A Practical Approach to Scientific Molding

Figure 1.1 Injection unit

There is mechanical heating, caused by the friction or shear inside the barrel, from the plastic pellets being rubbed against the barrel wall and compressed inside the flights of the screw.

There is electrical heating, from the heater bands on the barrel. They are used from a cold start to heat up the barrel and plastic. After a proper amount of soak time (30 minutes), start to rotate the screw. The barrel heater bands are to maintain the temperature in the barrel so the plastic does not hit any cold spots.

Once the barrel is up to heat, start to extrude plastic through the barrel. About 80% of the heat comes from the shearing process and 20% from the electrical portion. In Figure 1.2 you can see the shaded sections representing the different sections of the screw.

In Figure 1.3 it is shown how the plastic in each section has a circular motion inside the flight. There is a melt pool on the back side so that as the screw rotates the melt pool pushes the unmelted pellets forward and up against the barrel wall. As the unmelted pellets rub against the barrel wall it creates friction, and that friction causes the pellet to melt and go into the melt pool.

There is a hydraulic unit and valves that provide the oil flow and pressure needed to inject the plastic.

The injection velocity set point will give and maintain the speed of the ram coming forward and it must have ample pressure and flow to push the plastic. To ensure the injection high limit pressure set point is never reached (pressure limited) the valve is either restricted or opened depending on the feedback it receives from the linear transducer on what velocity or injection speed is desired. Also understand the influence the injection velocity has on the rheological properties of the material: plastics typically show non-Newtonian behavior, which means the faster the material is shot or the faster the flow rate of the plastic, the thinner the material becomes and the easier it will flow.

The non-return valve (check ring) is what pressurizes the melt. It creates a seal on the inside of the barrel through the use of a sliding check ring, ball-check screw tip, and/or poppet check ring. There are also plunger-style screws that inject the plastic into the molds: there are no moving parts to this design.

And as the plastic is pushed forward the non-return valve seals off, not allowing any plastic to return behind it. If it does there is either a worn non-return valve or possible wear in the barrel. This will be discussed later in the book.

There are many different screw styles available today, with a multitude of materials available. The reciprocating screw provides the function of conveying the material, and compressing and heating it to prepare it for the next shot.

Length (L) is measured from the front of the screw to the end of the flights. Diameter (D) is measured from the highest point on the flight of the screw to the corresponding other side (see Figure 1.6). Keep in mind the value of L/D for the screw in the press: too short of an L/D results on non-melted pellets, while too long of an L/D and the result is too much residence time, which can burn or degrade the plastic.

This refers to the depth of the feed section flight (Figure 1.8) divided by the depth of the metering section flight (Figure 1.7). If there is a 3:1 compression ratio screw this means that the depth of the feed section flight is three times the depth of the metering section flight. The measurement is taken from the root of the screw to the top of the flight.

Compression ratios for materials:

• Low compression screws range from 1.5:1 to 2.5:1, and are for shear sensitive materials

• Medium compression screws range from 2.5:1 to 3:1, and are for general-purpose materials

• High compression screws range from 3:1 to 5:1, and are for crystalline materials

One way of determining whether the compression ratio is correct for the material is to check if the standard cycle creates black streaks or non-melts in the parts. If one of these two conditions exist, then the machine can have the wrong compression ratio screw for the application.

The profile of the screw refers to the number of flights in each section of the screw (see Figure 1.9). Some screws will have a profile of 10-5-5, which...