What is injection molding?

What is injection molding?

Injection Mould

On thermoplastics,the injection molding machine make plastic particulate material through melting,injection, packing, cooling cycle into final plastic parts.

Thermoplastic injection molding machine usually uses clamping tonnage or shot size s a simple machine specification identification.Other parameters also includes injection rate, injection pressure, screw design, mold thickness and spacing of the guide bar and so on.

According to functional distinction, injection molding machines generally have three categories:
(1) general purpose injection molding machine;
(2) precision,tight injection machine;

injection molding machine structure

(3) High-speed, thin flesh injection molding machine.The auxiliary equipment injection molding machine mainly includes resin dryers, material handling and transportation equipment, grinder, mold temperature controller and condenser, plastic mold of a manipulator, and plastic processing equipment.
Injection assembly a typical injection molding machine includes injection system,mold system,hydraulic system,control system and clamping system.

2-1-1 Injection system

Injection mould is a tool for producing plastic products, and it is also a tool for giving plastic products complete structure and precise size. Injection moulding is a processing method used in mass production of some complex parts. Specifically, melted plastics are injected into the cavity by high-pressure injection moulding machine. After cooling and solidification, the formed products are obtained.

(1) hopper
Thermoplastics are usually supplied in small particles.Injection molding machine hopper can store plastic rubber particles.Plastic particles by gravity through the hopper neck into the barrel and screw combination.
(2) cylinder
Injection molding machine barrel can accommodate rotary screw, and use electric heater bands to heat the plastic.

(3) rotary screw
Rotary screw can compressor plastic,melt plastic and convey plastic.Screw includes feeding zone,compression zone or transition zone,and metering zone three segments.
The outer diameter of the screw is a fixed value, the depth of flight gradually decreases from the feeding zone to the metering zone starting point.The depth of flight change makes plastic with respect to a cylinder inner diameter of the plastic compress,causes shear heat, provides the main heat to melt plastic.

(4) Nozzle
When the cylinder moves to the forefront of the molding position, the outer diameter of the nozzle must be covered within a vertical runner positioning ring to form a seal.Nozzle temperature should be set at the material supplier recommendations of the plastic melt temperature, or slightly lower temperature.Thus, when cleaning cartridge, as long as the cartridge back away from the vertical runner, cleared plastic from the nozzle can fall freely.

Nozzle connecting with the mould gate

2-1-2 mold system

Clamping system is used for opening / closing the mold, supports and move mold assembly to produce enough power to prevent mold being pushed by injection pressure. Clamping mechanism may be a toggle mechanism locks, hydraulic mechanism locking, or a combination of these two basic patterns.

Injection Molded Systems

A typical injection molded system include melt delivery system and shaping member. Melt delivery system let melt flow from the injection machine nozzle to cavity channels, which generally include: sprue, (cold slug well, the main road, branching runner, and gates.)Design of conveyor system for filling model and plastic parts quality has a significant impact. Therefore, the runner system should be designed for maintaining the desired fill mode to deliver the melt to the mold cavity.After completion of the injection molding, cold runner conveyor system will be cut to become recycling waste, so the delivery system should be designed to produce minimal waste.

Hot Runner or runnerless forming process maintain the flow channel at a high temperature,so that the melt maintain in a molten state.Because the hot runner doesn’t release with plastic parts,so it doesn’t produce waste and saves secondary removal processing firmware process.

Injection molding machine operating sequence

Injection molding machine operating sequence
Plastic injection molding process is suitable for high-speed production of precision components processing method.Granular plastics in the barrel melt, mix,move (3 M’s: Melt, Mix, and Move), and then in the mold cavity flow,form, freeze (3F’s: Flow, Form, and Freeze).
Its operation can be divided into plastic plastics, filling, packing, cooling, ejection phases of the circulation process, including the basic operation actions as the following:
(1) Close the mold, so that the screw begin to move forward.
(2) The same as plunger-type injection molding machine,promote rotary screw to fill the mold cavity.
(3)  When the cavity cool,the gate solidify, the screw begin to recede, and plastics material ready for the next shot.
(5) Open the mold, push up the plastic parts.
(6) Closing the mold, ready for the next cycle.

Plastic which in barrel is squeezed by screw to produce a large amount of frictional heat to form molten state. Melt glue deposit in the front of the barrel, and heater is used to maintain the melt temperature.At the starting of the filling stage,injection molding machine opens nozzle,and the screw advances through the nozzle to deliver melt glue to closed cavity to complete the filling.
When the melt into the mold cavity,pressurized gas escape from ejector pin,parting line and vent.Good filled depends on the design of plastic components , gate location and a good exhaust.If the plastic is illiquid, or injection pressure is insufficient may cause short shot phenomenon; on the contrary, if the plastic mobility is good , likely to cause flash at plastic parts parting line.After melt completely fills the mold cavity, pressure should be continued to inject more melt glue to make up plastic volume contraction because of cooling,and ensure that the cavity is completely filled.

At the end of filling and packing stage,melt completely solidify in the mold, then open cavity to remove the plastic.Cooling time throughout the molding cycle accounted for a very high proportion, about 80%.Molded article cooling time according to nature of the plastic molded article shape, size, dimensions, and different accuracy.When moving template goes backward so that ejector pin touches rear plate and stop the movement.

To further illustrate the process cycle of injection molding machine operation,the following figure draws at different stages of the hydraulic cylinder pressure, cavity pressure,male and female mold separation distance and position of a screw schematic , which number represents:
1– filling (injection phase)
2 – packing and cooling
3 – Open Mold
4 – push up plastic parts
5 – Close Locks
Injection molding cycle time according to the process of plastic parts weight, flesh, plastic nature, the machine setting parameters changes. A typical cycle time may be from a few seconds to tens of seconds.

Screw Operation

According to the needs,speed of rotary screw can be set to plasticizing plastic granules.And extrude melt into mold cavity at a set screw speed, injection volume and injection pressure pressing.The main control parameters of rotary screw injection molding machine injection molding as the following:

(1) back pressure
Back pressure is acting on the top of the screw pressure value when screw push back in preparation for the next injection.
When the injection molding machine ready to inject, the screw will push the front of the plastic into mold cavity.After ejected plastic cooling in the mold, then injection machine at screw retral stage to start a new cycle. Typically, injection molding machine can adjust back pressure maximum.When the screw moves to back-pressure preset position,the screw retral stage is over.This preset screw stop position is based on the amount of plastic filling and runner what is set by manual mode.

(2) injection speed (or injection time)
Injection speed or ram speed refers to the screw goes forward speed in the injection operation.For the majority of engineering plastics should be under technical conditions of plastic part design and economic conditions of permit process to set the fastest injection speed.
However, in the initial stage of injection, we should adopt a lower rate of fire to avoid jetting or spoiler.Near the accomplishment,it should reduce the rate of fire to avoid plastic parts overflow,and at the same time it can help form a homogeneous sutures.
Injection time is the required time to fill melt into cavity which is controlled by injection speed.While the preferred filling rate depends on the plastic parts geometry,gate size and melt temperature,but in most cases melt will be injected into the mold as soon as possible.Because the mold temperature is usually less than resin freezing point,so long injection time will increase the possibility of early solidification of the plastic.
Thin flesh plastic parts use a high injection speed to prevent solidification occurring before filling the cavity.Sometimes, thick plastic or small gate will reduce the fill rate,this time we must maintain continuous melt flowing through the gates to prevent solidification of the gate, and then cavity fully charged.

(3)screw rotation speed
The screw rotation is the speed of the plasticizing screw.The faster the speed is,the more the plastic screw groove compression intensely, resulting in a greater amount of shear heating.

(4) Cushion
Cushion is the difference of screw between the maximum allowable advanced position and the most distal forward position.If allowed screw stroke is set the maximum,the buffer is zero,and the screw will advance to meet the nozzle then stop.Typically, the buffer amount is set 3 ~ 6 mm (1/8 ~ 1/4 inches).

(5) melt temperature
Melt temperature should be in accordance with (a) the type of resin (b) injection molding machine characteristics (c) the amount of injection, interworking.The melt temperature which initially set should refer data recommended by the resin supplier.Usually choose the melt temperature that higher than the softening temperature and below the melting point of the resin to to prevent overheating and cracking.

(6) Mold temperature
The limitation of mold temperature is in order to avoid plastic freezing in the sectional of mold cavity and cooling properties of plastic (for example, crystallization, etc.). Therefore, the mold temperature should be chosen between melt fluidity and mold temperature.

(7) Injection and holding pressure
The upper limit of injection pressure is injection machine capacity, the mold clamping force and mold structure.Typically,the set of injection pressure and holding pressure will not cause short shot lowest pressure.Injection pressure and holding pressure should be high enough, maintaining enough time so that the contraction phase of plastic parts continues filling plastic, and minimize the amount of shrinkage.However,the injection pressure is too high will cause potential stress of plastic parts. Two-stage pressure can be applied in a number of processes.The first phase is a high pressure filling, and the second is a reduced pressure of packing.

(8) pressure holding time
The time that after completion of filling the cavity, injection molding machine still put pressure on the mold is called pressure holding time.The pressure is aimed at maintaining the dimensional accuracy of the component.

(9) Remaining cooling time
The time from relieving pressure to opening the model is called the remaining cooling time. The purpose is that allow plastic parts is hardening enough to push out.If plastic parts is not cooled enough then pushed out that will cause warpage of plastic parts.

(10) mold-opening time or dead time
Mold-opening time includes the time of opening mold,pushing out plastic part,closing mold.Mold-opening time has a relationship with operating efficiency of injection molding machine,ease of product taking out,whether using release agent.The mold which is artificial resettlement insert embedded parts will reduce operational efficiency. The direction of best mold-opening time is maintaining minimum of human intervention at the injection mold machine operation time.

Secondary processing

After pushing out the plastic parts, removal of melt delivery system (vertical sprue, runner, gate) processing is called secondary processing.Some plastic parts need secondary processing to combine or decoration.Secondary processing details should be available from the material supplier design manual.

(1)combination
A combination of plastic parts secondary processing includes:
Bonding welding inserting staking swaging assembly with fasteners

(2) Decoration
Secondary processing of decorative plastic parts includes:
Surface treatment: heat or pressurization
Printing:plastic surface process in order to decorative or provide information.

(3) Other secondary processing
Other secondary processing includes:
Paint hard plating metal layer / masking layer surface treatment annealing Turning

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