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Plastic Injection Molding Process


1. Definition

Plastic Injection Molding refers to the method that the fully molten plastic material is stirred by the screw at a certain temperature, injected into the mold cavity with high pressure, cooled and solidified to obtain the plastic injection molding products.


This method is suitable for mass production of complex parts and is one of the important machining methods. Plastic injection gallon pots are made by this process.


2. Plastic Injection Molding Process

The injection molding process of plastic parts mainly includes four stages: filling, pressure maintaining, cooling and demolding. These four stages directly determine the molding quality of products, and these four stages are a complete continuous process.

plastic injection molding process

2.1 filling stage

Filling is the first step in the whole injection cycle. The time starts from the closing of the mold to the filling of the mold cavity to about 95%. Theoretically, the shorter the filling time is, the higher the molding efficiency is, but in practice, the molding time or injection speed is restricted by many conditions.


2.2 pressure maintaining stage

The function of the pressure holding stage is to continuously apply pressure, compact the melt and increase the plastic density (densification) to compensate for the shrinkage behavior of the plastic. In the pressure maintaining process, the back pressure is high because the mold cavity has been filled with plastic.


In the process of pressure maintenance and compaction, the screw of the injection molding machine can only move slightly forward slowly, and the flow speed of plastic is also relatively slow. At this time, the flow is called pressure maintaining flow.


In the pressure maintaining stage, the plastic is cooled and solidified by the mold wall, and the melt viscosity increases rapidly, so the resistance in the mold cavity is very large.


In the later stage of pressure maintenance, the material density continues to increase and the plastic parts are gradually formed. The pressure maintaining stage should continue until the gate is cured and sealed. At this time, the mold cavity pressure in the pressure maintaining stage reaches the highest value.


2.3 cooling stage

In the injection mold, the design of the cooling system is very important. This is because only when the molded plastic products are cooled and solidified to a certain rigidity, can the deformation of the plastic products caused by external forces be avoided after demolding.


Because the cooling time accounts for about 70% ~ 80% of the whole molding cycle, a well-designed cooling system can greatly shorten the molding time, improve the injection molding productivity and reduce the cost. Improper design of the cooling system will prolong the forming time and increase the cost; Uneven cooling will further cause warpage and deformation of plastic products.


2.4 demolding stage

Demoulding is the last link in an injection molding cycle. Although the product has been cold-formed, demolding still has a very important impact on the quality of the product. The improper demolding method may lead to uneven stress of the product during demolding and deformation of the product during ejection.


There are two ways of demolding: ejector rod demolding and stripper plate demolding. When designing the mold, the appropriate demolding method should be selected according to the structural characteristics of the product to ensure product quality.


3. Injection Molding Process Conditions

The most important factors in injection molding process conditions are temperature, pressure and time.


3.1 temperature. 

The temperature to be controlled in the injection molding process mainly includes barrel temperature, nozzle temperature and mold temperature.


3.1.1 barrel temperature

The selection of barrel temperature is related to the variety and characteristics of plastics. The barrel temperature is too low and the plasticization is insufficient; If the barrel temperature is too high, the plastic may overheat and decompose.


The temperature distribution of the material is generally based on the principle of high in front and low in the rear, that is, the temperature at the rear end of the material barrel is low and the temperature at the front end near the nozzle is high, so as to prevent the degradation of plastic due to sheer friction heat.


3.1.2 nozzle temperature

The nozzle temperature is generally slightly lower than the maximum temperature of the barrel to prevent the molten material from salivating at the nozzle.


3.1.3 mold temperature

Mold temperature has a great influence on the filling flow of melt, cooling rate and the properties of plastic parts after molding. The mold temperature depends on the crystallinity of the plastic, the size, structure and performance requirements of the plastic parts, and other process conditions (such as melt temperature, injection speed and injection pressure).


3.2 pressure

Pressure in the injection molding process includes plasticizing pressure and injection pressure, which directly affect the plasticization of plastics and the quality of plastic parts.


Plasticizing pressure, called back pressure, refers to the pressure on the molten material at the head of the screw when the screw rotates and retreats when the screw is used.


Generally, on the premise of ensuring the quality of plastic parts, the lower the plasticizing pressure, the better. Injection pressure refers to the pressure exerted by the plunger or screw head on the plastic melt.


The injection pressure depends on the type of plastic, type of injection machine, mold structure, the wall thickness of plastic parts and other process conditions.


3.3 time

The time required to complete an injection molding process is called the molding cycle. It includes mold filling time, pressure holding time, cooling time in the mold and other times. Other time includes mold opening, demolding, applying release agent, placing inserts and mold closing.


4. Precautions for Injection Molding Process

4.1 injection pressure

The existence of pressure is to overcome the resistance in the melt flow process, or conversely, the resistance in the flow process needs to be offset by the pressure of the injection molding machine to ensure the smooth progress of the filling process.


4.2 injection molding time

Reasonable injection time is conducive to the ideal filling of melt, and is of great significance to improve the surface quality of products and reduce dimensional tolerance.


4.3 injection temperature

The injection temperature must be controlled within a certain range. The temperature is too low and the melt plasticization is poor, which affects the quality of formed parts and increases the process difficulty; The temperature is too high and the raw materials are easy to decompose.


4.4 cooling stage

The cooling time accounts for about 70% ~ 80% of the whole molding cycle. A well-designed cooling system can greatly shorten the molding time, improve injection molding productivity and reduce the cost.


4.5 demolding stage

There are two ways of demolding: ejector rod demolding and stripper plate demolding. When designing the mold, the appropriate demolding method should be selected according to the structural characteristics of the product to ensure product quality.


5. Common Injection Molding Materials

5.1 PS(polystyrene)

PS, also known as polystyrene, is a thermoplastic. Common products: plastic seed trays, stationery, cups, food containers, home appliance shells, electrical accessories, etc


Properties: transparent when not shaded. When the product falls to the ground or is knocked, there is a metallic crisp sound. It has good luster and transparency. It is similar to glass. It is brittle and easy to break. You can mark the surface of the product with your fingernails. Modified polystyrene is opaque.


5.2 PP(Polypropylene)

Polypropylene is a thermoplastic polymerized from propylene. It is usually colorless, translucent solid, odorless and non-toxic. Its density is 0.90 ~ 0.919 g / cm3. It is the lightest general-purpose plastic. Its surface density is also relatively small and easy to be scratched


Its outstanding advantages are that it has the characteristics of cooking resistance in the water, corrosion resistance, strength, rigidity and transparency are better than polyethylene, while its disadvantages are poor low-temperature impact resistance and easy aging. Polypropylene is suitable for blow molding and injection molding. PP products are also widely used, such as injection molded nursery pots, cups, lunch boxes and.


5.3 PE(polyethylene)

Common products: low pressure polyethylene is suitable for making corrosion-resistant parts and insulating parts; high pressure polyethylene is suitable for making films, etc; UHMWPE is suitable for shock absorption, wear resistance and transmission parts.


Characteristics: excellent corrosion resistance and electrical insulation (especially high-frequency insulation). It can be chlorinated and modified by irradiation. It can be reinforced with glass fiber.


5.4 PVC(polyvinyl chloride)

Common products: PVC is one of the most widely used plastic materials.


Injection speed: the injection speed should be slow, otherwise excessive shearing will degrade the material.


Characteristics: chemical and physical properties: rigid, PVC material is an amorphous material. In practical use, PVC materials often add stabilizers, lubricants, auxiliary processing agents, pigments, impact agents and other additives. PVC material has non flammability, high strength, weather resistance and excellent geometric stability.


5.5 AS styrene acrylonitrile copolymer

Common products: Electrical (socket, shell, etc.), daily commodities (kitchen appliances, refrigerator devices, TV base, cassette box, etc.), automobile industry (headlight box, reflective environment, instrument panel, etc.), household products (tableware, food knives, etc.), cosmetics packaging, etc.


Melting temperature: 200 ~ 270 ℃. If thick wall products are processed, a melting temperature lower than the lower limit can be used.

Injection pressure: 350 ~ 1300bar.

Injection speed: high speed injection is recommended.


Characteristics: as is a hard and transparent material. Styrene makes as hard, transparent and easy to process. As has the strong load-bearing capacity, chemical reaction resistance, thermal deformation resistance and geometric stability.


5.6 ABS material is an engineering plastic

Common products: materials for making small toys, such as small alarm clocks, are also used on the shell of mobile phones when they are available.


Characteristics: ABS resin is composed of copolymers of acrylonitrile (a), butadiene (b) and styrene (s).


5.7 PC is the abbreviation of polycarbonate and PC engineering plastics

Common products: its strength can meet various needs from mobile phones to bulletproof glass.


Characteristics: PC is an amorphous thermoplastic resin with excellent comprehensive properties, with excellent electrical insulation, extensibility, dimensional stability, chemical corrosion resistance, high strength, heat resistance and cold resistance; It also has the advantages of self extinguishing, flame retardant, non-toxic and colorable.


The characteristics of large-scale industrial production and easy processing also make its price extremely low. The disadvantage is that compared with metal, it has an insufficient hardness, which makes its appearance easier to scratch, but its strength and toughness are very good. Whether it's heavy pressure or general wrestling, as long as you don't try to hit it with a stone, it will live long enough.


Plastic injection molding related products: plastic gallon planters, plastic greenhouse pots, thermoformed pots, heavy duty seed starting trays.


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