminum of important customers’ products?
Try to change the ratio of new material to recycled material?
Is the recycled material clean?
Die casting parameters and operating process factors
Do you choose process parameters based on different products?
(Temperature of die-casting aluminum liquid is 630-670C ℃) Reasonable selection of die-casting process parameters, especially injection speed. Adjust the starting point of high-speed switching.
Is there a way to reduce the water content of the release agent? Is there a release agent with a small amount of air?
Is the alloy compounding temperature too high?
How to measure the temperature of molten aluminum? Is the thermometer accurate?
Is there a timely adjustment of the injection speed and the conversion point of the injection speed according to the product?
Is there any big machine die-casting small parts, and the pressure chamber is too small?
Envirnmental Factor
Is the air humidity in the die-casting environment high?
Under normal circumstances; the hydrogen content in the surrounding air is not much; But if the relative humidity in the air is high; it will increase the solubility of the air in the molten aluminum and form seasonal porosity. For example, in the rainy season, due to the high humidity of the air, the phenomenon of porosity during aluminum alloy melting is more serious. Of course, the air humidity is high. At this time, aluminum alloy ingots, smelting equipment, tools, etc. will also increase the amount of surface moisture adsorption due to humid air, so more attention should be paid to adopting strong preheating and drying protective measures to reduce the generation of porosity.
Does gravity die casting or pressure die casting produce more porosity
Gravity Die Casting
Gravity die casting very less porosity, due to it fill the die under force of gravity. it is natural flow and fill.
Pressure Die Casting
Pressure die casting is easy to have porosity; due to it inject the light metal material into die very fast; plus high pressure. the air very easy to trap. also plus other effector, the pressure die casting parts is easy to have porosity. that’s why the pressure die casting part hard to run heat treatment. due to the porosity will inflate after heating. will cause bigger bump exist on the part surface.