Blog 37: Introduction to Blow molding

Introduction:

Blow molding is the process of inflating a viscous plastic tube, or parison, to the point where it fills a mold and takes the required shape. Hollow plastic parts are created by this process. Consider it similar to putting a balloon into a water bottle.

The process of blow molding involves creating hollow objects by inflating or blowing a thermoplastic molten tube known as a “parison” into the cavity of a mold. The procedure entails “dropping” or extruding a parison onto which the female mold halves are sealed. The shape of the final product is contained in the female mold’s two halves. The female mold halves that are closing compress the parison’s bottom hole shut. The heated mixture is blown out against the cavity walls by a pressured gas, often air, creating the final product.

Injection blow molding, stretch blow molding, and extrusion blow molding are some of the primary types of blow molding. Water bottles, cosmetic containers, and other small hollow pieces are frequently produced using injection molding and stretch blow molding. DEF tanks, HVAC ducts, and lawnmower seats are examples of medium to large hollow parts with complex geometries or highly technical requirements that are produced using extrusion blow molding.

The History

Glass blowing, where a craftsman would heat the glass to its melting point and then blow through a tube to inflate the glass, is where the technique of blow molding first emerged. As far back as the 1800s, this method has been in use. Using celluloid polymer in the method is depicted in a patent from the era. These primitive techniques weren’t appropriate for mass production.

They created industrial equipment for the production of blow-molded bottles in the 1930s, enabling mass production. The procedure could not be used to generate significant numbers due to the brittleness of the available materials and the length of time it needed to produce them.

With the development of low- and high-density polyethylene, blow molding surged into industrial predominance. The soft drink bottling industry and the car industry were two of the many industries that underwent a transformation as a result.

The Process

There are a few different types of blow molding. Their differences lie mostly in how they form the parison, the size of the parison, and how the parison moves between the molds. The main types of blow molding are:

  • Extrusion Blow Molding (EBM)
  • Injection Blow Systems (IBS)
  • Injection Stretch Blow Molding (ISBM)

Extrusion blow molding (EBM): 

In order to create a shape inside a hollow tube, plastic, or parison, is melted and expelled or extruded. Thus, it creates a mold.

A metal parent mold is used to contain and seal the parison. The mold is then filled with air, which helps it take on the desired shape. The mold is released, and the part is taken out after the plastic has cooled. DEF tanks, HVAC ducts, and lawnmower seats are examples of medium to large hollow parts with complex geometries or highly technical requirements that are produced using extrusion blow molding.

Injection Blow Systems (IBS):

When a plastic preform is injection molded, it is transported to the blow mold station on a core rod. There, blow air enters through the core rod, lifts the hot preform material off the core rod, and shapes it by air pressure to the design of the female blow mold. The blow molds open and the core rod holding the blown plastic bottle moves to the stripper station where the blown bottle is removed off the core rod once it has cooled to the correct shape. Next, the process is repeated.

Injection Stretch Blow Molding (ISBM):

Stretch blow molding is a technique used to create the bottles that are frequently used for juice, water, and other similar products. In this procedure, a preform—which is normally manufactured on an injection molding machine—is put into an automatic PET stretch blow molding machine, where it is heated and blown into a finished container. PET is the primary material utilized in this process, but the PET stretch blow molding machine can also be used with other materials, such as PP and triton.

Blow Molding Materials

  • Plastics that are suited for this process include:
  • PVC
  • PET
  • Nylon
  • ABS
  • EVA
  • TPE
  • Low and High-Density Polyethylene
  • Polypropylene
  • Co-polyester
  • COP and COC
  • Polystyrene

The wide variety of materials available for use in blow molding means that you can use the process to develop parts to fit your exact needs.

Process:

  • The creation of a parison, or hollow plastic tube, is the first step in the process. When the tube has reached the bottom of the mold, it is extruded downward. Then, the gap between them closes.
  • A blow pin at the top of the part or a needle in the middle of the part is used to introduce air into the cavity, expanding the plastic and pushing it up against the walls of the mold.
  • The component then cools in the mold.
  • The part is then removed from the mold.

There are typically two types of extrusion blow molding: 1) Continuous extrusion blow molding and 2) Intermittent extrusion blow molding.

Continuous Extrusion Blow Molding

  • The parison is continuously extruded throughout the cycle.
  • It can only be used with materials that have good melt strength.
  • It also can only be used for short cycle times.

Intermittent Extrusion Blow Molding

  • The plastic material that is extruded is stored in an accumulator.
  • When the mold opens, the accumulator pushes the material out of the die to form a parison.
  • It is used for parts with long cycles and materials with low melt strength
  • There are two ways to get air into the parison.
  • Blow pins are used to bring the air in on the parting line of the mold. In bottles, it is inserted through the neck.
  • Blowing needles are used to create very small holes in a part. Such pins can be located anywhere in a mold

As it leaves the die, the plastic material expands beyond its thickness. Die swell is the term for this. The plastic memory causes it to expand. In a blow molding die, the plastic material is squeezed as it passes through the land. It wishes to return to the form it had before it traversed the land in its current form.

It grows heavier and longer as the Parison develops. Gravity begins to stretch the Parison as it becomes heavier. Parison sag is the term for this. As a result, the Parison has irregular wall thickness.

Process Advantages

  • Molds are inexpensive vs. injection molding. Since molding pressures are much lower, tooling can be produced in aluminum.
  • Irregular geometry can be easily molded if the mold can open without destroying the part.
  • There is the ability to combine several components into one part.
  • The inside volume of the part can be filled with foam while the mold is still closed.
  • By using specialized polymers, better barrier properties can be had.
  • Recycled materials can be layered in the middle.
  • Less costly materials can be used in a layer to reduce the part’s cost

Process Disadvantages

  • There are problems filling corners and deep sections because the parison will be stretched too thin.
  • In injection molding, the part is cooled and constrained by two sides of the mold. With extrusion blow molding, the part only touches the mold on one side.  This helps to create wider tolerance than is seen with injection molding.
  • On long flat surfaces there is a tendency for the flat surfaces to warp.
  • To ensure proper filling and a strong weld line, the parison is extruded larger than the tool. Hence, there is always some flash created as excess material is squeezed between parting lines during the blowing operation.
  • Secondary operations are always needed to remove flash. Holes and other features are put in after the part is removed from the mold.
  • There is wall thickness variation because of parison sag and part shape. The parts tend to be thinner at the top because of sag.  When the parts have a bigger local diameter, the parts will be thinner in the thicker regions.  The further the parison has to travel; the thinner it gets.

Disadvantages

  • Any surface defect that results from the extrusion of the parison or sweating of the mold will cause surface appearance problems on the part.
  • If there is a change in parison thickness or if the pinch-off is not designed correctly, there could be a weak weld leading to part failure.

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