Gas-assisted molding show and related tips sharing.

The products shown in the picture are manufactured and produced by Meike mold with injection molding machine of 250 tons (currently available equipment with 1000 tons maximum clamping force), the molding cycle time is 240 seconds, and the product weighs 310 grams.

Gas Assistant molding technology developed in recent 30 years which is the second generation of injection molding technology. At present, this technology is mainly used in molding parts for automobile, home appliances and others in larger size.

The main principle is to inject a certain amount of molten plastic (usually 90% -98%, in terms of the total amount of the product) that can be analyzed and experienced, then inject high pressure nitrogen into the molten plastic along with preset path (preferably consistent with the flow direction and of course the specific circumstances you decide). Fill the whole mold cavity with molten plastic under 100% of required, holding pressure, cooling at the same time, venting, and stripping finally. High pressure nitrogen enters into molten polymers it will naturally pass through the low viscosity (high temperature) to press parts by high gas pressure to maintain pressure when cooling.

In addition to the traditional injection molding equipment, this technology also requires the gas-auxiliary injection molding control system (CAD-IT has the MDI controller) which has the following advantages over the traditional one:

1.Reduce the residual stress inside, thus weakening or even completely eliminating the warping or deformation on parts, while increasing its mechanical strength and rigidity.

2.The center of the thick part of the finished wall is hollow, which can reduce the raw materials consumption, especially the short-shot and hollow molds, and the plastic can save up to 30%.

3.Ease or eliminate the shrinkage caused by reinforcement.

4. Molding parts more evenly and precisely.

5.Lower clamping force consumption.

6.Use the airway to form reinforcing structures and strengthen the molded parts.

7.Less gating design as well as tooling cost.

8.Shorten the manufacturing cycle.

9.Products with a large thickness ratio can also be formed once by gas assistance.

10.Change the traditional concept of finished product design so that we can design more complex products.

The disadvantages:

1.Because the gas has compression characteristics, it is not easy to accurately control and sensitive to the surrounding operating environment, so the repetition and stability of the process are worse than the traditional process.

2. Developing technology and poor experience lead to higher wastage rate.

Currently it is usually used on molding such as car door handle, seat, bumper, door panel, TV outside passenger, air conditioning, refrigerator, toilet…

Previously molding car door handle, door panel, snowmobile front cover three types of 7 products in total.

The pre-injection of the gas-assistant molding, like traditionally, by adding a nitrogen filling and recovery system, as different press mode of gas filling, there are two kinds of filling devices used:

1. Generating pressure discontinuously is a way of volume control method, such as introduced by the company Cinpres, that at first filling a cylinder with gas (usually nitrogen) to a certain volume, and compressing it to a set value by a hydraulic device then injecting it into the channel and cavities so that the gas can reach everywhere we designed and desired. Most of injection molding machines with gas assistance were made in this way but it cannot maintain a constant high pressure.
2. Generating pressure continuously is a way of pressure control, designed by Battenfeld using a special compression device to produce a high-pressure gas flow constantly. This method can keep the pressure stable in the channel, cavities and all sections of part, gas pressure distribution is controlled by a special device.

Gas assistance technology makes more easier than traditional one to mold large parts with better quality for cars, home appliance, furniture, electronics, daily supplies, office supplies, building materials and so on. At present we mostly molding following parts with gas assistance,

Tubular, rods: such as handle, hook, chair armrest, shower head, etc., they are usually with hollow structure design, even though, it cannot affect the function and performance of products, but greatly save raw materials, shorten the cooling time and production cycle.

Large flat panels: such as automotive instrument panel, interior parts grille, commercial machine cover and parabolic sanitary antenna, etc. By setting the built-in airway in the assembly, the stiffness and surface quality of the product can be significantly improved, the warping or deformation and surface irregularities can be reduced with less clamping force, namely forming bigger parts by smaller machine.

The parts with wall thickness extremely uneven: such as TV, computer printer shell and its internal support and external decoration they usually cannot be formed once with the traditional injection molding process, the use of gas auxiliary technology to improve the freedom of mold design, is conducive to the integration of accessories.

In addition, for large plastic parts molded by traditional technology, they often appear melt marks, shrinkages, warping or deformation and other defects, but it needs greater injection pressure and mold locking force and will bring adverse effects on the machine, mold and products. The introduction of gas assisted technology breaks through some limitations of CIM, overcomes various defects of CIM, reduces raw material consumption by 10-50%, shortens the molding cycle, more importantly, improves the apparent quality and mechanical performance. Therefore, GAIM has been widely valued and applied by enterprises as soon as it appears. At present, almost all thermoplastic and some thermosetting materials used for ordinary injection molding can be formed by GAIM method. GAIM plastic parts have also been involved in structural and functional fields, especially in large plastic parts it has more obvious advantages. For example, the C2 series gas assisted injection machine and gas assisted injection part (and including computer control system) developed by GIL it has been able to meet the needs of the production of large plastic parts.

Although GAIM has many advantages that ordinary injection molding does not has, it introduced terms of pre-injection, melt / gas injection delay time, inflation injection pressure, rate and time, new technology parameters, delay line, product breakdown and "finger" effect (due to the shrinkage of local volume, the melt between the airway and the wall of the component, etc., especially easily in large plate components) or gas back irrigation. Therefore, in China, mainly used in molding TV casings by Changhong, Hisense, Konka, Xiamen, Panda and many other TV. In the automotive industry, With introduction of new models and requirements for high-quality injection molded parts, The technology is also beginning to be applied, for example, Chengdu Aerospace Plastic Group has introduced injection machines with gas assistance from British to make interior decorative parts for Fukang car, and will develop other new products, but not for some of super-large parts as lacking certain theoretical guidance and practical experience. It can be said that GAIM has become an essential molding method to mold large, ultra-thick, high-precision or apparent and high-definition plastic parts in industrial developed countries and regions.

In recent years, some new gas-assisted injection technologies have been developed, such as Liquid Gas-Assisted Injection Molding, Vibrated Gas Assisted and Water-Assisted:

Liquid Gas-Assisted Injection Molding：

Liquefied gas assisted injection injects a preheated, special vaporized liquid from the nozzle into molten polymers. The liquid entering into cavities together with the plastic melt and vaporized simultaneously, vaporized gas in the mold cavity expands to hollow the parts and pushes the melt to the surface of the mold cavity. Gas cooling becomes a stable state and exists in the assembly and no longer condenses into liquid; compared with the conventional gas-assisted injection method, the hot liquid gasification method can produce a very low gas pressure (2MPa, the conventional gas-assisted injection method is 3MPa), can be applied to any thermoplastic, including those with low molecular weight and easily blown through.

Vibrated Gas Assisted Injection Molding：

Vibration gas-assisted injection is to apply the vibration energy of the compressed gas to the oscillation of the plastic melt, so as to control the microstructure of the component and improve the performance of the product.

Water-Assisted Injection Molding：

Water-assisted injection technology (WIT) is a new molding method of producing hollow or partially hollow products. The principle of this method for cavity formation is basically similar to the gas-assisted injection technology (GAIM). WIT has a unique advantage: it can cool directly inside the product. Since the thermal conductivity of water is 40 times that of the gas, and the thermal enthalpy of water is 4 times that of the gas, the cooling capacity of WIT can reduce the cooling cycle time by 25% of GAIM. In addition to significantly shortening the molding cycle, WIT can form hollow products with thinner wall thickness and more uniform, WIT can produce very smooth inner surfaces, which is difficult to achieve in GAIM.

In the early 1970s, the liquid media of injection process like water, oil or polymer solution was proposed as a method for the formation of the hollow body, the pressure of the plunger and the flow rate achieved was too low. Today, WIT, developed at the German Plastic Processing Institute (IKV) in Aachen, Germany, makes this ancient idea a reality. IKV begans developing the technology in 1998, Sulo, in Herford, Germany, was the first company to implement the technology for plastic processing. Still, it wasn't until late October 2000 that an all-plastic supermarket cart processed with WIT technology at Fakuma officially declared WIT in commercial use. Now, more and more manufacturers have participated in the research, development and improvement of WIT.

The limitations of gas-assisted injection molding, the gas-assisted injection molding technology breaks through the limitation of traditional injection molding, and can be flexibly applied to the molding of various manufacturing parts. It has significant advantages in saving raw materials, preventing shrinkage, shortening cooling time, improving surface quality, reducing internal stress, reducing mold locking force, improving production efficiency, and reducing production cost. However, for the production of a larger diameter dielectric catheter, the GAIM process still has a large residual wall thickness, which increases the cooling time due to the transfer of heat from the inside through the gas. With large residual wall thickness, the possibility of air bubbles inside the product wall increases; to avoid this disadvantage, the pressure retention time or pressure release time of gas is extended. Obviously, this increases the cycle time. When the diameter of the assembly exceeds 40mm, the danger of the melt flowing down the mold wall also exists after the airway is formed. Therefore, GAIM is difficult to apply to the molding of hollow products with a larger diameter. The lack of GAIM technology has stimulated the development of new injection molding methods, and WIT has emerged.

Gas assisted injection molding (referred to as gas auxiliary molding) is a new method in the field of plastic processing, was used in the 1980s, is widely used in Europe and North America, Asian Japan and South Korea have been applied successively, some manufacturers in China also began to apply this new technology.

Gas-assisted injection molding is one more gas injection stage than the traditional injection molding, and the plastic melt is driven by the gas to fill the mold cavity. Therefore, a clear airway must be provided to guide the gas-assisted molding products design and the mold design of the gas. The size of the airway geometry, the determination of the section shape and the arrangement of the position will all affect the penetration of the gas and the interference of the melt flow, which will ultimately affect the quality of the molding product.

According to the different volume of the molten plastic injected into the type cavity during the gas auxiliary molding, the gas auxiliary molding process can be roughly divided into three ways:

1. Hollow molding, that is, when the melt injection cavity is filled to 60-70% of the type cavity volume, stop the melt injection, start to inject the gas, until the pressure cooling is finalized.This process is mainly suitable for similar handles, handles and other large wall thick plastic products, the application effect is the most ideal.
2. Short shot, when the melt is filled to 90-98% of the cavity volume, air intake begins.The method is mainly used for thick wall or partial wall products with larger planes
3. Full emission, that is, the gas is only filled with the space caused by the melt volume contraction, and the gas pressure and melt pressure are used, so that the warping deformation of the product is greatly reduced, used for the forming of thin-wall products with a larger plane, the process control is more complex.The first two methods are also called short material gas-assisted injection method, while the latter is called full material gas-assisted injection method.

Principle of gas auxiliary process

Stage 1: Plastic injection: the melt enters the type cavity and encounters a lower temperature mold wall, forming a thin solidification layer.

Phase 2: Gas injection: Inert gas enters the molten plastic, pushing the unsolidified plastic in the center into the unfinished cavity.

Phase 3: End of gas incidence: The gas continues to push the plastic melt to flow until the melt fills the entire cavity.

Stage 4: Gas pressure preservation end: under the pressure preservation state, the gas in the airway compression melt, feeding to ensure the appearance of the assembly.

The advantages of gas auxiliary molding

l. Eliminate product surface shrinkage and improve product surface quality;

2. Reduce warping deformation and reduce flow strip marks;

3. Reduce the product internal stress and improve the product strength;

4. Save plastic raw materials, reduce the weight of products (generally can be reduced by 20-40%);

5. Improve the distribution of materials on the product section, and improve the rigidity of the products;

6. Reduce the mold locking force, glue shooting force and injection molding machine power consumption;

7. Shorten the molding time and improve the production efficiency;

8. Extend the service life of the mold and reduce the cost of the mold manufacturing;