PP (Polypropylene): Applications, Advantages & Injection Molding Guide

PP, or Polypropylene, is a semi-crystalline thermoplastic polymer known for its outstanding chemical resistance, low density, and cost efficiency. It is one of the most widely used plastics in the world, applied across packaging, automotive, consumer products, and medical industries.

What Is PP?
Polypropylene is produced through the polymerization of propylene gas in the presence of a catalyst. It is classified into two main types: homopolymers and copolymers. Homopolymer PP is stronger and more rigid, while copolymer PP offers better impact resistance. PP features a low specific gravity (~0.90 g/cm³), making it ideal for lightweight applications.

Key Applications of PP
Thanks to its versatility, PP is used across nearly every industry:

Packaging
– Food containers, yogurt cups, and bottle caps
– Plastic bags and films
– Lids, trays, and rigid containers

Automotive
– Battery cases and fender liners
– Interior trim and bumpers
– HVAC ducting and consoles

Consumer Products
– Toys and housewares
– Furniture components
– Storage containers and textiles

Medical and Laboratory
– Syringes and diagnostic containers
– Pill bottles and specimen vials
– Disposable pipettes and centrifuge tubes

Electrical
– Cable insulation and conduit
– Electrical housings and junction boxes

Advantages of PP
– Excellent chemical and moisture resistance
– Lightweight with high tensile strength
– Good impact resistance, especially in copolymer forms
– Low water absorption and high fatigue resistance
– Easily colored, filled, or modified with additives
– Cost-effective and recyclable

Injection Molding Guidelines for PP

1. Drying Requirements
PP typically does not require drying under normal storage conditions, but drying may be beneficial for improved finish.
– Drying temperature: ~100 °C (optional)
– Time: 1–2 hours if moisture is suspected

2. Melt and Mold Temperatures
– Melt temperature: 220–275 °C (do not exceed 275 °C)
– Mold temperature: 40–80 °C (recommended ~50 °C for balance of speed and appearance)

3. Injection Pressure & Speed
– Pressure: Up to 1800 bar depending on part complexity
– Speed: High-speed injection preferred for smooth surface finish and good fill

4. Runner and Gate Design
– Circular runners and larger gates promote proper fill
– Gate diameter: 0.7–1.5 mm
– Submarine gates and edge gates commonly used
– Hot runners compatible with PP; avoid material degradation

Common Molding Issues
– Warping: Use uniform wall thickness and avoid sharp corners
– Sink marks: Optimize packing pressure and holding time
– Flash: Use proper clamping force and gate size
– Flow lines: Increase injection speed and gate location accuracy

Final Thoughts
Polypropylene stands out as a cost-effective, versatile material that offers solid performance in a broad range of conditions. Its easy moldability, chemical resistance, and recyclability make it an essential thermoplastic for modern product design and manufacturing.
Whether you are molding a food container, an automotive component, or a medical device, PP is a reliable material that balances durability, processability, and cost.
Need guidance on selecting the right PP grade or improving cycle times in molding? Our team can provide technical support and custom recommendations.