Blow Molding Details

Blow molding

is a specific manufacturing process by which hollow plastic parts are formed and can be joined together. It is also used for forming glass bottles or other hollow shapes. Theses blow molding details provide an overview. Call us for more information.

In general, there are three main types of blow molding: extrusion blow molding, injection blow molding, and injection stretch blow molding.

The blow molding process begins with melting down the plastic and forming it into a parison. The parison is a tube-like piece of plastic with a hole in one end through which compressed air can pass.

The parison is then clamped into a mold and air is blown into it. The air pressure then pushes the plastic out to match the mold. Once the plastic has cooled and hardened the mold opens up and the part is ejected. The cost of blow molded parts is higher than that of injection-molded parts but lower than rotational molded parts.

HISTORY

The process principle comes from the idea of glassblowing. Enoch Ferngren and William Kopitke produced a blow molding machine and sold it to Hartford Empire Company in 1938. This was the beginning of the commercial blow molding process. During the 1940s the variety and number of products was still very limited and therefore blow molding did not take off until later. Once the variety and production rates went up the number of products created soon followed.

The technical mechanisms needed to produce hollow bodied workpieces using the blowing technique were established very early on. Because glass is very breakable, after the introduction of plastic, plastic was being used to replace glass in some cases. The first mass production of plastic bottles was done in America in 1939. Germany started using this technology a little bit later, but is currently one of the leading manufacturers of blow molding machines.

In the United States soft drink industry, the number of plastic containers went from zero in 1977 to ten billion pieces in 1999. Today, an even greater number of products are blown and it is expected to keep increasing.

For amorphous metals, also known as bulk metallic glasses, blow molding has been recently demonstrated under pressures and temperatures comparable to plastic blow molding.

Typologies

In extrusion blow molding (EBM), plastic is melted and extruded into a hollow tube (a parison). This parison is then captured by closing it into a cooled metal mold. Air is then blown into the parison, inflating it into the shape of the hollow bottle, container, or part. After the plastic has cooled sufficiently, the mold is opened and the part is ejected.

Examples parts made by the EBM process: most polyethylene hollow products, milk bottles, shampoo bottles, automotive ducting, watering cans, and hollow industrial parts such as drums.

Advantages

  • Low tool and die cost
  • Fast production rates
  • Ability to mold complex parts
  • Handles can be incorporated in the design

Disadvantages

  • Limited to hollow parts
  • Low strengthTemplate:Of what
  • Parisons are often made of mixed (multilayer) materials, to increase their barrier properties, and are thus not recyclable
  • To make wide neck jars spin trimming is necessary.

Straight EBM

Straight EBM is a way of propelling material forward similar to injection molding whereby an Archimedean screw turns, then stops and pushes the melt out.

With an Accumulator

With the accumulator method, an accumulator gathers melted plastic and when the previous mold has cooled and enough plastic has accumulated, a rod pushes the melted plastic and forms the parison. In this case the screw may turn continuously or intermittently. With continuous extrusion the weight of the parison drags the parison and makes calibrating the wall thickness difficult. The accumulator head or reciprocating screw methods use hydraulic systems to push the parison out quickly reducing the effect of the weight and allowing precise control over the wall thickness by adjusting the die gap with a parison programming device.

Continuous extrusion blow molding

Continuous Extrusion Blow Molding is a variation of Extrusion Blow Molding. In continuous extrusion blow molding, the parison is extruded continuously and the individual parts are cut off by a suitable knife.

Continuous extrusion equipment

  • rotary wheel blow molding systems
  • shuttle machinery

Intermittent extrusion blow molding

Intermittent Extrusion Blow molding is a variation of Extrusion Blow Molding.

Intermittent extrusion machinery

  • reciprocating screw machinery
  • accumulator head machinery

Spin trimming

Containers such as jars often have an excess of material due to the molding process. This is trimmed off by spinning a knife around the container which cuts the material away. This excess plastic is then recycled to create new moldings. Spin Trimmers are used on a number of materials, such as PVC, HDPE and PE+LDPE. Different types of the materials have their own physical characteristics affecting trimming. For example, moldings produced from amorphous materials are much more difficult to trim than crystalline materials. Titanium coated blades are often used rather than standard steel to increase life by a factor of 30 times.

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