Injection molding, compression molding and extrusion molding of rubber

Injection molding, compression molding and extrusion molding of rubber

Injection molding process

1. Process flow: First, the rubber raw materials are dried, preheated and other pretreatments are carried out to ensure the stability of the raw material performance. The pretreated rubber raw materials are put into the barrel of the injection molding machine and melted by the heating device. The screw of the injection molding machine pushes the molten rubber forward and quickly injects it into the closed mold cavity under high pressure. After injection into the mold, a certain pressure and time are maintained to allow the rubber to be fully formed in the mold. After molding, the rubber product is cooled by the cooling system inside the mold to solidify and shape it. When the product is completely cooled, the mold is opened and the finished product is taken out of the mold by a robot or manually. Finally, the product is subjected to post-processing processes such as trimming, deburring, and inspection to remove excess flash, burrs, etc., and to check whether the size, appearance and performance of the product meet the requirements.
2. Equipment and molds: The equipment mainly includes injection molding machines, which are composed of injection systems, mold clamping systems, hydraulic systems, electrical control systems, etc. The mold is a key component of injection molding, generally consisting of a fixed mold and a movable mold, and contains complex geometric structures such as gates, runners, and molding cavities.
3. Process parameter control: The melt temperature should be selected according to the type and characteristics of the rubber raw materials. Too high a temperature may cause rubber degradation, while too low a temperature will affect fluidity. The injection pressure determines the filling speed and degree of the rubber melt in the mold. Insufficient pressure will cause product shortages, while excessive pressure may cause defects such as flash. The holding time has an important influence on the dimensional accuracy and internal quality of the product, and should be reasonably set according to the wall thickness and structure of the product. The cooling time must ensure that the rubber product is fully cooled and solidified. Too short a time may cause the product to deform after demolding.
4. Application areas: Commonly used in the manufacture of rubber products with complex shapes and high dimensional accuracy requirements, such as automobile tires, rubber seals, rubber buttons, and rubber toys.

Compression molding process

1. Process flow: First, plasticize the raw rubber to reduce its elasticity and increase its plasticity. Then mix the plasticized rubber with various compounding agents (such as vulcanizers, accelerators, fillers, etc.) on a rubber mixer to make mixed rubber. Cut or weigh the mixed rubber into rubber blanks of certain specifications according to the shape and size requirements of the product. Put the rubber blank into the preheated mold cavity, and after the mold is closed, apply a certain pressure and temperature through equipment such as a flat vulcanizer to vulcanize and shape the rubber blank in the mold. After vulcanization is completed, open the mold, take out the product, and carry out post-processing procedures such as cooling, trimming, and inspection.
2. Equipment and molds: The equipment mainly includes a flat vulcanizer, which provides pressure and temperature to vulcanize and shape the rubber in the mold. The mold usually consists of an upper mold and a lower mold, with a relatively simple structure, and is designed and manufactured according to the shape and size of the product.
3. Process parameter control: The vulcanization temperature directly affects the vulcanization speed of the rubber and the performance of the product. Different rubber materials have different suitable vulcanization temperature ranges. The vulcanization pressure helps the rubber to flow and fill in the mold, ensuring the compactness and dimensional accuracy of the product. The pressure should be determined according to the characteristics of the rubber and the structure of the product. The vulcanization time determines the degree of the rubber vulcanization reaction. If the time is too short, the vulcanization is incomplete and the product performance is poor; if the time is too long, it may lead to over-vulcanization, making the product hard and brittle.
4. Application areas: Widely used in the manufacture of large rubber products, such as automobile tires, rubber soles, rubber seals, rubber shock absorbers, rubber pads, etc., as well as some rubber products with relatively simple shapes and large batches.

Extrusion molding process

1. Process flow: Add the rubber raw material into the hopper of the extruder, and push the raw material forward through the rotation of the screw. During the pushing process, the raw material is heated by the barrel and sheared by the screw, and gradually melts and plasticizes. The plasticized rubber is squeezed by the screw and extruded through the die to form a continuous rubber product with a certain shape and size. The extruded rubber product is cooled and shaped by a cooling device, which can be air-cooled or water-cooled. The cooled product is then subjected to subsequent treatments such as traction, curling, and cutting as needed to obtain the final product.
2. Equipment and molds: The equipment is mainly a rubber extruder, which consists of a barrel, a screw, a feeding device, a die, a cooling device, etc. The mold is mainly a die and a mouth die. The die converts the spiral motion of the rubber into a linear motion and provides the necessary pressure before extrusion. The mouth die determines the cross-sectional shape and size of the extruded product.
3. Process parameter control: Extrusion temperature includes barrel temperature and die temperature, which should be set according to the characteristics of rubber raw materials and extrusion process requirements. Too high or too low temperature will affect the extrusion quality. The screw speed determines the extrusion speed and output of the rubber compound, and also affects the plasticization effect of the rubber compound and the quality of the product. It needs to be adjusted according to the properties of the rubber compound and the size of the die. Extrusion pressure affects the flow and molding of the rubber compound in the die and die. Unstable pressure will lead to fluctuations in product size or poor surface quality.
4. Application areas: Mainly used in the production of rubber products with continuous cross-section shapes, such as rubber pipes, rubber sealing strips, rubber wire and cable sheaths, and rubber special-shaped strips.

• How to choose a suitable molding process for rubber products?

Choosing a suitable molding process for rubber products requires comprehensive consideration of multiple factors. The following are some key points

The shape and complexity of the rubber product

• High shape complexity

If the rubber product has a complex internal structure, thin-walled parts, fine details or multiple protrusions in different directions, injection molding is usually the best choice. For example, there are many tiny buttons and marks on the rubber keypad, and injection molding can accurately fill the mold cavity to make the product details complete. Because injection molding injects molten rubber into a closed mold under high pressure, it can well replicate complex mold shapes.

• The shape is relatively simple

For rubber products with regular shapes and simple shapes, such as round or square rubber gaskets, simple pipes, etc., extrusion or compression molding may be more suitable. For example, extrusion molding can efficiently produce continuous pipes, and the desired cross-sectional shape can be obtained by adjusting the shape of the die; compression molding can also handle flat or simple block products well, as long as the mold shape can match the appearance of the product.

Product size requirements

• Large products

Compression molding has advantages in manufacturing large rubber products. Since it is to put the rubber blank into the mold and then directly press vulcanize it, it can better guarantee the internal quality and overall performance of large products. For example, large rubber industrial pads or automobile tires (part of the process), compression molding can ensure that the rubber is uniformly vulcanized within a large size range, avoiding the problem of uneven molding due to over-sizing.

• Small to medium-sized products

Injection molding and extrusion molding are widely used in the production of small to medium-sized products. Injection molding can accurately control the dimensional accuracy of small products and is suitable for the production of small rubber seals, toys, etc. Extrusion molding can continuously produce medium-sized products with certain length requirements, such as medium-sized rubber sealing strips, and can be cut into suitable lengths as needed, and the dimensional stability is also good.

Production batch and efficiency requirements

• High volume production

Injection molding and extrusion molding are suitable for mass production. The production cycle of injection molding is relatively short. Once the mold is debugged, it can be quickly injected into batches, such as a large number of rubber mobile phone accessories. Extrusion molding is a continuous production process. As long as the raw material supply is sufficient and the equipment is operating normally, products with the same cross-sectional shape can be continuously produced, such as a large number of wire and cable sheaths.

• Small batch production

Compression molding is more flexible for small batch production. Although the single production cycle of compression molding may be longer, the mold cost is relatively low. For some small batch rubber products with less complex shapes and less high size requirements, such as customized small rubber shock absorbers, it is easier to achieve economical and efficient production.

Product performance requirements

• High physical performance requirements

If there are strict requirements on the physical properties of rubber products (such as strength, hardness, elasticity, etc.), compression molding and injection molding may be more suitable. These two molding processes can better control the vulcanization degree and internal structure of the rubber during the molding process, so that the product has better physical properties. For example, for rubber seals that require high strength, injection molding or compression molding can accurately control the vulcanization conditions, so that the rubber molecular chains are better cross-linked and the performance of the product is improved.

High appearance quality requirements

Injection molding excels in producing products that require high appearance quality. Because it is molded in a closed mold, the surface finish of the product is high and surface defects can be effectively reduced. For example, for rubber toys or decorative rubber products that need to have a good appearance, injection molding can produce products with smooth surfaces and no obvious defects.

Cost Factor

• Mold cost

Injection molding molds are usually more complex and costly. If the product output is not enough to spread the mold cost, it may lead to excessive production costs. Compression molding molds are relatively simple and low-cost, suitable for some products that do not require high mold precision. Extrusion molding molds are mainly heads and mouth dies, and the cost is relatively low. Especially when producing pipes or profiles of various specifications, the cost of replacing the mouth die is lower than remaking the injection mold or compression mold.

• Equipment cost and operating cost

Injection molding equipment (injection molding machines) has a high cost, but also a high degree of automation, high production efficiency, and the unit cost may be reduced in large-scale production. Compression molding equipment (flat vulcanizers, etc.) has a relatively low cost, but the production speed is slow. Considering the production efficiency, the operating cost may increase due to the long production cycle. Extrusion molding equipment has a moderate cost, and its operating cost mainly depends on the raw material consumption and energy consumption. In the continuous production process, if the process parameters can be reasonably controlled, better cost-effectiveness can be achieved.

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