Profile extrusion
From Plastics Wiki, free encyclopedia
acactaelpas To manufacture plastic pipe, industry uses a process known as Profile Extrusion. This process is used to manufacture plastic products with a continuous cross-section such as; drinking straws, plastic evestroughing, decorative molding, window trimming and a wide variety of other products polymer melt into the hollow mold cavity under high pressure.
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Process
The plastic is fed in pellet form into the machines hopper (this machine is known as an Extruder), the material is conveyed continuously forward by a rotating screw inside a heated barrel being softened by both friction and heat. The softened plastic is then forced out through a die and directly into cool water where the product solidifies. From here it is conveyed onwards into the take-off rollers, which actually do the pulling of the softened plastic from the die.
The die is a metal plate placed at the end of the extruder with a section cut out of its interior, this cutout, and the speed of the take-off rollers, determines the cross-section of the product being manufactured. A simple way to understand this concept is to consider squeezing a toothpaste tube, the product comes out in a solid rod because of the opening at the end of the tube, if that opening had a different cross-section than the product produced would take on that new cross-section.
Raw Materials
Most common thermoplastic polymers can be used for extrusion and the material choice is dependent on both the performance requirements and on the economic constraints. It is here that the designer should seek specialist advice from the extrusion company or material suppliers.
Typical Materials for Plastic Profiles
- HDPE (High Density Polyethylene)
- LDPE (Low Density Polyethylene)
- LLDPE (Linear Low Density Polyethylene)
- PETG
- Flexible PVC
- Butyrate
- Polypropylene
- Polystyrene
- ABS
The most commonly used material for general purpose extrusions is PVC. The wide application of this material is due to cost, chemical resistance and its availability in various hardness and colours. The hardness of PVC can vary from the rigid type used for windows (Shore ‘A’ hardness of 100 or British Standard softness of 0) to the plasticized or soft version used for garden hoses (generally Shore ‘A’ 80 deg or BSS 38) and even down to very soft materials of Shore ‘A’ 60 deg (BSS 75) which have limited uses. The colour can be either matched to a colour sample or chosen from several hundred standard colours. PVC is a very versatile material but, as with all materials, there are limitations and again specialist advice should be sought for critical applications.
Tooling
Steel dies are typically made by a wire EDM process. Some "downstream" tooling may be necessary to ensure shape of profile.
Cost
Dies and parts are relatively inexpensive.
Advantages
- Equipment widely available in all geographical areas. Short lead times.
- Relatively low tooling costs
- Inexpensive process
- Product combinations possible
- Design freedom
Disadvantages
Design possibilities severely limited because of linear nature of process.
Applications
Process: Sheet extrusion • Profile extrusion • Pipe extrusion • Blown film extrusion • Cast film extrusion • Foam extrusion • Twin screw extrusion • Adiabatic extrusion • Coextrusion
Equipment: Plastic extruder • Feedscrew • Single screw extruder • Twin screw extruder • Extrusion machine manufacturers
See also: Plastic thermoforming • Blow molding • Plastic welding • Injection molding • Rotational molding
