Plastic Molding Method: Injection Molding

Nowadays, injection molding has become the most common method of manufacturing plastic parts, it is ideal for mass production.

Many things can be made by injection molding, such as buckle, packaging, bottle caps, automotive parts and components, toys, mechanical parts (including gears), components of medical products, one-piece chairs and small tables, storage containers, etc. 

In order to facilitate the molding process, engineers must be very carefully to consider all molding conditions, below elements must be taken into account while designing:
1. The material that injected
2. The material of the mold
3. The desired shape and features of the part
4. The properties of the molding machine

How:

Heat the plastic material until it becomes fluid, deliver the molten material with plunger or screw to injection nozzle at the same time, and then inject the material into mold cavities, after the molded plastic article cooling down and curing, eject it out with ejector pin. 


Advantages:

1. Can be used to manufacture plastic parts in complicated shape or different material
2. Suitable to be used for mass production because of the efficiency
3. Fine dimensional stability
4. Can be fully automatic
5. With less material wasted


Disadvantages:

1. Unavoidable results including parting line, ejector pin marks, or gate marks are usually present on the finish products
2. The cost of mold and equipment is high
3. The overall structure of the mold is complicated, thus it's not easy to do further processing


Notices:

The quality of the mold is one of the important parts during production, in order to have nice performance and benefit from the production, it is necessary to select suitable die steel.

Ideal die steel should have below features:
1. Easy to polished
2. Corrosion resistance
3. Crack resistance
4. Transcalent
5. Abrasion resistance


Here're two different kinds, including Plunger Injection Molding and Screw Injection Molding:

1. Plunger Injection Molding

The crucial factors during plastic injection including the pressure of the mold interior / exterior and the temperature of molten plastic. As the temperature goes higher, the viscosity of molten resin decreases and becomes more fluid, which means it can be easily filled up all parts of the mold cavity.

The torpedo-like spreader in the injection machine conduces to the spreading of plastic material, the molten plastic material can be evenly distributed and molded quickly.


How:
Heat up the thermoplastic until it becomes fluid, force the molten plastic into mold cavity with pressure. After the material has been shaped by the mold, waited for few times until the plastic molded article cooled down and cured, finally, eject it out with ejector pin.


2. Screw Injection Molding

The screw injection molding is the enhanced type of plunger injection molding, it contributes to the temperature cycling during injection molding.


How:
Place the material into the funnel, it will then drop into the heated and rotating barrel. With the high temperature in the barrel, the material plasticized gradually and go forward through the rotating screw, the molten material accumulated till it achieves fixed quantity, then the plunger goes down to make the screw stops rotating, meanwhile, the plunger is raised by hydraulic pressure to inject the molten plastic into the mold cavity.

Before the plastic material injecting to cavity, in order to prevent the polymerization in cylinder, it is necessary to cool down the front side of cylinder by cooling water, and do not eject the plastic article out until it is totally cooled down.