Injection Moulding

INJECTION MOULDING:

          This is the most common method of producing parts made of plastic. The process includes the injection or forcing of heated molten plastic into a mold which is in the form of the part to be made. Upon cooling and solidification, the part is ejected and the process continues. The injection molding process is capable of producing an infinite variety ofm part designs containing an equally infinite variety of details such as threads, springs, and hinges, and all in a single molding operation. 

          A plastic is defined as any natural or synthetic polymer that has a high molecular weight. There are two types of plastics, thermoplastics and thermosets. Thermosets will undergo a chemical reaction when heated and once formed cannot be resoftened. The thermoplastics, once cooled, can be ground up and reheated repeatedly. Thus, the thermoplastics are used primarily in injection molding.

There are four major elements that influence the process. They are:
• the molder
• the material
• the injection machine
• the mold

          Of these four, the injection machine and the mold are the most varied and mechanically diverse. Most injection machines have three platens. Newer models use just two platens and may be electrically operated as opposed to the traditional hydraulic models. They can range in size from table top models to some the size of a small house. Most function horizontally, but there are vertical models in use. All injection machines are built around an injection system and a clamping system.
          The injection system mechanism may be of the reciprocating screw type or, less frequently, the two-stage screw type. Also included is a hopper, a heated injection barrel encasing the screw, a hydraulic motor, and an injection cylinder. The system’s function is to heat the thermoplastic to the proper viscosity and inject it into the mold. As the resin enters the injection barrel, it is moved forward by the rotation of the screw. As this movement occurs, the resin is melted by frictional heat and supplementary heating of the barrel encasing the screw. The screw has three distinct zones which further processes the resin prior to actual injection.

          Injection is accomplished through an arrangement of valves and a nozzle, all acted upon by the screw and the hydraulic pump that pushes the resin into the mold. This so-called “packing action” occurs at pressures from 20,000 to 30,000 psi and higher. The temperature of the resin at this time is between 320o and 600o F. The clamping system’s function is to keep the plastic from leaking out or “flashing” at the mold’s parting line. The clamping system consists of a main hydraulic pressure acting on the mold platens and a secondary toggle action to maximize the total clamping pressure.
          The platens are heavy steel blocks that actually hold the mold tightly closed during the injection phase. Most injection machines have three platens. The “stationary” platen has a center hole that receives the injection nozzle and holds the cavity half of the mold. This platen also anchors the machine’s four horizontal tie bars. The “movable” platen holds the core half of the mold. This platen moves back and forth on the tie bars and as the mold opens, the mold’s ejection system of pins and posts expel the finished part. The “rear stationary” platen holds the opposite ends of the tie bars and anchors the whole clamping system. 

          All injection machines have some sort of safety interlock system that prevent access to the molds during the clamping and injection phases when the machine is operating semi-automatically. The operator removes the finished part, closes the door or gate, which sets in motion the next molding cycle. In full automatic operation, finished parts fall into a container, conveyor, or are removed by robot mechanisms.

ADVANTAGES:

1. Injection molding allows for high production output rates.

2. When producing your product you may use inserts within the mold. You may also use fillers for added strength.

3. Close tolerances on small intricate parts is possible with Injection Molding.

4. More than one material may be used at the same time when utilizing co-Injection Molding.

5. There is typically very little post production work required because the parts usually have a very finished look upon ejection.

6. All scrap may be reground to be reused, therefor there is very little waste.

7. Full automation is possible with Injection Molding.

DISADVANTAGES:

1. High set up costs - Moulds etc.

2. Complicated process.

3. can only be used for large quantities due to costs.