Mold processing

Metal casting mold

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  • Date : 25-11-2021 23:12:47

Metal casting mold


I. Definition.

Metal casting mold is a product manufacturing technology by pouring liquid material into a mold to create a product with a molded shape where the material is metal.

The casting solidifies under the influence of gravity without any other action.


II. Structure.


The mold consists of 2 main parts:

  • Mold cavity: Has the same shape as the outer shape of the casting.
  • Thao (core): has the same shape as the inner shape of the casting.

III. Classification of metal casting molds.


1. Casting in sand mold

Đúc trong khuôn cát
a, Concept:

It is a very old casting method and is still very popular today. The mold is made of a mixture of silica sand, binder, filler, and mold paint. And the mold is only used once.

c. Advantages:

  • The casting method is simple, low cost with single quantity.
  • It is possible to cast complex parts that can be cored.
  • Casting parts with large volume, casting a variety of materials.
  • Mechanical properties of castings are relatively good due to an annealing process after casting.

d. Defect:

  • The molding cycle is long because the mold can only be used once.
  • The quality is not good, The casting error is large.
  • Thin-walled castings cannot be cast.

e. Common failures

  • Concave concave: bar shape due to the volume of metal shrinking due to cooling. Usually forms at the top as the metal solidifies last. How to handle: create more hot beans on the mold
  • Porosity: there is a gas dissolved into the metal when cooking or following the metal flow into the mold. Measures: need to have suitable pouring parameters to avoid mixing gas into the flow.
  • Celestial : Due to uneven crystallization, alloys are deposited. Hard to fix. Turn it into an advantage.

2. Gravity casting (casting in metal molds)

Đúc trong trọng lực ( đúc trong khuôn kim loại)

Đúc trong trọng lực ( đúc trong khuôn kim loại) -1
a, Concept:  The casting method is similar to casting in a sand mold the mold materials are metals.
b, Advantages:
  • The mold is used many times to shorten the molding cycle
  • Reduced cost with bulk casting.
  • Can cast complex parts but still poor in sand mold.
  • Good surface quality, small casting error.
  • Can cast castings with an average wall thickness of 3-4 mm
c Defect:
  • Chế tạo khuôn phức tạp, đắt tiền chỉ phù hợp với sản xuất hàng loạt trở lên.
  • Chỉ hiệu quả khi đúc vật liệu có nhiệt độ nóng chảy thấp.
  • Không đúc được vật đúc có khối lượng lớn.
  • Dễ xảy ra khuyết tật do thoát khí kém.

d. Common failures:
  • Air porosity: Gas is mixed into the flow when pouring and the gas in the mold cannot escape. Measures: suitable pouring mode, need to design air outlet.
  • Cracked, no bond due to uneven cooling rate, so there is a solidifying area before a later solidifying area, these two do not stick together. Fix: adjust pouring parameters: speed, melting temperature, cooling mode. …

3. Centrifugal casting

Đúc li tâm

a, Concept: The process of filling liquid metal into the cavity of a rotating mold. Thanks to the centrifugal force, the metal will evenly adhere to the wall and solidify there. Mainly cast round, hollow parts.

b, Advantages:

  • The metal organization is smooth and tight, and there are no pitting and shrinking defects.
  • Create hollow castings without manipulation.
  • No complicated pouring system is used, so there is less metal waste.
  • Creates a casting consisting of several distinct metals, only the metal layer

c. Defect:
  • There is a phenomenon of regional bias according to the horizontal area of the casting, because each element of different mass is subjected to different centrifugal forces.
  • When casting pipes, the hole diameter is less precise and the surface quality is poor.


4. Melt casting (wax casting, thin shell molding, fire casting)

Đúc mẫu chảy (đúc sáp, đúc khuôn vỏ mỏng, đúc mẫu cháy)
Đúc mẫu chảy (đúc sáp, đúc khuôn vỏ mỏng, đúc mẫu cháy)-1
Đúc mẫu chảy (đúc sáp, đúc khuôn vỏ mỏng, đúc mẫu cháy) - 2

a, Definition: A special form of disposable mold similar to sand casting, molds made of fusible materials. After the mold shell is shaped, the mold shell and sample will be fired, then the mold will melt, leaving the hollow part of the mold.
b, Advantages:

  • Precise casting is achieved by eliminating the need to disassemble the mold.
  • Castings have very complex shapes that other methods cannot do
  • The surface of the casting is good due to the smooth surface of the mold, not burning.
  • Good casting mechanical properties due to uniform cooling.
  • Can cast difficult-to-melt materials

c. Defect: The molding cycle is slow because molds and templates are only disposable, automation is needed to reduce mold and prototyping time.
d, Common damages: Porosity, gas inside the product.

5. High pressure casting

Đúc áp lực cao

a, Concept: The metal in the chamber is pressed under the high pressure of the piston into the metal mold cavity and cooled rapidly in the mold by the cooling water pump.
b, High pressure casting machines are divided into 2 types:
  • Hot chamber casting machine
  • Cold chamber casting machine
  •  Hot chamber casting machine: The cylinder chamber is placed in the crucible, the molten metal is always contained in the compression chamber. As the piston presses down the liquid metal is pushed into the die.
  • The hot chamber casting machine is suitable for casting metals and alloys with low melting point such as tin, magnesium. With the scientific development of the hot chamber casting machine, it can be applied to the casting of higher fusion metal such as aluminum and aluminum alloy with the V-line machine.
  •  Cold chamber casting machine: The metal is melted separately from the cylinder. The process of transferring molten metal from the crucible to the cylinder is carried out by mechanisms, robots, and metal scoops.

c, Advantages:
  • For casting complex castings, thin-walled (1¸5mm) holes can be molded with small sizes.
  • High gloss and precision.
  • High casting mechanical properties due to high casting density.
  • High productivity thanks to fast filling and favorable mechanization.
d. Defect:
  • The average weight of castings is < 40kg. Because it requires the machine to have a large pressing force.
  • Need high automation, expensive lines.
  • Can not be used for sandblasting because the flow is pressurized. Therefore the hole or inside shape should be simple.
  • The mold is quickly worn out by the pressurized flow of the alloy at high temperature.
  • Complicated mold calculation, high mold making cost.
  • Details can only be heat treated when there is a suitable casting mode.

e. Common failures
  • Porosity: Due to casting under high pressure, the metal stream sprays into a jet, mixing the air inside. Remedy : Special casting mode.
  • Cracked, welded seams : Due to uneven cooling, liquid metal does not bond with solidified metal Remedy : adjust pouring temperature.
  • Concave, shrink : Uneven wall thickness. The dense wall solidifies later than it contracts. Remedy: design the part of the right thickness.
  • Product jam in mold : Non-standard parting surface or small slope. Remedy : Fix the mold.

6. Low pressure casting

a. Concept: Similar to high-pressure casting, the liquid metal stream is introduced into the mold cavity under the action of pressing force, but with a smaller force through compressed air is blown into the closed crucible to make pressure in the drum, pushing liquid flow into the mold. Or through the vacuum of the mold cavity to help suck the liquid metal into the mold.
b, Advantages:
  • Can cast castings with mass < 70kg
  • The quality of castings is highest after heat treatment due to less gas in the part.
  • Cores can be placed for casting parts with complex cores.
  • Short casting cycle is only larger than high pressure casting.
d. Defect: Hệ thống máy đồng bộ, đắt tiền.
e. Common failures: Product jam in the mold : The parting surface is not standard or the slope is small. Remedy : Fix the mold.

IV. Materials for making molds


Materials used to make molds. Metal molds are made from metal (cast iron, steel, alloy steel), stone mold made of stone, earth mold made of clay, ceramic mold made from silicate suspension, thin shell mold from sand - thermosetting plastic , sand-clay molds from casting sand mixed with clay, chemical curing molds from sand mixed with binders and additives according to a certain composition.

V. Application of die casting


Widely applied, however, mainly for details with complex shapes such as engine covers, etc. Besides, there are also many simple products that are applied casting technology depending on the technical requirements as well as the requirements. as the quantity of the product.

Technical requirements
In precision mold machining, there are many issues to pay attention to, but the following basic factors are common when machining all types of molds.

  • Precise requirements on the size and shape of the profile: the tolerance ratio for the error in the size and shape of the profile is very small. Deviations in detail and mold size affect the entire mold making process. On the other hand, the allowable tolerance ratio is calculated as 1/1000mm. Therefore, CNC and EDM/WC stitches have a lot of important notes.
  • Requirements on mold making materials: input materials for mold making, details for molds must be reasonable in terms of hardness, durability, wear resistance during use and must be easy to process. Choosing good materials, suitable for the working properties of the new mold, ensures the quality of the product created from the mold and the operating life of the mold.
  • Polishing requirements: in fact, the polishing stage is extremely simple, but on the contrary, this job requires extremely high technical factors. The gloss of the surface of the mold cavity and the core determines the gloss of the extruded plastic product. Just carelessly polishing the inside of the mold a little, the quality of the mold was unsatisfactory and reworked.
  • · Requirements on the relative position of the two halves of the mold when assembling: when the mold is put into plastic injection operation, the higher the tightness of the two mold faces, the more perfect the details are created. This is a technical element related to the CNC machining and assembly stages.
  • The design and machining requirements are highly uniform to ensure that the product is easily removed from the mold. Specifically, this depends greatly on the fit of the two halves of the mold and the gloss of the mold.
  • Requirements on hardness and positions of mold parts: during the working process (checking during the testing process of the product) whether there is any deformation or deviation from the original position when subjected to a large pressure. If not, it means the mold has reached the required item quality.

In the mold processing process, the technical factors are carried out in accordance with the correct process and the higher the accuracy is ensured, the other stages also become smooth and the final product from the mold will be perfect.