Blow Molding http://www.krk.co.jp/en/tech/blow.html Basics The principal methods used to process thermoplastic materials into finished or semifinished products are, in order of importance: screw extrusion, injection moulding, blow moulding and calendaring Blow moulding and calendering are often postextrusion operations. Hollow articles, such as bottles and other containers, can be manufactured by the blowmoulding process. The most common procedure is to extrude vertically downwards, from a crosshead die, a hollow thick walled tube or pari son of melt. This tube is surrounded by a split mould of the appropriate shape, one end being clamped around a spigot incorporating an inlet for compressed air, which is used to blow the parison into contact with the water-cooled mould. When solidified, the moulding is removed and its ends are trimmed of excess material. While blowing and cooling are being carried out, the mould unit, of which there may be several associated with one extruder, is removed from beneath the extrusion crosshead to allow a new parison to form. Figure 2.8 shows a simplified view of this process. In figure 2.8a, the two halves of the mould are about to close on the parison, thereby severing it at the top and squeezing it into contact with the spigot at the bottom. Figure 2.8b shows the parison blown into contact with the mould. Both the elastic properties of the melt and the effect of gravity on the suspended parison affect its thickness distribution, and only relatively modest dimensional accuracy can be achieved. Also, the amount by which the diameter of a parison can be increased by blowing is limited. In addition to extrusion blow moulding, there is also an injection blowmoulding process, in which the material to be blown is first injection moulded. Although better dimensional accuracy than in extrusion blow moulding can be achieved, the process is more expensive Blow Molding Modified extrusion and injection-molding process Characteristics Hollow thin-walled parts of various sizes High production rates Low cost for making beverage and food containers General Process The process can be simplified into 3 steps 1) Melting the resin 2) Forming a preform tube 3) Blowing the preform tube into the shape desired Steps 1 and 2 can be done either in an extruder or in an injection molding machine. http://upload.wikimedia.org/wikipedia/en/9/97/Recip_Machine_Trimmer.png Methods There are two types of blow molding Extrusion Blow Molding Injection Blow Molding Injection Molding gives rise to a third process called stretch blown molding There are fundamental differences in the these two types of blow molding. Extrusion Blow Molding Extrusion blow molding: (1) and (2) parison is pinches at the top and sealed at the bottom around a metal blow pin as the two halves of the mold come together (3) the tube is inflated so that it takes the shape of the mold cavity (4) mold is opened to remove the solidified part. http://www.sinotech.com/products/plastic/blowMolding.htm Problems The continuous nature of the extrusion process is a problem with extrusion blow molding. Six ways to solve the problem of interference: 1. 2. 3. 4. 5. 6. Intermittent Extrusion Blow Molding Continuous Extrusion Blow Molding Accumulator Head (Ram Extension) Rising Mold System Parison Transfer System Multiple-Mold System Figure: Parison transfer system for continuous extrusion blow molding. Solving Interference Problems Intermittent Extrusion Blow Molding • Stop the extrusion of the parison while the part is cooling in the mold. Continuous Extrusion Blow Molding • An accumulator receives the material from the extruder and the creates the parison as needed. Accumulator Head or Ram Extrusion • The extruder output flows into a reservoir that is external to the extruder itself. A ram pushes the resin out of the reservoir and through the die that is mounted at the end of the accumulator to create the parison. http://upload.wikimedia.org/wikipedia/en/4/40/Rotary_Wheel_Blow_Molder.png Solving Interference Problems Rising Mold System • When the mold closes on the parison and air injection has occurred, the mold is moved off to the side. The pinching off the mold creates a break point in the parison so that when the mold moves aside, the parison is not disturbed. Parison Transfer System • The parison is cut from the die by a knife and transferred by a mechanical arm to a mold where it is blown, cooled, and ejected. Multiple-Mold System • Several Molds are mounted to a rotating wheel. While one mold is closing the capture the parison, the mold ahead is in a position for the blowing of the part, and other parts are closed for cooling, and further up a mold opens to eject the part Plastics Materials and Processing Injection Blow Molding http://www.sinotech.com/products/plastic/blowMolding.htm Injection blow molding: 1. 2. 3. 4. Parison is injection molded around a blowing rod Injection mold is opened and parison is transferred to a blow mold Soft polymer is inflated to conform to a blow mold Blow mold is opened and blown product is removed. Extrusion vs. Injection Blow Molding Extrusion • • Bottles over ½ pound • Tooling costs are 50% to 75 % less • Generates 20 to 30% scrap Additional machinery needed to grind scrap • Total cycle time is shorter • Wider choice of resins • Greater flexibility in part design Injection • Long runs and smaller bottles • No trim scrap • Higher accuracy in the final part • Uniform wall thickness • No seam lines or pinch marks • Better transparencies • Improved mechanical properties Stretch Blow Molding http://plastics.turkavkaz.ru/processes/molding/blow-molding/stretch/ A: Stretch-Blown Pin; B: Air Entrance; C: Mold Vents; D: Preform; E: Stretch Rod Extended; F: Cooling Channels http://plastics.turkavkaz.ru/processes/molding/blow-molding/stretch/ Stretch blow molding produces a part with biaxial molecular alignment. In the process a preform, or parison, elongated mechanically in the mold and than expanded radially in a blowing process. A desirable resulting molecular orientation yields a material with increased strength. This means that products that are strength-based designs can be produced using less material than if they were to be produced using simpler blow molding techniques. Materials Considerations One critical requirement is that the polymer must have good melt strength. The common blow molded plastics are: • Polyolefins – HDPE – LDPE – Polypropylene (PP) • PVC • PET http://upload.wikimedia.org/wikipedia/en/2/2a/Soft_drink_shelf.JPG PET bottles are excellent barrier materials and are widely used for soft drinks Polyolefins ADVANTAGES The polyolefins are easy to process and have the advantage of good electrical properties, moisture resistance, and low cost. DISADVANTAGES Sensitive to oils and can have stress crack problems PROCESSING PARAMETERS These resins are usually blow molded in the range of 320o – 410oF. Mold temperatures should range from 50o – 100oF. Polyvinyl Chloride (PVC) ADVANTAGES Very desirable properties of clarity, smooth surface low glass and aroma permeation, high stiffness with low wall thickness, high chemical resistance, and easy labeling and printing DISADVANTAGES Very temperature sensitive, therefore not normally injection blown because the process requires two heat cycles PROCESSING PARAMETERS Typical melt temperature range for PVC is 375o – 420oF. Polyethylene terephthalate (PET) ADVANTAGES Easily recycled, very lightweight, it makes a good gas and fair moisture barrier. It is strong and impact-resistant. It is naturally colorless and transparent. DISADVANTAGES Have to be careful to avoid a temperature that is two high or acetaldehyde forms. PROCESSING PARAMETERS Temperature range for PET to blow mold is 200o – 212oF. Mold temperatures should be in the range of 35o – 40oF Conclusion – Wrap Up ADVANTAGES RAW MATERIALS APPLICATIONS This process lends itself to any designs involving hollow shapes. Equipment availability is good in most geographical locations. Can save tooling dollars over injection molding. Most commodity grade and engineering grade resins may be blow molded. The viscosity of the plastic must be high to keep the parison from stretching too much prior to mold closure. All types of bottles, toys, air ducts for automobiles, chemical & gasoline tanks, household goods. Can be obtained from these blow molding processes. DISADVANTAGES Cycle times are slower than injection molding. Piece prices are higher than injection molding.
