What is blow molding?

Blow molding: It is a method of clamping molten thermoplastic raw materials extruded from an extruder into a mold, and then blowing air into the raw materials. The molten raw materials expand under the action of air pressure, adhere to the wall of the mold cavity, and finally cool and solidify into the desired product shape.

Blow molding can be divided into two types: thin film blow molding and hollow blow molding:

1. Film blow molding

Thin film blow molding is the process of extruding molten plastic into a cylindrical thin tube from the annular gap of the extruder head mold. At the same time, compressed air is blown into the inner cavity of the thin tube from the center hole of the head, and the thin tube is blown and expanded into a larger diameter tubular thin film (commonly known as a bubble tube). After cooling, it is rolled up.

2. Hollow blow molding

Hollow blow molding is a secondary molding technology that uses gas pressure to blow and expand the rubber like blank closed in the mold cavity into hollow products. It is a method for producing hollow plastic products. Hollow blow molding has different manufacturing methods according to the blank, including extrusion blow molding, injection blow molding, and stretch blow molding.

1) Extrusion blow molding: Extrusion blow molding is the process of using an extruder to extrude a tubular billet, clamp it in the mold cavity while it is hot, seal it, and then blow compressed air into the inner cavity of the billet to form it.

2) Injection blow molding: The blank used is obtained by injection molding. The blank is left on the core mold of the mold, closed with a blow molding mold, and compressed air is introduced from the core mold to blow and cool the blank. After demolding, the product is obtained.

Advantages:

The product has uniform wall thickness, small weight tolerance, less post-processing, and small waste corners; Suitable for producing small precision products in large quantities.

3) Stretch blow molding: Place the billet that has been heated to the stretching temperature in the blow molding mold, use a stretching rod for longitudinal stretching, and use compressed air blown in for transverse stretching and blowing to obtain the product.

Application:

1. Film blow molding is mainly used to manufacture plastic films;

2. Hollow blow molding is mainly used to make hollow plastic products (bottles, packaging barrels, spray cans, fuel tanks, cans, toys, etc.).

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Compression injection molding

Compression injection molding is a relatively traditional injection molding method

Principle:

First, inject the molten material into the mold cavity, and when the molten material enters the mold lung. The mold opens slightly under its pressure blade; After the molten material fills the mold cavity, use a high-pressure locking mold to obtain the required product. The second step is compression molding. Due to the fact that the molten material enters the mold cavity when the mold has been slightly opened, the required allowable mold labor is relatively small.

During the molding process, the screw no longer injects material into the mold cavity, but relies on high-pressure locking of the mold to force Lf against the plastic, resulting in a smaller product orientation and lower internal stress. This method is particularly suitable for products with high transparency requirements that are formed and have a small volume;

Advantages:

It can increase the flow length ratio of injection molded parts; Adopting smaller locking force and injection pressure; Reduce internal stress of materials; And improve processing productivity.

Injection compression molding is suitable for products made of various thermoplastic engineering plastics; Such as large-sized curved parts, thin-walled, miniaturized parts, optical lenses, and parts with good impact resistance requirements

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Low pressure injection molding

Low pressure molding is a process that injects packaging materials into molds at very low injection pressures and rapidly solidifies them into shape.

Low pressure molding process is a packaging process that injects packaging materials into molds at a very low injection pressure (2-20 bar) and rapidly solidifies (5-50 seconds) to achieve insulation, temperature resistance, impact resistance, vibration reduction, moisture resistance, waterproof, dustproof, and chemical corrosion resistance.

This process is currently mainly applied to the packaging and protection of precision and sensitive electronic components, including printed circuit boards (PCBs), automotive electronic products, mobile phone batteries, wire harnesses, waterproof connectors, sensors, microswitches, inductors, antennas, and so on.

Low pressure molding is a process method that falls between high-pressure injection molding and sealing.

Features and advantages

The traditional injection molding process has defects due to high pressure, as low-pressure molding only requires a small amount of pressure to allow the melt to flow into a small mold space, thus not damaging the fragile components that need to be packaged, greatly reducing the scrap rate.

The low-pressure forming process can effectively seal, moisture-proof, waterproof, dust-proof, and chemically resistant the encapsulated components. In addition, it can also balance performance including high and low temperature resistance, impact resistance, insulation, and flame retardancy.

Shorten product development cycle

Low pressure forming molds can use cast aluminum molds instead of steel, making them very easy to design, develop, and manufacture, which can shorten the development cycle. In addition, compared to the time-consuming two-component sealing process, the process cycle of low-pressure hot melt injection molding can be reduced to a few seconds to tens of seconds, greatly promoting production efficiency.

Save total production costs

Firstly, the equipment cost of low-pressure injection molding process is low. Traditional injection molding equipment systems generally have higher costs, including purchasing high-pressure injection molding machines, as well as having a water-cooling system and expensive steel molds. The low-pressure injection molding process equipment system is generally relatively simple, consisting only of three parts: a hot melt glue machine, a work console, and a mold.

Secondly, due to the extremely low injection pressure, aluminum molds can be used for mold design, development, and manufacturing, which can save material costs and development cycles. If low-pressure injection molding technology is used to replace traditional sealing technology, it can also save the cost of sealing the shell and post heating curing.

Finally, due to low pressure and low temperature, the scrap rate of finished products can be greatly reduced, avoiding unnecessary waste.

Therefore, choosing low-pressure injection molding technology can not only significantly improve production efficiency and reduce the defect rate of finished products, but also help production enterprises establish cost advantages overall

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Gas assisted injection molding

Gas assisted injection molding, also known as gas injection molding, is a new injection molding process. It is one of the most important developments in the injection molding industry since the introduction of reciprocating screw injection machines.

Gas assisted injection molding is an extension of injection molding, which is developed on the basis of injection molding technology and structural foam injection molding; It can also be considered as a combination of injection molding and hollow molding, in this sense, it can also be called "hollow injection molding". The principle is to use relatively low pressure gas to replace the resin in the mold cavity during the pressure holding stage of the original injection molding. 

Gas assisted injection molding can be achieved by adding a gas supply device to the existing injection molding machine. The gas supply device consists of an air pump, a high-pressure gas generation device, a gas control device, and a gas nozzle. The gas control device uses a special compressor for continuous gas supply, and is controlled by an electric control valve to maintain a constant pressure. Pressure usually has three levels. A gas control device can be equipped with multiple injection molding machines.

The gas commonly used in gas assisted injection molding is nitrogen. The gas pressure and gas purity are determined by the shape of the molding material and the product. The pressure is generally between 5~32MPa, with a maximum of 40MPa. High pressure gas is injected from the gas nozzle at a set pressure timing during each injection. One or more gas nozzles, located on the injection molding machine nozzle, mold runner or cavity.

Advantages:

Gas assisted injection molding technology has many incomparable advantages. It not only reduces the manufacturing cost of plastic products, but also improves certain properties; When the parts can meet the same usage requirements, using gas assisted injection molding can greatly save plastic raw materials, with a savings rate of up to 50%. On the one hand, the reduction in plastic raw material consumption leads to a reduction in the time of each link in the entire molding cycle

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Water-assisted injection molding

Water-assisted injection molding technology is an advanced injection molding process that injects part of the melt into the mold cavity, then injects high-pressure water into the melt through equipment to finally shape the workpiece.

Advantages:

Due to the incompressibility of water, the front end of the water forms a solid interface, squeezing the inner wall of the product into a cavity. The front end of the water also plays a role in rapid cooling. Therefore, water-assisted has many advantages that gas-assisted cannot match. Research and application show that water-assisted can generate thinner and more uniform cavity walls, and the inner wall surface of the flow channel is very smooth. Especially for thick-walled workpieces, the cooling time of water-assisted can be greatly reduced compared with gas-assisted.

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What is injection molding?

 Injection molding, also known as injection molding, is a molding method that combines injection and molding. The advantages of injection molding are fast production speed, high efficiency, automated operation, a wide variety of colors, shapes from simple to complex, sizes from large to small, and precise product size. The product is easy to update and can be made into parts with complex shapes. Injection molding is suitable for mass production and molding processing fields such as complex-shaped products.

At a certain temperature, the completely molten plastic material is stirred by a screw, injected into the mold cavity with high pressure, and cooled and solidified to obtain a molded product. This method is suitable for mass production of parts with complex shapes and is one of the important processing methods.

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History of Injection Molding

 In 1868, Hyatt developed a plastic material he named celluloid. Celluloid had been invented in 1851 by Alexander Parks. Hyatt improves it so that it can be processed into finished shapes. Hyatt and his brother Isaiah registered the patent for the first plunger injection machine in 1872. This machine is relatively simpler than those used in the 20th century. It operates basically like a giant hypodermic needle. This giant needle (diffusion barrel) injects plastic into the mold through a heated cylinder.

In the 1940s World War II created a huge demand for cheap, mass-produced products. , low-priced, mass-produced products.

In 1946, American inventor James Watson Hendry built the first injection molding machine, which allowed for more precise control of injection speed and quality of produced items. This machine also enables material mixing before injection, so that colored or recycled plastics can be thoroughly mixed into virgin matter. In 1951, the United States developed the first screw injection machine. It did not apply for a patent, and this device is still in use.

In the 1970s, Hendry went on to develop the first gas-assisted injection molding process and allowed the production of complex, hollow products that cooled rapidly. This greatly increases design flexibility as well as the strength and endpoints of manufactured parts while reducing production time, cost, weight and waste.

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Injection molding temperature control

 ⒈ Barrel temperature: The temperatures that need to be controlled during the injection molding process include barrel temperature, nozzle temperature and mold temperature. The first two temperatures mainly affect the plasticization and flow of plastics, while the latter temperature mainly affects the flow and cooling of plastics. Each type of plastic has a different flow temperature. The same type of plastic has different flow temperatures and decomposition temperatures due to different sources or brands. This is due to different average molecular weights and molecular weight distributions. The plasticization process of plastics in different types of injection machines is also different, so the barrel temperature is also different.

⒉ Nozzle temperature: The nozzle temperature is usually slightly lower than the maximum barrel temperature. This is to prevent the "drooling phenomenon" that may occur in the straight-through nozzle. The nozzle temperature cannot be too low, otherwise it will cause premature coagulation of the melt and block the nozzle, or the premature coagulation of the melt will be injected into the mold cavity, affecting the performance of the product.

⒊ Mold temperature: The mold temperature has a great influence on the intrinsic performance and apparent quality of the product. The mold temperature is determined by the presence or absence of plastic crystallinity, the size and structure of the product, performance requirements, and other process conditions (melt temperature, injection speed and injection pressure, molding cycle, etc.).

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Injection molding pressure control

 The pressure in the injection molding process includes plasticizing pressure and injection pressure, which directly affects the plasticization of plastics and the quality of products.

⒈ Plasticizing pressure: (back pressure) When using a screw injection machine, the pressure on the top of the screw when the screw rotates backward is called plasticizing pressure, also known as back pressure. The size of this pressure can be adjusted by the overflow valve in the hydraulic system. In injection, the size of the plasticizing pressure needs to be changed according to the design of the screw, the requirements of product quality and the type of plastic. If these conditions and the speed of the screw remain unchanged, increasing the plasticizing pressure will strengthen the shearing effect, that is, it will increase the temperature of the melt, but it will reduce the efficiency of plasticization, increase backflow and leakage, and increase the driving power.

In addition, increasing the plasticizing pressure can often make the temperature of the melt uniform, the colorant mixes evenly and discharges the gas in the melt. In general operation, the plasticizing pressure should be determined as low as possible while ensuring the quality of the product. Its specific value varies with the type of plastic used, but it usually rarely exceeds 20 kg/cm2.

⒉ Injection pressure: In current production, the injection pressure of almost all injection machines is based on the pressure applied by the plunger or screw top to the plastic (converted from the oil circuit pressure). The role of injection pressure in injection molding is to overcome the flow resistance of plastic from the barrel to the cavity, increase the speed of molten material filling the mold, and compact the molten material.

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Injection molding cycle

 The time required to complete an injection molding process is called the molding cycle, also known as the molding cycle. It actually includes the following parts: Molding cycle: The molding cycle directly affects labor productivity and equipment utilization.

Therefore, in the production process, the relevant time in the molding cycle should be shortened as much as possible under the premise of ensuring quality. In the entire molding cycle, the injection time and cooling time are the most important, and they have a decisive influence on the quality of the product.

The filling time in the injection time is directly inversely proportional to the filling rate. The filling time in production is generally about 3-5 seconds. The holding time in the injection time is the pressure time of the plastic in the mold cavity, which accounts for a large proportion of the entire injection time, generally about 20-120 seconds (special thick parts can be up to 5~10 minutes). Before the molten material is frozen at the gate, the amount of holding time has an impact on the dimensional accuracy of the product. If it is later, there is no effect.

The holding time also has a most favorable value. It is known that it depends on the material temperature, mold temperature, and the size of the main channel and gate. If the size of the main channel and gate and the process conditions are normal, the pressure value with the smallest fluctuation range of the product shrinkage rate is usually used as the standard. The cooling time is mainly determined by the thickness of the product, the thermal and crystal properties of the plastic, and the mold temperature.

The end point of the cooling time should be based on the principle of ensuring that the product does not cause changes when demolding. The cooling time is generally between 30 and 120 seconds. It is unnecessary to have a cooling time that is too long, which will not only reduce production efficiency, but also cause demolding difficulties for complex parts, and even produce demolding stress when forced demolding.

The other time in the molding cycle is related to whether the production process is continuous and automated, as well as the degree of continuity and automation.

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