Blow mould Introduction Blow Moulding There are three
Blow mould
Introduction: Blow Moulding • There are three main processes used by the blow moulding industry : – Injection blow moulding, – Extrusion blow moulding, and – Stretch blow moulding. 2
Injection blow moulding • Used for small bottles of less than 500 ml volume. • The process is scrap‐free, with accurate control of neck finish. • It is particularly useful for oval bottles. • However, this is not suitable for containers with handles. 3
Extrusion blow moulding • Used for bottles of 250 ml in volume or larger. Tanks as large as 1000 liter weight can be moulded. • Mould is less expensive. • Containers with handles are easily fabricated. 4
Stretch blow moulding • Used for bottles of 250‐ 2000 ml in size, and some times as large as 25 liter. • The molecular bi‐axial orientation of certain resins enhances stiffness, and barrier properties. 5
INJECTION BLOW MOULD 6
Injection Blow Mould • Injection blow moulding requires two moulds: – preform mould – blow mould. 7
• The blow mould consists of the bottle cavity, neck‐ring insert, and bottom‐plug insert. 8
Injection mould & blow mould: 9
Preform cavity • The preform mould consists of the preform cavity, injection nozzle, neck‐ ring insert, and core‐rod assembly. 10
• The preform cavity design is based on the following parameters: – length‐to‐diameter ratio (10: 1 or less), – ratio of preform size to maximum bottle size (3: 1 or less), – the parison wall thickness (2‐ 5 mm), – neck‐ring. 11
Neck design • A neck‐ring insert forms the finish or threaded neck section of the bottle. • The core‐rod assembly forms the interior of the preform and supports the parison or the bottle during transfer. 13
• During the blowing process, the neck‐finish area of the parison must be cooled to retain its shape; the remainder of the parison is kept hot for expansion in the bottle cavity. 14
• The neck‐ring insert is used in the bottle cavity in a manner similar to its use in the parison cavity. • The thread diameter dimensions in the bottle cavity are 0. 05– 0. 25 mm larger than in the preform cavity. 15
Core rod: 16
Mould Cooling • Normally, a blow‐moulded part is cooled externally in the mould. • Some internal cooling systems have been designed to speed up the product cooling process. 17
• There are three basic systems of mould cooling: –liquefied gas –Super‐cold air with water vapor –air‐ exchange method 18
1. liquefied‐gas system • Non‐inflammable liquefied gas (such as CO 2, or N 2) is atomized through the blow pin into the bottle immediately after the parison expansion. • This method has increased production rates by 25– 35%. 19
2. Super cold air system • A stream of very dry air expands the parison and circulates through the bottle. • Immediately after the parison expansion, a fine mist of water is injected into the cold air stream and turns into snow. 20
• As the snow circulates through the container, it melts and vaporizes. • At the end, the water mist is stopped first. The circulating air dries the interior before the mould is opened. • Production rates can be improved as much as 50%. 21
3. Air‐exchange system • In the air‐exchange system the plant air is circulated through the bottle and is exhausted after the parison expansion. • Production rate increases by only 10– 15%. 22
EXTRUSION BLOW MOULD 25
Extrusion blow mould • The extruded hot parison is captured between two mould halves. • For blowing the parison, air enters through a blow pin and expands the visco‐elastic stage plastic into the mould. 26
Parison formation: 27
Pinch‐off • Pinch‐off is a raised edge around the cavity in the mold that seals off the part and separates the excess material. • Pinch‐off land is the width of the pinch‐off blade that seals the parison. 28
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Cavity venting • All blow moulds are properly vented. • The air that first occupies the product area escapes rapidly when parison blows. 30
• In case of poor venting, the entrapped air in the mould prevents good contact between the parison and the mould cavity surface and the surface of blown part becomes rough. • Moulds are easily vented by means of their parting line through small holes. 31
Effect of vent size on the product wall: 32
Parison thickness control • Parison programming is required to control the wall thickness of the parison. 33
• There are two methods to control the parison wall thickness. – By varying the orifice size in the extrusion die – By varying the rate of extrusion. 34
1. Variable orifice method • Variable orifice method is suitable for the continuous extrusion blow moulding. • In this method the programming system varies the opening of the orifice. The orifice opening can be changed either by moving the mandrel or the bushing. 35
Die head of the variable orifice : 36
2. Variable rate of extrusion method • The variable rate of extrusion method is suitable for intermittent extrusion blow moulding. • This is suitable for a ram accumulator blow moulding machine. 37
Extrusion blow moulding 38
Parison formation 39
Parison “swell” and “sag” 40
Stretch blow mould 41
Comparison of blow moulding techniques 42
- Slides: 39