Injection Molding

Injection Molding

August 19, 2024

wanplas

A plastic processing method in which the plastic is plasticized in the heated barrel of the injection molding machine and then injected into the cavity of a closed mold by a plunger or a reciprocating screw to form a product. This method can process products with complex shapes, precise dimensions or with inserts, and has high production efficiency. Most thermoplastics and some thermosetting plastics (such as phenolic plastics ) can be processed by this method. The material used for injection molding must have good fluidity to fill the mold cavity to obtain the product. Since the 1970s, a type of injection molding with chemical reactions, called reaction injection molding, has emerged and developed rapidly.

Introduction

Injection molding is the process of allowing the molten plastic from an injection machine to enter a mold under the thrust of a plunger or screw and obtain a finished product after cooling.

Main equipment

Injection molding machine

That is, injection molding machine. It consists of three parts: injection device, clamping device and injection mold. There are two ways to express the specifications of injection molding machine: one is the maximum injection volume or weight per time, and the other is the maximum clamping force. Other main parameters of injection molding machine are plasticizing capacity, injection rate and injection pressure.

Injection device

The main part of the injection molding machine. The plastic is heated and plasticized into a fluid state, and then injected into the mold under pressure. The injection methods include plunger type, pre-plasticization type and reciprocating screw type. The latter has the advantages of uniform plasticization, small injection pressure loss, and compact structure, and is widely used.

Clamping device

It is used to close the fixed mold and movable mold of the mold, realize the opening and closing action of the mold and eject the finished product.

Injection mold

Injection mold for short. It consists of pouring system, molding parts and structural parts.

The pouring system refers to the plastic flow channel from the injection machine nozzle to the cavity;

The molding parts refer to the parts that constitute the mold cavity, which are composed of the female mold and the male mold;

The structural parts include various parts such as guide, demolding, core pulling, and parting.

The mold is divided into two parts: the fixed mold and the movable mold, which are fixed on the fixed plate and the movable plate of the mold clamping device respectively. The movable mold moves with the movable plate to complete the opening and closing action. The mold can be heated or cooled as needed.

How to operate

It varies depending on the material being processed. The injection molding of thermoplastics includes processes such as feeding, plasticizing, injection, pressure holding, cooling, and demolding. The molding of thermosetting plastics and rubber also includes the same process, but the barrel temperature is lower than that of thermoplastics, the injection pressure is higher, the mold is heated, and the material needs to undergo a curing or vulcanization process in the mold after injection, and then demold while hot.

Injection molding refers to a model with a certain shape, which is formed by injecting a molten colloid into a mold cavity through pressure. The process principle is: melt the solid plastic at a certain melting point, and inject it into the mold at a certain speed through the pressure of the injection machine. The mold is cooled by water channels to solidify the plastic and obtain a product that is the same as the designed mold cavity.

Injection molding is a method in which thermoplastic or thermosetting molding materials are first plasticized uniformly in a heated barrel and then pushed into the mold cavity of a closed mold by a plunger or a moving screw.

Injection molding is applicable to almost all thermoplastics. Injection molding is also successfully used to mold some thermosetting plastics. The molding cycle of injection molding is short (a few seconds to a few minutes), the mass of the molded product can range from a few grams to tens of kilograms, and it can mold molded products with complex shapes, precise dimensions, and metal or non-metal inserts in one go. Therefore, this method has strong adaptability and high production efficiency.

Injection molding machines are divided into two categories: plunger injection machines and screw injection machines. They consist of three major parts: injection system, clamping system and mold. The molding methods can be divided into:

(1) Exhaust injection molding. The exhaust injection molding machine used in exhaust injection molding has an exhaust port in the middle of the barrel, which is also connected to the vacuum system. When the plastic is plasticized, the vacuum pump can extract the water vapor, monomers, volatile substances and air contained in the plastic through the exhaust port; the raw materials do not need to be pre-dried, thereby improving production efficiency and product quality. It is particularly suitable for the molding of hygroscopic materials such as polycarbonate, nylon, plexiglass, cellulose, etc.

(2) Flow injection molding. Flow injection molding can be performed using an ordinary moving screw injection machine. That is, the plastic is continuously plasticized and squeezed into the mold cavity at a certain temperature. After the plastic fills the cavity, the screw stops rotating. The thrust of the screw keeps the material in the mold under pressure for an appropriate time, and then cools and shapes. Flow injection molding overcomes the equipment limitations for producing large products, and the quality of the product can exceed the maximum injection volume of the injection machine. Its characteristic is that the plasticized object is not stored in the barrel, but is continuously squeezed into the mold, so it is a method that combines extrusion and injection.

(3) Co-injection molding. Co-injection molding is a method of using an injection molding machine with two or more injection units to inject plastics of different types or colors into a mold simultaneously or successively. This method can produce composite products of multiple colors and (or) multiple plastics. Representative co -injection molding methods are two-color injection and multi-color injection.

(4) Runnerless injection molding. This is a molding method in which the extended nozzle of the injection machine directly injects the molten material into each mold cavity without setting a branch runner in the mold. During the injection process, the plastic in the runner remains in a molten flow state and does not come out with the product when demolding, so there is no runner residue on the product. This molding method not only saves raw materials and reduces costs, but also reduces the number of processes and can achieve fully automatic production.

(5) Reaction injection molding. The principle of reaction injection molding is to pump the reaction raw materials into the mixing head after metering by the metering device, collide and mix in the mixing head, and then inject them into the closed mold at high speed, quickly solidify, demold, and take out the product. It is suitable for processing some thermosetting plastics and elastomers such as polyurethane, epoxy resin, unsaturated polyester resin, silicone resin, alkyd resin, etc. It is mainly used for the processing of polyurethane.

(6) Injection molding of thermosetting plastics. Granular or lumpy thermosetting plastics are plasticized into a sticky plastic state by the action of the screw in a barrel with strictly controlled temperature. Under high injection pressure, the material enters the mold within a certain temperature range for cross-linking and curing. In addition to physical state changes, thermosetting plastic injection molding also has chemical changes. Therefore, compared with thermoplastic injection molding, there are great differences in molding equipment and processing technology. The following table compares the differences between thermosetting and thermoplastic injection molding.

Comparison of injection molding conditions for thermosets and thermoplastics

Process conditions	Thermosetting plastics	Thermoplastics
Barrel temperature	Low temperature, below 95℃, strict temperature control	High temperature, above 150℃, and poor temperature control
Residence time in barrel	short	Longer
Barrel heating method	Liquid medium (water, oil)	Electric heating
Mold temperature	150-200℃	Below 100℃
Injection pressure	100-200MPa	35-140MPa
Injection volume	The injection volume is small and the residual material in the front of the barrel is small	The injection volume is large, and there is more residual material in the front of the barrel

Precautions

Injection molding is an engineering technology that involves converting plastics into useful products that maintain their original properties. The important process conditions of injection molding are the temperature, pressure and corresponding action time that affect plasticization flow and cooling.

Temperature control

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 the plastic, while the latter temperature mainly affects the flow and cooling of the plastic. Each plastic has a different flow temperature. The same 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 molding machines is also different, so the barrel temperature is also different.
Nozzle temperature: The nozzle temperature is usually slightly lower than the highest temperature of the barrel to prevent the molten material from ” drooling ” in the straight-through nozzle. The nozzle temperature should not be too low, otherwise it will cause premature coagulation of the molten material and block the nozzle, or the premature coagulation of the material will be injected into the mold cavity, affecting the performance of the product.
Mold temperature: 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.).

Pressure control

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

Plasticizing pressure: (back pressure) When a screw injection machine is used, 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 does not change with the speed of the screw. Increasing the plasticizing pressure will increase the temperature of the melt, but will reduce the speed of plasticization. In addition, increasing the plasticizing pressure can often make the temperature of the melt uniform, the colorant mixes evenly, and discharge 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. The specific value varies with the type of plastic used, but it usually rarely exceeds 20 kg/cm2.
Injection pressure: In current production, the injection pressure of almost all injection machines is based on the pressure exerted by the plunger or the top of the screw on the plastic (converted from the oil circuit pressure ). The role of injection pressure in injection molding is to overcome the flow resistance of the plastic from the barrel to the cavity, increase the speed of the molten material filling the mold, and compact the molten material.

Injection pressure is divided into injection pressure and holding pressure, usually 1 to 4 injection pressures + 1 to 3 holding pressures. Generally, the holding pressure is less than the injection pressure. It is adjusted according to the actual plastic material used to achieve the best physical properties, appearance and size requirements.

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: 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 while ensuring quality. In the entire molding cycle, 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 to 5 seconds.

The holding time in the injection time is the time of pressure on the plastic in the cavity. It accounts for a large proportion of the entire injection time, generally about 20 to 120 seconds (for extra thick parts, it can be as long as 5 to 10 minutes). Before the melt freezes at the gate, the holding time affects the dimensional accuracy of the product. If it is later, it has 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 properties and crystallization 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 about 30 to 120 seconds. It is unnecessary to have a too long cooling time, which not only reduces production efficiency, but also makes demolding difficult for complex parts, and even generates 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 dualization.