5 Must-Have Features in a ODM copper forgings

Introduction:

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This article dives into the fascinating world of copper alloy jewelry and its production methods. It’s divided into four main sections:

1. Overview: Learn about copper’s history, properties, and its role in jewelry making. Copper alloys like brass, bronze, and cupronickel are highlighted for their versatility and beauty.

2. Pure Copper and High Copper Alloys: Discover the different types of pure copper, such as oxygen-free copper and micro-alloyed copper, and how they’re used in jewelry for their durability and unique colors.

3. Copper Alloys: Explore popular alloys like brass (copper-zinc), bronze (copper-tin), and cupronickel (copper-nickel), each offering distinct colors, corrosion resistance, and workability for crafting stunning pieces.

4. Production Techniques: Get insights into advanced methods like lost-wax casting, stamping, and electroforming, along with surface treatments such as gold plating and polishing to create high-quality, eye-catching jewelry.

Oxygen does not dissolve in copper and forms high-melting-point brittle compounds Cu2O with copper. When oxygen-containing copper condenses, oxygen precipitates as an eutectic (Cu+Cu2O), distributed along the grain boundaries. The eutectic temperature is very high (℃ )and does not affect thermal deformation performance, but it is hard and brittle, making cold deformation difficult and leading to “cold brittleness” in the metal. When oxygen-containing copper is annealed in a hydrogen or reducing atmosphere, “hydrogen embrittlement” occurs. The essence of “hydrogen embrittlement” is that during annealing, hydrogen or the reducing atmosphere easily penetrates the copper and reacts with the oxygen in CuO to form water vapor or CO2. Therefore, clear process specifications must be established and implemented during smelting.

Pure copper can be smelted using a reflection or core induction electric furnace. During smelting in a reflection furnace, dense ingots can be obtained through refining processes using iron or copper molds for casting, and semi-continuous or continuous casting can also be performed using a holding furnace. The following process flow can be referenced for the induction smelting process.

① First, preheat the crucible to a dark red color, then add a layer of dry charcoal or covering agent ( 63% borax + 37% crushed glass) with a thickness of about 30~50cm at the bottom of the crucible, followed by the corner scraps, waste blocks, and rods materials, and finally add pure copper.

② The added alloying elements can be preheated on the furnace platform, and adding cold materials to the molten metal is strictly prohibited. The charge should be stirred frequently during the entire melting process to prevent bridging.

③ After the alloy is completely melted due to heating, when the temperature reaches ~℃, add phosphorus copper deoxidizer, which accounts for 0.3%~0.4% of the weight of the molten alloy. Phosphorus reacts with cuprous oxide as follows:

5Cu2O + 2P = P2O5 + 10Cu
Cu2O + P2O5 = 2CuPO3

The generated gas P2O5 escapes from the alloy and copper phosphate can float on the surface, allowing for slag removal to achieve the purpose of deoxidation. In addition, continuous stirring is required during the deoxidation process.

④ Finally, the slag is removed, and the pouring temperature of the alloy liquid is generally ℃~℃.

Due to pure copper’s poor mechanical and casting properties, most copper materials used for popular jewelry are copper alloys. There are many categories of copper alloys, and currently, there are no specific technical standards for copper alloys used in jewelry, both domestically and internationally. Industrial copper alloy grades are typically used, and the application is quite chaotic, affecting product quality. Therefore, copper alloys for jewelry need further standardization. Copper alloys for jewelry are not entirely the same as industrial copper alloys and have unique requirements.

(1) The alloy must meet the usage requirements for jewelry. It should have certain mechanical properties, meet setting requirements, possess good corrosion resistance, have no tendency for stress corrosion cracking, and have certain colors, etc.

(2) The alloy should meet various process requirements, including: ① Good casting performance. The copper alloy should have good fluidity and minimal solidification shrinkage when producing ornaments using the lost-wax casting process. ② Welding performance. It should not easily produce cracks, oxidation, gas absorption, and color differences during welding. ③ Machinability. The hardness should be moderate; if it is too high, tool wear will be significant, and it will be difficult to achieve a high surface brightness if it is too low. ④ Surface treatment performance. Most copper ornaments require surface treatment, which should facilitate coloring and anti-corrosion treatment with good color quality.

Copper alloys used for jewelry mainly include several types, such as brass, cupronickel, and bronze.

Therefore, the structure of special brass usually corresponds to the structure of ordinary brass with an increased or decreased zinc content. The phase α and phase β in complex brass are multi-component complex solid solutions with a greater strengthening effect. In contrast, the phase α and phase β in ordinary brass are simple solid Cu-Zn solutions with a lower strengthening effect. Although the zinc equivalents are comparable, a multiple solid solution’s properties differ from those of a simple binary solid solution. Therefore, a small amount of multi-strengthening is a way to improve the performance of the alloy.

In special brass, a copper-based imitation gold alloy is known as “rare gold,” widely used in jewelry and crafts. It is well known that gold has a brilliant golden color, good chemical stability, does not change color when heated, and has excellent oxidation resistance, making it a long-standing choice for decorative art pieces. However, its high price makes low-cost alloys with similar properties widely used as substitutes. In recent years, researchers, both domestically and internationally, have been competing to develop copper-based imitation gold alloys to replace gold and significant progress has been made. The gold color of these materials can rival 16K~22K gold and have good corrosion resistance and workability.

In rare metal copper-based imitation gold alloys, zinc, aluminum, silicon, and rare earth elements are generally used as alloying elements, and the effects of each element on color and oxidation resistance are as follows.

① Zinc. Zn can change copper from red to yellow, the main element forming a golden-yellow luster. Zn can improve the discoloration resistance of alloys, and as the Zn content increases, the discoloration resistance improves.

② Aluminum. Al is another major element that contributes to the color of alloys. The aluminum content significantly affects the color of the alloy; as the aluminum content increases, the main wavelength of light reflected by the alloy decreases, and the hue changes from red to yellow. Further increasing the aluminum content noticeably weakens the yellow hue of the alloy, leading to a greater color difference between the alloy and pure gold. The alloy structure becomes more uniform when aluminum is added to brass alloys. It promotes the formation of the phase β, which helps reduce dezincification corrosion in brass and improves the anti-discoloration performance of gold-like alloys in artificial sweat. The reason is that when the aluminum content is sufficiently high, a dense and firmly attached mixed oxide protective film of copper and aluminum forms on the surface of the alloy, and this film has self-healing capabilities when damaged. The anti-discoloration performance is poor when the aluminum content is too low to form a dense protective film.

③ Silicon. Si can improve the color and discoloration resistance of alloys. When 0.05%~2.50% Si added to the alloy, compared to the same alloy without Si, the discoloration resistance time in artificial sweat increases 50%~100%; at the same heating temperature, the discoloration resistance time increases 50%. Adding Si can also improve the fluidity and wear resistance of the alloy.

④ Rare earth. Adding rare earth elements to brass alloys can enhance the brightness of the alloy, improve its color, and provide good wear resistance, hardness, and a color similar to gold that does not easily fade. In the jewelry industry, it is commonly referred to as “rare gold material.” Jewelry made from rare gold material can have a color resembling 18K or 20K gold, is not easily oxidized or faded, making it suitable for daily wear, and is inexpensive, becoming a material for producing higher-end imitation gold jewelry.

Table 2-10 shows several common imitation gold-copper alloys, which can be classified into the copper-based alloy Cu-Al system and the Cu-Zn system.

(1) Corrosion Resistance Performance

Brass has poor corrosion resistance in high temperature, high humidity, and salt mist atmospheres and can also experience “dezincification corrosion” in flowing hot seawater (zinc dissolves first, leaving a porous sponge-like pure copper on the surface of the workpiece). In humid atmospheres, especially those containing ammonia and SO2 , brass undergoes stress corrosion cracking. As newly polished brass ornaments, the surface will become dull or develop dark spots in certain areas even after being exposed to air for some time. Therefore, brass ornaments generally require surface coloring or electroplating treatment to improve their corrosion resistance.

(2) Casting Process Performance

The solidification range of brass is very small, so the fluidity of the liquid metal is good, the filling ability is excellent, and the tendency for shrinkage cavities is low. During melting, zinc generates a large vapor pressure, effectively removing gases from the copper liquid, making it difficult for pores to form in brass. The melting temperature is lower than tin bronze, and casting is relatively convenient, allowing for the easy casting of small jewelry pieces. It is also commonly used for the casting of copper crafts.

(3) Mechanical Properties

Due to the different zinc content in brass, the mechanical properties also vary. For α brass, as the zinc content increases, both σb and δ continuously rise. For (α+β) brass, the room temperature strength continuously improves when the zinc content increases to about 45%. If the zinc content is further increased, the strength sharply decreases due to the appearance of a more brittle phase γ (a solid solution based on compounds Cu5 Zn8 ) in the alloy structure. The room temperature plasticity of zinc content. On the other hand, (α+β) brass consistently decreases with the increase of the content of Zinc. Therefore, copper-zinc alloys with a zinc content exceeding 45% has no practical value.

(4) Machinability

Single-phase α brass (from H96 to H65) has good plasticity and can withstand cold and hot processing. However, single-phase α brass is prone to medium-temperature brittleness during hot processing such as forging, with the specific temperature range varying depending on the Zn content, generally between 200~700℃. Therefore, the temperature during hot processing should be above 700℃. The main reason for the medium-temperature brittleness zone in single-phase α brass is the presence of two ordered compounds Cu3 Zn and Cu9 Zn within the ordered phase α region of the alloy Cu-Zn system , which undergo ordered transformation during medium to low-temperature heating, making the alloy brittle; additionally, trace amounts of lead and bismuth harmful impurities form low-melting-point eutectic films distributed at the grain boundaries with copper, causing intergranular cracking during hot processing. Practice shows that adding trace amounts of cerium can effectively eliminate medium-temperature brittleness.

Two-phase brass (from H63 to H59) has, in addition to the ductile phase α in its alloy structure, a solid solution β based on the electronic compound CuZn. The phase has high ductility at high temperatures, while the phase β’ (ordered solid solution) is hard and brittle at low temperatures. Therefore, (α+β)brass should be forged in a hot state. Β Brass with a zinc content greater than 46%~50% is hard and brittle due to its properties and cannot be processed by pressure.

For relatively delicate jewelry, brass is generally processed using cold working. Brass materials such as wire, sheet, and plate materials can be used to obtain the final product through cold processing. Of course, during the processing, intermediate annealing is used to restore the plasticity of the brass and prevent cracking due to work hardening. Figure 2-7 shows a lobster clasp made of brass, and Figure 2-8 shows a bracelet made of brass. Brass plates can also be used for engraving, employing various manual techniques such as pushing, drilling, picking, twisting, and pulling to carve images on the surface of the copper plate. The engraved images are then electroplated with a 24K gold protective layer, resulting in the “gold sculpture painting.”

(5) Welding Performance

The welding performance of brass is good. For larger crafts, gas welding is usually used; for delicate jewelry, torch welding is generally employed.

(6) Polishing Performance

The cutting performance of brass is good, and it can withstand operations such as correction, polishing, and finishing. The jewelry can be polished to a very bright finish using conventional jewelry finishing methods.

The invention of cupronickel is an outstanding achievement in ancient China’s metallurgy technology. In ancient China, cupronickel was referred to as “Gan.” The “Old Book of Tang • Treatise on Clothing” states: “Only the oxen pulling the carriages of first-rank officials can be adorned with cupronickel.” This means that during the Tang Dynasty, it was stipulated that only the oxen of first-rank court officials could be decorated with cupronickel, indicating that cupronickel was quite valuable during that time. The people of Yunnan invented and produced cupronickel, making them among the earliest in China and the world, which is recognized by the academic community domestically and internationally. The cupronickel produced in ancient Yunnan was also the most famous, known as “Yun Cupronickel.”

The cupronickel artifacts manufactured in ancient China were sold throughout the country and exported abroad. According to research, as early as the Qin and Han dynasties, cupronickel coins were cast in the Daxia Kingdom, located west of Xinjiang, containing nickel up to 20%. Based on their shape, composition, and the historical conditions of the time, it is very likely that they were transported from China. During the Tang and Song dynasties, Chinese nickel cupronickel was already being exported to the Arab region, where the Persians referred to cupronickel as “Chinese stone.” After the 16th century, Chinese cupronickel was sold worldwide and received widespread acclaim. It was exported through Guangzhou and sold in Europe by the British East India Company. The English term “Paktong” or “Petong” is a transliteration of the Cantonese “cupronickel,” meaning cupronickel from China, specifically referring to the copper-nickel alloy produced in Yunnan.

In the 17th to 18th centuries, nickel cupronickel was widely introduced to Europe and was regarded as a precious item. It was called “Chinese silver” or “Chinese cupronickel,” and it significantly impacted the modern chemical industry in the West. After the 16th century, some European chemists and metallurgists began to study and imitate Chinese cupronickel.

In , the German Heineger brothers successfully replicated Yunnan cupronickel. Soon after, the West began large-scale industrial production and renamed this alloy “German silver” or “nickel silver,” while the genuine Yunnan cupronickel became obscure. After Western countries successfully replicated Yunnan cupronickel, the export quantity of Chinese cupronickel significantly decreased. By the late 19th century, German silver had replaced Chinese cupronickel in the international market, leading to the decline of China’s cupronickel mining and metallurgy.

Nickel cupronickel has many excellent properties as a material for jewelry, but it also has some drawbacks. Since the main additive element, nickel is a scarce material, the price of cupronickel is relatively high. Additionally, due to the widespread concern about the harmful effects of nickel in various countries, products made for contact with human skin, such as zippers, eyeglass frames, coins, cutlery, and jewelry, may cause skin allergic reactions. Therefore, nickel-cupronickel materials have faced challenges in recent years, making the development of new nickel-free cupronickel alloys particularly important.

So far, most of the research on nickel-free cupronickel has been focuses on the Cu-Mn-Zn alloy, and the main roles of each alloying element are as follows.

(1) Manganese

Manganese is the main additive element in nickel-free cupronickel alloys. It can reduce the yellow and red components in the color of copper’s surface, acting as a bleaching or fading agent, changing the color of the alloy from colored to colorless. Manganese can improve the mechanical properties of the alloy by strengthening the solid solution. Partially replacing zinc with manganese can improve aging crack conditions. Manganese can suppress the evaporation of zinc during smelting and reduce material costs. However, if the manganese content exceeds 15%, the alloy will exhibit a α+β multiphase structure, leading to poorer processing performance. Manganese is detrimental to the casting performance of the alloy; during smelting, manganese easily oxidizes to form high-melting-point manganese oxide inclusions, which have a high density and are difficult to float out of the molten metal, making it easy for castings to have inclusion defects. Additionally, manganese increases the shrinkage rate of the alloy, reducing its fluidity, and high manganese content can worsen the processing performance of the alloy. Therefore, from the perspective of process performance, the manganese content should not be too high.

(2) Zinc

Zinc can improve the strength and hardness of alloys through solid solution strengthening, lower the melting point of alloys, enhance forming performance, and reduce the cost of alloys. When the zinc content is too low, the strengthening effect is poor; increasing the zinc content can improve the strengthening effect. However, zinc significantly reduces the corrosion resistance of copper, especially when the zinc exceeds 22%, causing the alloy to transform into a α+βmultiphase structure, which deteriorates processing performance and is prone to aging crack issues induced by residual stress. When the zinc content is less than about 30%, increasing the zinc content reduces the red component in the color of the Cu-Mn-Zn alloy while increasing the yellow component and brightness value. Zinc also has an important impact on the color stability of alloys; as the zinc content increases, the alloy’s resistance to discoloration in artificial sweat decreases.

(3) Aluminum

Aluminum is one of the most important coloring elements in imitation gold alloys. As the aluminum content increases, the Cu-Zn-Al ternary alloy’s brightness value and yellow component increase while the red component decreases. The zinc equivalent coefficient of aluminum is very high; every 1% aluminum is equivalent to 6% of zinc, so the α-phase region is significantly reduced after adding aluminum. Aluminum can form a dense oxide film on the surface of the alloy, which can improve the aging cracks and dezincification corrosion issues of the alloy, and it also produces solid solution strengthening, which is beneficial for improving the mechanical properties of the alloy. When the aluminum content is too low, the strengthening effect is insufficient and not enough to resist aging cracks. However, if its content exceeds 4%, it becomes difficult to purify the molten metal during alloy smelting, and a complex α+β phase structure appears, deteriorating cold working performance.

(4) Tin

The zinc equivalent coefficient of tin is 2, so adding a small amount of tin has little effect on the structure, and the alloy remains single-phase. Tin has a certain solid solution-strengthening effect. Still, if its content exceeds a certain level, it is prone to form low melting point phases at the grain boundaries, which is detrimental to mechanical properties. A small amount of tin also has little effect on the color of the Cu-Mn-Zn alloy; its main role is to form a SO2 protective film on the surface of the alloy, which can greatly improve the alloy’s resistance to discoloration. Tin can increase the fluidity of the alloy and improve casting performance, but it increases the cost of the alloy.

(5) Rare Earth

Trace amounts of the rare earth element cerium can refine grain size, improve the tensile strength and elongation of the alloy, and enhance the cold working performance of the alloy.

By comprehensively utilizing these elements, researchers at home and abroad have developed a series of multi-element nickel-free white Cu-Mn-Zn alloys, such as Cu- 12Mn -8Zn – 1Al – 0.04%Ce, Cu – 15Mn – 15Zn – 1Al, Cu – 20Mn – 20Zn – 0.3Al – 0.2Sn – 0.05Mg, etc.

(2) The Role of Alloying Elements in Tin Bronze

① Zinc. Adding zinc to tin bronze can reduce the crystallization temperature range of tin bronze, improve the fluidity of the alloy, and decrease the tendency to produce shrinkage cavities. Additionally, zinc has a relatively high vapor pressure during melting, and the zinc vapor formed can prevent the oxidation of copper and tin elements, purifying the alloy and reducing the tendency to form pores. The effect of zinc on the structure and properties of tin bronze is similar to that of tin, with the addition of 2% zinc being equivalent to the role of 1% tin. However, the price of zinc is much lower than that of tin, so zinc can be used to replace tin to reduce costs. If the zinc content exceeds 5%, it can make the patterns unclear, increase susceptibility to corrosion, and make it difficult to generate an elegant green outer layer.

② Lead. Lead has a very low hardness and is distributed in a particulate form in tin bronze, improving the wear resistance of the alloy and facilitating the processing of bronze. At the same time, lead’s low melting point enhances the fluidity of tin bronze. During solidification, lead accumulates in the gaps between dendrites, reducing shrinkage and preventing leakage, with the best anti-leakage effect generally achieved at a lead content of around 5%. Lead has a relatively high specific gravity in bronze, and excessive lead can cause gravitational segregation, so it is important to stir the lead-containing tin bronze before pouring and to use water cooling or metal molds to accelerate cooling and prevent segregation.

③ Nickel. Nickel is infinitely soluble in the solid solution of bronze, promoting the development of α dendrites; thus, adding a small amount of nickel can reduce the segregation of tin and lead. Adding 1%~2% nickel can refine the grains, improve mechanical properties, corrosion resistance, and thermal stability, and enhance the casting performance of bronze. A larger amount of nickel will make the bronze appear whiter.

④ Iron. The main function of iron is similar to that of nickel; it can refine grains, increase strength, and improve coloring performance. However, the content must be controlled below 5%; otherwise, it will make bronze brittle and reduce corrosion resistance.

⑤ Aluminum. In tin bronze, aluminum is a harmful impurity that makes coloring difficult. As long as 0.5% aluminum is present, the surface changes from dark red to golden yellow and then to silver white. However, aluminum can improve strength, corrosion resistance, and casting performance in lead-free bronze.

⑥ Phosphorus. 0.03%~0.06% phosphorus must be added to tin bronze to deoxidize it and improve casting performance; excessive amounts can easily produce a brittle phase Cu3 P and reduce coloring effects.

⑦ Silicon. Adding silicon to bronze will deteriorate its mechanical and casting properties but can increase corrosion resistance. Silicon gives the surface a dark red to brown color, sometimes appearing purple, due to a very dense SiO2 film covering the surface, making coloring difficult.
Tin bronze has a beautiful appearance and excellent processing performance. It has been widely used in casting crafts from ancient times. Table 2-13 lists some commonly used tin bronze materials for artistic castings.

(3) Fine Finishing

Fine finishing is based on rough finishing and further processing to make the entire wax sample more refined and aesthetically pleasing. First, a compass is used to take the dimensions of each part on the wax sample template and draw some auxiliary lines. Based on these auxiliary lines, remove the excess wax with a finishing bur, then use a steel bur to smooth out the rough marks left from the previous process. Use large and small spatulas to level off any corners or protruding parts on the wax sample, and refine it with a scalpel. Finally, large and small files smooth the overall wax sample.

(4) Removing Bottom Weight

The purpose of removing bottom weight is to reduce the weight of the workpiece. Install the ball bur and wheel bur on the electric hanging flexible shaft grinder, and use the ball bur to remove excess wax material at the bottom of the pattern head or the inner circle of the ring shank (Figure 2-14). Generally, the reserved bottom thickness for pave setting is 1.1 mm; for light gold and flush setting, it is 0.7 mm; for bezel setting and channel setting, it is 1.6 mm. Then, use a dental bur, drill bur, surgical knife, etc., to trim the wax sample’s bottom frame. During the bottom weight removing, it is important to frequently measure the dimensions at the light gold position, pave setting position, channel setting position, etc., using internal calipers to prevent deviations.

(5) Make the Stone Setting Position

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According to the size of the stone and the setting method, open the stone position, use appropriate diamond drills for channel setting and bezel setting, drill holes at the designated stone position, and then use steel burs, small files, scalpels, etc. for adjustments; steel burs can also be used to directly open the stone position.

(6) Repair

Repair involves adjusting certain details to ensure that the repaired workpiece better meets the order requirements. During the repair, attention should be paid to adjusting and coordinating the relationship between wax weight and size according to the order’s requirements for product weight and dimensions.

(7) Polishing

Wipe the surface of the wax plate with nylon cloth to make it smooth and fine.

(8) Replicating Silver Model

After the hand-carved wax model is completed, it must be cast into a silver model to replicate the rubber mold. The surface of the cast silver model is then refined (Figure 2-15) to ensure a good surface finish, avoiding any defects on the silver model from being replicated onto the casting. The silver model’s shape, dimensions, and weight are checked to meet the order requirements. Additionally, some processes that the hand-carved wax model cannot complete are supplemented, such as planting prongs, creating buckles and buttons, dangling earrings, etc.

(9) Welding Sprues

The sprue is designed to leave a channel for the flow of molten metal during the casting process. In jewelry casting, because there is no set risers on the workpiece to make up for the contraction, the sprue has become a channel for the liquid metal filling but also needs to bear the liquid metal solidification contraction within the makeup for the contraction of the task, the correct setting of the sprue is to ensure that the casting quality of the basic conditions of the casting of many of the defects of the casting of the molten mold directly or indirectly by the setting of the sprue caused by the irrationality of the defects, such as insufficient filling, loosening, porosity, and other common defects.

Due to the fine nature of jewelry, when making wax molds, it is necessary to use the pressure of a wax injection machine to inject the wax liquid into the cavity of the rubber model. Many wax injection machines exist, including ordinary air-pressure wax injection machines, vacuum wax injection machines, and digital automatic wax injection machines. Place the wax material in the wax tank. The wax material must be kept clean, and the wax tank and nozzle’s temperature must be adjusted to the required temperature.

Before applying the wax, open the silicone mold and check its integrity and cleanliness. Spray release agent in the mold’s smaller, more complex areas (or sprinkle a small amount of talcum powder) to facilitate the removal of the wax mold.

Start the vacuum pump during wax injection and check if the wax temperature is between0~75℃. Adjust the injection time and air pressure according to the complexity of the wax parts in the mold, then evenly clamp the mold to perform the wax injection operation (Figure 2-19). The wax piece can be removed from the mold after cooling for about 1 minute. Care should be taken when removing the mold to avoid breaking or deforming the wax piece.

After removing the wax mold, carefully inspect it. If there are defects such as flash, clamping marks, unclear flower heads, or overlapping flower heads, they need to be trimmed with a surgical blade; for sand holes and broken claws, they can be repaired with a wax welder; small holes that are blocked can be penetrated with a welding needle; for deformation of the wax mold, it can be corrected in hot water at 40~50℃. Finally, cotton soaked in alcohol removes the wax shavings from the mold.

Due to the static electricity generated on the wax model tree, which easily attracts dust, it can be immersed in a surfactant or diluted detergent before grouting, washed with distilled water, and dried. During the powder mixing and grouting process, attention should be paid to appropriately controlling the setting time of the gypsum slurry. If it sets too quickly, the gas cannot be expelled in time; if it sets too slowly, the powder can easily settle in the slurry, resulting in a local change in the solid-liquid ratio, causing different roughness on the top and bottom of the jewelry.

After the casting mold is completed and the vacuum operation is performed, it should be allowed to stand for 1.5~2 hours to fully solidify and harden the gypsum mold. Then, remove the rubber base, the wrapping material around the steel flask, and the splattered slurry, and make marks on the side and surface of the casting mold.

(2) Wax Removal from the Mold

After the slurry solidifies, there are two different methods to remove the wax: steam dewaxing or drying dewaxing in a burnout furnace.

Steam dewaxing can remove wax more effectively and benefit the environment. Note that the boiling of water should not be too vigorous, and the time for steam dewaxing should be controlled; otherwise, splashed water may enter the mold and damage the surface of the mold. Additionally, in wax-setting casting, using steam dewaxing may dilute the boric acid protectant in the casting powder, leading to issues such as cloudy or discolored gemstones.

Burnout dewaxing is a method that directly uses a burnout urnace to heat the mold, allowing the wax material to melt and flow out of the mold. Due to the low boiling point of the wax material, when using this method, if the wax liquid boils violently, it can damage the surface of the mold, or if the wax liquid is not discharged smoothly, it can seep into the surface layer of the mold, both of which will deteriorate the surface quality of the casting. Therefore, it is important to control the heating temperature and speed during the dewaxing stage and to set up a corresponding insulation platform.

(3) Molding Burnout

The purpose of burnout is to eliminate the moisture from the gypsum mold and the residual wax, achieving the desired high-temperature strength and mold’s air permeability and meet the mold temperature requirements during pouring. The burnout system and equipment largely influence the final performance of the gypsum mold.

The gypsum burnout furnaces used in the jewelry industry generally adopt resistance furnaces, and some use oil-fired furnaces. Regardless of the type of furnace, the temperature distribution inside the furnace must be as uniform as possible. The resistance burnout furnace is commonly used, which generally adopts three-sided heating, and some use four-sided heating. They usually come with temperature control devices and can achieve segmented temperature control. Still, the temperature distribution inside the furnace is not uniform enough, and it is also difficult to adjust the atmosphere inside the furnace during burnout. In recent years, some advanced burnout technologies have continuously emerged, focusing on achieving uniform temperature distribution inside the furnace, eliminating wax residues, and automating control furnace. For example, a type of furnace uses a rotating bed method, with heating on all four sides, providing uniform and stable heat. The gypsum mold can be evenly heated, making it particularly suitable for the requirements of wax-set casting processes.

When roasting the mold, a suitable roasting system must be established, and a heat preservation platform should be set up during several sensitive stages. The mold is burn out at the highest temperature for 3~4 hours. After all residual carbon is burned off, the mold temperature needs to be lowered to a certain temperature to prevent defects such as shrinkage and porosity in the casting due to excessive mold temperature; however, since jewelry pieces are generally quite delicate and difficult to form, cold mold pouring is not used to ensure complete filling. Otherwise, the surface of the casting is prone to roughness and unclear contours. Generally, depending on the structure of the workpiece and the amount of casting, the mold temperature during pouring is between 520~650℃.

(3) Pouring

Because jewelry pieces are relatively delicate, solidification occurs quickly during the casting process, resulting in a loss of fluidity. Therefore, conventional gravity casting struggles to ensure proper shaping, and it is necessary to introduce some external force to promote the rapid filling of the mold cavity with molten metal, achieving castings with complete shapes and clear contours. Based on the method of using external force, it can be divided into two main categories: centrifugal casting and static casting; based on the degree of automation in the casting process, it can be divided into manual casting machines and pressure casting machines.

① Manual pouring. Manual pouring is generally carried out with torch or induction melting. After the metal liquid is smelted and refined, the temperature is adjusted to the pouring temperature range. Then, the mold is removed from the burnout furnace to prepare for pouring. Depending on the type of equipment used, manual pouring mainly includes centrifugal casting and vacuum suction casting.

Figure 2-27 is a simple mechanical transmission centrifugal machine in some small jewelry processing factories. It does not come with an induction heating device, uses gas-oxygen to melt metal, or uses an induction furnace to smelt metal and pour it into a crucible. The gypsum mold is placed flat in the rotating arm’s mold base, and the rotating arm is started. Under the action of centrifugal force, the molten metal enters the mold cavity, completing the pouring process. Many factors affect quality during the operation, making it suitable for pouring small jewelry items, such as links, ear studs, etc.

Electrochemical degreasing involves hanging the jewelry on the cathode or anode of an alkaline electrolyte (Figure 2-38). During electrolysis, the bubbles escaping from the surface of the jewelry have a strong tearing effect on the grease film on the surface, and the agitation caused by the rising bubbles continuously carries away the oil, further enhancing the degreasing effect. The speed of electrochemical degreasing exceeds that of chemical degreasing, resulting in good effectiveness.

Jewelry must be cleaned and weakly etched before entering the electroplating tank. The purpose of cleaning is to remove the adhering liquid on the surface of the jewelry, promote the deposition of metal ions, and avoid contaminating the electroplating solution. Copper jewelry is generally cleaned before plating using a multi-stage countercurrent cleaning production line, as shown in Figure 2-39. The main purpose of etching is to neutralize any alkaline solution that may remain on the surface of the workpiece, dissolve the oxide film on the surface of the workpiece, and activate the surface to ensure a strong bond between the plating layer and the base metal. The concentration of the etching solution is generally quite dilute, 1%~5%, and will not damage the surface finish of the material, with the time usually only a few seconds to 1 minute.

The typical electroforming process for copper jewelry includes sculpting the mold, replicating the mold, wax mold injecting, wax mold finishing, applying silver plate, electroforming, finishing, wax removing , and polishing.

(1) Wax Mold Making

Making wax molds involves designing wax material, sculpting the mold, replicating the mold, wax molds injecting, and wax molds finishing. When making large workpieces, clay carving templates are also used, which are copied into silicone and wax molds.

① Sculpting the wax mold. Carved into a wax template using high relief, low relief, openwork, and line carving techniques. First, initial rough carving is performed, which is done according to the design intent and process conditions, using carving tools to shape the wax material into a certain form to determine its basic shape; after the initial rough process, fine carving and detailed finishing are carried out to address various shortcomings from the previous steps and to make the surface of the wax mold smooth and polished.

② Replicating molds. Copy the qualified carved wax model into a rubber mold to achieve the purpose of mass production.
For small copper jewelry, first, fix the wax model on a glass surface, surround it with sandpaper, leaving a certain distance between the model and the sandpaper tube, then vacuum the uniformly mixed silicone and inject it into the sandpaper tube (Figure 2-41), then vacuumed, and glue is injected according to the actual situation. After filling it with silicone, please place it in the vacuum machine to vacuum, and put the last vacuumed sandpaper tube in a suitable and stable position to let it dry naturally.

For large copper ornaments, the method of first applying glue on the template and then using plaster for replication is generally adopted. Fix the model on a disc, and use a brush to apply the prepared silicone on the template. After the first layer is qualified, repeat the brushing two more times until the thickness reaches 3~5 mm. Use the oil clay to fill the larger concave hole. Then, use the right amount of water to mix a good plaster paste, with a flat shovel and hand (wearing rubber gloves) scraping and wiping plaster mud on the mold plate, a thickness of about 20 ~ 30mm. Scraping and wiping, depending on the complexity of the shape of the model, is broken down into several parts of the production, divided into two pieces, complex divided into 3 ~ 4 or several pieces, to facilitate the removal of the rubber mold and the model. After the work, the entire mold is placed to dry naturally, with a rubber hammer to knock the decomposed plaster layer.

③ Mold cutting. Use a scalpel to cut the silicone layer at the appropriate position and remove the model. When cutting the mold, choose areas that are easy to repair so that the poured wax mold is easy to finish (scrape) on the plate. Avoid cutting through the facial features of sculptures of people or animals. After cutting the mold, check the quality of the silicone mold to see if there are any bubbles and whether the silicone molds fit together tightly. For large decorative items, close the cut silicone mold, use plaster to decompose the mold to hold the silicone mold together, fix it, and then use glue lines and glue tape to secure it tightly.

④ Wax mold injecting. Use the gas from an air compressor to blow away impurities inside the rubber mold, then place the mold in an electric oven to preheat for about 5 minutes, allowing the mold’s temperature to reach 60~65℃ and remove moisture. Remove the mold from the oven, close it, and ensure the joints are completely sealed, securing them with rubber bands. Use an iron spoon to scoop the wax from the electric heating tank and pour it into the rubber mold (Figure 2-42), then place it in a vacuum-vibrating machine to vacuum for 1~2 minutes, remove the wax supplement, and vacuum again for 1~2 minutes. After completing the injecting, supplementing, and vacuuming, place the rubber mold on the workbench to cool naturally. Once the pouring opening has solidified, stand the mold in a plastic basin filled with cold water to accelerate the wax solidification. The solidification time depends on the volume of wax, generally over 30 minutes and sometimes up to 1 day. Once the wax model inside the rubber mold has completely solidified, loosen the rubber bands and tape and open the rubber mold to remove the wax figure.

⑤ Wax mold finishing. Use a wax scraper or surgical knife to remove the flash, wax marks, sprues, etc., from the wax mold (Figure 2-43), and make the entire surface of the wax mold beautiful and smooth. Use an electric soldering iron to dot wax to fill small holes and other defects on the wax mold or connect several wax parts. Wipe the surface of the wax mold with gasoline to make it clean and smooth.

③ Open reserved holes. To remove wax and silver plate and ensure the purity of the metal in the ornament, it is necessary to leave reserved holes for post-processing. This avoids increasing metal loss and the likelihood of product scrap due to opening holes in the finished product. Opening reserved holes should follow two points: first, it should not affect aesthetics and should be placed in a relatively concealed position; second, the quantity and size should be appropriate. Therefore, it must be coordinated with various processes such as wax carving, engraving characters, inserting rods, and post-processing, and cannot be done independently.

④ Weighing. Place the wax mold with the iron hanging rod on the electronic scale to understand and control the weight of the casting.

⑤ Place the mold into tank for electroforming. The wax mold must be cleaned with pure water before placed into tank to remove dust from the surface; otherwise, the casting may develop perforations due to dust. Areas with more recesses in the wax mold should face the metal mesh of the casting cylinder, which ensures a faster-pouring speed in the recesses and a more uniform casting layer. Otherwise, the pouring speed in the recesses will be slow, resulting in a thin casting layer after demolding, which may lead to perforations during grinding and wax removal.

At the expected start time, take out the casting for weighing. If the weight meets the required range, it can be started and cleaned, and then the hanging rod from the casting can be registered and handed over to the next process operation.

(3) Surface Treatment

Copper electroformed parts generally require gold plating on the surface. For plush sand products, a thin layer of gold is usually plated immediately after copper electroforming. The gold plating process occurs after surface treatment of the copper electroformed parts for water and products. Typical surface treatment tasks include finishing, drilling, wax removal, polishing, and gold plating.

① Finishing. Perform preliminary treatment on the cast surface to remove burrs.

② Wax removal. Remove the wax inside the casting to make the casting a complete metal body, a hollow, multi-layered jewelry craft casting.

First, place the jewelry in the 150~300 ℃ electric oven for 20~30 minutes to burn out the wax from the jewelry. Remove it while hot and place it in the ultrasonic wax removal machine to eliminate any remaining wax. After removing the wax, remove the jewelry and pour the water inside, then place the casting in the ultrasonic cleaning machine for cleaning. Rinse the surface of the casting with tap water, use an air gun to blow away the water droplets inside and outside the casting, and let it air dry naturally on the workbench.

③ Baking in the oven. After cleaning the surface of the accessory, please place it in the oven of 750℃ for about 10~20 minutes to bake at degrees to remove moisture and impurities in the sand, preventing the appearance of red spots, eliminating internal stress, and changing the brittleness of the accessory.

④ Polishing. Polishing certain areas of the castings to make the product appear more striking, dazzling, and noble.

⑤ Gold plating. The main purpose is to enhance the surface protection of the workpiece and prevent discoloration of the workpiece surface. The workpiece is placed in a chemical degreaser and cleaned in an electro-degreasing tank to remove surface oil. After cleaning the water, the gold plating operation is performed.

Copper is a versatile and widely-used metal in various industries, thanks to its excellent electrical and thermal conductivity, corrosion resistance, and ductility. Machining copper can be challenging due to its softness and tendency to produce long, stringy chips. To achieve a high-quality finish and prolong tool life, it's critical to select the appropriate Surface Feet per Minute (SFM) for your machining operations. In this blog post, we will explore the factors that influence SFM for machining copper, discuss different machining techniques, and provide practical tips to optimize your processes.

Understanding Surface Feet per Minute (SFM)

Surface Feet per Minute (SFM) is a measure of how fast the cutting edge of a tool moves across the workpiece's surface during machining. It plays a crucial role in determining the cutting speed, which directly impacts the quality of the machined surface, tool wear, and overall efficiency of the machining process. Selecting the appropriate SFM for copper machining is essential to avoid excessive tool wear, work hardening, and poor surface finish.

Factors Influencing SFM for Copper Machining

Several factors influence the optimal SFM for machining copper, including:

1. Tool Material:The choice of cutting tool material significantly affects the SFM. For example, high-speed steel (HSS) tools typically operate at lower SFMs compared to carbide or diamond-coated tools. Carbide tools are generally preferred for machining copper due to their increased wear resistance and ability to withstand higher cutting speeds.

2. Copper Alloy:The specific copper alloy being machined also impacts the SFM. Pure copper is softer and more ductile than most copper alloys, which can lead to a higher SFM. However, harder and more abrasive alloys, such as beryllium copper or tellurium copper, may require lower SFMs to minimize tool wear and achieve a good surface finish.

3. Machining Operation:Different machining operations, such as turning, milling, drilling, or tapping, have varying SFM recommendations. The complexity of the operation, tool geometry, and required surface finish all contribute to determining the optimal SFM for each specific process.

4. Coolant and Lubrication:The use of appropriate coolants and lubricants can significantly impact the SFM during copper machining. Proper lubrication helps reduce friction, heat generation, and tool wear, allowing for higher SFMs and improved surface finishes.

Machining Techniques for Copper

There are several machining techniques commonly used for copper, each with its own SFM considerations:

1. Turning:Turning operations, such as facing, grooving, and parting, require careful selection of SFM to achieve a smooth surface finish and minimize tool wear. For turning copper with carbide tools, a general SFM range of 600 to 1,000 is recommended, while HSS tools should operate at lower speeds, typically between 200 and 400 SFM.

2. Milling:Milling copper involves removing material using rotating cutting tools, such as end mills or face mills. The recommended SFM range for milling copper with carbide tools is between 500 and 800, while HSS tools should operate at speeds between 150 and 300 SFM. Using climb milling techniques can help reduce the formation of long, stringy chips and improve surface finish.

3. Drilling:Drilling copper requires careful consideration of SFM, feed rate, and tool geometry to prevent work hardening and excessive tool wear. For drilling with carbide tools, a general SFM range of 400 to 800 is recommended, while HSS drills should operate at speeds between 100 and 250 SFM.

4. Tapping:Tapping copper involves creating internal threads using a rotating tap. The SFM for tapping copper is generally lower than other machining operations to minimize the risk of tap breakage and ensure accurate thread formation. For carbide taps, a recommended SFM range of 50 to 150 is suggested, while HSS taps should operate at speeds between 30 and 100 SFM.

Practical Tips for Optimizing Copper Machining

To achieve the best results when machining copper, consider the following practical tips:

Select the appropriate cutting tool material, such as carbide or diamond-coated tools, for increased wear resistance and higher SFMs.

Use proper coolant and lubrication to reduce friction, heat generation, and tool wear.

Opt for climb milling techniques to minimize chip formation and improve surface finish.

Adjust your SFM based on the specific copper alloy, machining operation, and desired surface finish.

Monitor tool wear and adjust SFM accordingly to prolong tool life and maintain a high-quality finish.

In conclusion, selecting the appropriate SFM for machining copper is crucial for achieving a high-quality finish, minimizing tool wear, and optimizing overall machining efficiency. By understanding the factors that influence SFM and implementing the recommended techniques and tips, you can ensure successful copper machining operations and improve your production processes.

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