Stretch Blow Moulding

Stretch blow molding

Stretch blow molding produces a part with biaxial molecular alignment. In the process a preform, or parison, elongated mechanically in the mold and than expanded radially in a blowing process. A desirable resulting molecular orientation yields a material with increased strength. This means that products that are strength-based designs can be produced using less material than if they were to be produced using simpler blow molding techniques.

A goal in stretch blow molding is a designed work material developed by producing desirable molecular orientation. In order to produce and retain desired structure and specified properties the stretching and blowing processes need to be carried out at temperature lower than in other blow molding processes and the allowable temperature range will be smaller and so more difficult to control. A temperature conditioning station in-line is required, or a re-heating operation needed for preforms allowed to cool before use or for purchased preforms. This increased the difficulty of process design and operation and material specification since polymer properties depend on temperature history, e.g., on temperature, time at temperature and number of temperature cycles.

Important polymer properties to be considered:

• Tensile strenth and yield above Tg
• Effect of orientation on gas permeability through the polymer

In the Stretch Blow Molding (SBM) process, the plastic is first molded into a "preform" using the Injection Molded 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 an EBM 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. Usually the preform is stretched with a core rod as part of the process. 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. The Injection blow molding process produces bottles of superior visual and dimensional quality compared to extrusion blow molding. The process is ideal for both narrow and wide-mouthed containers and produces them fully finished with no flash. A sign of injection blow molding is the seam where the two halves of the mold meet.

This picture shows what happens inside the blow mold. The preform is first stretched mechanically with a stretch rod. As the rod travels down low-pressure air of 5 to 25 bar (70 to 350 psi) is introduced blowing a 'bubble'. Once the stretch rod is fully extended, high-pressure air of up to 40 bar (580 psi) blows the expanded bubble into the shape of the blow mold.

Injection Blow Moulding

Injection blow molding

Injection blow molding is a two stage process since the parison is produced in a separate operation. In the first process molten plastic is injected into a heated preform mold around a hollow mandrel blow tube or core rod. This is similar to insert injection molding. The workpiece for the second, blow molding, process is the preform-mandrel assembly. The preformed parison is placed in a larger mold cavity for blow molding. Between the preform production and blow blow molding processes a heated preform may be held in a temperature conditioning stage or a cooled preform re-heated. After blow molding the part is stripped from the core rod at an ejection station.

Raw Materials

• Polyethylene (Low Density) LDPE, LLDPE
• Polypropylene PP
• Polyethylene - Terephthalate PET
• Polyvinyl chloride PVC
• Polyethylene (High Density) HDPE

These factors are critical to this process:

• Shear & temperature dependent viscosity
• Temperature-dependent tensile strength on the pin
• Tensile elongation during inflation
• Crystallization kinetics on the core pin
• Crystallization kinetics during blowing and cooling

Examples of Application

• Bottles
• Jars
• Roll-on containers

Injection blow molding

The process of Injection Blow Molding (IBM) is used for the production of hollow glass and plasticbottles. The process is divided into three steps: injection, blowing and ejection. 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

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.

Another application of injection blow molding is in the production of soft elastic gelatin capsule for pharmaceutical applications. Two strips of gelatin are pressed together in a rotary die which cuts out the desired shape of capsule while the fill liquid is injected. Afterwards, they are cooled and dried to yield a firm, strong capsule.

Extrusion Blow moulding

Extrusion Blow Molding

In extrusion blow molding the parison is formed by forcing molten plastic through an annular orifice in a die that is part of the die head assembly. The orifice is formed by the space between the mandrel and the die. Extrusion may be directly from an extruder, or for large parts for which more material is needed than the extruder can continuously provide an accululator is used. The parison is extruded and drops to between the mold halves and when the mold closes the parison is sealed. Air injected into the parison inflates it to the shape of the mold cavity. After cooling and solidification the mold is opened and the part removed.

Raw Materials
This process usually use commodity materials such as:

• Polypropylene PP
• Polyethylene PE
• Polyethylene - Terephthalate PET
• Polyvinyl chloride PVC

Important factors one should consider for extrusion blow molding include the following:

• Polymer viscosity at high & low shear rates
• Melt strength (important for uniform wall thickness, no holes)
• Strain recovery (MW & Distribution)
• Crystallization rate (slow rate desired)
• Thermal properties (thermal diffusitivity, thermal conductivity, specific heat, etc.)

Advantages of Extrusion Blow Molding:

• Low initial mold tooling costs.
• Flexibility of tooling. Molds can accommodate interchangeable neck finishes and body sections.
• Flexibility in production: Neck inner diameters (I.D.) can be easily controlled to varying requirements. Bottle weights are adjustable.
• Container sizes can range from less than 1 oz. to 55 gallons and up. (Custom Bottle's equipment is most efficient producing containers up to 1 liter in capacity.)
• Container shape is not restricted by blow-up ratios. Bottles can be long and flat or have handles.
• Wide selection of machine sizes: Molds can be geared to volume requirements.

Applications

• Bottles and containers
• Automotive fuel tanks
• Venting ducts
• Watering cans
• Boat fenders etc

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.

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

• rotary wheel blow molding systems
• shuttle machinery

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.

Basic polymers, such as PP, HDPE, PVC and PET are increasingly being coextruded with high barrier resins, such as EVOH or Nylon, to provide permeation resistance to water, oxygen, CO2 or other substances. In dairy applications, it is possible to extrude a black light-blocking layer in the center layer of containers, with opaque white resin used in the inner and outer layers.

Compared to injection molding, blow molding is a low pressure process, with typical blow air pressures of 25 to 150 psi. This low pressure process allows the production of economical low-force clamping stations, while parts can still be produced with surface finishes ranging from high gloss to textured. The resulting low stresses in the molded parts also help make the containers resistant to strain and environmental stress cracking.

Blow Moulding

Blow Molding Process

Process of inflating a hot, hollow, thermoplastic preform or parison inside a closed mold so its shape conforms to that of the mold cavity. A wide variety of hollow parts, including plastic bottles, can be produced from many different plastics using this process.

Main steps:

• parison is formed between mold halves
• mold closes around the parison
• sealing one end of the parison
• closing the parison around a mandrel at the other end
• parison is inflated by air blown through hollow mandrel or needle in side of parison
• cooling and solidification of the part
• mold opening and part ejection

During this process the resin raw material is melted in the machine barrel, forced over a spreader and through the die head into the mold. The mold halves are held on platens which ride on tie rods or tie bars.

Raw Materials
Most commodity grade and engineering grade resins may be blow molded,. but the most common is polyethylene, which is used for food or chemical or detergent bottles. PET or polyester is used for clear beverage bottles such as water bottles or the familiar 2-liter beverage bottles. EVA is a rubber-like material used for blow molded elastomer parts. Generally the list includes: HDPE, PET, Polypropylene, LDPE, PVC, Polycarbonate, ABS, EVOH, LLDPE, TPO, PBT,Nylon, TPE, ABS/PC Blend, Polystyrene, K-Resin®, MDPE, PUR, PETG and PPO. The "melt index", or viscosity, of the plastic must be high to keep the parison from stretching too much prior to mold closure. The resin is in the form of pellets before processing.

Tooling
Machined or cast aluminum is traditional.

Cost
Part prices are generally higher than injection molded parts, but lower than rotationally molded parts. Tooling costs are moderately expensive.

Advantages
This process lends itself to any designs involving hollow shapes. Equipment availability is good in most geographical locations. Can save tooling dollars over injection molding.

Disadvantages
Cycle times are slower than injection molding. Piece prices are higher than injection molding.

Applications
All types of bottles, toys, air ducts for automobiles, chemical & gasoline tanks, household goods.

There are three general types of blow molding: extrusion blow molding, injection blow molding, and stretch blow molding. Extrusion blow molding is usually used to make items of weight greater than 12 oz. such as containers for food, laundry, or waste. Injection blow molding is used to achieve very accurate wall thickness, high-quality neck finish, and to process polymers that cannot be extruded. Usual applications include pharmaceutical, cosmetic, single serving liquor bottles that weighs less than 12 oz. Stretch blow molding is only used for difficult to blow crystalline and crystallizable polymers such as polypropylene and polyethylene terephthalate.

Profile Extrusion

To manufacture plastic pipe, industry uses a process known as Profile Extrusion. This process is used to manufacture plastic products with a continuous cross-section such as; drinking straws, plastic evestroughing, decorative molding, window trimming and a wide variety of other products polymer melt into the hollow mold cavity under high pressure.

The plastic is fed in pellet form into the machines hopper ( this machine is known as an Extruder ), the material is conveyed continuously forward by a rotating screw inside a heated barrel being softened by both friction and heat. The softened plastic is then forced out through a die and directly into cool water where the product solidifies. From here it is conveyed onwards into the take-off rollers, which actually do the pulling of the softened plastic from the die.

The die is a metal plate placed at the end of the extruder with a section cut out of its interior, this cutout, and the speed of the take-off rollers, determines the cross-section of the product being manufactured. A simple way to understand this concept is to consider squeezing a toothpaste tube, the product comes out in a solid rod because of the opening at the end of the tube, if that opening had a different cross-section than the product produced would take on that new cross-section.

Raw Materials
Most common thermoplastic polymers can be used for extrusion and the material choice is dependent on both the performance requirements and on the economic constraints. It is here that the designer should seek specialist advice from the extrusion company or material suppliers.

Typical Materials for Plastic Profiles:

• HDPE (High Density Polyethylene)
• LDPE (Low Density Polyethylene)
• LLDPE (Linear Low Density Polyethylene)
• PETG
• Flexible PVC
• Butyrate
• Polypropylene
• Polystyrene
• ABS

The most commonly used material for general purpose extrusions is PVC. The wide application of this material is due to cost, chemical resistance and its availability in various hardness and colours. The hardness of PVC can vary from the rigid type used for windows (Shore ‘A’ hardness of 100 or British Standard softness of 0) to the plasticized or soft version used for garden hoses (generally Shore ‘A’ 80 deg or BSS 38) and even down to very soft materials of Shore ‘A’ 60 deg (BSS 75) which have limited uses. The colour can be either matched to a colour sample or chosen from several hundred standard colours. PVC is a very versatile material but, as with all materials, there are limitations and again specialist advice should be sought for critical applications.

Tooling
Steel dies are typically made by a wire EDM process. Some "downstream" tooling may be necessary to ensure shape of profile.

Cost
Dies and parts are relatively inexpensive.

Tolerances
While plastics extrusions can be produced to consistent tolerances the designer must be aware that these are not the same as for machined parts or for metals extrusion and are generally greater. The tolerance bands applicable vary with the relevant dimension, the material used and with the manufacturer but in general BS 3734:1978 for extruded rubber products (Table 2 Class E 2) can be used as a guide. Specific tolerances for critical areas and non-critical tolerances must be discussed and agreed between customer and producer. Inevitably, the unit price increases with the number of tolerance dimensions and the tightness of the tolerances specified.

Advantages

• Equipment widely available in all geographical areas. Short lead times.
• Relatively low tooling costs
• Inexpensive process
• Product combinations possible
• Design freedom

Disadvantages
Design possibilities severely limited because of linear nature of process.

Examples of Applications

• Typical applications/design possibilities
  The following application examples have been chosen to illustrate possibilities and the same ideas and techniques can, obviously, be used in many fields such as:
• Window profiles
  The basic frame of the window is an extruded, un-plasticized PVC section. This section contains air gaps or chambers which are carefully designed to give the necessary thermal and sound insulation. The normal colour is white and the polymer is UV stabilized to prevent fading. New developments with co-extrusion and printing techniques allow the profile to be produced with wood-effect or coloured finishes. This basic profile is mitre cut and welded into a frame to fit the windows of the house exactly. Extrusions are also used to provide the essential sealing lips on the profile. By skilled design a system of extrusions is built up to provide outward opening windows, tilt and turn windows, patio doors, roller shutters and other elements of the glazing system of the house.
• Sealing sections
  Extrusions are applied in many sealing applications where the designer has considerable choice in fixing method. A co-extrusion of hard and soft materials will allow the hard material to be screwed, nailed, stapled or glued to one sealing face and the soft material will still provide the required seal. A single hardness soft extrusion can be punched or stapled but may need a reinforcing rod. Alternatively, it may be clipped into one sealing face using a groove in the face as a location/fixing area. The designer can choose between these varied options and the extrusion manufacturer can provide advice on the technology available. Typical application areas are refrigerator door seals (which incorporate a magnetic extrusion for an airtight seal), car door and boot seals, acoustic cabinet seals and the window seals described above.
• Modular drawer profiles
  Drawer systems utilizing extrusions are available both as 'Do-It-Yourself' and professional kits. These illustrate important options for the designer: the ability to use an extrusion to provide variable length and width and the use of injection moulded corner pieces to provide the necessary jointing. The requirements for light weight and easy assembly rule out the use of welding and the assembly is built up using the clip-in corner pieces which give rigidity and professional finish.
• Decorative trim
  The decorative trim strips seen on bedroom and other furniture are examples of two important techniques available. One is the ability to apply a foil to the extruded PVC to give a bright and attractive finish (an option which is often used in the automobile industry for trim and bumper strips although, in this case, special exterior foils and techniques are necessary). The other is the use of double sided tape for rapid and strong mounting of the profile. For fixing to smooth, flat surfaces i.e. furniture, a film tape is used but when the surface is not regular then a foam tape may be used to give the necessary surface conformance and adhesion.

More examples are possible but the engineering designer is seeking to innovate and, hopefully, those examples outlined above can help this innovation through increased awareness of the process and its capabilities.

Sheet Extrusion

Sheet extrusion is a technique for making flat plastic sheets from a variety of resins. The thinner gauges are thermoformed into packaging applications such as drink cups, deli containers, produce trays, baby wipe containers and margarine tubs. Another market segment uses thick sheet for industrial and recreational applications like truck bed liners, pallets, automotive dunnage, playground equipment and boats. The third primary use for extruded sheet is in geomembranes, where flat sheet is welded into large containment systems for mining applications and municipal waste disposal.

Thermoplastic sheet production is a significant sector of plastics processing. Thermoplastic sheets are flat, plastic materials with a gauge of at least 250 microns and which include both flexible and rigid materials, as well as solid, foamed, and hollow materials.

General

Solid sheet extrusion units consist of at least one extruder and one sheet extrusion die. They are followed by the polishing stack, in general comprising 3 calenders, calibrating and cooling the sheet with their surfaces or calender nips. Behind this the roller conveyor and the draw-off rolls for air cooling are located. The sheet is finally cut and stored. Sheet extrusion characteristics:

• width in excess of 2 m
• thicknesses ranging from approx. 0.5 to 15 mm
• no limitations as to length
• setup as multilayer sheets with functional surfaces (colour, haptics, UV-protection ...)
• grain/structured surfaces
• easier forming possible (corrugated panels, folding, thermoforming ...)

Materials

Polystyrene continues to be the most common polymer for use in sheet extrusion. It is the dominant material for thermoformed packaging and competes with ABS and PP in technical markets. End use applications include tubs and pots for yogurt, margarine, and desserts. Thermoformed packaging is also used in many other applications in the food industry.

There are three primary techniques used to manufacture thermoplastic sheet. These are:

1. Extrusion through a flat die onto casting rolls.
2. Extrusion through an annular die onto a sizing mandrel. The pipe-like cross section that is extruded will be slit in one or more places and then flattened and handled as sheet.
3. Resins and additives will be plasticated between large rolls and then sized through a series of additional rolls into a flat sheet. This process is known as calendering.

Each of these methods has advantages and disadvantages depending on factors such as type of polymer being processed, thickness and width of sheet, and surface quality desired.

Single Layer Flat Sheet extrusion is the most common technique used in extruding plastic sheet for the thermoforming industry. The classic machinery components for this process can be described as follows:

Resin is fed into an extruder where it is plasticated into a melt.

The extruder, consisting of a heated barrel with an internal rotating screw, pumps the melted resin into a flat sheet die which sizes the sheet (thickness and width).

The sheet exits the die in a semi-viscous state and travels through a series of rolls to cool. These rolls also determine final sheet size, thickness, and width.

The flat sheet may then be wound onto continuous rolls, or "pre-sheared" into discrete lengths.

Coextrusion is a process that allows the combination of different materials and colors in a single sheet. This is done to achieve special properties which are specific to a certain polymer, or for aesthetic effects with color, or for economic reasons where an inexpensive material "sub-strata" is combined with a more expensive material "cap".

Applications

Within the building and construction industries, sheet extrusion is used for a variety of applications. One of the main uses of extruded PS sheet is for thermal insulation materials for walls, roofs, and under floors.

In the automotive industry, sheet is currently used to produce interior trim, panels, and dashboards. Foamed polyolefin sheet, both cross-linked and non-cross-linked, is also used in automotive applications.

There are a number of other applications where thermoformed sheet plays a significant role. These include the manufacturing of luggage, refrigerator liners, and shower units.

Types of plastic extrusion

• Sheet Extrusion
  Sheet extrusion is a technique for making flat plastic sheets from a variety of resins. The thinner gauges are thermoformed into packaging applications such as drink cups, deli containers, produce trays, baby wipe containers and margarine tubs. Another market segment uses thick sheet for industrial and recreational applications like truck bed liners, pallets, automotive dunnage, playground equipment and boats. The third primary use for extruded sheet is in geomembranes, where flat sheet is welded into large containment systems for mining applications and municipal waste disposal.
• Profile Extrusion
  Rubber Profile Extrusion is accomplished by forcing uncured rubber through a die, under heat and pressure, to form a part with a uniform cross section. This uncured rubber is then run through a heating unit to initiate the chemical cross linking reaction that causes the rubber to cure.
• Pipe extrusion
  Pipe extrusion is defined as a process of forcing the polymer melt through a shaping die (in this case: circular). The extrudate from the die is sized, cooled and the formed pipe is pulled to the winder or a cut off device with the aid of haul off device.
• Co-extrusion
  The process of extruding two or more materials through a single die with two or more orifices arranged so that the extrudates merge and weld together into a laminar structure before chilling.
• Blown Film Extrusion
  In film blowing a tubular cross-section is extruded through an annular die (usually a spiral die) and is drawn and inflated until the frost line is reached. The extruded tubular profile passes through one or two air rings to cool the material.
• Cast Film Extrusion
  The cast film process differs from the blown film process through the fast quench and virtual unidirectional orientation capabilities. These characteristics allow a cast film line to operate at higher production rates while producing amazing optics. Applications in food and retail packaging take advantage of these strengths.
• Foam Extrusion
  During the chemical foam extrusion process plastic resin and chemical foaming agents are mixed and melted. The chemical foaming agent decomposes liberating gas which is dispersed in the polymer melt and expands upon exiting the die. Typically foamed profile extrusions require more intense cooling than solid profiles due to the insulation properties of the foam structure.
• Pultrusion
  Similar to extrusion but with much higher Strengths- even used to make road bridges. Glass or other fibres are incorporated into the extrusion and so loadings of up to 60% glass can be achieved with very good fibre alignment. Materials are generally thermosetting type materials such as epoxy.
• Calendering
  Calendering is a process that usually uses four heated rolls rotating at slightly different speeds. Again the material is fed into the rolls, heated and melted, and then shaped into sheet or film. PVC is the most commonly calendered material.