The importance of reciprocating screw in injection molding machine
The biggest innovation to improve plastic injection molding in the 20th century came from one of the most primitive and basic machine types in history: the screw. Let us take a closer look at the clever application of this simple design and how it improves the plastic injection molding process.
How was the plastic injected in the first place? Injection Molding Screw Barrel

Before the 1950s, all injection molding was done using a relatively simple barrel and plunger system. Use the basic temperature control system to simply heat solid plastic pellets in the barrel. When the plastic is fully liquefied by heating, the plunger pushes the molten plastic into the barrel and into the mold. But this technique has many disadvantages.

Schematic diagram of the early patent application injection molding machine engineering
Early injection molding machines used simple heating barrels and plungers.

What are the disadvantages of early injection molding machines?
The main problem with the original design was uneven heat distribution throughout the molten plastic. In order for the internal mass of the plastic to reach the injection temperature, more heat is applied to the barrel wall. It is difficult to conduct this heat uniformly and consistently, and there is always a risk of overheating the external mass. If the plastic is left in an overheated environment for too long, it will degrade rapidly.

Graphic image showing thermal imbalance in a plastic injection molding machine
Uneven heat distribution of the barrel system

In order to overcome this problem, the plastic is kept low in the barrel to avoid degradation, so higher injection pressures and longer injection cycles are required. Both of these compromises impose restrictions on the mold design and the finished products made from them, as well as the chemical composition of the plastic raw materials themselves.

When was the reciprocating screw introduced into the injection molding machine?
Reciprocating screws were developed in the mid-1950s, and by 1960 they soon began to replace the old systems in Europe and the United States. One of the great advantages of the reciprocating screw design is that it helps to manage temperature in three key ways.

How does the reciprocating screw eliminate the core heating problem?
Since the screw passes through the center of the barrel of the injection molding machine, it replaces the inner core of the raw material, thereby eliminating an uneven heating source.

Diagram showing the improved heat transfer performance of modern plastic injection molding machines
The presence of the screw in the barrel eliminates the colder plastic area in the core.

If the core does not overheat, there is less risk of plastic degradation during processing.

What is the function of the thread on the reciprocating screw?
The threads on the reciprocating screw perform a number of important functions.

First, they help move plastic through the barrel. When the particles are fed from the hopper into the barrel, the screw rotates, pushing the material forward while adding more particles. Secondly, the propeller provides continuous mixing, so that the heat is evenly distributed throughout the material.

3D illustration showing plastic pellets entering the barrel of an injection molding machine
When the plastic is fed along the heated barrel, the rotating thread helps to mix the plastic.

By the way, this mixing also helps to clear the mechanism of different materials and colors left over from any early production process on the same machine.

How does the reciprocating screw help heat the plastic resin?
The reciprocating screw is responsible for providing most of the heat for the thermoformed plastic. This is because the diameter of the screw increases as it approaches the tip. The plastic particles pulled by the scraper are compressed into a tighter space and cut or cut by the action of the rotating scraper. This action creates friction, mixes the particles into a uniform consistency, and heats them to the proper temperature.

Diagram showing the reciprocating screw in a plastic injection molding machine
The cutting action of the thread can provide up to 90% of the heat required for forming.

When the plastic reaches the tip, 60% to 90% of the heat required to melt it comes from the screw, not the heated barrel.

How does the reciprocating screw maintain a consistent injection pressure?
The last advantage of the screw is the form of a non-return ring, which can maintain a consistent pressure at the tip of the screw entering the nozzle.

Graphic image showing the non-return ring and nozzle of the plastic injection molding machine
The non-return ring maintains pressure in the barrel.

When enough molten plastic fills the barrel in front of the tip, the reciprocating screw is pushed forward to inject the resin into the mold. When the mold is filled, it will try to resist this compression, which will force the non-return ring to rest on the thrust ring. This effectively locks in any back pressure, so the screw can now completely fill the mold.

Are all reciprocating screws the same?
The bimetallic reciprocating screw can resist the deterioration of abrasive plastics such as polystyrene or corrosive plastics such as PVC, which may generate chlorine when heated. Bimetal screws are more expensive, but more durable, allowing production machines to run longer without downtime for maintenance. In addition, the screw will wear out over time, which will adversely affect the temperature and pressure in the Conical Screw Barrel . Bimetallic screws can better maintain process control parameters to achieve consistent production.
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