MACHINE SELECTION of injection moulding

Selecting an Injection molding machine with proper design is critical to molding a quality part and ensuring economic success.

Below are 3 important molding machine points to consider when moulding operation with Polypropylene. These issues are discussed in detail here.

• Proper Screw and Barrel selection.
• Proper nozzle tip type.
• Clamp requirement.
  

Screw and Barrel Selection

The screw performs the following function in the injection molding process.

1. Conveys the material through the barrel.

2. Mixes the material to the proper molten state.

3. Compresses the material to maximum density.

4. Forces the material into the mold.

Adjustments to the injection process often involve the screw to some degree. The screw, when perform its function provides important contribution to the overall process.

The screw speed for the polypropylene material ranges 50-125 rpm.

Below are several process parameters that are affected by screw design.

• Material melting profile.
• Melt Temperature.
• Material Mixing.
• Shearing of the resin.
  

Therefore it is important to be aware of the type of screw that is being used for each applied specific Barrel and screw designs are recommended for maximum machine wear resistance and proper material plasticization.

While moulding Polypropylene products, should be aware of two critical parameters are L/D ratio and compression Ratio.

(L/D)= (Flight Length of screw)/ (Outside diameter of screw)

Compression Ratio= (Depth of feed section)/ (Depth of metering section)

The recommended L/D ratio and compression ratio for Polypropylene are tested. This will ensure proper melt dispersion.

Polypropylene material type L/D Compression ratio

Unreinforced homopolymers mm, 20:1 3:1

Reinforced material mm, 20:1 2.5:1

Barrel Sizing:

When choosing a press in which to run a mold, it is important to check the shot size as a % of the total barrel capacity. Most barrels are rated ounces of general-purpose system (GPPS) to know the shot capacity of any material other than GPPS, the GPPS once rating must be converted to the density of the other material.

For polypropylene resins one must insert the specific gravity (S.G. - a measure of density) as shown in formula.

Barrel Size X S.G (PP) = Barrel size

(Tons GPPS) S.G (GPP) (Tons PP)

Nozzle Tip Type

Reverse Taper "Steel" tip nozzles recommended when molding Poly propylene products. The reverse taper will minimize material drool and stringing, which can be encountered when molding crystalline resins. The reverse taper design is suggested when molding unreinforced polypropylene, and may be used with reinforced grades.

However the nozzle bore diameter are recommended to be approximately 25% larger for reinforced Poly propylene products than for unreinforced polypropylene due to the higher material viscosities encountered.

Typically, general-purpose nozzles are recommended when using reinforced grades. Temperature control on the nozzle should be controlled separately from other barrel zones. Extended nozzles may require two or more zones of control.

Clamp Requirement:

The injection moulding machine clamp is used to close the mold, hold it closed during the injection and curing of the plastic material, and open the mold for removal of the formed part. The requirements in terms of clamp area, tonnage, clamp opening force, open Day light closed daylight. There are three different types of clamp designs; any one of them can be used for clamping purpose during moulding.

1. Straight Hydraulic Clamp.

2. Toggle Clamp.

3. Hydro- mechanical Clamp.