During die casting and injection molding processes, pores are caused by shrinkage of material. While most pores are round, most shrinkage is irregular. High porosity will reduce strength, stiffness, corrosion resistance and air tightness of die castings, thereby affecting product performance.

 

(Porosity inside die casting)
Few and small pores are not a problem and are normal phenomena. However, if pores are connected in series due to machining or changes in temperature and pressure, it may cause product leakage.

 

(Porosity inside injection molded parts)
Therefore, it is necessary for us to clearly define acceptable standards for pores in terms of product technical requirements.
First of all, let’s first understand the reasons why pores are generated during die casting process:
1. During punching and solidification process, gas evaporated from coating invades molten metal, causing round holes on the surface or inside casting.
2. If gas content in alloy liquid is too high, round holes will also be formed during solidification process.
3. During solidification process of liquid alloy, a shrinkage cavity occurs due to volume reduction or solidified part cannot be filled with liquid metal.
4. During heating process, due to uneven thickness or local overheating, casting solidifies slowly in some places, and casting surface becomes concave when volume shrinks.
Secondly, cause of pores in injection molded products is similar to that of die-cast products. Most of them are caused by temperature and shrinkage. It is also possible that raw materials themselves are not fully dried, have high moisture content, and introduction of gas, coupled with insufficient mold exhaust, can also cause trapped gas, resulting in dents or pores on the surface or inside of product.
Finally, let us explain technical standard requirements for air holes when molding die castings and injection molded parts:
Die Casting:
Let’s first look at some definitions of standards:

 

Generally, our electronic control products are divided into machined surfaces and blank surfaces, which correspond to different air hole standards.
Machined surface: D ≦ 0.4; h ≦ 0.4. Quantity and distance are not considered.
Blank surface: D ≦ 0.75; h ≦ 0.75; quantity ≤ 4, of which pores with diameter ≤ 0.4 and depth ≤ 0.4 are not counted. (If target area is small, use area scale and ignore values below decimal point); 2.0 ≤ distance mm, where pores with diameter ≤ 0.4 and depth ≤ 0.4 are not considered.
Injection mold:
Injection molded parts usually rarely have internal pores defined, but recently when studying air tightness, through X-RAY inspection, we found that there are also many pores inside plastic products. If pores are relatively close to product surface, strength issues may result in internal and external penetration holes, which may cause product leakage under application conditions with large pressure differences.
Generally speaking, pores on outer surface of injection molded parts can be referenced: if surface area is within 100 square centimeters, number of pores shall not exceed 3 and diameter shall not exceed 1 mm; if surface area is between 100 and 500 square centimeters, number of pores shall not exceed 5 , diameter shall not exceed 1 mm; if surface area is more than 500 square centimeters, number of pores shall not exceed 10, and diameter shall not exceed 2 mm.
As for size, location, and quantity standards of internal air holes in injection molded parts, I have not yet encountered them. Usual practice is to conduct air tightness and strength environment tests to confirm whether appearance of product is cracked and whether it will cause poor sealing to judge quality of injection molded product.