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.