Blow molding


Blow molding
The blow molding process

Blow molding (also known as blow moulding or blow forming) is a manufacturing process by which hollow plastic parts are formed. In general, there are three main types of blow molding: extrusion blow molding, injection blow molding, and stretch blow molding. The blow molding process begins with melting down the plastic and forming it into a parison or preform. The parison is a tube-like piece of plastic with a hole in one end in which compressed air can pass through.

The parison is then clamped into a mold and air is pumped 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.

Contents

History of blow molding

There has been evidence found suggesting that Egyptians and Babylonians blew plastic materials[citation needed], but Enoch Ferngren and William Kopitke were the first verified people who used the blow molding process. The process principle comes from the idea of glassblowing. Ferngren and Kopitke produced a blow molding machine and sold it to Hartford Empire Company in 1937. 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 followed soon afterwards. In the United States soft drink industry, the number of plastic containers went from zero in 1977 to ten billion pieces in 1999. Today, even a greater number of products are blown and it is expected to keep increasing.

For amorphous metals, also known as bulk metallic glasses (BMGs), blow molding has been recently demonstrated under pressures and temperatures comparable to plastic blow molding.[1] This technique allows molding BMGs with an about 50 times higher strength than plastics into shapes that were previously not achievable with crystalline metals.

Typologies of blow molding

Extrusion blow molding

Extrusion blow molding

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. Continuous and Intermittent are two variations 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. In Intermittent blow molding there are two processes: straight intermittent is similar to injection molding whereby the screw turns, then stops and pushes the melt out. 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.[2]

EBM processes may be either continuous (constant extrusion of the parison) or intermittent. Types of EBM equipment may be categorized as follows:

Continuous extrusion equipment

Intermittent extrusion machinery

  • reciprocating screw machinery
  • accumulator head machinery

Examples of parts made by the EBM process include dairy containers, shampoo bottles, hoses/pipes, and hollow industrial parts such as drums.

Injection blow molding

The process of injection blow molding (IBM) is used for the production of hollow glass and plastic objects in large quantities. In the IBM process, the polymer is injection molded onto a core pin; then the core pin is rotated to a blow molding station to be inflated and cooled. This is the least-used of the three blow molding processes, and is typically used to make small medical and single serve bottles. The process is divided into three steps: injection, blowing and ejection.[3]

The injection blow molding 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 mold. The preform mold 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.

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

After a cooling period the blow mold 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 mold 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 molding and ejection.

Stretch blow molding

Stretch blow molding process

In the stretch blow molding (SBM) process, the plastic is first molded into a "preform" using the injection molding process. These preforms are produced with the necks of the bottles, including threads (the "finish") on one end. These preforms are packaged, and fed later (after cooling) into a reheat stretch blow molding machine. In the SBM process, the preforms are heated (typically using infrared heaters) above their glass transition temperature, then blown using high pressure air into bottles using metal blow molds. The preform is always stretched with a core rod as part of the process. In the single-stage process both preform manufacture and bottle blowing are performed in the same machine. The stretching of some polymers, such as PET (polyethylene terephthalate) 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.The main applications are bottles, jars and other containers.

Advantages of blow molding include: low tool and die cost; fast production rates; ability to mold complex part; produces recyclable parts

Disadvantages of blow molding include: limited to hollow parts, wall thickness is hard to control.

Using a deformable die

To overcome problems with radial wall thickness, Harald Feuerherm patented PWDS.[4] The system consisted of a flexible steel die that could be squeezed into a series of shapes as the moulding emerged. This gave a degree of control over wall thickness. The system is currently used to produce complex extruded shapes such as automotive fuel tanks and is used widely for UN drums and jerry cans. An enhancement of PDS is the 3DX deformable die system for radial wall thickeness control [5]

Twin Moog servo valves being used to deform the shape of the die on this blow molding accessory

Spin trimming

Containers such as jars often have an excess of material due to the molding process. This is trimmed off by spinning the container while a blade cuts the material away. This excess plastic is then recycled to create new mouldings. Spin Trimmers are used on a number of materials, such as PC, PET, PVC, HDPE, PE+LDPE and LLDPE. 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.[6]

See also

References

  1. ^ Jan Schroers, Thomas M. Hodges, Golden Kumar, Hari Raman, Anthony J. Barnes, Quoc Pham, Theodore A. Waniuk (February 2011). "Thermoplastic blow molding of metals". Materials Today 14: 14–19. doi:10.1016/S1369-7021(11)70018-9. 
  2. ^ Extrusion Blow Molding Technology, Hanser Gardner Publications, ISBN 1-56990-334-4
  3. ^ Blow molding Guide
  4. ^ Harald Feuerherm Patent List
  5. ^ 3DX deformable die
  6. ^ Example of Spin Trimmer

Bibliography


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