Posted tagged ‘type’

BLOW MOULDING PROCESS

August 23, 2011

Today, when walking in your supermarket, it is increasingly difficult to find items packed in glass and jars.  Packaging for soft drinks, healthcare and beauty products, household chemicals and medicines, among other products, have switched from glass or metal to plastics.  Today the Blow Molding industry has expanded from simple plastic containers to plastic drums, gas tanks, automobile parts and toys in all shapes and sizes.

01-blow-molding-extrusion blow molding-injection blow molding-parison extrusion


Blow Molding (BM) process makes it possible to manufacture molded products economically, in unlimited quantities, with virtually no finishing required.  The basic process of blow molding involves a softened thermoplastic hollow form which is inflated against the cooled surface of a closed mold.  The expanded plastic form solidifies  into a hollow product.

Blow molded components are now seen all over the markets and industries for traditional materials, particularly in liquid packaging applications.  The last few decades saw the introduction of  Poly Ethylene (PE) squeeze bottles for washing liquids, Poly Vinyl Chloride (PVC) for cooking oil and fruits squash bottles, and Poly Ethylene Terephthalate (PET) for carbonated beverage bottles.  Nowadays, it is also used for the production of toys, automobile parts, accessories and many engineering components.

There are basically four types of blow moulding used in the production of plastic bottles, jugs and jars. These four types are:

  1. Extrusion blow molding,
  2. Injection blow molding,
  3. Stretch blow molding and
  4. Reheat and blow molding.

Extrusion blow molding is perhaps the simplest type of blow molding, whereby a hot tube of plastic material is dropped from an extruder and captured in a water cooled mold. Once the molds are closed, air is injected through the top or the neck of the container; just as if one were blowing up a balloon. When the hot plastic material is blown up and touches the walls of the mold the material “freezes” and the container now maintains its rigid shape. There are various types of shuttle, reciprocating and wheel style machines for the production of extrusion blown bottles. Shuttle or reciprocating type machines can be used for small, medium and high volume production with wheel machines being the most efficient for huge volume production of certain resins.


01-petblow-plastic products manufacturing-PET Preform-PET bottles-stretch blow molding

A typical apparatus consists of following major components i.e. blow pin, plunger, accumulator and lastly a mold.

Actually the process utilizes air pressure to inflate softened thermoplastic tube which is sealed at one end (also called as parision). This parision is constantly inflated and extruded. Then later on it is cut according to required dimensions. The temperature in Accumulator is maintained around 400 degree Celsius or so.

Stretch_blow_mold-dies-PET Pre form mold-household appliance mold

The mold consists of two split parts which have a semi-circular cross-section. Usually the air pressure which is applied in low pressure molding is about 50 to 250 psi. Various forms of blow molding used in industry today on a wide scale are Injection Blow Molding.

Injection Blow Molding though not used in industry, has very limited and specific applications like making small medicine plastic bottles etc. Extrusion blow molding is the simplest form of blow molding. A tube of plastic material which is generally maintained hot, is dropped from an extruder only to be captured in a water cooled mold. Once the molds are closed, air is injected through the top or the neck of the container and the tube is inflated just like a balloon. When the hot plastic material is blown up and touches the walls of the mold the material is cooled and the container now maintains a solid, rigid shape.

Now Stretch blow molding, this process requires the raw material to be formed in a pre-form using injection molding and later on stretch blow molding process can be applied.

The product range varies from various cylindrical components like bottles, cans, floats heater ducts in automobile parts and various small pipe fittings and hollow cylindrical parts can be produced in mass production.

The advantages are many like the tooling costs are very less as compared to injection molding, the part performance is excellent under pressure. Then the products have excellent environmental stress crack resistance. The products also perform excellently in high speed impact strength than even the metal components the process can be automated and used in mass production.

The disadvantages mainly raise environmental concerns. It depends on petroleum industry as any plastic industry depends. Also the cylindrical shapes are delicate so if the dimensions are not accurate then they result in scrap.

INTERVIEW QUESTIONS:GERNAL:

August 22, 2011
  • Explain the Second Law of Thermodynamics.
    The entropy of the universe increases over time and moves towards a maximum value.

 

  • How do you measure temperature in a Wet Bulb Thermometer?
    Wet bulb temperature is measured in a wet bulb thermometer by covering the bulb with a wick and wetting it with water. It corresponds to the dew point temperature and relative humidity.

 

  • What is Bending moment?
    When a moment is applied to bend an element, a bending moment exists in the element

 

  • What are the points in the Stress Strain curve for Steel?
    Proportional limit, elastic limit or yield point, ultimate stress and stress at failure.

 

  • Define Reynolds number.
    Reynolds number is the ratio of inertial force and viscous force. It is a dimensionless number. It determines the type of fluid flow.

 

  • What is a Newtonian fluid?
    A Newtonian fluid possesses a linear stress strain relationship curve and it passes through the origin. The fluid properties of a Newtonian fluid do not change when any force acts upon it.

 

  • How many Joules is 1 BTU?
    1 BTU is equal to 1055.056 joules.

 

  • What is PS?
    PS is Pferdestarke, the German unit for Horsepower.

 

  • Explain Otto cycle.
    Otto cycle can be explained by a pressure volume relationship diagram. It shows the functioning cycle of a four stroke engine. The cycle starts with an intake stroke, closing the intake and moving to the compression stroke, starting of combustion, power stroke, heat exchange stroke where heat is rejected and the exhaust stroke. It was designed by Nicolas Otto, a German engineer.

 

  • Explain the nomenclature of a 6203-ZZ bearing.
    6 is the type code, which shows it is a single-row ball bearing, 2 is the series, means light, 03 is the bore, which is 17 mm and ZZ is the suffix meaning double shielded bearing.

 

  • What is Gear ratio?
    It is the ratio of the number of revolutions of the pinion gear to one revolution of the idler gear.

 

  • What is Annealing?
    It is a process of heating a material above the re-crystallization temperature and cooling after a specific time interval. This increases the hardness and strength if the material.

 

  • Define Torque.
    Torque is defined as a force applied to an object that results in rotational motion.

 

  • What is Ductile-Brittle Transition Temperature?
    It is the temperature below which the tendency of a material to fracture increases rather than forming. Below this temperature the material loses its ductility. It is also called Nil Ductility Temperature.