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.
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.
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)
- Flexible PVC
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.
Steel dies are typically made by a wire EDM process. Some “downstream” tooling may be necessary to ensure shape of profile.
Dies and parts are relatively inexpensive.
While plastics extrusions can be produced to consistent tolerances the designer must be aware that these are not the same as for machined parts or for metals extrusion and are generally greater. The tolerance bands applicable vary with the relevant dimension, the material used and with the manufacturer but in general BS 3734:1978 for extruded rubber products (Table 2 Class E 2) can be used as a guide. Specific tolerances for critical areas and non-critical tolerances must be discussed and agreed between customer and producer. Inevitably, the unit price increases with the number of tolerance dimensions and the tightness of the tolerances specified.
- Equipment widely available in all geographical areas. Short lead times.
- Relatively low tooling costs
- Inexpensive process
- Product combinations possible
- Design freedom
Design possibilities severely limited because of linear nature of process.
Examples of Applications
Typical applications/design possibilities
The following application examples have been chosen to illustrate possibilities and the same ideas and techniques can, obviously, be used in many fields such as:
The basic frame of the window is an extruded, un-plasticized PVC section. This section contains air gaps or chambers which are carefully designed to give the necessary thermal and sound insulation. The normal colour is white and the polymer is UV stabilized to prevent fading. New developments with co-extrusion and printing techniques allow the profile to be produced with wood-effect or coloured finishes. This basic profile is mitre cut and welded into a frame to fit the windows of the house exactly. Extrusions are also used to provide the essential sealing lips on the profile. By skilled design a system of extrusions is built up to provide outward opening windows, tilt and turn windows, patio doors, roller shutters and other elements of the glazing system of the house.
Extrusions are applied in many sealing applications where the designer has considerable choice in fixing method. A co-extrusion of hard and soft materials will allow the hard material to be screwed, nailed, stapled or glued to one sealing face and the soft material will still provide the required seal. A single hardness soft extrusion can be punched or stapled but may need a reinforcing rod. Alternatively, it may be clipped into one sealing face using a groove in the face as a location/fixing area. The designer can choose between these varied options and the extrusion manufacturer can provide advice on the technology available. Typical application areas are refrigerator door seals (which incorporate a magnetic extrusion for an airtight seal), car door and boot seals, acoustic cabinet seals and the window seals described above.
Modular drawer profiles
Drawer systems utilizing extrusions are available both as ‘Do-It-Yourself’ and professional kits. These illustrate important options for the designer: the ability to use an extrusion to provide variable length and width and the use of injection moulded corner pieces to provide the necessary jointing. The requirements for light weight and easy assembly rule out the use of welding and the assembly is built up using the clip-in corner pieces which give rigidity and professional finish.
The decorative trim strips seen on bedroom and other furniture are examples of two important techniques available. One is the ability to apply a foil to the extruded PVC to give a bright and attractive finish (an option which is often used in the automobile industry for trim and bumper strips although, in this case, special exterior foils and techniques are necessary). The other is the use of double sided tape for rapid and strong mounting of the profile. For fixing to smooth, flat surfaces i.e. furniture, a film tape is used but when the surface is not regular then a foam tape may be used to give the necessary surface conformance and adhesion.
More examples are possible but the engineering designer is seeking to innovate and, hopefully, those examples outlined above can help this innovation through increased awareness of the process and its capabilities.