The BOPP Film Production Process

What is BOPP Film?

BOPP stands for Biaxially Oriented Polypropylene. The term “biaxially oriented” refers to the unique manufacturing method where the polypropylene (PP) film is stretched in two directions:

1. Machine Direction (MD): The direction of the manufacturing line.
2. Transverse Direction (TD): The direction perpendicular to the machine flow.

This dual-stretching process aligns the polymer chains, significantly improving the film’s mechanical properties, optical clarity, and barrier resistance compared to non-oriented cast PP (CPP) or blown films.

Step-by-Step: The BOPP Film Production Process

The most common method for producing BOPP film is the Stenter Frame Process. Below is a detailed technical breakdown of the stages involved.

1. Raw Material Feeding and Dosing – BOPP Film

The process begins with the precise dosing of raw materials. The primary resin is Homo-Polypropylene (PP). However, to enhance performance and reduce production costs, manufacturers mix the resin with functional additives and fillers.

• Primary Resin: Isotactic Polypropylene.
• Additives: Slip agents, anti-block agents, and antistatic agents.
• Filler Masterbatch: High-quality CaCO3 Filler Masterbatch is often introduced here. At US Masterbatch, we recommend specific grades with ultra-fine particle sizes to ensure dispersion without breaking the film during high-speed stretching.

2. Extrusion and Filtration – BOPP Film

The mixture is fed into an extruder where it is melted at temperatures ranging between 200°C and 230°C.

• Filtration: The molten polymer passes through a filtration system to remove impurities. This is critical because even a microscopic particle can cause film breakage (web breaks) during the orientation phase.
• Co-Extrusion: Modern lines often use co-extrusion (3 to 5 layers) to create a skin layer for heat sealing and a core layer for mechanical strength.

3. Casting (The Chill Roll) – BOPP Film

The molten polymer is extruded through a flat die onto a casting drum (Chill Roll).

• Function: Rapid cooling solidifies the melt into a thick sheet (cast sheet).
• Importance: The cooling rate determines the crystallization of the $PP$. Controlled crystallization is vital for the subsequent stretching phases.

4. Machine Direction Orientation (MDO) – BOPP Film

This is the first stage of “Biaxial” orientation. The thick sheet passes through a series of heated rollers rotating at increasing speeds.

• Process: The film is stretched lengthwise (in the direction of the machine flow).
• Ratio: Typically, the sheet is stretched 4 to 5 times its original length.
• Result: The polymer chains align in the machine direction, increasing tensile strength and clarity.

5. Transverse Direction Orientation (TDO) – BOPP Film

After MDO, the film enters the “Stenter” oven.

• Process: The edges of the film are gripped by clips on a moving chain. As the film travels through the heated oven, the rails diverge (widen).
• Stretching: The film is stretched widthwise (transversely) up to 8 to 10 times its original width.
• Result: This creates the final thickness of the film (usually 12 to 60 microns) and balances the mechanical properties across the film’s surface.

6. Annealing and Surface Treatment – BOPP Film

• Annealing: The film is heat-set to relieve internal stresses, preventing shrinkage during later use.
• Corona Treatment: A high-voltage discharge is applied to the film surface. This increases surface energy (dyne level), enabling the film to accept inks, adhesives, and coatings effectively.

7. Winding and Slitting – BOPP Film

Finally, the film is wound onto large “master rolls” (jumbo rolls). These rolls are later slit into smaller widths according to customer specifications for printing, lamination, or bag making.

Optimizing BOPP Production with Filler Masterbatch

One of the biggest challenges in the BOPP film production process is balancing cost with quality. This is where US Masterbatch plays a strategic role.

Using Calcium Carbonate (CaCO3) Filler Masterbatch specifically designed for BOPP applications offers substantial benefits:

 

Benefit	Technical Explanation
Cost Reduction	Replacing a portion of virgin PP resin with CaCO3 significantly lowers raw material costs.
Improved Stiffness	CaCO3 increases the modulus of elasticity, making the film stiffer and easier to process in packaging machines.
Higher Productivity	The high thermal conductivity of CaCO3 allows the film to cool faster on the chill roll and in the stenter, enabling higher line speeds.
Better Printability	The filler creates a micro-roughness on the surface, improving ink adhesion (matte effect) and reducing the need for excessive Corona treatment.

Technical Note on Dispersion

Not all Masterbatches are suitable for BOPP. The stretching ratio in BOPP lines is extremely high (5 X 9 = 45 times area expansion). If the Filler Masterbatch contains coarse particles or poor dispersion, it will cause “cavitation” voids or film breakage.

At US Masterbatch, our BOPP-grade Filler Masterbatch utilizes ultra-fine CaCO3 powder coated with special processing aids to ensure perfect dispersion and high compatibility with PP resin.

Conclusion

The BOPP film production process is a testament to modern engineering, transforming simple pellets into a versatile packaging material. However, the efficiency of this process relies heavily on material science.

By integrating high-quality additives like those from US Masterbatch, manufacturers can not only reduce production costs but also enhance the physical properties of their final product.

Looking to optimize your BOPP film line?

Contact US Masterbatch today for a consultation on our specialized Filler Masterbatch solutions tailored for the plastics industry.