Used Injection Moulding Machines for Sale — 2,926

PROCESS OF INJECTION MOULDING

Injection molding is perhaps the most common and important of all plastic processing processes as the process is extremely versatile and can produce very complex shaped parts, with the help of multi-sided molds. Injection molding is one of the more common methods used to mold thermoplastics however, this method can also be used to mold thermosets and elastomers. But, modifications in equipment and operating parameters must be considered in order to avoid premature cross-linking of these materials. The injection molding process is capable of producing numerous varieties of part designs with intricate details such as threads, springs, and hinges, that too all in a single molding operation. Plastic pellets are fed into a heating chamber and are pushed along by a plunger until they are homogenized and fluidized. The fluid plastic is then forced under high pressure through a nozzle into a relatively cold mold. The fluid plastic conforms to the shape of the clamped mold and upon cooling and solidification, the part is ejected and the process continues.

INJECTION MOULDING VS EXTRUSION

Injection molding is a cyclic process similar to blow molding but in contrast to extrusion, which is a steady state process. The injection molding process is almost similar to the extrusion method except that in extrusion, the molten material is forced through a die. Hence, higher back pressures may be generated in single screw extrusion machines compared to injection molding machines. The primary function of the screw extruder in injection molding is simply to obtain a melt whereas in an extrusion machine, the screw must mix, homogenize and melt the material. Also to achieve better mixing of the polymers used, the screws used in extrusion process are longer than the ones in injection molding machines. Injection molding has a major advantage over extrusion process as the injection-molded products are discrete items with varying cross sections in each axis. Extruded products are very long and continuous, and have a cross section that is usually constant with respect to the axis of production. Melt viscosity of the plastic is one characteristic which often differentiates extruded resins from injection-molded resins at normal processing temperatures. Extruded plastics often have a higher melt viscosity.

INJECTION MOULDING MACHINE

The two principal units of an injection moulding machine are: Injection unit & Clamping unit. The injection unit which functions more or less like an extruder is responsible for the melting the polymer to the proper viscosity and injecting it into the mould. It consists of the following major components:

• Hopper
• Screw
• Barrel
• Hydraulic Motor
• Injection Cylinder

As the resin is fed from the hopper to the injection barrel, it is plunged forward by the rotation of the screw. During this rotational movement, the resin is primarily melted by the frictional heat supplemented by the heating of the barrel encasing the screw. The screw has three distinct zones (Feed zone, Compression zone and Metering zone) just like in an extruding machine which further processes the resin prior to actual injection. The screw also acts as a ram to inject the molten plastic into the mould. Non-return valve provided at the tip of the screw prevents the back flow of the melt along the screw threads. The most common injection system mechanism used is the reciprocating screw type, but the two-stage screw type is also used rarely in few cases. The clamping unit performs the following functions: holds two halves of mold in proper alignment with each other, opens and closes the mould at the appropriate time and finally keeps the mould closed by the application of a clamping force during the moulding cycle. The clamping system may be either hydraulic, mechanical or a combination of the two.

INJECTION MOULD

Injection mould is the important part of the system as it decides the size and shape of the part to be produced. They are custom designed and fabricated based on the requirements of the final product. The basic elements of an injection mould are: Cavity (Stationary platen), Core (Moving platen) and Distribution channel. The distribution channel consists of the passage through which the polymer melt flows from the nozzle to the mold cavity. Sprue, Runners and Gates form the distribution passage. Ejection pins are built onto the moving half of the mould to eject the molded part from cavity at the end of the molding cycle. Cooling system is made up of passages in the mold that are connected to an external pump through which water is circulated to remove heat from the hot plastic. The injection mould is also equipped with air vents to permit evacuation of air from cavity as the polymer melt rushes in. Various types of mould used are:

• Two-plate mold
• Three-plate mold
• Hot-runner mold

ADVANTAGES AND LIMITATIONS

The major advantages of the injection moulding process include its versatility in moulding a wide range of products, the ease with which automation can be introduced, high production rates, low labor costs, high reproducibility of complex details and manufacturing of articles with close tolerances and excellent surface finish. Through this technique, even parts with metal inserts can be produced. High tool and die cost is one of the limitation of this process. Although injection molding dies are expensive to produce, each die can be used to make several thousands of components at a very rapid pace, so that the cost per piece is very low. Process is economical only in mass production as the cost of mould is very high and hence not suitable for small runs. It is a precision technique capable of producing complicate and intricate shapes but the major challenge is to design the mould in such a way that the part is ejected out successfully. Medical components, air fix kits, toys, plastic cutlery, machine housings, washing bowls are some of the injection moulded parts.

Buying Used Injection Moulding Machines

You can find a large variety of used  injection moulding machines for sale from top class manufactrues such as Ferromatik Milacron, NETSTAL, Otto Männer ARBURG, HUSKY Injection Molding Systems, ENGEL, TOSHIBA, Stork, Haitian International, Billion, REP and Sumitomo (SHI) Demag  on TradeMachines.