Extrusion blow molding

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Pinch seam on the bottom of the container

The extrusion blow molding , and blow molding called, is a method of plastic processing for manufacturing hollow bodies from thermoplastic plastics . The melted polymer is pressed through the nozzle via a screw conveyor so that a tubular preform is created ( extrusion ). This is transferred to a blow mold and adapted to the internal contours of the mold using internal pressure ( blow molding ). Hollow bodies produced in this way are used both as primary packaging such as canisters or barrels and as technical components such as fuel tanks or ventilation ducts . Extrusion blow molding allows the production of hollow bodies with a volume of around one milliliter ( eye drop ampoules ) to ten cubic meters ( oil tanks ). A typical distinguishing feature for extrusion blow-molded packaging is a pinch seam on the underside. Injection stretch blow molding is usually used for bottles subject to pressure (e.g. for carbonated beverages) .

Origin and spread of technology

Preform in extrusion blow molding

The manufacture of hollow bodies by melting and blowing was first practiced by glass blowers . An important development step was the introduction of hollow forms made of wood, the so-called models , which guaranteed a reproducible article shape even with large quantities. In the 19th century the wooden molds were replaced by metal molds. Since the beginning of the 20th century, glass machines for the production of hollow glass took over the manual production of everyday objects.

In a US Patent on 24 June 1851, the author ST Armstrong entitled describes "Improvement in Making gutta-percha Hollow Ware" (improvement in the production of hollow bodies of gutta-percha ) "the formation of a tubular preform by internal pressure to the Die wall is blown ”. Gutta-percha is a rubber product and is obtained from the milky sap of the gutta-percha tree . Developments with celluloid and rubber followed. Due to the severely limited availability of suitable materials, the development of blow molding stagnated.

The plastic polyvinyl chloride (PVC) was discovered in the 1930s . The glass industry in the United States was very interested in this new material, as it promised less fragile containers. It is therefore understandable that in the early days between 1938 and 1945 many patents for blow molding of plastics were registered in America by the glass industry. In 1939, the first series production was launched plastic bottles by the Owens-Illinois Glass Company . The process used for this corresponded to injection blow molding .

The development of blow molding technology in Europe began independently of its development in the USA. Inspired by the developments of the nearby company Dynamit Nobel in Troisdorf , which produced the plastic compound celluloid from nitrocellulose , the Reinold and Norbert Hagen brothers ( Kautex-Werke Bonn) constructed the first molded parts from plastic sheets. The processing technique used for this came from sheet metal working and was derived from bending and welding. To simplify the process, the Hagen brothers developed their first extrusion blow molding machine in 1949, which made it possible to manufacture bottles, containers and other hollow bodies from plastic. The blow molding made it possible to produce much more complex geometries than would have been possible with metal or sheet steel. From 1955 their blow molding machines were also exported to America.

With the market maturity of various types of polyethylene at the end of the 1950s, the importance of blow molding of plastics increased more and more, especially for the packaging industry. With the development of more and more powerful plastics, their use in technical products also increased.

Manufacturing process

Representation of the various process steps in extrusion blow molding

The screw shaft in the extruder pushes the molten polymer out vertically downwards through a nozzle located on the deflection head. A tube-like preform is created. The two-part tool, which is still open at this point, has the negative shape of the finished workpiece on the inside. When the preform has reached the desired length, the mold closes and the lower end of the preform is squeezed together. The pinch seams that are typical for extrusion blow molding are created on the bottom of the molded part.

In a second process step, a mandrel plunges into the tube from above, through which compressed air then flows in, with which the preform is inflated and pressed against the inner contour of the blow molding tool. The mandrel also has the task of shaping the neck area of ​​the workpiece. At this moment, the tool and thus the plastic begin to cool down. To shorten the cooling time, cooled purging gases can be used in the interior of the hollow body. After the molded part has cooled down sufficiently and has sufficient strength, the tool opens and the molded part can be removed.

Are formed on the neck part, at the bottom and at the interface between the two tool halves of slugs of plastic, some of which are sheared off when opening the same; post-processing is sometimes necessary. As a rule, these are called regenerated returned to the extruder.

In order to keep cycle times short, the blow molded articles are often removed from the mold when they have reached a minimum strength. Because the article wall is usually cooled on one side from the outside, the plastic is still very hot on the inside and can give off heat to the outside, which could make the entire article soft again. To prevent this, post-cooling is used, with cool air being blown into the interior of the article through a thin post-cooling mandrel to further reduce the temperature. The heated air can exit again on the side of the mandrel.

Wall thickness control

Variable nozzle on the hose head for wall thickness control during extrusion

The wall thickness of the preform is controlled by an adjustable exit area on the nozzle of the hose head. A servo-hydraulic shifts the two conical parts of the nozzle against each other and controls the flow rate over the remaining gap width. This enables the wall thickness in the finished product to be set on the one hand, and the wall thickness of the hose to be continuously adapted on the other during its creation. Without a corresponding control, the wall would be thinner in more inflated and thus more stretched sections of the end product than in narrower, less inflated sections.

In the example of a bottle shape shown here, the upper and lower parts of the preform must be made thicker in order to achieve a uniform wall thickness in the end product. The material consumption and the cooling time can be significantly reduced through a high wall thickness constancy.

In the case of blow molded parts with a strongly oval cross section, the shape of the nozzle and mandrel usually creates a tube with a wall thickness that varies over the cross section in order to take into account the different radial stretching paths in the blow mold.

The adaptive wall thickness control has an additional advantage with large preforms: the larger the mass of the preform and the longer it hangs on the hose head, the more the upper hose area is stretched by its own weight. The mass flow in the upper area is increased via the wall thickness control in order to compensate for the consequences of the elongation and thus counteract a reduction in hose thickness.

Continuous and discontinuous extrusion

Construction of a simple storage head for discontinuous extrusion

With continuous extrusion , plastic melt constantly comes out of the nozzle. As soon as the tube reaches the required length, the blow molding tool closes and takes over the preform. It moves from the nozzle area to the blowing position and the preform is inflated. After the article has cooled down and ejected, the blow molding tool moves back under the nozzle to take over the next tube (shuttle process) . The cycle times of extrusion and cooling time must be precisely coordinated.

Alternatively, a second continuous process is available in which the hose is removed from the nozzle via a gripper (hose feeder) and transferred to the blow molding tool. This procedure is particularly useful if the blow molding tool is difficult to move due to its size or weight, or if there is little space available to set up the machine. In addition, several clamping units (tools) can be served by one extrusion unit in a machine equipped with a hose transport gripper. The continuous tube formation is limited by the stiffness of the melt, the required tube length and the cycle time. Typical limits are at a volume of around 100 liters or with clock cycles of less than 120 seconds.

In discontinuous extrusion , the polymer melt is first collected in a battery or storage head. Only when the filling volume is reached does a piston push the entire mass out of the nozzle head relatively quickly. This guarantees compared to the slower, continuous extrusion less sagging of the tube (Sagging) and a lower cooling of the lower end of the preform. The method is therefore particularly suitable for long, heavy preforms that are required for the production of larger containers such as oil tanks. In the case of sensitive polymers such as polyamide, a ring buffer is required which guarantees that the polymer is processed in the order in which it is melted ( first in - first out ). This prevents long dwell times, which result from the fact that not all of the melt is completely ejected from the reservoir.

Multilayer or coextrusion blow molding

Coextrusion makes it possible to produce hollow bodies with a multilayer wall structure. The plastics melted in individual extruders are brought together in the storage head and ejected as a common, multi-layer hose. Up to seven layers can be arranged on top of each other. The inflation takes place in the same way as normal blow molding. The reasons for multiple layers can be varied.

  • Coextrusion is often used to improve tightness . The wall contains one or more barrier layers with a low diffusion rate . An adhesion promoter layer is normally arranged between the base layer and the diffusion barrier. Such coextrusion layers are used in fuel tanks in order to reduce the diffusion of hydrocarbons or in food packaging in order to reduce the permeability of oxygen or aromatic substances. Polyamide (PA) or ethylene vinyl alcohol (EVOH) can be used for this purpose .
  • The use of plastic waste (regenerated material) in the middle class helps to save costs.
  • In the case of cosmetic articles, particularly hard and scratch-resistant outer layers made of polyamide (PA) protect the surface from damage.
  • Foamed middle layers increase the rigidity and / or thermal insulation of polyethylene packaging.
  • Additional layers can also be used as improved antistatic equipment or to improve printability.

3D extrusion blow molding

3D extrusion blow molding: soft segment in the bellows area through sequential blow molding

In 3D extrusion blow molding, also known as the tube insertion process, the preform is inserted into the tool along the product contour with the aid of mechanical components or air. Components with complex geometry and multi-axis curved components can be easily implemented. Advantages are, in particular, a reduction in the proportion of slugs (squeezed off material on the outer edge) and a reduction in pinch seams (particularly in stressed component areas). The 3D technology also allows the sequential (successive) extrusion of different materials. This makes it possible to manufacture a component that has different properties depending on the area. Typical is e.g. B. the targeted use of soft segments in the flexible bellows area.

The implementation of 3D technology differs depending on the manufacturer. In the “laydown” process, the preform is deposited either by a movable extruder head or a corresponding nozzle or by moving the open mold half below the stationary extruder. The hose remains partially filled with supporting air to prevent it from collapsing. In the so-called “manipulation process”, the preform is gripped by a robot gripper and placed in the open mold.

With the 3D suction process, the hose is extruded into the already closed mold and a vacuum is simultaneously applied to the lower end. The preform is guided through the tool with the aid of the negative pressure. This process is particularly suitable for products with a small diameter where only slight changes in diameter or cross-section are intended.

Quality assurance and further processing

Leak tightness is an important property for packaging items and a number of technical products. In many leak test methods, all openings in the article are closed and the interior of the hollow body is provided with overpressure. This pressure must not fall below a specified limit value during a specified holding time. Underwater tests or helium tests are used for plastic fuel tanks . Losses of a few ppm can be detected here.

Scales are often used for quality assurance to check the net weight. Containers of dangerous goods must be subjected to a special drop test.

Some in-line machines contain all process steps from extrusion to the finished product and can also take on filling, sealing, labeling, printing and packaging.

Types of plastic and products used

Thermoplastics are processed for extrusion blow molding . The raw material must meet various requirements for processing:

  • The extensional viscosity must be high enough so that the freely hanging hose does not begin to flow uncontrollably due to its own weight during extrusion. On the other hand, polymers with too high a tensile viscosity cannot be inflated and can tear or burst in thin places before the mold wall is reached. By fillers or reinforcing materials , the characteristics of the materials can be modified within certain limits.
  • The processing temperature window of the raw materials must be large enough so that the temperature of the plastic compound enables further processing over the entire length of the preform, especially in the case of large molded parts. The wider this temperature window, the more time is available before the mold halves have to be closed. Too much cooling can lead to deformations during the inflation process and to poor welding of the pinch seam at the lower end of the molding. The width of the processing temperature window is significantly higher for polyolefins with around 30  Kelvin than with polyamides (PA) or polycarbonates (PC) with around 10, at most 15 Kelvin.

The most frequently processed plastics for extrusion blow molding are the polyolefins polyethylene (PE) and polypropylene (PP). Due to its neutral taste and odor, good barrier properties against water and chemical resistance to solvents, high-density polyethylene (PE-HD) is particularly suitable for the production of packaging containers of different sizes (e.g. canisters, tank containers, IBCs ), but also many technical products (e.g. fuel filler pipes, seats, panels, air ducts). Larger children's toys (e.g. slide vehicles) are also often made using this technology.

Polypropylenes are used to manufacture food packaging (e.g. bottles for juices, syrups, sauces) or for packaging pharmaceutical and cosmetic products. It is also used for technical blown parts in automotive engineering (e.g. cooling water tanks and air duct components).

Due to their transparency and impact resistance, blow molds made of polycarbonate (PC) are used for water bottles in drinking water dispensers. Polyamides (PA 6, PA 66) have a comparatively high heat resistance and are processed into technical parts (e.g. intake lines in the engine compartment of motor vehicles) that can later be exposed to higher temperatures. Other thermoplastics for technical applications such as bumper beams, spoilers, etc. are modified polyphenylene oxide (PPO), acrylonitrile butadiene styrene (ABS) or polymer blends made from different materials.

The use of polyvinyl chloride (PVC) as a blow molding material continues to decline. For some technical applications in the construction industry, PVC is still important because of its long-term stability. The processing of PVC by blow molding is quite complex and requires special experience, as hydrochloric acid can be released if it is not processed properly.

Polyester (PET) is not suitable for extrusion blow molding due to its poor melt stability. Stretch blow molding is suitable for this material.

  • Examples of products made by extrusion blow molding

  • Petrol can

  • Children's toys

  • Water bottles for drinking water dispensers

  • White water kayak

Properties of extrusion blow molded articles

Due to the principle of extrusion blow molding, only the outer surface of the article is precisely defined by the shape of the tool. The wall thickness and the inner surface can only be adjusted indirectly using the wall thickness control. Additional structuring of the inside of the component is not possible. In particular, no reinforcements in the form of ribs, as are customary in injection molding, can be attached. Groove-shaped depressions (so-called beads ) are used to increase the rigidity of flat component areas during blow molding . Flat panels are given additional rigidity by pressing them together in certain areas and thus connecting the opposite walls to one another (through-hole plating).

Pinch-off edges, on which excess material (slugs) are separated off, are typical of extrusion blow-molded articles. These can be found at at least one end, often also at two ends of the component; in the case of very complex shapes, they are partially or completely arranged around the component. They remain visible despite post-processing. Pinch edges can also represent weak points in the blow molded part. In addition, fine lines can be seen at the contact points of the molding tools.

During blow molding, air must be introduced into the blow molded part to inflate it. If the product does not have any openings that can be used, such as B. is the case with open containers, the wall must be pierced with a blow needle. This leaves a small hole that is then sealed by welding. Since this point remains visible, it should be arranged in an inconspicuous area. The formation of sharp edges and corners has process-related limits. Corner and edge radii smaller than two millimeters can hardly be manufactured reproducibly.

literature

  • Michael Thielen, Klaus Hartwig, Peter Gust: Blow molding of plastic hollow bodies. Processes, machines, tools , Hanser, Munich 2006, ISBN 3-446-22671-0
  • Werner Knappe, Alfred Lampl, Otto Heul: Plastics processing and tool making. An overview , Hanser, Munich 1992, ISBN 3-446-16270-4
  • Friedrich Johannaber (Ed.): Kunststoff-Maschinenführer , Hanser, Munich 2003, ISBN 978-3446220423 , pp. 433-467

Web links

Commons : Blow Molding  - Collection of images, videos and audio files

Individual evidence

  1. ↑ Thomas Brinkmann, Volker Lessenich-Henkys and Walter Michaeli : Constructing plastic components suitable for the material , Hanser Verlag, 1995, ISBN 978-3446175358 , p. 21 ( online preview on GoogleBooks).
  2. ↑ The German glass industry. Archived from the original on April 29, 2013 ; Retrieved July 26, 2013 .
  3. ↑ a b c Michael Thielen, Klaus Hartwig, Peter Gust: Blow molding of plastic hollow bodies , Hanser Verlag, 2006, ISBN 978-3446226715 , section History of blow molding of hollow bodies , pp. 7-11 ( online preview on GoogleBooks).
  4. ↑ Franz-Josef Vossebürger, Leo Wolters, Walter Michaeli and Helmut Greif: technology of plastics: learning and working book , Hanser Verlag, 2008, ISBN 978-3446415140 , pp 103-105 ( online preview on Google Books)
  5. ^ A b Walter Michaeli , Thomas Brinkmann and Volker Lessenich-Henkys: Constructing plastic components suitable for the material , Hanser Verlag, 1995, ISBN 978-3446175358 , p. 21 ( online preview on GoogleBooks).
  6. ↑ a b c d Michael Thielen, Klaus Hartwig, Peter Gust: Blow molding of plastic hollow bodies , Hanser Verlag, 2006, ISBN 978-3446226715 , section History of blow molding of hollow bodies , pp. 106-110 ( online preview on GoogleBooks).
  7. ↑ a b Blow molding of plastic hollow bodies. Xeel GmbH, accessed on February 3, 2013 (detailed, illustrated description of the extrusion blow molding of plastic containers (with video)).
  8. ↑ a b c Michael Thielen, Klaus Hartwig, Peter Gust: Blow molding of plastic hollow bodies , Hanser Verlag, 2006, ISBN 978-3446226715 , section Continuous / discontinuous extrusion , pp. 41-44 ( online preview on GoogleBooks).
  9. ↑ Günter Mennig: Tool making for plastics processing: types of construction, production, operation , Hanser Verlag, 2008, ISBN 3-446-18257-8 , pp. 114–119 ( online preview on GoogleBooks).
  10. ↑ a b Processing of Grilamid and Grilon by extrusion blow molding. (PDF; 323 kB) Technical data sheet. EMS-GRIVORY, 1998, accessed July 16, 2013 .
  11. ↑ a b c d e Michael Thielen, Klaus Hartwig, Peter Gust: Blow molding of plastic hollow bodies , Hanser Verlag, 2006, ISBN 978-3446226715 , section Special process variants, pp. 110-133 ( online preview on GoogleBooks).
  12. ↑ Erwin Baur, Sigrid Brinkmann, Tim A. Osswald and Ernst Schmachtenberg: Saechtling Kunststoff Taschenbuch , Hanser Verlag, 2007, ISBN 978-3446403529 , pp. 284–286 ( online preview on GoogleBooks).
  13. ↑ a b Blow molding instructions. (PDF; 376 kB) DuPont, accessed on July 21, 2013 (instructions with the basics of polyamide).
  14. ↑ a b c d e f Michael Thielen, Klaus Hartwig, Peter Gust: Blow molding of plastic hollow bodies , Hanser Verlag, 2006, ISBN 978-3446226715 , section plastics for extrusion blow molding , pp. 22-24 ( online preview on GoogleBooks).
  15. ↑ Thomas Schweizer: Polymers II, Part 2: Technology of Polymers. (PDF; 2.6 MB) 2nd lesson: Primary molding I, extrusion / blow molding. ETH Zurich, 2008, accessed on August 11, 2013 (slides, bachelor's degree in materials science).
  16. ↑ a b c Michael Thielen, Klaus Hartwig, Peter Gust: Blow molding of plastic hollow bodies , Hanser Verlag, 2006, ISBN 978-3446226715 , section Blow-mold-compatible construction , pp. 219-223 ( online preview on GoogleBooks).
This version was added to the list of articles worth reading on August 17, 2013 .