Blog 28: Parts of an Injection Molding machine.


Injection molding, One of the most crucial processes in the mass manufacture of items made of thermoplastics. It is frequently used for processes that create products, without the need for additional finishing steps. Currently, the majority of injection molding machines are universal models that can accept all mold types within certain parameters. 

In the process of injection molding, a thermoplastic polymer is heated past its melting point, turning it from a solid to a fluid that has a relatively low viscosity. This melt is mechanically injected, or pressed, into a mold that mimics the final shape desired. As long as the item is not cooled down below the polymer’s freezing point, the low viscosity of the molten polymer allows for the complete filling of the mold. In the case of semi-crystalline polymers, in-mold cooling of the product at a predetermined cooling rate often controls the crystallinity of the object, which controls its mechanical and aesthetic attributes. The mold is opened and the part is ejected in the last stage.

The two primary mechanisms of injection molding are pressure flow and heat transfer. The two pieces of the necessary equipment in an injection molding machine are an injection molding machine, sometimes known as a press, and a mold, which is also often called a tool or a die. The end result of the process is molding, which is occasionally incorrectly and confusingly referred to as a mold.

How does an Injection Molding Machine work?

Parts of the Injection Molding machine.

1. Hopper

Before the injection molding process can start, the plastic material is placed into the hopper. For the purpose of keeping moisture away from the plastic material, the hopper typically has a dryer unit. Additionally, it might be equipped with tiny magnets to keep any dangerous metallic particles out of the device. The plastic material is then put into the barrel, the key component that comes after it in the assembly line.

2. Barrel

To allow the plastic to flow through the barrel, the barrel or the material tube and barrel must be heated until the plastic melts. The clamping units inside the screw inject plastic into molds or cavities. In order to maintain the proper temperature for various types of plastic material, the temperature inside the barrel must be properly controlled. Before the plastic reaches the injection mold, the cylinder transports, compacts, melts, agitates, and presses it.

3. Screw Motion or Reciprocating Screw

Plastic is fed through the barrel by the screw. Initially, the screw is turned as the pellets are fed from the hopper into the barrel, moving the material ahead as additional pellets are introduced. The flights also offer a constant mixing action that evenly disperses heat throughout the pile. The removal of different materials and any colors left over from a previous manufacturing cycle on the same injection molding machine is another benefit of mixing.

The thermoforming plastic receives the majority of its heat from the revolving screw. This is due to the screw’s diameter decreasing as it nears the tip. Plastic pellets are then moved by the flights, squeezed into a smaller area, and cut by turning flights as a result. Friction produced by this motion uniformly mixes the pellets and raises their temperature.

4. Heaters

Different kinds of heaters can be used in an injection molding machine to maintain temperature in conduits and nozzles as well as to heat molds and platens. The molding material in the hopper can be melted and turned into liquid by attaching a heating element to the barrel. Band heaters, coil/nozzle heaters, cartridge & strip heaters, and insulated fabric heating jackets are a few of the several types of injection molding heaters.

5. Nozzle

The machine’s ejector system’s bottom section houses the nozzle, an injection molding component. It forces the liquid plastic into the mold from the barrel. The sprue bushing and locating ring, a feature on the mold, against which the nozzle rests help center the nozzle on the mold. Today, nozzles may perform a number of tasks, such as filtering, mixing, and stopping the flow of melt.

Nozzle filters can reduce the likelihood of foreign particles or pollution in the melt stream clogging gates and hot runner tips. Mixing nozzles can improve additive dispersion and mixing, which improves the quality of molded parts while lowering additive volume and cost. In injection molding processes where the press often disengages from the mold, such as in many two-shot molding applications, shut-off nozzles help lessen drool.

6. Extraction Pins or Ejector Pins

Ejector pins are essential while making components. They are a crucial part of the ejection mechanism in molds, which controls how goods turn out when injection molding is done.

The A and B sides make up the two portions of the metal injection mold. Both sections are separated to extract the solid plastic when the molten material in the mold has cooled. When an injection mold is opened, the A-side half is lifted, leaving the produced portion and the B-side behind.

On the B-side of the injection mold, there are extraction pins that are used to extract the produced part from the mold (or extract it). The pin mark is frequently visible as a dent on completed goods.

Ejector pins come in a variety of designs. Ejector pins that are through hard are heat-treated to maintain consistency in the hardness throughout the pin’s diameter. A case-hardened ejector pin is appropriate for die-casting ejection systems since it is substantially harder than through-hardened pins. Black surface treatment is applied to a black ejector pin, enabling it to self-lubricate and endure temperatures of up to 1000°C.

7. Split Molds

A parting line in injection molding is the place where two mold halves converge when the mold is closed, particularly on a split mold. The line dividing the two molds along which the plastic product produced by the injection mold is divided into two sections is known as the “parting line.” Split molds are one kind of injection mold in which the mold cavity is formed by the jaws. When the mold opens with a pull tab, the jaws are first injected diagonally on the nozzle side and then transferred diagonally to the outside. The injection-molded component is then let go.

The ejector side can also be used to guide the jaws. Then, either before or after the mold is opened, they are moved, typically using hydraulic cylinders or mechanically with springs or air.

8. Clamping Unit

The clamping unit’s functions include opening and closing an injection mold and ejecting the items that have been injection molded. The hydraulic and toggle designs are the two primary types of clamping mechanisms. While the toggle clamp system uses a number of links, the hydraulic clamp system uses one or more hydraulic cylinders.

The clamping mechanism holds the injection mold using two sizable clamping plates. Two steel components are fastened to each of the substantial plates on the clamping unit to form a mold. The clamping device shuts the two separate plates when the machine is ready to inject plastic into the mold or cavity. Building the part, allows the plastic to flow into the hollow. The plastic part is then solidified by cooling. The clamping unit opens the injection mold when the plastic is cool enough, and the part drops out of the mold halves and is collected in a container.

The clamping unit also includes tie bars, a stationary platen, a moving platen, and machine ejectors.

9. Injection Unit

The injection unit, which is made up of various pieces, is a crucial part of injection molding equipment. The raw material is to be melted and directed into the mold by the injection equipment. The hopper, barrel, and screw make up the injection unit. The coloring pigment or other reinforcing additives are mixed with the dried and placed in the hopper polymer grains.

The granules are heated, combined, and driven toward the mold by the screw motion as they are fed into the barrel. In order to help raise the pressure to the proper levels and melt the material, the screw and barrel have the same geometry.

10. Hydraulic Unit

For machinery used in plastic injection molding, a hydraulic system or component is essential. It’s possible that the system runs nonstop throughout production cycles. There are a lot of motion-activated sub-circuits needed for the nozzle approach, injection of the plunge screw, extruder screw rotation, and mold closure. When flowing into the mold during the screw rotation and plunge phase, granular plastic material needs to travel through the heated plasticized state smoothly, requiring a very steady motion. Any inconsistencies in the hydraulic motions could affect the quality of the injection-molded product.

In order to prevent the mold from opening and to maintain the position of the nozzles along the sprue of the injection mold, force control sub-circuits are also necessary. These processes are essential for injection molding. The mechanical pressure is adjusted and monitored by the hydraulic pressure.

A complex mechanism that calls for accurate hydraulic seals throughout the injection and packing process of the mold is the screw mechanism, which is driven by the hydraulic motor, connected with the cylinder that drives the screw forward. Modern advancements in seal technology, however, result in leak-free hydraulic machines suited for molding plastic goods that can even be employed in the food and medical sectors.

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