Injection molding uses thermophysical properties of plastics to add materials from hopper into barrel. Outside of barrel is heated by a heating ring to melt material. A screw driven by an external power motor is installed in barrel. Material is transported forward and compacted along screw groove. Material is gradually plasticized, melted and homogenized under dual effects of external heating and screw shearing. When screw rotates, material pushes melted material to head of screw under friction and shearing force of screw groove. At the same time, screw retreats under reaction of material, so that head of screw forms a storage space to complete plasticization process; Then, under action of piston thrust of injection cylinder, screw injects molten material in storage chamber into mold cavity through nozzle at high speed and high pressure. After molten material in cavity is maintained under pressure, cooled, solidified and shaped, mold is opened under action of clamping mechanism, and shaped product is ejected from mold through ejection device.

 

According to configuration of clamping parts and injection parts, there are three types: horizontal, vertical, and angle.
(1) Horizontal injection molding machine: Horizontal injection molding machine is the most commonly used type. Its characteristics are that center line of injection assembly is concentric or consistent with center line of clamping assembly and parallel to installation ground. Its advantages are low center of gravity, stable operation, convenient mold installation, operation and maintenance, large mold opening, and small space height; but it occupies a large area, and large, medium and small machines are widely used.
(2) Vertical injection molding machine: Its characteristics are that axis of clamping device and injection device are arranged in a straight line and are perpendicular to the ground. It has advantages of small footprint, convenient mold assembly and disassembly, easy installation of inserts, relatively uniform plasticization of materials falling from hopper, easy automation and automatic line management of multiple machines. Disadvantage is that ejected product is not easy to fall off automatically, and often needs to be taken out manually or by other methods, which makes it difficult to achieve fully automated operation and injection of large products.
(3) Angle injection molding machine: Axes of injection device and clamping device are arranged perpendicular to each other. According to relative position of center line of injection assembly and installation base, there are horizontal vertical type, vertical horizontal type, and horizontal horizontal type: ① Horizontal vertical type, injection assembly line is parallel to base surface, and center line of clamping assembly is perpendicular to base surface; ② Vertical horizontal type, injection assembly center line is perpendicular to base surface, and center line of clamping assembly is parallel to base surface. Advantage of angle injection molding machine is that it has advantages of both horizontal and vertical injection molding machines, and is particularly suitable for molds with asymmetric geometric products with side gates.

According to requirements of injection molding process, injection molding machine is a highly mechatronic machine, mainly composed of injection parts, clamping parts, machine body, hydraulic system, heating system, control system, feeding device, etc.

Working principle is: during pre-plasticization, screw in plasticizing part drives main shaft to rotate through hydraulic motor. One end of main shaft is connected to screw key, and the other end is connected to hydraulic motor key. When screw rotates, material is plasticized and plasticized melt is pushed into storage chamber at the front end of barrel. At the same time, screw retreats under reaction of material, and thrust seat retreats through thrust bearing.
Piston rod is pulled back linearly through nut to complete metering. During injection, rod chamber of injection cylinder pushes piston rod through bearing to complete action, rod chamber of piston pushes piston rod and screw to complete injection action.

Plasticizing parts are of two types: plunger type and screw type. Screw type is introduced below.
Screw type plasticizing part is mainly composed of a screw, a barrel, a nozzle, etc. Plastic changes its physical state during continuous advancement of rotating screw, and is finally injected into mold cavity in a molten state. Therefore, plasticizing component is core component to complete uniform plasticizing and achieve quantitative injection.
Working principle of screw plasticizing component: During pre-plasticization, screw rotates to continuously push forward material falling into screw groove from material port, and heating ring transfers heat to material in screw groove through barrel wall. Solid material is plasticized and melted under dual effects of external heating and screw rotation shearing, and undergoes thermal history of each functional section of screw. Molten material pushes check ring and flows into front end of screw through surrounding channel of screw head, and generates back pressure, pushing screw backward to complete metering of molten material. During injection, screw acts as a plunger, and under action of oil cylinder, it moves forward quickly to inject melt in storage chamber into mold through nozzle.
Screw-type plasticizing components generally have following characteristics:
① Screw has two functions: plasticizing and injection;
② When plasticizing, screw is only used for pre-plasticization;
③ Thermal history of plastic in plasticizing process is longer than that of extrusion;
④ Screw will undergo axial displacement during plasticizing and injection, and at the same time, screw is in an intermittent working state of turning and stopping, thus forming instability of screw plasticizing process.
(1) Screw
Screw is a key component in plasticizing component. It is in direct contact with plastic. Plastic passes through effective length of screw groove and undergoes a long thermal history. It must undergo transformation of three states (glass state, viscoelastic state, and viscous flow state). Length, geometry, and geometric parameters of each functional segment of screw will directly affect conveying efficiency and plasticizing quality of plastic, will ultimately affect injection molding cycle and product quality.

 

Compared with extrusion screw, injection screw has following characteristics:
① Aspect ratio and compression ratio of injection screw are relatively small;
② Screw groove of homogenizing section of injection screw is deeper;
③ Feeding section of injection screw is longer, while homogenizing section is shorter;
④ Head structure of injection screw has a special form.
⑤ When injection screw is working, plasticizing capacity and melt temperature will change with axial displacement of screw.
(i) Classification of screws
Injection molding screws can be divided into general screws and special screws according to their adaptability to plastics. General screws are also called conventional screws. They can process most thermoplastics with low and medium viscosity, crystalline and non-crystalline civil plastics, engineering plastics. They are the most basic form of screws. Corresponding to them are special screws, which are used to process plastics that are difficult to process with ordinary screws;
According to screw structure and its geometric shape characteristics, they can be divided into conventional screws and new screws. Conventional screws are also called three-stage screws. They are basic form of screws. There are many types of new screws, such as separation screws, diversion screws, corrugated screws, screws without metering sections, etc.
Effective length of thread of a conventional screw is usually divided into a feeding section (conveying section), a compression section (plasticizing section), and a metering section (homogenizing section). According to different properties of plastics, they can be divided into gradual type, sudden type and general type screws.
① Gradual screw: Compression section is longer, and energy conversion during plasticization is gentle, which is mostly used for plastics with poor thermal stability such as PVC.
② Mutant screw: Compression section is shorter, and energy conversion during plasticization is more intense, which is mostly used for crystalline plastics such as polyolefins and PA.
③ Universal screw: Universal screw with strong adaptability can adapt to processing of various plastics, avoid frequent screw replacement, and help improve production efficiency.
Length of conventional screw segment is as follows:

(ii) basic parameters and structure of screw
Basic structure of screw is mainly composed of effective thread length L and connecting part of tail.
ds——screw outer diameter, screw diameter directly affects size of plasticizing capacity, which directly affects size of theoretical injection volume. Therefore, injection molding machine with a large theoretical injection volume has a large screw diameter.
L/ds——screw aspect ratio. L is effective length of screw thread. The larger aspect ratio of screw, the longer thread length, which directly affects thermal history of material in screw and ability to absorb energy. Energy source has two parts:
One part is transmitted by external heating ring of barrel, the other part is friction heat and shear heat generated when screw rotates, which is converted from external mechanical energy. Therefore, L/ds directly affects melting effect and melt quality of material. However, if L/ds is too large, transmission torque will increase and energy consumption will increase.
L1 - length of feeding section. Feeding section is also called conveying section or feeding section. In order to improve conveying capacity, surface of screw groove must be smooth. Length of L1 should ensure that material has enough conveying length, because too short L1 will cause material to melt prematurely, making it difficult to ensure conveying conditions of stable pressure, it is also difficult to ensure plasticizing quality and plasticizing capacity of subsequent sections of screw.
Plastic slides from hopper into screw groove under its own gravity. When screw rotates, material is compressed into a dense solid plug nut under friction of thrust surfaces formed by barrel and screw groove, moves relative to screw. In this section, plastic is in a solid state, that is, a glassy state.
h1 - depth of screw groove in feeding section. The deeper h1 is, the more material can be accommodated, which increases feeding amount and plasticizing capacity, but it will affect plasticizing effect of material and shear strength of screw root. Generally, h1≈(0.12～0.16)ds.
L3 - length of melting section. Melting section is also called homogenization section or metering section. Melt is further homogenized in screw groove of L3 section, with uniform temperature and composition, forming a better melt quality. Length of L3 helps fluctuation of melt in screw groove, has effect of stabilizing pressure, and makes material squeezed out from screw head with a uniform amount of material, so it is also called metering section.
When L3 is short, it helps to improve plasticizing ability of screw. Generally, L3 = (4-5) ds.
h3 - depth of screw groove in melting section. When h3 is small and screw groove is shallow, plasticizing effect of plastic melt is improved and it is beneficial to homogenization of melt. However, if h3 is too small, shear rate will be too high and shear heat will be too large, causing degradation of molecular chain and affecting melt quality. On the contrary, if h3 is too large, reflux effect generated by screw back pressure during pre-plasticization will be enhanced, which will reduce plasticizing ability.
L2 - thread length of plasticizing section (compression section). Material is continuously compressed, sheared and mixed in this conical space. Starting from entry point of L2 section, molten pool continues to increase. At exit point, molten pool has filled the entire screw groove. Material completes transition from glass state to viscoelastic state to viscous flow state. That is, in this section, plastic is in a state of coexistence of particles and melt.
Length of L2 will affect transformation process of material from glassy state to viscous flow state. If it is too short, material will not be able to transform in time, and solid material will be blocked at the end of L2 section to form a very high pressure, torque or axial force; if it is too long, it will increase torque of screw and unnecessary consumption. Generally, L2 = (6~8) ds. For crystalline plastics, melting point of material is obvious and melting range is narrow. L2 can be shorter, generally (3~4) ds. For thermosensitive plastics, this section can be longer.
S——pitch, its size affects helix angle, thereby affecting conveying efficiency of screw groove, generally S≈ds.
ε——compression ratio. ε=h1/h3, that is, ratio of screw groove depth h1 in feeding section to screw groove depth h3 in melting section. A large ε will enhance shear effect, but will weaken plasticizing ability. Generally speaking, it is better to have a slightly smaller ε to improve plasticizing ability and increase adaptability to material. For crystalline plastics, compression ratio is generally 2.6~3.0. For low-viscosity and heat-stable plastics, a high compression ratio can be selected; while for high-viscosity and heat-sensitive plastics, a low compression ratio should be selected.

 

(2) Screw head
In injection screw, function of screw head is: during pre-molding, it can release plasticized melt into storage chamber, and during high-pressure injection, it can effectively seal melt in front of screw head to prevent backflow.
Screw heads are divided into two categories, with and without check rings. For those with check rings, during pre-molding, melt in homogenizing section of screw pushes check ring open and flows into storage chamber through gap formed with screw head. During injection, melt pressure at screw head forms a thrust to push check ring back to runner to block and prevent backflow.
For some high-viscosity materials such as PMMA, PC, AC or materials with poor thermal stability such as PVC, in order to reduce shearing and material retention time, check rings can be omitted, but such injection will produce backflow and extend holding time.
Requirements for screw head:
① Screw head should be flexible and smooth;
② Clearance between check ring and barrel should be appropriate, that is, to prevent melt from flowing back and to be flexible;
③ There should be enough flow cross section and end face of check ring should have return force to ensure rapid closure during injection;
④ Structure should be easy to disassemble and clean;
⑤ Screw head thread is opposite to screw thread to prevent screw head from loosening during pre-molding.
(3) Barrel
(i) Barrel structure
Barrel is an important part of plasticizing component, with a screw inside and a heating coil outside, which bears effects of composite stress and thermal stress. 1-front barrel; 2-electric heating coil; 3-screw hole; 4-feeding port
Screw hole 3 is equipped with a thermocouple, which should be in close contact with thermocouple to prevent floating, otherwise it will affect temperature measurement accuracy.
(ii) Feeding port
Structural form of feeding port directly affects feeding effect and feeding capacity of plasticized parts. Most injection molding machines rely on deadweight of material in hopper to feed. Commonly used feed port cross-section form is simple to manufacture, but it is not conducive to feeding. Now, asymmetric forms are mostly used. This type of feed port is conducive to improving feeding efficiency due to large contact angle and contact area between material and screw, and it is not easy to open a bridge hole in hopper.
(iii) Wall thickness of barrel
Barrel wall thickness requires sufficient strength and rigidity, because barrel must withstand pressure of molten material and gas, and barrel has a large aspect ratio. Barrel requires sufficient heat capacity, so barrel wall must have a certain thickness, otherwise it is difficult to ensure temperature stability; but if it is too thick, the barrel is bulky, wastes materials, has large thermal inertia, heats up slowly, and has a large lag in temperature regulation.
(iv) Barrel gap
Barrel gap refers to single-sided gap between inner wall of barrel and outer diameter of screw. If this gap is too large, plasticizing capacity will be reduced, injection backflow will increase, and injection time will be prolonged, causing partial degradation of material in process; if it is too small, thermal expansion will intensify friction between screw and barrel, increase energy consumption, and even cause jamming. This gap Δ= (0.002~0.005) ds.
(v) Heating and cooling of barrel
Barrel heating methods of injection molding machine include resistance electric heating, ceramic heating, and cast aluminum heating. They should be reasonably set according to application occasion and processed materials. Commonly used ones are resistance heating and ceramic heating. In order to meet requirements of injection molding process, barrel should be controlled in sections, with 3 sections for small machines and 5 sections for large machines.
Cooling refers to cooling feeding port. If temperature at feeding port is too high, solid material will "bridge" at feeding port and block feeding port, thereby affecting conveying efficiency of feeding section. Therefore, a cooling water jacket is set here to cool it. Our factory cools feed port by cooling circulating water.
(4) Nozzle
(i) Function of nozzle
Nozzle is an important component connecting plasticizing device and mold runner. Nozzle has multiple functions:
① During pre-plasticization, it establishes back pressure, drives out gas, prevents melt drooling, improves plasticizing capacity and metering accuracy;
② During injection, it forms contact pressure with main mold sleeve, maintains good contact between nozzle and sleeve, forms a closed runner, and prevents plastic melt from overflowing under high pressure;
③ During injection, it establishes melt pressure, increases shear stress, and converts pressure head into a velocity head, increases shear speed and temperature rise, enhances mixing effect and homogenization effect;
④ Change nozzle structure to match mold and plasticizing device to form a new runner type or injection molding system;
⑤ Nozzle also undertakes functions of temperature adjustment, heat preservation and material cutting;
⑥ Reduce viscoelastic effect and eddy current loss of melt at inlet and outlet to stabilize its flow;
⑦ When pressure is maintained, it is convenient to add material to mold product, when cooling and shaping, backflow resistance is increased to reduce or prevent melt in mold cavity from flowing back.
(ii) Basic forms of nozzles
Nozzles can be divided into straight-through nozzles, locked nozzles, hot runner nozzles and multi-runner nozzles.
Straight-through nozzles are the most commonly used nozzles. Their characteristics are that nozzle spherical surface directly contacts spherical surface of mold main sprue. Arc radius and runner of nozzle are smaller than those of mold. During injection, high-pressure melt is directly filled into mold cavity through mold runner system. Speed is fast, pressure loss is small, manufacturing and installation are convenient.
Locked nozzle mainly solves drooling problem of straight-through nozzle and is suitable for processing of low-viscosity polymers (such as PA). Nozzle runner can be closed during pre-molding to prevent melt from drooling, and it can be opened under action of injection pressure during injection to allow melt to be injected into mold cavity.

 

Working principle is: when injection cylinder is filled with oil, piston drives piston rod and bearing placed in thrust seat to push screw forward or backward. Axial position of two parallel piston rods and axial position of injection screw can be adjusted synchronously through nut on piston rod head.

During injection, thrust seat pushes screw through thrust shaft for injection; and during pre-molding, thrust shaft is driven by oil motor to drive screw to rotate to achieve pre-molding.

When seat shift cylinder is filled with oil, injection seat is moved forward or backward, and injection nozzle is ensured to be in close contact with arc surface of main casting of mold, generating an injection seat pressure that can seal melt.

After assembly, the whole injection part should be placed on frame. Nozzle must be tightly connected with main casting sleeve of mold to prevent overflow. Center line of injection part must be concentric with center line of its mold clamping part. In order to ensure matching accuracy between injection screw and inner hole of barrel, parallelism of two injection cylinder holes and barrel positioning center hole and symmetry of center line must be ensured;
For horizontal machines, parallelism of two guide holes of seat-shifting cylinder and symmetry of its center must also be ensured. For vertical machines, parallelism of two seat-shifting cylinder holes, barrel positioning center hole and symmetry of center line must be ensured. Factors affecting above position accuracy are dimensional accuracy, geometric accuracy, manufacturing accuracy and assembly accuracy of holes and shafts of associated components.