- The parison as a test bed for recycled materials in blow molding
- Recycled HDPE and PP for bottles: rheological properties and stretching behavior
- Melt homogeneity and internal mixing in the blowing of recycled containers
- Thickness control and mass distribution in the blown bottle
- Sagging, Collapse, and Parison Instability: Causes and Corrective Strategies
- Localized fragility, welding line and mechanical resistance in recycled bottles
- Typical defects of post-consumer blown containers: bubbles, opacity and shape memory
- Optimize molds, thermal profiles and additives for blow molding with recycled materials
How to Use Recycled HDPE and PP in Bottle Blow Molding While Ensuring Melt Homogeneity, Controlled Wall Thickness and Reliable Performance
Essay. Recycling of Post-Consumer Plastics. Chapter 21: Blow Molding of Containers in Recycled Plastics. Parison, Defects and Process Control
Blow molding of containers is one of the most delicate processes when working with recycled materials, especially HDPE and PP. Unlike the extrusion of pipes or profiles, blow molding requires a specific quality from the material: the ability to maintain its shape in an intermediate state between viscosity and solidity, the so-called parison state, that hot and still unstable cylinder which, a few moments after leaving the extrusion head, must be inflated and brought to its final geometry. It is in this phase that the limits and potential of recyclates become evident, because the parison does not forgive inhomogeneities, changes in viscosity, solid residues or incompatible phases. Every irregularity appears as a difference in wall thickness, lateral collapse, deformation, opacity or poor weld strength in the most stressed areas.
Recycled HDPE is the most common choice for blow molding, thanks to its inherent impact strength, good chemical stability and relatively predictable behaviour under shear. But in post-consumer materials, predictability becomes a relative concept: viscosity may show non-linear fluctuations, the chain length distribution may be broader, and the lack of bound additives—no longer present in recyclates—makes the matrix more sensitive to heat. The parison, which in virgin materials descends as a regular, coherent cylinder, may show sagging, sudden neck-downs, twisting or oscillations. The operator perceives it immediately: a parison that is too soft tends to elongate under its own weight; one that is too stiff has an internal memory that prevents a uniform distribution of wall thickness once blown.
Recycled PP, on the other hand, poses different challenges.
More crystalline and less ductile, it behaves in a more brittle way, especially when it comes from mixed streams that include homopolymer polypropylene, random copolymers, block copolymers and sometimes traces of foreign polymers. The parison in recycled PP can stiffen too quickly, creating difficulties during blowing and a less uniform distribution of wall thickness. Some areas may cool earlier than others, creating differences that, after blowing, appear as opaque bands, thin zones or unusual rigidity in the shoulders of the container.....© Reproduction Prohibited
