Blow Molding Process

Blow molding products

Blow molding is used to make simple parts like plastic bottles, containers, tanks in very high volumes at very low cost. There are three types of blow moldings as follows,

1. Extrusion Blow molding

Extrusion blow molding

This method is generally used to manufacture Plastic bottles of various shapes of from the volume of 10 ml   to 10000 l. This is very simple method compared to the others. This method can be used to process different types of plastics like HDPE, PET, PP and PVC.

The steps of the extrusion blow molding process are,

• The melted polymer is extruded as a hollow pipe which is known as Parison
• Then Parison is clamped in to the bow mold
• Then Parison is blown inside a mold using the air blasting
• Then cooling and ejection is done

2. Injection Blow molding

Injection blow molding

Injection blow molding is used for the production of hollow objects in large quantities with volumes in a range from 10 ml to 100 l. Generally bottles, jars and other containers are manufactured using this method. The appearance and the dimensional quality are higher when compared with the extrusion blow molding.

The steps of the extrusion blow molding process are,

• The melted polymer is injected in to the cavity after the core is inserted
• Then the core is moved in to the mold and the Blowing done using air
• After the cooling mold the ejection is done

3. Stretched Blow molding

This method is used for producing high quality containers. Generally pet bottles which are used to store water and beverages are manufactured by stretch blow molding.

The steps of the stretch blow molding process are,

• The melted polymer is injected in to the cavity as in injection blow molding
• Then Conditioning is done, that mean heated it up to the required temperature.
• Then it is inserted in to the mold and Stretching and Blowing is done at the same time

Stretching blow molding

• After cooling the discharge is done.

Rubber Processing

 Mixing, shaping and curing are the main three stages in rubber processing. Mixing with different compounding ingredient is done to increase chemical and physical properties of the rubber. That also known as rubber compounding. There are three general processes for shaping called molding, extrusion and calendering. after shaping curing is done for improve hardness of rubber product  by forming cross links in rubber chains. That is also called vulcanizing.  

 Now a day’s both natural rubber and different synthetic rubbers are used for rubber processing.

• Natural rubber(NR)

Chemical structure of Natural Rubber

Natural rubber is latex taken from the tree Heavea Brazilianzis

which is generally known as rubber tree. NR has excellent tensile strength and elongation. Its abrasion resistance, tear resistance, fatigue resistance and electrical resistance also high. NR formula has unsaturated double bonds, so its weather resistance is poor. Not only that natural rubber is dissolved in non polar chemicals like oils, petroleum based fuels etc.

Natural rubber is used as both liquid (latex rubber) and solid form (dry rubber) for processing. Some applications of these two types, some processing methods and advantages & disadvantages are summarized below.

	Applications	Processes	Advantages
Latex rubber	Gloves, Balloons	Dipping, tubing	Simple machinery can use,low energy  consumption
Dry rubber	Tires,  o rings, engine mounts	Extrusion, molding	Easy to transport and store

• Synthetic rubbers

Synthetic rubbers are made by polymerisation of different petrolium based monomers. Some of synthetic rubbers are Isoprene rubber(IR), Styrene Butadiene Rubber(SBR), Isoprene Isobutaline Rubber(IIR), Ethelene Propelene Rubber(EPR), Chloroprene Rubber(CR), Silicon Rubber etc.

 Those synthetic rubbers have different properties, so they have different applications and different processing methods. they will be explained in future.

Size reduction of waste plastic 2

Densification process

Plastic wastes which are coming as packaging films, fibers and foams have lower density. So previously mentioned cutting or crushing methods can’t use for those wastes to size reduction. Because of the lower density many problems can arise while transporting, conveying, and feeding. In this method density is increased by using agglomeration process.

Generally there are three main methods are used to do agglomeration by densifying disks, by compression, and by agitation.  

 

densifying disks

• Densifying disks

Mostly films which are made up from PP, LDPE, HDPE and fibres made from polyamides are converting in to granulate form using this method. In here plastic is slightly heated by friction disks and densification is happen without happening thermal degradation.

• By compression

In this method compression is used to extrude plastic and form pellets continuously. Compressed material is coming out through a die and it is chopped using a rotary cutter.  

• By agitation

In this method waste plastic is vigorously agitated causing the plastic material to agglomerate in to densified particles. This is happening because high shear generated by rotating blades brings plastic to a near melting state.

Advantages of densification are reduced storage volume, reduced the transporting cost, improved flow properties etc.   

  

Pulverization processes

pulverization mill

Pulverization mills like disk mills and pin mills are used in this method for converting plastic waste in to fine powder. Recycle polymer powders including sintered powder coatings, molding powders, fillers are produced using this method.  

Mainly this method is used for size reduction of PVC. After pulverization that powder has high bulk density, excellent flow characteristics, and homogeneous composition.

Products design with plastics

 

some plastic products

There are different advantages as same as some disadvantages when we are using plastic for product designing. 

Advantages

• No corrosion
• Light weight
• Low processing temperature (compare to the metal)
• Low energy requirement for processing
• High strength can be achieve if needed
• Non toxic, non smell, non taste etc

Disadvantages

• Plastic has temperature dependent characteristics
• Less dimension stability

If we are going to design a plastic product we have to consider about some factors. They are application of the product, service life, service temperature, environment concerns and other specific customer requirements. 

Now a day’s some commercial plastics are there widely used for product design.  

Polyethylene (PE)

It is generally milky white brittle material which can easily crack. It has different classifications according to its density and branching. LDPE (low density polyethylene), HDPE (high density polyethylene), LLDPE (linear low density polyethylene) are the most common and important types of them. Mainly they are used for film and packaging applications.

Polypropylene (PP) 

More ductile material than polyethylene and it is not easily cracked. It has good resistance to fatigue. Mainly used in plastic furniture applications.

Polyvinyl chloride (PVC)

Generally there are two types called UPVC (un-plasticizer) and PPVC (plasticizer). Each one has different properties and different applications as follows.

PPVC	UPVC
Relatively soft	Relatively hard
Used for cable insulation, soft bottle squeezed etc.	Used for piping, roofing, doors, window frames etc.

Polystyrene (PS)

It has good resistance to water, but swells in solvents. It is brittle, not strong and cheaper material. Generally used for packaging and food containers like yogurt containers.

Nylon

Nylon has high strength as same as dimension stability. Generally used for cloths and mechanical engineering applications like aircraft propellers, gear wheels.

When we design a plastic product we have to consider about different modes of deformations can be happen in plastics. Some of them are tension, compression, torsion, simple shear, bending. They will discuss in detail future.  

Plastic Flat Film Extrusion Process

There are two major types of flat extrusion processes called film extrusion and sheet extrusion. Those processes used for production of film and sheets up to 4m width and thicknesses up to 0.25mm for films and 30mm  for sheets. In this post I will specially explained about flat film extrusion process. Polyolefin such as LDPE(low density poly ethylene), LLDPE(linear low density poly ethylene),HDPE(high density poly ethylene) ethylene and propylene co polymers, and poly propylene  are the most commonly used plastic for film extrusion. Generally three main equipments can be identified in film extrusion process. They are for cooling, taking off and winding up.

Flat film extrusion process can be divided in to two mainly according to the cooling method as follows.

Water quench process

 

This process required low operating cost compared to the chill rolled cooling process. In this method it is easy to maintain good optical properties.

Chill roll process

Now a day’s chill rolled cooling process is commonly using process. Matt finished rolls and high gloss finished rolls are used

There are three types of flat film dies containing manifold to spread the flowing polymer across the width of the die. 

• Manifold die V hanger type

Melt is distributed across V channel.

• Manifold die T type

This is used when processing low viscosity plastics that are not thermally sensitive.

• Manifold die Fish Tail type