What is the principle of injection molding of foamed plastics?

Foamed plastics are thermoplastic or thermosetting resin-based plastics with numerous tiny pores inside. Foaming is one of the most important methods of plastic processing, and the foam obtained from plastic foaming contains both gas and solids - gas and solid. The gas exists in the foam body in the form of vesicles, and the vesicles that are isolated from each other are called closed pores, and the ones that are connected are called open pores, so there are closed pore foam and open-pore foam. The foam structure of open or closed pore is determined by the performance of the raw material and its processing technology.

 

The technology of plastic foam has a long history. The earliest was foam mylar in the early 1920s, made in a similar way to foam rubber; rigid polyurethane foam and polystyrene foam in the 1930s; polyethylene, PVC, epoxy resin, phenolic foam in the 1940s; and polystyrene foam and flexible polyurethane foam in the 1950s.

 

Now, basically, all plastics, including thermoplastic and thermosetting can be foamed into foam. The industrial preparation methods are extrusion foam, injection foam, molding foam, calendering foam, powder foam, and spray foam, etc. Among them, injection molding foam is one of the most important molding methods, and the focus here is on injection molding foam.

Foaming principle

The foaming method of plastics can be divided into two categories, physical foaming method and chemical foaming method, according to the different foaming agents used. Here, we will first briefly introduce the foaming agent.

 

Foaming agent

The blowing agent can be divided into two categories: physical and chemical blowing agents. The requirements of the physical blowing agent are: non-toxic, odorless, non-corrosive effect, non-combustion, good thermal stability, no chemical reaction in the gaseous state, the diffusion rate in the plastic melt in the gaseous state is lower than the diffusion rate in the air. Commonly used physical blowing agents are air, nitrogen, carbon dioxide, hydrocarbons, Freon, etc.

 

The chemical blowing agent is a kind of heat that can release gas such as nitrogen, carbon dioxide, and other substances, the requirements of the chemical blowing agent is: the decomposition of the gas released should be non-toxic, non-corrosive, non-combustible, no impact on the molding and physical and chemical properties of the product, the speed of gas release should be able to control, the blowing agent in plastic should have good dispersion. More widely used inorganic foaming agents such as sodium bicarbonate and ammonium carbonate, organic foaming agents such as azo formamide and azo ...

 

Physical foaming method

Simply put, is the use of physical methods to make plastic foaming, there are generally three methods.

 

(1) first inert gas dissolved in the plastic melt or paste under pressure, and then released under reduced pressure, thus forming pores in the plastic and foaming.

(2) foaming by evaporation of the low-boiling liquid dissolved in the polymer melt to make it vaporize.

(3) in the plastic by adding hollow balls and the formation of foaming body and foaming, etc.

 

Physical foaming method used in the physical foaming agent cost is relatively low, especially the cost of carbon dioxide and nitrogen is low, and can be flame retardant, non-polluting, so the application value is high; and physical foaming agent foaming without residue, the impact on the performance of foamed plastic is not significant. But it requires a special injection molding machine and auxiliary equipment, technical difficulties.

 

Chemical foaming method

The chemical foaming method is the use of chemical methods to produce gas to make plastic foaming: the chemical blowing agent added to the plastic is heated to make it decompose and release the gas and foaming; in addition, it can also use the chemical reaction between the plastic components to release the gas and foaming.

 

The process of plastic injection molding using chemical blowing agents is basically the same as the general injection molding process. Plastic heating and heating, mixing, plasticizing and most of the foam expansion are done in the injection molding machine.

 

In short, no matter which plastic raw material is selected, and no matter which foaming method is used, the foaming process generally goes through the formation of bubble nuclei, bubble nuclei expansion, bubble body curing, and shaping stages.

 

Structural foam molding

StructralFoamMolding is a chemical foaming method, which is a revolution in injection molding technology. It retains many advantages of the traditional injection molding process and avoids some of the problems encountered in the traditional injection molding process, such as insufficient product strength, too long a production cycle, and low molding rate.

 

In addition, the use of structural foam technology can also mold large and complex products, the use of the low-cost mold, multiple cavities can be operated at the same time, thereby reducing the production cost of products. Structural foam products are a kind of dense surface of the continuous foam material, its unit weight strength and stiffness than the same kind of material without foam 3 ~ 4 times higher.

 

The most important feature of the structural foam method is that it can be produced without additional equipment and can be injection molded by ordinary injection molding machines, but the high-pressure structural foam injection molding machine with expanded cavity method adds a secondary clamping device compared with ordinary injection molding machines.

 

In recent years, structural foam injection molding technology has been widely developed, and there are many molding methods, but in summary, they can be divided into three kinds: low-pressure foaming method; high-pressure foaming method (Note: low and high pressure here refers to the pressure in the mold cavity); two-component foaming method.

 

Low-pressure foaming method

The difference between low-pressure foam injection molding and ordinary injection molding is that the mold cavity pressure is lower, about 2~7Mpa, while ordinary injection molding is between 30~60Mpa. Low-pressure foam injection molding generally adopts the under-injection method, that is, a certain amount of plastic melt (containing blowing agent) is injected into the mold cavity, and the gas from the decomposition of the blowing agent makes the plastic expand and fill the mold cavity. For low-pressure foam injection on ordinary injection molding machines, the chemical blowing agent is usually mixed with plastic and plasticized in the barrel, and a self-locking nozzle must be used.

 

When injecting, the injection speed of the injection machine should be fast enough because the diffusion of a gas is fast, which will cause the surface roughness of the products. A booster is generally used to increase the injection speed and injection volume so that the injection action is completed in an instant.

 

High-pressure foaming method

The high-pressure foaming method of injection mold cavity pressure between 7~15Mpa, using the full injection method, that is, one injection volume is exactly equal to the volume of the mold cavity. In order to make the parts get foam expansion, the mold cavity can be forced to expand, or make part of the plastic shunt out of the mold cavity. The cavity enlargement method is generally used.

 

When the molten mixture of plastic and blowing agent is injected into the mold cavity, it is delayed for a period of time, and then the moving template of the mold closing mechanism moves backward for a small distance so that the moving and fixed molds of the mold are slightly separated, and the mold cavity is expanded, and the plastic in the mold cavity starts to foam and expand.

 

After the product cools down, dense skin is formed on the surface of the product. Since the foaming and expansion of the plastic melt are controlled by the moving mold plate, the thickness of the dense skin of the product can also be controlled. The movement of the moving template can be the whole movement or partial movement to make partial foaming, so as to obtain products of different densities. The high-pressure foaming method requires high precision of mold manufacturing, high mold cost, and has secondary clamping pressure requirements for injection molding machines.

 

Two-component foaming method

Two-component foam injection molding is a special high-pressure structural foam injection molding method, which uses a special two-component injection molding machine. This kind of injection molding machine has two sets of injection devices: one set is used to inject the core part of the product, and one set is used to inject the surface layer of the product. During the injection, the skin material is injected first, and then the core material mixed with the foaming agent is injected through the same gate.

 

Since the core material flows in laminar flow, this ensures that the core material is evenly covered inside the skin layer, so that the cavity is completely filled. After the cavity is filled with melt, a small amount of melt without blowing agent is injected to close the gate. After the product is removed, the gate is removed to obtain a lightweight product with a non-foaming dense surface layer and a foaming core.

 

Microcellular foam molding

Microcellular foam molding is a physical foaming method. The diameter of the pores of conventional foam is generally larger than 50 mm, and the density of pores (the number of pores per unit volume) is less than 106/cm3. These large pores often become the source of initial cracks when stressed, which reduces the mechanical properties of the material.

 

Injection molding is one of the main molding methods for microporous plastic products. After the plastic raw material is added to the barrel of the injection molding machine, it is plasticized by the sheer force of the screw and the external heating of the heating ring, and the foaming agent is injected directly into the molten section of the injection screw at the end and mixed evenly with the melt, and then injected into the mold cavity at high pressure and high speed.

 

In the mold cavity sudden pressure drop, so that a large number of supersaturated gas in the melt from the precipitation, foaming, expansion, molding, shaping the formation of microporous plastic products. In addition, the method of changing temperature can also be used to form bubble nuclei, which is easier to control compared with the method of changing pressure but is not applicable to plastics whose gas solubility is not sensitive to temperature.

 

It is more difficult to develop microporous plastic injection molding technology because the size of bubble holes is much smaller compared with conventional foam. To get good microporous plastic products, it is necessary to ensure accurate metering of the supercritical fluid entering the melt in the barrel, require that the plastic melt must be fully mixed, homogenized, and dispersed to form a homogeneous mixture, ensure that the nucleation point in the melt must be more than 109 bubbles/cm3, timely control of Nucleation bubble expansion and so on. This has very high requirements on the equipment itself and the injection process parameters.

 

The features of using this technology are.

Approximately 50% reduction in product weight.

Reduction of injection pressure by about 30-50%.

A 20% reduction in clamping force.

A 10-15% reduction in cycle time.

The use of a set of gas input devices with metering control devices.

 

Conclusion.

Foamed plastic has the characteristics of lightweight, material saving, ability to absorb impact load, heat, and sound insulation performance, high specific strength due to the presence of bubbles, especially in terms of material saving, energy reduction, and cost reduction, which makes it possible to promote the application vigorously.

 

In addition to the above advantages of general foam, the microcellular foam has more excellent mechanical properties. Therefore, the foam has a wide range of applications, especially in the production of automobiles, aircraft, and a variety of transport equipment and other fields that have special application value. It can be expected that foam injection molding, especially microcellular foam injection molding, will be a hot spot for plastic processing machinery and product processing.