Casting defects are defects in castings that occur in the casting process. There are various defect phenomena such as cracks in castings, rough surfaces, and casting nests with porosities inside. Casting defects are classified into several types, each with different causes. In addition, since there are several pre-casting processes in the casting process, and even defects in the previous process lead to casting defects, there are complex cases where casting defects are not limited to one, and there are actually many complex causes.
Typical defects fall into six categories. This time, we will look at the causes and countermeasures of "casting cracking".
目次
What is Casting Cracking?
During solidification, cracks occur in the final coagulation part of the casting body and at the tip of the molten metal.
In the case of jewelry such as rings with a high copper content as a discount (such as pink gold or red gold), cracks are not seen immediately after casting, but cracks may occur when the ring is resized and the ring is tapped with a ring bar, or when the metal is heated again with a burner.
Why Cracks in Castings occur, and What are the countermeasures?
There are three main causes of casting cracking. We will introduce the causes and countermeasures, respectively.
１ Cracking due to Cold Shut
At the point where molten metal flowing from multiple directions meets during casting, a cold shut occurs at the collision confluence surface due to the lack of temperature when the molten metal tip merges from the front of each other. It is characterized by a rounded tip at one end and a round shape at the other.
Increase the casting temperature of the casting mold or the casting temperature of the bullion to increase the solidification time during casting.
２ Intergranular Cracking ( including brittleness)
Polycrystalline metals (alloys) contain gases and foreign dioxide (intermetallic compounds and oxides) between crystal grains. Due to the low adhesion between crystals, cracks occur in places where intrinsic gases and impurities tend to accumulate, such as places where shrinkage stress is concentrated during solidification or where molten metal merges.
Change the melting point by installing a vent without gas, changing the thickness, mounting location, and angle of the sprue.
In alloys containing copper, copper is oxidized and deposition during crystallization of solidification and extruded into the surface layer of the cast. When copper content such as pink gold or red gold, segregation occurs during solidification, and brittleness may increase due to alloys containing intermetallic compounds and their crystal structure, and brittle fracture may occur due to external forces. Care must be paid to the alloy of gold and copper because it creates a regular lattice during solidification. In addition, it is necessary to be careful in the same way because reverse segregation is often caused even in bronze.
In the case of these alloys, the mold is immediately cooled after casting to reduce crystal growth by reducing the time to solidification. This prevents intergranular cracking during secondary heating.
４ Shrinkage Cracking
Directional solidification is not secured, and the solidification part is broken so that it is torn by the stress when it solidifies and contracts due to a time difference.
What is directional solidification?
In general, hot objects (substances) expand at higher temperatures.
Metal is no exception (temperature shrinkage ants in some metals), especially in casting, because the metal is dissolved, so it expands greatly, but shrinks when the state of the metal (phase) is going from dissolution to solidification.
Because of this property, casting is called directional solidification, in which it is solidified sequentially from the tip of the casting to the source of the molten metal. In order to perform this directional solidification, the "thickness", "length", and "location" of the sprue are decided.
No directional solidification adapted directional solidification
Summary of Casting Cracking
We have introduced the causes and countermeasures, but we will summarize what to do when casting cracks occur.
Check List
- Increase the casting temperature of casting molds and bullion to increase the solidification time
- Install a vent for degassing
- Change the thickness, mounting location, and angle of the sprue
- For pink gold or red gold, cool the casting mold immediately after casting to reduce solidification time
There is not always one cause. Review all the items again to see how they work.
宝石などが石留めされる宝飾品やアクセサリーで、主役となる宝石の周囲や周辺に留められる宝石を指す。
カーボンルツボを使って高周波誘導加熱を行う場合、ルツボと加熱コイルの接触を避けるために高周波誘導の影響を受けない素材で保護をする。
焼成した鋳型をつかむ工具。
埋没材を手で攪拌する際に使用するゴム製の容器。
ラバーキャストで使用する器具。
熱加硫式のシリコーンゴムを加硫する際にシリコーンゴムが熱膨張で枠からはみ出る。
ブローチ針を装着する工具。
ワックス型修正用の工具。
鋳型の周囲にツバが付けられた吸引鋳造用のフラスコ。
チタンは、ルツボの素材と反応して金属組成が変化するため、チタン鋳造では一般的なルツボを使った高周波誘導加熱溶解は行わない。この代わりに銅製のルツボを使用し、アーク溶解を行う。
シリカ系埋没材などの無結合型埋没材の脱水を行うために使用する専用紙。
耐火性・耐熱性の強い手袋。
埋没材を手で攪拌する場合に使用する専用のヘラ。
ルツボの中の高融点金属を攪拌するための石英製の棒。
ラバーキャストで使用する器具。
ロストワックス鋳造のブロックモールド法（ソリッドモールド法）のゴム型にインジェクションワックスを射出する際に使う器具。
黒鉛製の棒。
金以外の割り金の種類や配合を変えて、イエローゴールド以外の発色をする金合金の総称。
黒鉛製の棒。
硫酸などに浸けた金属を陽極（+）として電気を流し、金属の電子を取り除くことで表面を酸化させること。これにより不動態を形成する。アルマイトはアルミの陽極酸化。
ロストワックス鋳造（ブロックモールド法）で、ワックス型の作製工程でゴム型に射出する専用のパラフィンを主成分とするワックス。通常60℃前後で溶解する。