Blow Moulding

Blow mould: Blow moulding is a technique employed for manufacturing products which are essentially followed. Blow moulding process consists of two stages of operation producing a pre-form called parson and processing of the parson. It is most popular method of plastic. It is widely used for producing bottles or other hallows objects from thermoplastic method. It is a method of fabrication in which a worm plastic is blacked between the two holes of a mould and forced to assume the shape of that mould cavity by use of pressure.

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Blow moulding is a method of fabrication in which a warm plastic (hollow tube) is placed between the two halves of a mould (Cavity) and forced to assume the shape of that mould cavity by use of pressure. 

Blow moulding is used for producing bottles or other hollow objects from thermoplastic materials

Blow moulding: Blow Moulding process is widely used for producing bottles or other hollow objects, due to its least expensive & simplest process to manufacture. It offers a number of processing advantages, such as moulding of irregular curves, low stresses, variable wall thickness, the use of polymers with high molecular weight & favorable moulding cost.

An extruded person tube of heated thermoplastic is positioned between two halves of an open split mould and expanded against the sides of the closed mould via air pressure. The mould is opened and the part ejected. Low tool and die costs, rapid production rates, and ability to mould fairly complex hollow shapes in one piece. Generally limited to hollow or tubular parts; some versatile mould shapes, other than bottles and containers. Extrusion Blow moulding, Injection Blow moulding, Stretch Blow moulding.

Process:

1. In this process, firstly the thermoplastic is fed into a heated barrel of an extruder and then with the help of a screw rotation the plastic is melted and homogenised. 

2. The melt is forced to pass through a set of die to form a tube or parison. 

3. The soft, deformable parison is introduced into a mould; the mould closes and pinches off at one open end of the parison. 

4. The blow pin is inserted through the open end of the parison to form a neck. 

5. Finally the air is introduced through the blow pin to inflate the parison inside the mould and at this stage the molten polymer copies the design and details of the mould. 

6. Lastly the moulded product is cooled, then mould opens and the hollow container is ejected by mechanical or pneumatic force. 

7. In the finishing stage, the bottle undergoes trimming, finishing, printing, labelling and decorating.  

Raw Materials:

HDPE, PET, Polypropylene, LDPE, PVC, Polycarbonate, ABS, EVOH, LLDPE, TPO, PBT, Nylon, TPE, ABS/PC Blend, Polystyrene, MDPE, PUR, PETG and PPO etc. 

1. Injection Blow moulding (IBM): An ideal combination of Injection & blow moulding. It includes three stages: Formation of perform by using injection moulding. Transferring the perform into the blow moulding. Passage of air or gas pressure through the core rod to blow the thermoplastic resin into the cavity walls of the blow mould.

1. Accurate shaping of the neck.

2. A high level of dimensional accuracy.

3. Minimal weight tolerances.

4. The highest quality surface finish.

5. Production absolutely free from waste.

Injection blow moulding is a two stage process since the parison is produced in a separate operation. 

In the first process molten plastic is injected into a heated preform mould around a hollow mandrel blow tube or core rod. This is similar to insert injection moulding. 

In the second, blow moulding process is the preform-mandrel assembly. The preformed parison is placed in a larger mould cavity for blow moulding. 

Step 1. Injection 

The injection blow moulding machine is based on an extruder barrel and screw assembly which melts the polymer. The molten polymer is fed into a manifold where it is injected through nozzles into a hollow, heated preform mould. The preform mould forms the external shape and is clamped around a mandrel (the core rod) which forms the internal shape of the preform. The preform consists of a fully formed bottle/jar neck with a thick tube of polymer attached, which will form the body. 

Step 2. Blowing

The preform mould opens and the core rod is rotated and clamped into the hollow, chilled blow mould. The core rod opens and allows compressed air into the preform, which inflates it to the finished article shape.

Step 3. Ejection 

After a cooling period the blow mould opens and the core rod is rotated to the ejection position. The finished article is stripped off the core rod and leak-tested prior to packing. The preform and blow mould can have many cavities, typically three to sixteen depending on the article size and the required output. There are three sets of core rods, which allow concurrent preform injection, blow moulding and ejection.

Materials: LDPE, LLDPE, PP, PET, PVC, HDPE

These factors are critical to this process: 

1. Shear & temperature dependent viscosity 

2. Temperature-dependent tensile strength on the pin 

3. Tensile elongation during inflation 

4. Crystallization kinetics on the core pin 

5. Crystallization kinetics during blowing and cooling 

Application

Bottles , Jars , Roll-on containers

2. Stretch Blow moulding (SBM): It is mass production process, Stretch blow moulding is a process on which stretching is done before blowing. The stretch blow moulding method can be applied to both injection and extrusion blow moulding process. Each plastic material has a temperature limit at which it can be stretched to get optimum orientation and properties. Biaxial stretch blow moulding is the method in which the stretching is done in both longitudinal and transverse direction. Biaxial orientation increases the tensile strength, gas barrier properties, drop impact strength and clarity of plastics. The various materials suitable for stretch blow moulding are PET, PVC, PS, SAN, POM, and PP. 

1. In the Stretch Blow Moulding (SBM) process, the plastic is first moulded into a "preform" using the Injection Moulded Process.

2. These preforms are produced with the necks of the bottles, including threads (the "finish") on one end. 

3. These preforms are packaged and fed later (after cooling) into an EBM blow moulding machine. 

4. The preforms are heated (typically using infrared heaters) above their glass transition temperature, then blown using high pressure air into bottles using metal blow moulds. 

Usually the preform is stretched with a core rod as part of the process.  

5. The stretching of some polymers, such as PET results in strain hardening of the resin, allowing the bottles to resist deforming under the pressures formed by carbonated beverages, which typically approach 60 psi.

6. The picture given below shows the preform is first stretched mechanically with a stretch rod. As the rod travels down low-pressure air of 5 to 25 bar (70 to 350 psi) is introduced blowing a 'bubble'. Once the stretch rod is fully extended, high-pressure air of up to 40 bar (580 psi) blows the expanded bubble into the shape of the blow mould.

Single stage process: low investment, requires no copping ring, higher weight bottle, low volume, low efficiently. 

Single stage process: high investment requires copping ring & bumper holes, high barrier properties, high efficiently.

Stretch blow moulding: Biaxial orientation blow moulding. The container is stretch radially by the blow air and vertically by a mechanical stretch rod.

3. Extrusion Blow Moulding (EBM):

1. In EBM, plastic is melted and extruded into a hollow tube (a parison)

2. The parison is formed by forcing molten plastic through an angular orifice in a die that is part of the die head assembly. 

3. The orifice is formed by the space between the mandrel and the die. 

4. The parison is extruded and drops between the mould halves and when the mould closes the parison is sealed. 

5. Air injected into the parison inflates it to the shape of the mould cavity.

6. After cooling and solidification the mould is opened and the part removed.

Process steps

Step 1. Parison Extrusion. Parison denoted by arrow.

Step 2. Mould Halves close onto parison. 

When the parison has reached a sufficient length a hollow mould is closed around it. The mould mates closely at its bottom edge thus forming a seal. The parison is cut at the top by a knife prior to the mould being moved sideways to a second position where air is blown into the parison to inflate it to the shape of the mould.

Step 3. Parison inflated against inflated internal mould walls.

The parison is cut at the top by a knife prior to the mould being moved sideways to a second position where air is blown into the parison to inflate it to the shape of the mould.

Step 4. Mould halves open, Blow moulding part ready for ejection

After a cooling period the mould is opened and the final article is ejected. To speed production several identical moulds may be fed in cycle by the same extruder unit.

Raw Materials

This process usually uses commodity materials such as: PP, PE,  PET,  PVC 

Important factors one should consider for extrusion blow moulding include the following: 

1. Polymer viscosity at high & low shear rates 

2. Melt strength (important for uniform wall thickness, no holes) 

3. Strain recovery (MW & Distribution) 

4. Crystallization rate (slow rate desired) 

5. Thermal properties (thermal diffusitivity, thermal conductivity, specific heat, etc.) 

Advantages of Extrusion Blow Moulding: 

1. Low initial mould tooling costs. 

2. Flexibility of tooling: Moulds can accommodate interchangeable neck finishes and body sections. 

3. Flexibility in production: Neck inner diameters (I.D.) can be easily controlled to varying requirements. Bottle weights are adjustable. 

4. Container sizes can range from less than 1 oz. to 55 gallons and up. (Custom Bottle's equipment is most efficient producing containers up to 1 litre in capacity.) 

5. Container shape is not restricted by blow-up ratios. Bottles can be long and flat or have handles. 

6. Wide selection of machine sizes: Moulds can be geared to volume requirements. 

Applications

Bottles and containers , Automotive fuel tanks , Venting ducts , Watering cans , Boat fenders etc 

The main applications are bottles, jars and other containers. The Injection blow moulding process produces bottles of superior visual and dimensional quality compared to extrusion blow moulding. The process is ideal for both narrow and wide-mouthed containers and produces them fully finished with no flash. A sign of injection blow moulding is the seam where the two halves of the mould meet.

Advantages: Better strength, better clarity, increased impact strength, improved gas and water vapour barrier, reduced creep.

Blow molding material: The most commonly used material for blow moulds are aluminum alloy, beryllium-copper and cast iron.

Blow up ratio: The blow up ratio or bottle/parsion diameter ratio normally should be below 3:1 but can be increased for special design requirements. Hoop ratio is defined as the ratio of the largest inside diameter D, of the blow articles to the inside diameter D2 of the preform (D/D2).

Axial ratio: is defined as the ratio of length (A1) from where stretching starts to the inside bottom of the blown article to the some measurement (A2) on the preform (A1/A2).  Total blow-up ratio=Hoop ratio X axial ratio.

Significance of BUR: In designing a stretch blow holding pressure, the hoop ratio is the most important.