What are the techniques of jewelry making?
The history of human adornment with jewelry dates back to ancient times, reflecting the innate human desire for beauty and the pursuit of aesthetic expression. Various civilizations and cultures have employed different forms of jewelry for decorative purposes. For instance, the ancient Inca people adorned young individuals by piercing their ears and inserting gold decorative panels, while other ethnic groups incorporated jewelry in the form of wooden sticks, metal strips, or animal bones inserted into pierced noses or lips.
Throughout history, jewelry has held different symbolic meanings. In the medieval period, it symbolized power and authority. During the Renaissance, jewelry represented spiritual and intellectual elevation. In the 18th and 19th centuries, jewelry was closely associated with wealth and elegance. In the 20th century and beyond, jewelry has become a means for individuals, particularly women, to express their individuality and enhance their personal beauty.
The advancement of jewelry processing technology has been instrumental in the evolution of the industry. Traditional handicraft techniques have transitioned into large-scale modern production, resulting in the accumulation of extensive expertise in craftsmanship, design, quality control, and management. As a result, contemporary jewelry products exhibit remarkable levels of craftsmanship and technological sophistication.
In present-day jewelry processing enterprises, wax loss casting technology is predominantly employed. The specific process involves steps such as creating rubber molds, injecting wax into the molds, repairing and refining the wax molds, assembling wax trees, encasing them in gypsum cylinders, vacuuming the gypsum, allowing for natural solidification, baking the gypsum, melting the chosen metal, casting the molten metal, removing the gypsum, and performing post-casting processes including blasting, washing, acid treatment, cleaning, shearing, and final mold execution.
Plate Making:
- The production of wax plates is a crucial step in the lost wax casting process. It involves creating smooth and flawless silver plates using rubber molds pressed with silver. There are three main techniques used: hand-carved wax plates, computer-carved wax plates, and handcrafted silver plates.
Hand-carved wax plates use paraffin to carve shapes based on design drawings. Computer wax carving combines 3D software and a wax spraying machine, while handcrafted silver plates are made directly by the plate maker. Each technique has its advantages and is used based on specific requirements.
When hand carving wax plates, attention should be paid to filing down rough surfaces and avoiding deformations. Removing excess wax from the bottom and ensuring precise nail positions are important.
For silver plates, careful removal of waterlines and achieving smooth finishes using files are essential. Rings should be checked for size, and any necessary repairs should be made. After production, silver plates are sent for stone inlay adjustments if needed.
Jewelry casting：
Rubber Mold Compression
Precautions and Procedure for Rubber Mold Compression:
- Ensure cleanliness of the mold frame and raw film; avoid direct contact between hands and the raw film surface.
- Apply a layer of silver plating on copper plates to prevent adhesion between the original and rubber.
- Determine the appropriate vulcanization temperature and time, which depend on the thickness, length, and width of the rubber mold as well as the complexity of the original model. Typically, the mold temperature is set around 150 degrees Celsius. For a rubber mold thickness of 3 layers (approximately 10mm), the vulcanization time is generally 20-25 minutes. For 4 layers (approximately 13mm), the vulcanization time can be extended to 30-35 minutes. The vulcanization temperature should be adjusted for complex or small original models, lowering it and extending the vulcanization time accordingly.
- Ensure a gap-free fit between the original model and the raw film. Use methods such as plugging, wrapping, and patching to fill gaps, recesses, and stone inlay positions on the first plate. Small rubber particles should be used for filling, and firm pressure should be applied with sharp tools (e.g., tweezers).
- Preheat the mold before compression. After the vulcanization time has elapsed, promptly remove the rubber mold. The compressed rubber mold should be smooth, without deformation, and with no skewed water lines. Allow it to cool naturally until it is no longer hot to the touch. Then, carefully open the hot rubber mold using a sharp surgical knife.
Opening the Rubber Mold and Wax Injection Molds
Opening the Rubber Mold:
The process of opening the rubber mold is crucial as it directly impacts the quality of both the wax mold and the final metal piece. The tools required for opening the mold are relatively simple and include surgical knives, tweezers, scissors, and pointed nose pliers. Rubber molds are typically opened using a four-legged positioning method. This means that the rubber mold is opened by cutting either straight lines or curves between the four legs. Careful inspection of the opened rubber mold is necessary to ensure there are no defects such as visible scratches, missing corners, or adhesions that may affect the wax mold. Any defective areas should be repaired by cutting open uncut sections or using a wax welder to fix broken or missing parts.
The general steps for opening the mold are as follows:
- Once the pressed rubber mold has cooled down to a temperature that is no longer hot to the touch, remove the water outlet block using pointed pliers and detach the coke shell.
- Stand the rubber mold’s water outlet upright and cut along the centerline of the four sides of the mold starting from one side of the water outlet. The depth of the cut should be 3-5 mm (adjustable based on the mold’s size). Cut open all four sides of the mold.
- Begin by cutting the first leg from the initial cutting point. Cut two straight edges with a depth of 3-5 mm (adjustable based on the mold’s size). Then, pull open the already cut straight edge and create a diagonal edge at a 45-degree angle to form the leg of a right-angled triangle. The two halves of the opened mold should now have corresponding triangular legs that fit together. Repeat this process to cut the remaining three legs sequentially.
- Pull open the first-cut leg and use a blade to cut inward along the centerline. If the cutting rule is curved, swing the blade in a specific curve to create a fish scale or wavy cut. Simultaneously, pull the rubber mold outward. Be cautious when approaching the water outlet line, gently lift the rubber mold with the tip of the blade to release the water outlet. Then, cut the ring along one end face of the outer ring until reaching the flower head and inlay positions. Cutting the flower head is a challenging and complex step. If the main stone is claw-set, cut the flower head along the axis of the two claws on one side, and then cut towards the outer ring end face on the other side until reaching the water outlet position.
- At this point, the rubber mold has been cut in half, but the silver plate cannot be removed. Cut the portion of the rubber mold containing inlays and flower heads. Cut along the axis of the two already cut claws on the outer side of the main stone inlay, between the main stone and the secondary stone inlay, until the other two claws are symmetrical. Then, cut along the remaining direction on the outer side of the main stone inlay, opposite to the previously cut surface, to make the main stone inlay upright. Cut the claw roots of the main stone inlay and the auxiliary stone inlay horizontally, effectively separating the flower head into two parts. If there are any elongated glue threads (usually formed by holes in the secondary stone inlay and the hollow part of the flower head), they should be cut off.
- Remove the silver plate and check for any adhesive residue between the silver plate and the rubber mold. If present, it must be removed.
- Open the bottom of the mold. Cut along the inner ring of the ring’s circumference and the marks on the inlays and flower heads. These marks are caused by incomplete cutting and result in slightly concave shallow marks formed by stretched rubber. Follow these marks to the corresponding position of the water outlet and cut a strip with a width of 8-12 mm in the same direction along the water outlet, with a length close to the water outlet. This will create a mushroom-like shape at the bottom, allowing the inner part of the ring to be pulled out from the cut bottom. The rubber mold can be smoothly removed after waxing.
Rubber molds should be stored in a low-temperature, dark environment, away from oil and acidic substances. If not frequently used, rubber molds can be used for up to ten years. However, if used frequently, they generally have a lifespan of two years. The average lifespan of a rubber mold is between 2-3 years.
Wax Injection Molds:
After opening the rubber mold, the wax injection process can be carried out using a wax injection machine. Several factors need to be considered, including wax temperature, pressure, and the tightness of the adhesive film.
The wax injection machine is equipped with a heater and temperature sensor to maintain a consistent wax temperature. Typically, the wax temperature should be maintained between 70-75 degrees Celsius to ensure proper fluidity. If the temperature is too low, the wax may not fill the mold completely, resulting in incomplete wax molds. Conversely, if the temperature is too high, the wax may overflow from the mold or injection port, leading to burrs or burns.
The pressure inside the wax injection machine cylinder is provided by an external air pump. The recommended air pressure ranges from 0.5-0.8 kg/cm² for wax samples with flat surfaces and simple shapes, and 1.0-2.0 kg/cm² for samples with thin walls, multiple stone inlay positions, or narrow gaps. The wax extraction time is typically 4 seconds for large wax samples and 2 seconds for fine wax samples.
Before wax injection, the opened rubber mold should be checked for integrity and cleanliness. If it is a used mold, a dewaxing agent can be sprayed on it or a small amount of talcum powder can be sprinkled to facilitate mold removal. Dewaxing agents and talcum powder should not be used simultaneously to avoid rough surfaces on the wax models. Stone powder can be used to remove 3-6 wax pieces at once. Preheat the wax injection machine, turn on the air pump, and adjust the pressure and temperature.
To inject wax, start the vacuum pump and ensure the wax temperature is within the recommended range. Check the air pressure and adjust the wax injection time based on the complexity of the wax parts in the mold. Clamp the mold in the clamp plate using both hands, ensuring even compression. Align the glue mold’s nozzle with the wax injection nozzle and push them together parallelly. Hold the wax injection nozzle firmly and gently tap the wax injection switch with your foot, releasing it immediately. The indicator light on the wax injection machine will change from yellow to red and then to green, indicating the completion of the wax injection process. Remove the glue mold from the wax nozzle. After making 6-7 consecutive glue molds, the first mold can be opened (if it has a bottom, remove the bottom first) to remove the wax mold. Inspect the wax mold carefully for any defects such as missing edges, broken claws, deformation, severe flash, or multiple bubbles. Defective wax molds should be considered scrap, while minor defects can be repaired.
In summary, the wax injection process involves maintaining the appropriate wax temperature and pressure, ensuring the cleanliness and integrity of the rubber mold, and properly injecting the wax into the mold using a wax injection machine.
Trimming Wax Molds and Building Wax Trees:
Trimming Wax Molds: After the wax molds are removed following the wax injection process, they may have various defects that need to be addressed. Surgical blades can be used to smooth out burrs, multiple edges, pinch marks, unclear flower heads, or overlapping edges. For sand holes and broken claws, repair welding can be done using welding wax. Small inaccessible holes can be penetrated with welding needles. Deformed wax molds can be corrected by immersing them in hot water at 40-50 ℃. If the hand size is incorrect, the finger circle needs to be directly changed during wax repair.
Building Wax Trees: Wax molds are sequentially welded onto a wax stick in a circular direction using a wax soldering tool, creating a wax tree that resembles a large tree in shape.
The first step in building wax trees is to dip the head of the wax stick into melted wax liquid and insert it into the concave hole of the wax tree base while it is hot.
The second step involves layer-by-layer welding of the wax molds onto the wax rod, starting from the head of the wax stick and working downwards. This top-down method helps prevent melted wax from dripping onto the already welded wax molds, avoiding the need for rework.
The key requirement for building wax trees is to arrange the wax molds in an orderly manner, ensuring they do not touch each other. The goal is to weld as many wax molds as possible onto the wax trees to meet the demands of mass production.
Additionally, before building wax trees, the weight of the rubber base should be measured. After the wax trees are built, the weight should be measured again. The weight of the wax trees can be obtained by subtracting the initial and final weights. By converting the weight of the wax trees based on the density ratio of paraffin to the cast metal, the amount of metal required for casting can be estimated.
Grouting and Baking Gypsum:
Gypsum Injection: Cover the wax tree with a stainless steel cylinder and base plate. Prepare gypsum slurry by mixing the appropriate weight of gypsum powder with water (usually in a ratio of 2-2.5:1, using distilled water). Slowly inject the gypsum slurry along the inner wall of the steel cylinder after the first vacuum pumping. Avoid pouring the slurry directly onto the wax tree. Perform a second vacuum pumping when the slurry is about 1cm below the wax tree. Allow the gypsum to naturally solidify for 6-12 hours.
Baking Gypsum: Baking gypsum molds is a crucial process for proper casting, involving dewaxing, drying, and providing insulation.
For example, in the case of baking 18K gold, the dewaxing temperature ranges from 0-350℃ with a 2-hour insulation time. The drying temperature ranges from 350-700℃ with a 2-3 hour insulation time. The casting temperature ranges from 500-600℃ with a 1-2 hour insulation time.
The general baking process is as follows: Preheat the electric furnace to the initial temperature and place the gypsum mold with the nozzle facing down to facilitate wax evaporation. After maintaining a constant temperature for one hour in the initial temperature zone, gradually increase the temperature and maintain constant temperatures at intervals of 1-2 hours. Pay attention to maintaining a heating (or cooling) rate of 100-200℃/hour to prevent cracks in the gypsum mold. Excessive heating can damage or scrap the gypsum mold. Slow heating can lead to wax residue or incomplete drying, affecting casting quality. The baking time of gypsum molds depends on the size and complexity of the wax tree and can be adjusted accordingly.
Casting Molten Gold:
As the gypsum mold baking nears completion, the molten gold is prepared and kept in a liquid state. Once the gypsum mold insulation is finished, the molten gold is injected through the water outlet to complete the casting process. Two common methods for casting are vacuum suction casting machines and vacuum induction centrifugal casting machines.
Blast Washing, Cleaning, and Shearing of Gypsum Molds:
After casting, the gypsum mold is in a high-temperature state. It is left to stand naturally for 10-30 minutes after being removed from the casting machine, followed by immersion in cold water for quenching and washing.
Once the gypsum has disintegrated due to shrinkage, the gold tree is taken out. Large pieces of gypsum are removed using a steel brush, and then it is soaked in 30% hydrofluoric acid for 10 minutes. Afterward, it is rinsed by clipping it out. The gold tree is then sprayed with a high-pressure cleaning gun to ensure the removal of any remaining gypsum until the surface is clean.
The jewelry is cut from the gold tree along the bottom of the water outlet, air-dried, weighed, and then either delivered to the warehouse or moved on to the next process.
