In extrusion, thermoplastics are melted to a viscous mass in a screw and then pressed into shape through a shaping die.
Machine parameters for the success with Effect pigments
Masturbates or compounds are usually used to color the molten mass with effect pigments. For a satisfactory result in plastic extrusion with effect pigments, a balanced ratio must be maintained between the mixture energy and pigments which are as undamaged as possible. Excessive shear from mixing sections or inappropriate screws or filters destroy effect pigments and dramatically decrease the pearl luster effect.
The orientation of the pigments is a requirement for an even effect. This has to be ensured in the process through an corresponding engineering and design of the machinery.
Special possibilities in Co-extrusion
Co-extrusion is used to combine different materials or the same materials in different colors or effects. The two materials are combined into one flow in the co-extrusion die. If an effect pigments is used in the surface layer, it is possible to increase the effect strength and by the same time saving cost due to a thin layer and the perfect orientation of the effect pigment on the visible surface However, it must be taken into account that, due to the much thinner co-extrusion layer, a higher concentration of effect pigment must be used than in solid layers in regular extrusion – this could mean up to ten percent in the plastic, depending on the layer thickness and desired effect. Less pigment is needed when the entire layer mass is colored.
The inner layer in co-extrusion usually uses high coverage. It is also called the substrate layer because of its much higher layer thickness. Functional materials can be used in this, such as to improve the barrier or other properties, or internal recycling materials can be used to save raw materials and boost waste reduction and economy. Effect pigments are seldom used in this layer.
This technique can be used in all extrusion processes, including extrusion blow molding.
The size, shape, alloy, extrusion ratio, tongue ratio, tolerance, finish, and scrap ratio are interrelated in the extrusion process as are extrusion speed, temperature of the billet, extrusion pressure and the alloy being extruded.
In general, extrusion speed varies directly with metal temperature and pressure developed within the container. Temperature and pressure are limited by the alloy used and the shape being extruded. For example, lower extrusion temperatures will usually produce shapes with better quality surfaces and more accurate dimensions. Lower temperatures require higher pressures. Sometimes, because of pressure limitations, a point is reached where it is impossible to extrude a shape through a given press.
The preferred billet temperature is that which provides acceptable surface and tolerance conditions and, at the same time, allows the shortest possible cycle time. The ideal is billet extrusion at the lowest temperature which the process will permit. An exception to this is the so-called press-quench alloys, most of which are in the 6000 series. With these alloys, solution heat-treat temperatures within a range of 930°-980° F must be attained at the die exit to develop optimum mechanical properties.
At excessively high billet temperatures and extrusion speeds, metal flow becomes more fluid. The metal, seeking the path of least resistance, tends to fill the larger voids in the die face, and resists entry into constricted areas. Under those conditions, shape dimensions tend to fall below allowable tolerances, particularly those of thin projections or ribs.
Another result of excessive extrusion temperatures and speeds is tearing of metal at thin edges or sharp corners. This results from the metal’s decrease in tensile strength at excessively high-generated temperatures. At such speeds and temperatures, contact between the metal and the die bearing surfaces is likely to be incomplete and uneven, and any tendency toward waves and twists in the shape is intensified.
As a rule, an alloy’s higher mechanical properties means a lower extrusion rate. Greater friction between the billet and the liner wall results in a longer time required to start the billet extruding. The extrusion ratio of a shape is a clear indication of the amount of mechanical working that will occur as the shape is extruded.
Extrusion Ratio = area of billet/area of shape.
When the extrusion ratio of a section is low, portions of the shape involving the largest mass of metal will have little mechanical work performed on it. This is particularly true on approximately the first ten feet of extruded metal. Its metallurgical structure will approach the as-cast (coarse grain) condition. This structure is mechanically weak and shapes with an extrusion ratio of less than 10:1 may not be guaranteed as to mechanical properties.
As might be expected, the situation is opposite when the extrusion ratio is high. Greater pressure is required to force metal through the smaller openings in the die and extreme mechanical working will occur. Normally acceptable extrusion ratios for hard alloys are limited to 35:1 and for soft alloys, it is 100:1. The normal extrusion ratio range for hard alloys is from 10:1 to 35:1, and for soft alloys is 10:1 to 100:1. These limits should not be considered absolute since the actual shape of the extrusion can affect results. The higher the extrusion ratio, the harder the part is to extrude which is the result of the increased resistance to metal flow. Hard alloys require maximum pressure for extrusion and are even more difficult because of their poor surface characteristics which demand the lowest possible billet temperature.
Difficulty factor is also used to determine a part’s extrusion performance.
Factor = Perimeter of Shape/ Weight per Foot.
Weight per foot is of primary importance because of the consideration for profitable press operation. As might seem obvious, a lighter section normally requires a smaller press to extrude it. However, other factors may demand a press of greater capacity such as a large, thin wall hollow shape. Though it has low weight per foot it may take more press tonnage to extrude it. The same reasoning applies to the factor as with the extrusion ratio. A higher factor makes the part more difficult to extrude consequently affecting press production.
The tongue ratio also plays an important role in determining a part’s extrusion performance. The tongue ratio of an extrusion is determined as follows: square the smallest opening to the void, calculate the total area of the shape, and then divide the opening squared by the area.. The higher the ratio, the more difficult the part will be to extrude.
In order to help us understand your needs and requirements and service you better, the following is a check list of things to consider when submitting items to an extruder for quoting or new business:
- Description or drawings of the part- talk to the extruder early before the design is finalized.
- Specifications to be met; Federal specs, military, ASTM, etc.
- Alloy and temper; if unknown, indicate requirements for strength, corrosion resistance, machinability, finish, weldability, to aid the extruder in making a recommendation.
- End use length and purchase length.
- Tolerances; commercial, per drawing, other.
- Surface Finish; mill, anodize, paint, exposed surfaces, etc.
- Packaging; acceptable maximum and minimum weight per package and shipping and handling requirements.
- Secondary fabrication requirements-mitering, punching, bending, anodizing, drilling, etc.
- Product end-use.
- Quantity needed; this order and on an annual basis.
- Shipping date.
- Special quality considerations.
Coextrusion is the process of pressing two or more materials through the same die to produce a single piece. When multiple plastics are combined, the result can yield properties distinct from those of a single material. Coextrusion has opened up new frontiers in material engineering and addressed several previously difficult manufacturing needs.
Coextruding a stripe of radiopaque plastic into a catheter, for example, improves x-ray quality as the catheter moves through a vein without compromising the effectiveness of the catheter itself. Coextrusion can also reduce costs by using recycled and reground scrap inside virgin material for handrails, fences and other applications. The process can be seen in projects as diverse as tubing and structural components or air blown food containers.
The Coextrusion Process
In standard extrusion, solid plastic pellets are gravity fed into a forming mechanism, where jacketed compression screws melt and feed the materials into a die. By contrast, coextrusion involves multiple extruders forming layered or encapsulated parts. Sometimes five or more materials are used in a single cycle, with each extruder delivering the precise amount of molten plastic needed for the operation..
Unlike ordinary plastic mixing, each individual plastic retains its original properties, but is combined into a compound-material part. If mixed prior to extrusion, the characteristics of the individual materials may be altered, but the end result is a homogeneous product.
Not all plastics are suitable for coextrusion because some polymers will not adhere to others, although introducing a conductive middle layer can often solve this problem. Plastics with drastically different melting temperatures are also unsuitable for the process, as degradation will occur in the lower melting material. Finally, PVC and acetals should never be coextruded together because of the potentially violent reactions that can occur when they are joined.
The multi-colored drinking straw is a good example of coextruded tubing’s design features. Striped tubing also serves many purposes in the medical field, in which stripes and colors can denote different chemicals. Coextrusion can produce internally hardened tubes through which a cable can be run while retaining the tube’s flexibility. Other tubes benefit from a high performance liner impervious to corrosives, or an inexpensive coating to add bulk and stability. In addition, plastic fiber optic cables are composed of a coextruded cable and jacket.
Coextruded Structural Units
Plastic is sometimes used as a substitute for wood. Manufacturers can create decking, boat docks, fences and dimensional components with “plastic lumber,” which has some advantages over natural wood. Coextrusion is a cost-effective method of fabricating many of these artificial materials. It can add titanium dioxide, a weather resistant material, to exterior structural plastics or produce decking with an inner layer of recycled plastic.
The particles of polymer produced in the dispersion polymerisation process are of the order of 0.2µm in size, whilst those from a granular polymerisation are hundreds of µm in size, built up from smaller particles.They are both highly crystalline – about 90 to 95%.
The dispersion particles can be studied directly in a conventional transmission electron microscope, providing the electron intensity is kept low. On raising the electron beam intensity the particles change rapidly in appearance to become transparent with a crumpled texture. At this stage the crystallinity has disappeared and the ‘particles’ probably consist of a shell of carbon. Electron diffraction patterns and dark field micrographs suggest that the particles are composed of a pile of small single crystals with the molecular axis along the axis of the brick-shaped particles . The particles also appear to have a striated surface structure generally parallel to the long axis. A replica of some dispersion particles. During coagulation, the dispersion particles aggregate to form a larger particle, made up of a loose structure of agglomerates of the primary particles. During cold, lubricated extrusion the agglomerated particles are highly distorted, with their primary particles becoming aligned and also drawn into fibrous material.
During the early stages of polymerisation granular particles form as aggregates of smaller particles. This process continues and large irregular fibrous structures are produced.This material is then modified mechanically to reduce it to the familiar form suitable for processing.