Blow Molding 101

Overview of Blow Molding:

The blow molding of glass was going on in Egypt thousands of years ago.  The blow molding of plastic materials (Cellulose) started in the late 1880’s for dolls and their heads.  In the 1930’s and 1940’s Hartford Empire Company and then the Plax Corporation began making blow molded parts such as Christmas tree ornaments.  These early attempts to make blow molded parts and their associated machinery would bear little resemblance to modern machines but the process is very much the same. First a hollow tube called a parison is created by heating plastic and pushing it out of an orifice. Mold halves then close around the tube and the tube is inflated with air which forces the material into the shape of the part.

The parison is the key to understanding the process. If we were trying to make this tube (parison) a perfectly uniform wall, gravity and thermodynamics would be busily working against us.  The heated material is hotter at the top (closer to the heat source) than at the bottom and it will therefore stretch resulting in a non uniform wall thickness.  Up until 1962 this was a serious problem; Denes Hunkar solved the problem by inventing the parison programmer.  This electronic device coupled with some hydraulics varies the size of the orifice and gets rid of the gravity problem by increasing the wall as the material is pushed out.

Now that we have created this perfect parison we are going to close the mold. Using either a blow stick or a needle (which results in a small hole in the part) we are able to put air into the parison forcing it against the walls of the mold. The resulting part will have a more or less nominal wall thickness. Thus solves the first of our problems.  Except! If an area of the part stretches our uniform tube more than another area then a non uniform wall will result.  As long as the non uniformity is symmetrical we can once again solve the problem by extruding more material in that particular area.  If the part happens to be square instead of round as our parison is, the second major problem occurs.  This can be fixed by shaping the orifice, resulting in the tube now having four thick ribs running from top to bottom.  When the air blows into the part the parison expands uniformly hitting the flats of the square first and then stretching into the corners.  As a result of shaping the orifice there is more material available to stretch into the corners thus leaving the part with a nominal wall thickness.

The next issue is the correct sizing of the machine to the particular part. This seems like it should be an easy task, however there is no standard sizes for these machines. Each maker of the machines has their own sizes and therefore we must be able to choose from the market place. To do this we must understand how.  For example many people understand the sizing of an injection machine which is done by the weight of the part and the projected area of the part.  In the blow molding process there is an extra step in which we must determine the output of the extruder verses the weight of the part verses the cycle time of the part.  This extra step means we must know the cycle time of the part before we can determine the size of the extruder needed. Otherwise we will be extruder limited and will have to run the part at a longer cycle than necessary.  The wall thickness of the part and temperature of the cool water will generally allow the cycle to be determined. Once this is done we can select the out put of the extruder. The size of the mold sets the size of the platen and the size of head equals the shot plus the flash.   FLASH!  Remember the parison has a top and bottom that is not included in the finished part weight which will be cut off and reground. This is depending on the part and can be as little as 1.35 times part weight to 10 times part weight (very unusual).  The head or heads of the machine determine the not just the weight of the parison but the size of the Parison. Many times the machine will have the right sized shot, the right sized platen, and extruder, but not enough head diameter to provide the right sized parison. The parison diameter is determined by how big the orfice is.  Each machine has an orifice range (head tooling) between very small (converging) to very large (diverging), but specific to the overall head size. 

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History of Blow Molding

U.S. Patent 237168 was issued on February 1, 1881, to Celluloid Novelty Co. and Celluloid manufacturing Company, New York. This was the first patent for the processing of extruded polymer into a parison for blow molding.

The first applications for blow molding were for cellulose nitrate, and later, in the 1930’s, for cellulose acetate. Blow molding remained a relatively small part of the plastics manufacturing scene until the introduction of Low Density Polyethylene (LDPE) in the 1940’s. The production of LDPE squeeze bottles by Monsanto caused a rapid expansion of the industry, with containers produced to replace glass bottles for shampoos and liquid soaps.

The mass production of high density polyethylene (HDPE) and polypropylene (PP) in the 1950’s led to a further increase in blow molding demand, for applications such as liquid detergents, motor oil, water and milk. The lightweight HDPE one gallon milk container revolutionized the dairy industry, as glass bottles and paperboard were quickly replaced.

The production of polyethylene terephthalate (PET) led to the viability of reheat stretch blow molding. The biaxial orientation properties of PET allowed the high volume production of bottles able to resist the carbonation pressure in soft drink applications. The high clarity and economics of PET stretch blow molding have made this a popular production method for bottles for water, detergents, and other products.

Types of 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.

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

1. Continuous Extrusion Equipment

  • Rotary wheel blow molding systems
  • Shuttle machinery

2. Intermittent Extrusion Machinery

  • Reciprocating screw machinery
  • Accumulator head machinery

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

Basic polymers, such as PP, HDPE, PVC and PET are increasingly being co-extruded 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.

Accumulator Head Machinery is used for the extrusion blow molding of large industrial hollow parts. Examples of parts produced on this machinery include drums, trash cans, automotive panels, playground equipment, and large containers, such as Jerry Cans, for liquid storage. Most parts produced on accumulator head machinery are single layer; however, specialized machinery is capable of producing parts with up to seven unique layers of plastic - these machines are used primarily to manufacture automotive gasoline tanks with barrier layers.

Accumulator Head Machinery is characterized by the accumulation of melted plastic resin in one or more extrusion heads. As extruders melt the plastic, it accumulates in the heads until the resin is ready to be extruded into parisons. An internal plunger is then activated, using hydraulic pressure, to extrude the parison through an extrusion die in between two open mold halves.
Unlike shuttle machinery or rotary wheel machinery, which is characterized by continuous extrusion, accumulator head machinery utilizes an intermittent extrusion process. This allows large, heavy parisons to be dropped in a few seconds, followed by the rapid closing of the molds. Due to the large, heavy weight of the parisons, it is not practical to slowly extrude the plastic while the prior parison is blown and cooled in the molds. Cycle times of 30 to 120 seconds or more are common in thick-walled parts, and the parisons would cool and sag if extruded slowly over this time period. The intermittent process also allows the machinery to function without shuttling the molds, which is not economical with large, heavy molds and clamping structures.

In some applications, the parison is extruded over one or more blow pins, which are used to form precise openings in the part, as well as provide an entry point for the blow air. In other applications, the blow air may enter the part through the center of the extrusion heads, or through needles, which puncture the parison.
Due to the size of parts produced, requiring large clamps, the extruders and flow-heads are typically positioned on an upper, "mezzanine" level. The clamp, electrical cabinets, operator station, and hydraulic system are typically positioned on the lower "ground" level.


  • In some cases, parts are dropped from the molds, and are removed manually from the machine without an extractor. In some cases, the parts drop onto angled trays, which then slide the parts away from the clamps, for manual removal. This approach may require longer cycle times, to allow the operators to remove the parts. The use of drop slides may also require the clamp of the machine to be elevated, increasing cost and required factory ceiling height.
  • In some cases, the parts are moved from the molds into secondary cooling stations. This approach allows the operator to reduce the overall cycle time required to manufacture the part. Most Jerrycans are manufactured using secondary cooling stations.


  • In 1949, Reinhold Hagen of Kautex, Siegburg Germany, develops the first blow molding machine for processing polyethylene.
  • In the 1960's, reciprocating screw blow molders were developed, with single or double heads with up to 10 lb (4.5 kg) plastic shot capacity. These were precursors of modern accumulator head machines.
  • In 1964, the first prototype plastic fuel tank was produced by Kautex. Uniloy introduced the first "unitized block" construction machine.
  • In 1972, Barr Polymer produced the first American accumulator machine. This technology was later sold to Uniloy.
  • In 1973, the first commercial polyethylene fuel tanks were produced by Kautex and installed in the production series Volkswagen Passat.
  • The first Sterling single 10 lb (4.5 kg) accumulator machine, with a 36 x 30 in (914 by 762 mm) press and MACO IV controller was demonstrated at NPE 1979.
  • In the 1980's the advent of modern "engineering plastics" such as Noryl® Modified Polyphenylene Oxide, spurred a generational leap in accumulator head machinery, as early generations of these materials had reduced melt strength. To be able to process these resins, machinery was developed including modern features such as:
  • High press closing speeds, > 1200 in. per minute.
  • Proportional valve hydraulics used with variable displacement pumps
  • Hydraulic pre-fill valves for fast clamp closing and lock-up.
  • Entire hydraulic system filtration to 3-10 micrometers, eliminating the need for a separate tank for parison programming.
  • In 1994, the first co-extruded (multilayer) fuel tank was utilized in series production. Milacron also produced their first commercial machines that year.
  • In 1994 Jackson Machinery produced its first new bottle blow molding machine.
  • In 1997 Jackson Machinery produced its first new accumulator head machine.

Water Bottle Machinery

This equipment was originally engineered to be simple, versatile, and capable of producing basic bottles and containers. However, without integral bottle trimming capability, most applications were centered on the production of various “niche” industrial, recreational, automotive, and novelty type products.

This family of machines utilizes a reciprocating screw extruder with a direct feed “flow through” die head design for forming the parison. Cantilevered clamp systems are non-shuttling for simplicity and provide open access for convenient part extraction.

In the mid to late 1970’s the 5-gallon polycarbonate water bottle market began to develop and Improved Blow Molding, working with GE Plastics, designed and supplied a number of special "B30" model blow molding machines to meet the need. That “niche” market enjoyed significant domestic growth in the ‘80’s and significant growth worldwide in the ‘90’s.
Both Single and Dual Head configurations are common in the industry. Hydraulic extruders are commonly used. The use of hydraulic compaction blow pin assemblies to allow the production of necks with unblemished sealing surfaces has improved the performance of the containers by greatly reducing the number of "leakers."


  • Lightweight Bottle Machinery
  • The Rocheleau Tool & Die Co., Inc. is established in 1938 by Leopold A. Rocheleau. They later develop a niche in reciprocating screw machinery for small containers.
  • In the 1950's, the Uniloy Division of Hoover Ball & Bearing becomes the first American supplier of blow molding machinery. They go on to develop machinery and patents for the production of "hollow-handle" plastic bottles for the dairy packaging market.
  • In 1985, Uniloy was purchased by Johnson Controls, and continues market dominance in light weight dairy bottle machinery.
  • APS Plastics Systems is formed by former Uniloy employees in 1997.
  • Uniloy became part of Milacron Inc. in 1998.
  • In 1998, Liberty Blow Molding also emerges, competing with Milacron in large bottle reciprocating screw blow molding.
  • Liberty is purchased by Uniloy in 1992, after achieving 40% market share in four years, and shipping some 100 machines.
  • Graham Engineering purchases the technology of APS Plastics Systems in 2003. GMG capitalizes on refinements such as shot pot technology as a viable alternative to reciprocating screw extruders.
  • Bottle Machinery
  • This equipment product line is relatively old, originating in 1960 and produced by IMPCO, a NH based injection molding machine manufacturer. IMPCO (a.k.a. Improved) subsequently became part of Ingersoll-Rand Co. in 1964.

Blow Molding Technology

More and more people are becoming involved in Blow Molding. As this happens, inevitably some need a quick primer as to what kind of machine does what kind of job. What follows is an explanation of how the process works and how the particular machine types differ from one another. Generally the accumulator head machines are used to make industrial parts and the continuous extrusion machines are used to make bottles. The reciprocating screw machines are generally cross over that can make containers as well as industrial parts (mostly dairy bottles). The IBM machines are generally make health and beauty aid products as well as medical bottles. The re-heat and blow machines (both one-step and two-step) make bottles out of PET (polyethylene terephthalate). Most of the PET products are bi-axially oriented and are used to contain carbonated beverages.

The advantages of the blow molding process are the creation of a hollow part in which the molds are usually made out of aluminum (although not in the injection blow process). Aluminum is of course much softer than steel, has a much higher heat transfer value and is far easier and quicker to machine. Thus the tooling cost for the blow molding process is far less than in the injection molding process.

The Actual Blow Molding Process

The machine’s barrel temperature is set for the parameters of the particular resin to be molded. The plastic material is placed into the hopper and fed into the mouth of the extruder. The extruder has a heated barrel with a screw inside to convey the pellets down the barrel while heating them. Soak times vary from 1 hour to several hours depending on the head size. The liquefied plastic is then forced around the mandrel which forms it into a tube which is then pushed out of the head by a plunger. The orifice of the head has tooling that is programmed in the original step to form the particular wall thickness of the part by increasing or decreasing the annulus. The parison (name for the tube) is now hanging in midair and the mold halves close on it. An air source either by a blow pin or a needle blow then blows the part to the shape of the mold cavity. The mold is then re-opened and the part is removed from the machine and secondarily trimmed or finished as necessary.

Types of Machines That Are Commonly Used in Blow Molding

Continuous extrusion machine (generally bottle machines).
Continuous extrusion means that the extruder is constantly producing a parison (tube) out of its head. The mold halves then grab the parison and then transfer it to a blow station where the air molds the part to the configuration of the mold.

Accumulator head blow molding machine.
These are generally used for industrial parts in which the melted material is accumulated in the head and then pushed out from the head by cylinders.

Reciprocating screw blow molding machine
These machines are a semi injection machine and a semi accumulator head machine. They first melt the plastic and collecting the shot in front of the screw and then push it out over the mandrel creating the parison and then form the part in the same way as type 1 and type 2.

Injection blow machine or IBM’s.
These are machines that are a cross between an injection molding machine and a blow molding machine. They first squirt liquefied plastic material into a closed mold (steel) forming a pre-form. The machine then opens its clamp and indexes the pre-form on a mandrel to a blowing station where the air is applied to form the part (usually a bottle) into the shape desired. The machine then indexes a second time to the ejection station.

Two-step re-heat and blow machine or RHB
In this process an injection molded pre-form is unscrambled and placed into a serpentine belt system and re-heated. (The re-heating is programmed by quartz heaters to allow the exact form to more easily be blown when it reaches the mold cavities.) When it reaches the mold cavities a rod pushes the parison thereby lengthening it and simultaneously blow air is supplied through the mouth of the container thereby stretching the pre-form in two directions at the same time. This produces a bi-axially oriented product which is capable of providing a co2 barrier thus making a typical pop bottle.

One-step blow molding machine
The process is similar to the above IBM process ( #4) in that a pre-form is molded in the first stage mold halves, indexed to the second stage and stretched and blown at the same time as in step #5.

The particular materials that are generally blow molded are:

PET (polyethylene terephthalate)
HDPE (high density polyethylene)
HWPE (high molecular weight polyethylene)
ABS (acrylonitrile butadiene styrene)
PVC (polyvinyl chloride)
Engineered materials such as zenoi, ppo and other common injection molded materials
Silicone gum rubber

As you can see from the above list most commonly injection molded and extruded materials can be blow molded. The particular materials are chosen for their physical properties, cost and environmental utilization properties.

The sizing of the particular machine is based on the weight of the part plus the flash of the part (if flashed) and the particular molecular weight of the resin. For example if one were going to run a 5 gallon gas can (the red ones in your garage). One would have to first know that it weighs approximately 1500 grams, cycles in about 48 seconds, is molded out of HDPE and generally is molded on a single cavity or double cavity machine. The key point in the above example is that the 1500 gram finished weight of the product has a flashed weight of about 2000 to 2100 grams. Thus a five pound machine (accumulator head) is necessary to mold the product. It can also be molded in a reciprocating machine as well as in a continuous extrusion machine. In the case of the continuous extrusion machine it becomes a bit more difficult to determine what size is necessary. In all cases the platens must be large enough to accommodate the mold. With continuous extrusion one must understand that the out-put is continuous and therefore one must know the cycle time of the product in order to determine the pounds per hour that the extruder must produce. In the case of the red gas can the machine must be capable of 350 to 400 pounds per hour to keep up with the cycle time.

All of the above is a very cursory explanation of the sizing of the machines versus the nature of the product. An additional factor to come into play is the capability of the head to accommodate the tooling which makes the parison the correct diameter. Each machine is capable of a tooling size range. In the case of the red gas can we would need a tooling size of approximately 7 inches in diameter. The way that this is calculated is to first use the circumference of the circle.

The circumference is then divided by two thereby giving us lay flat.

Lay Flat = C=pd
The lay flat is really the parison squeezed flat which is what happens when the mold halves close on the tube.

Parison Diameter x Pie (-3.14) ÷ 2 = Lay flat

In other words the diameter of the tooling determines the diameter of the tube and thus the size of the product that can be made by that head tooling. Each machine has a particular maximum and minimum tooling diameter thereby allowing the machine to be size to make the particular product.

As one might well understand a few paragraphs do an injustice to the technology. This information is provided to help the user in sizing and selecting the machinery necessary for their job. More information and help in sizing any machinery can be provided by speaking with Bob Jackson at Jackson Machinery.