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Blow Molding Overview

Blowing molding is the primary method to form hollow plastic objects such as soda bottles, jerry cans, carboys. The process includes clamping the ends of a softened tube of polymers which can be either extruded or reheated, inflating the polymer against the mold walls with a blow pin, and cooling the product by conduction or evaporation of volatile fluids in the container.

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 conot 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.

Extrusion Blow Molding (parison)

 (a parison is a plastic tubular form produced by extrusion or injection molding)

This process usually use commodity materials such as PVC, PS, PP, LDPE, HDPE. The extrusion part of the process is continuous and the rest is cyclic. In order to get around the problem, one can either do continuous parsion blow molding or intermittent parison blow molding.

For continuous parsion blow molding, extrudate is produced continuously which would achieve good melt uniformity. Several molds will be used to process the extrudate. Swift parison removal and control is required for this process.

For intermittent parison blow molding, the extra extruder must either feed the melt into a separate ram accumulator or the screw will reciprocate more waiting for the next batch. This process is used for small scale production because it is not as efficient as the continuous parison blow molding.

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.)

Injection Blow Molding

Injection blow molding include the following steps: at first, the polymer is injection molded onto the core pin; then the core pin is rotated to a blow molding station to be inflated and cooled.

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

Stretch Blow Molding (parison)

For Stretch Blow Molding, the polymer is first heated to aboe the glass transition temperature. Then polymer is inflated and stretched with a hollow core-rod. This process resembles that of a rubber balloon inflation.

Important polymer properties to be considered:

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

Guide to Plastic Resins

High Density Polyethylene (HDPE) is the most widely used resin for extrusion blown plastic bottles. This material is economical, impact resistant, and provides a good moisture barrier. HDPE is compatible with a wide range of products including acids and caustics but is not compatible with solvents. It is usually supplied in FDA approved food grade. HDPE is naturally translucent and flexible. The addition of color will make HDPE opaque although not glossy. Adding extra weight to the bottle will yield a rigid container. HDPE can be supplied electro-treated or flame-treated by request so that it can be ready for silk screen decoration. While HDPE provides good protection at below freezing temperatures, it cannot be used with products filled at over 180F or products requiring a hermetic seal. A glossy surface can be achieved with the use of the special copolymer resin, High Gloss HDPE.

Post Consumer Resin (PCR) is a blend of reclaimed natural HDPE (primarily from milk and water containers) and virgin resin. The recycled material is cleaned, ground and recompounded into uniform pellets along with prime virgin material especially designed to build up environmental stress crack resistance. PCR has no odor but exhibits a slight yellow tint in its natural state. This tint can be hidden by the addition of color. PCR is easily processed and inexpensive. However, it cannot come into direct contact with food or pharmaceutical products. PCR can be produced in a variety of recycled content percentages up to 100%.

Low Density Polyethylene (LDPE) is similar to HDPE in composition. It is less rigid and generally less chemically resistant than HDPE, but is more translucent. LDPE is used primarily for squeeze applications. LDPE is significantly more expensive than HDPE, but will yield a glossy bottle when produced in colors.

Medium Density Polyethylene (MDPE) combines the characteristics of low and high density polyethylene. Bottles are less translucent than LDPE but more flexible than HDPE. Like LDPE, MDPE is glossy when produced in colors.

Polypropylene (PP) is a naturally translucent material which provides contact clarity and an excellent moisture barrier. PP is easily processed via injection molding (jars and closures), and injection, extrusion, or stretch blow-molding (bottles). One major advantage of polypropylene is its stability at high temperatures, up to 200F. Therefore, it is used for hot fill products such as pancake syrup. Polypropylene is also autoclavable and offers the potential for steam sterilization. PP has excellent chemical resistance, but provides poor impact resistance in cold temperatures. Oriented PP offers improved impact resistance and clarity at low temperatures. Produced in color, PP exhibits a glossy finish.

K-Resin is ideally suited to a wide variety of packaging applications by virtue of its sparkling clarity, high gloss, and impact resistance.

K-Resin, a styrene derivative, is easily processed on polyethylene equipment. It is suitable for packaging many products but is specifically incompatible with fats and unsaturated oils or solvents. This material is frequently used for display and point-of-purchase packaging.

Polyvinyl Chloride (PVC) is naturally clear, has extremely good resistance to oils, and has very low oxygen transmission. It provides an excellent barrier to most gases and its drop impact resistance is also very good. This semi-rigid material is also very chemically resistant, but it is vulnerable to solvents.

PVC is available in general purpose grade, food grade, and fragrance-guard perfume grade. The occurrence of the blue tint in clear PVC can be modified by controlling the toner levels in each of these grades. General Purpose PVC will distort at 160F, making it incompatible with hot filled products. New PVC grades are able to withstand temperatures up to 190F and can be hot filled. Since it provides a good oxygen barrier, PVC is an excellent choice for salad oil, mineral oil, and vinegar. It is also commonly used for shampoos and cosmetic products.

PETG is a durable material with excellent gloss, clarity and sparkle desired for clear bottles. PETG can be processed via conventional extrusion blow molding methods, generally on machines designed to process PVC.

Applications include shampoos, soaps and detergents. PETG exhibits a good impact strength and gas barrier. The chemical resistance of PETG is fair and compatibility testing is recommended, especially with products that contain alcohol.

Resin Chart

This chart is a useful guide for performance of different materials, but is intended only as a guideline. Being certain that a specific type of plastic will serve all the needs of a specific product is a complicated matter and may not be determined from this chart.
Resin Polyethylene
Polyethylene PE
Low Density
Polyethylene PE
High Density
Polypropylene PP
Resin Density 1.35 0.91 to 0.925 0.94 to 0.965 0.89 to 0.91
Clarity Clear Hazy
Moderate Hazy
Moisture Barrier Fair to Good Good Good to Excellent Good to Excellent
Oxygen Barrier Good Poor Poor Poor
Acid Resistance Fair to Good Fair to Good Fair to Good Fair to Good
Grease & Oil Resistance Good Good Good Good
Distortion temperature 125o to
180o to
175o to
230o to
Rigidity (stiffness) Moderate to High Low Moderate Moderate to High
Impact Resistance Good to Excellent Excellent Good to Excellent Poor to Good
Heat Resistance Poor to Fair Fair Good Good
Cold Resistance Good Excellent Excellent Poor to Fair
Sunlight Resistance Good Fair Fair Fair

Resin Polystyrene PS Polyvinyl
Acrylonitrile Barex Polyethylene
glycol PETG
Resin Density 1.0 to 1.1 1.35 1.15 1.27
Clarity Clear Clear Hazy
Moisture Barrier Poor to Fair Fair Poor to Fair Fair
Oxygen Barrier Good Good Good Good
Acid Resistance Fair to Good Good to Excellent Good Fair
Grease & Oil Resistance Fair to Good Good Good to Excellent Good
Distortion temperature 200o to
140o to
165o to
140o to
Rigidity (stiffness) Moderate to High Moderate to High Moderate to High Moderate to High
Impact Resistance Poor to Good Fair to Good Poor to Good Good
Heat Resistance Fair Poor to Fair Fair Fair
Cold Resistance Poor Fair Fair to Good Good
Sunlight Resistance Fair to Poor Poor to Good Fair Good


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