Blow molding is a process for converting thermoplastics into hollow objects of desired shape and size. This is a batch process, involving a cyclic sequence of operations. The cycle is repeated automatically or semi-automatically with a short cycle time for mass production of a stream of molded parts. The sequence consists essentially of the following operations
Production of the preform or parison:
Positioning of the mould to surround and entrap the parison and cutting the parison from top with a knife/hot wire.
Forming the neck of the component if it is in the form of a container;
Introduction of air to blow the parison to the shape of the mould cavity, together with the simultaneous cooling of the mould and the moulding;
Opening of the mould to take out the moulded product;
Closing of the mould around the next parison;
A Blow moulding machine essentially consists of the following components:
1.THE EXTRUDER :
Blow molding process begins with an extruded tube or parison. The extruder is single-screw type with profile suitable for extrusion of polyolefins. The requirements of an extruder for processing HDPE are: grooved feed section barrel and 20-25: 1 L/D ratio screw with venting, shearing and mixing zones.
2. PARISON HEAD :
The Parison head, also known as die head, is a specialized from of tubular extrusion die. Its function is to deliver a straight parison in the correct diameter, length, wall thickness, and at the melt viscosity suitable for blow molding.Prior to being clamped in the mold; the parison is suspended unsupported in free air. To avoid undue deformation, it is necessary to extrude the parison vertically downwards.The Parison should carry as little evidence as possible of the weld line formed when the melt stream from the extruder flows around the torpedo.Continuous parison extrusion is used for containers up to a capacity of 60 litres. Accumulator heads are employed for containers ranging from 10 litres upto several cubic meters. A parison programming or profiling system is advisable to obtain finished product with constant wall thickness, eventhough it may have a variable cross section.
3. ACCUMULATOR SYSTEM :
The accumulator system is used for producing large-diameter thick-walled parisons for blow molding of large jerrycans and drums. It has been developed to overcome the problems of slow extrusion of such parisons which under the effects of gravity thin out or "sag" at the upper part adjacent to the die as the parisons approaches full length. In principle, an extruder is used to feed continuously an accumulator chamber from which the charge of the material is forced through the die by a piston. In this way, it is possible to produce heavy parisons at required intervals and at a sufficiently rapid speed to avoid thinning of the parison tube adjacent to the die.
4. BLOW MOULDS :
The blow mold consists of two halves, each containing cavities which define the exterior shape of the molding when the mold is closed. There are no cores to define the inner shape as it is blown by air. Single cavity molds are used when working with a single parison. When multiple parison are in use, a number of single cavity molds may be mounted on the machine platen, or the requirement may be met by a multiple cavity mold. Mold details will vary considerably according to the geometry of the product and the blow molding process in use.
5. PINCH-OFF-ZONE :
The pinch-off-zone performs two functions. It must weld the parison to make a closed vessel that will contain blowing air, and it must leave prinched-off waste materials in a condition to be removed easily from the blown article.
6. BLOWING AND CALIBRATING DEVICES :
The Blow pin introduces blowing air into the parison through what will became a neck or opening in the finished blow molding. The blow pin body has the secondary function of calibrating the bore of the bottle neck. If the blow pin is plunged into the neck after mold closing. It is possible to produce a flash-free bottle mouth. After the mold opens, the blow pin is stripped of by retracting through a stripper plate. The blowing pin contains channels for the circulation of blowing air and cooling water.
7. VENTING AND SURFACE FINISH :
When a parison is blown, a large volume of air must be displaced from the mold cavity in a short time. It is essential to provide venting to allow this air to escape. Unless a gloss finish is required on the molding, it is common practice to sandblast the cavity to a fine matte finish. This helps air to escape as the expanding parison touches the cavity face but it is not sufficient in itself. Vent slots may be cut at appropriate points into the mold parting face to a depth of 0.05 mm to 0.15 mm, and venting can also be provided within the mold cavity by means of inserts provided with vent slots, porous sintered plugs, or holes with a diameter not greater than 0.2 mm.
8. COOLING SYSTEM :
Efficient mold cooling is essential for economical blow molding. Typically up to 80% of a blow molding cycle is devoted to cooling. Molds are constructed as far as possible from high thermal conductivity alloys, and water cooling channels are placed as close as possible to the surface of cavities and pinch-off-zones.
The different techniques of blow moulding are illustrated below:
EXTRUSION BLOW MOLDING :
In extrusion blow molding, an extruder feeding a parison head is used to produce a parison vertically between the two halves of a blow mold. The mold halves are clamped to platens that are linked to a mold closing and clamping device. A blow pin is provided to inject air under pressure into the parison. Because blow molding is conducted at relatively low pressures, the construction of the machine and mold can be much lighter than is required for injection molding. Production rate can be increased by using multiple molds or multi-cavity molds with multiple parisons.
INJECTION BLOW MOLDING:
This process uses injection molding rather than extrusion to produce the perform. It has an integral injection unit and a multi-cavity mold assembly in which the mold cores are mounted on a rotary table. The cores also act as blowing pins and rotate in 120° steps between Injection, Blowing and Ejection stations.
Station 1 is the preform injection mold where the preform is formed over the core pin. The preform is like a test tube, with a hemispherical closed end an open bore on the other side, formed by the core pin. The thread and neck flange for a screw-top container, are directly produced by injection molding. While the preform is still hot and plastic, the injection mold is opened and the preforms, still on the core pins, are rotted to station 2, the blowing station. Here the preforms are enclosed within the blow mold, and are blown by introducing blowing air through the core pins. The blow mold is then opened, and the finished articles, still on the core pins, are roated to station 3, an ejection station where they are stripped off. The machine has three sets of core pins, so that the three stages take place simultaneously. The process is best suited for small containers (1-100ml).
In injection blow molding process the preform can be injection molded in a profiled shape similar to the finished product. As the neck is molded in injection stage, the quality is much superior to a blow molded neck. There is a no pinch-off scrap and no bottom seam. IBM bottles give better clarity and gloss compared to extrusion blow moulded bottles. Also, higher MFI injection moulding grades can be used by this process.
STRETCH BLOW MOLDING:
Stretch blow molding is designed to produce biaxial orientation in the blown articles by stretching the prefom axially before or during blowing. Conventional orientation, but there is no axial orientation. In SBM, biaxial orientation is accomplished by means of stretch rod that is advanced axially inside the preform at a controlled rate. These processes are in widespread use for producing PET bottles for carbonated drinks/mineral water and PVC bottles for mineral water Main advantage of SBM is to produce containers with higher gloss and clarity and with same stiffness/mechanicals at 30-40% reduction in weight compared to preform is injection molded or extrusion blow molded.Injection stretch B/M and Extrusion stretch B/M are the tow variations of this process depending on whether the preform is injection molded or extrusion blow molded.
BLOW / FILL / SEAL:
The Blow/Fill/Seal process is a complete packaging technique that integrates the blow molding and container filling steps. This provides for aseptic filling of the containers and is used for medical, pharmaceutical, food and cosmetics products. The process employs a two-part mold in which the container body mold cavity blocks are separate from the neck-forming members. The body mold closes on the parison which is blown normally by a neck calibrating blow pin. Immediately, with the mold still closed, the liquid products, precisely measured by the dosing unit are injected through the pin. The pin is then withdrawn and the neck is formed and sealsed under vaccum by the neck -forming members. Both mold parts are then open to eject a filled and sealed container. Small containers are formed by vacuum.Blow/Fill/Seal machines are specially adapted for clean and sterile working. MDPE and Polypropylene are leading materials for blow/fill/seal applications, in the form of high-purity food approved grades free of potentially harmful additives.
3D BLOW MOLDING:
As blow molding process is performed on a cylindrical parison, the process is not well suited to the production of technical articles with complex forms that deviate substantially form the parison axis. Such forms can be produced by conventional blow molding by using a very large parison which can blanket the entire mold cavity in its flattened form, thus resulting in excessive pinch off scrap generation.
Recent developments in parison handling equipment and in blow mould design make it possible to manipulate a relatively small parison into the complex mold cavity.
Hence the blow moulding is largely free from flash and scrap, offering considerable process savings. There are many such techniques, some of them proprietary and collectively they are known as 3D Blow Molding.
SEQUENTIAL COEXTRUSION BLOW MOULDING: This is a special multi-material technique used for production of technical articles. The different materials are chosen typically to contribute complementary mechanical properties and are present in distinct sequential zones in the finished part.
The equipment consists of two plasticizing units, each equipped with a melt accumulator working on "first in first out" principle and co-extrusion die. These are operated sequentially, typically in A-B-A sequence, to produce a parison with three distinct material zones in axial succession. The parison is subsequently blow moulded by normal techniques.
A typical example is an automobile air duct, in which, rigid end sections (PP) are joined by a central flexible zone (PP/EPDM).
The flexible zone allows for installation mismatches, accommodates thermal expansion and damps vibration noise. The rigid portions allow for direct connection to other mechanical elements in the assembly.