BLOW MOLDING: Single- or Two-Stage PET Bottle Making

Production of PET containers requires injection molding of preforms and subsequent stretching and blowing of these into bottles. Both operations can be combined in one machine (the single-stage process) or in two (the two-stage process). Both processes have their distinct advantages and disadvantages, and bottle producers and brand owners would be well advised to know those in order to make the right choice for their products.

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The advantages of single stage are:

 &#;  Blemish-free bottles;
 &#;  Transfer ring not necessary;
 &#;  Control over preform production;
 &#;  Good conditioning possibilities for oblong bottles on 
some machines;
 &#;  Thread start can be chosen to coincide with bottle shape;
 &#;  Compact and flexible.

In my view, these advantages makes this process a no-brainer for all non-beverage containers. However, there are some disadvantages as well, including:

 &#;  Long cycle times;
 &#;  Long changeover times;
 &#;  Uneven wall distribution;
 &#;  Quality problems with thermal-gated hot runners (valve gates 
are available and  recommended);
 &#;  Need to run machines run 24 hr/day to avoid higher scrap percentage;
 &#;  Longer learning curve for operators, as two processes and PET drying must be mastered;
 &#;  Inefficient blow station as the injection station always has precedent over the cycle time.

The latter issue has led to a subcategory of single-stage machines that I call integrated two-stage machines. These machines feature a multiple of two or three injection cavities per blow cavity, and the blow sections of them cycle two or three times for every injection cycle. This saves blow cavities and thereby reduces tooling costs, a critical issue for small- and medium-volume applications.

The uneven wall distribution is a result of viscous heating of the melt. As the melt moves through the barrel and hot-runner channels it heats up unevenly. When it is then divided into two streams (left and right, typically), warmer material flows to the back of the new channel. If you were to stand in front of the machine you might notice that finished bottles are often thinner at the back because they were blown with warmer material in the back panel. 

While PET is very good at self-leveling (the strain-hardening effect that compels initially warmer areas to blow out after cooler areas have blown), this effect is not significant enough to mask the uneven preform heat. A variety of measures have been employed over the years with varying success.

Most hot runners are also not naturally balanced. Natural balance means that the path of the melt to each cavity has the same length and number of turns. Because of geometry&#;all preforms in one row&#;this is often not possible, and as a result, preforms do not fill at the same speed, aggravating the problem. Changing nozzle diameters to allow slower-moving melt through larger openings is helpful, but usually can only be optimized for a very narrow weight range. Another solution is to add obstacles in the flow path of the faster-moving cavities.

Thermal-gated hot runners are inferior to valve-gated ones, in my opinion, but most machines still run with the former.

Now let&#;s look at the advantages of the two-stage process:
 &#;  Scalable from to 72,000 bottles/hr;
 &#;  Fast cycle times;
 &#;  Fast changeovers;
 &#;  Flexibility (preforms can be made elsewhere and stored);
 &#;  Very good wall distribution for 
round bottles;
 &#;  On average, lower gram weights possible for round bottles;
 &#;  Process can be stopped at any time.

With two-stage, the main disadvantage is the potential damage to the preforms that occurs when they tumble onto conveyor belts and into storage containers, and then again when they are dumped into the hoppers of blow molding machines. Many of the little nicks and scratches can be stretched out when high stretch ratios are used. But this is not always the case, particularly when a preform is chosen from a vendor and does not exactly fit the bottle to be blown. Wrap-around labels or sleeves are a good way to hide these marks, which is one reason they are so popular nowadays.

A lesser known problem with low-cavitation machines is that heat to each preform can be quite different when indexing machines are used. In these machines, two, four, or six preforms are loaded and blown together. A chain indexes them around the oven system.

Preforms are exposed to varying temperatures inside the ovens because of differences in lamp output and exposure to cooling air. This translates into different preform temperatures from cavity to cavity, and adjusting a process to suit them all can be quite a challenge. There are now a number of linear machines available that move preforms continuously and avoid most of these issues. Rotary machines of course do not have this problem at all, as each preform gets identical heat.

Today, more than 80% of all bottles produced are for beverages, and the vast majority of these are made using two-stage technology. It is the remaining 15% to 20% where either process is an option. Many decisions are actually driven by tool prices. Buying even a four-cavity single-stage tool can be hard to justify for volumes of fewer than 2 million/year, which many of the custom applications are. There is less capital expense involved to buy preforms and run them on a two-cavity reheat machine. The number of available preforms has increased dramatically over the last 10 years with suppliers spanning the globe, because preforms, unlike bottles, are cost-effective to ship.

In order to make an informed choice on which process to use, you need to understand the particulars of the application and make your decision either on the basis of bottle features or economics. If your bottle must be blemish-free, oblong in shape, and with a fixed thread, single-stage is the way to go. If none of those applies, the economics must be scrutinized. The preforms for single-stage are always custom-made (unless you plan to make a variety of shapes out of the same preform) so the fit is guaranteed. If preforms are bought however, the quest for the right one starts. Not every preform that has the right neck finish and weight is suitable for a particular bottle. An expert should evaluate the available preforms and choose the best fit.

Let&#;s look at a hypothetical application involving a round 1-liter bottle with a 33-mm neck finish and a weight of 42 g. The yearly volume in this scenario is 750,000 bottles. In two-stage, this will require a one- or two-cavity machine that will probably run around 600 to 800 bottles/cavity/hr. While machine builders publish much higher numbers, most custom applications run slower for a number of reasons. This will keep the one-cavity machine busy for hr, and the two-cavity for half of that. Capital cost is quite low as only one or two blow cavities have to be purchased.

In single-stage, cycle time will be about 13 to 16 sec&#;say 250 cycles/hr. A two-cavity system would then make the required bottles in hr, a four-cavity system in 750 hr. Capital cost is significant for each cavity, and of course, most machines will be able to run four or even six cavities. Here the decision would be based on cost per piece in different cavitations, and what future outlook the job holds. 

ABOUT THE AUTHOR: OTTMAR BRANDAU 

Ottmar Brandau has been working in the plastics industry since , and is the president of Apex Container Tech Inc., Wasaga Beach, Ont. His latest book, The Rapid Guide to Perfect PET Bottles, describes 30 common problems and their solutions. It can be found at blowmolding.org/shop. Contact: (705) 429-; ; blowmolding.org.

Structure and working principle of automatic Blow Molding Machine  

1. The main structure and principle of the machine

1.1, clamping part

The positioning rack is balanced and positioned, the front, middle and rear three templates, the double crank arm linkage mechanism, and the mold clamping cylinder. The cylinder is driven by the electromagnetic directional control valve to drive the crank arm to open and close the mold. The structure is reasonable in design, stable in operation and large in clamping force.

1.2, stretch the blowing part

It is composed of a stretch solenoid valve, a high-pressure blowing valve, a pull-body sealing cylinder, a movable sealing seat, and a blowing air reservoir. When working, the piston of the stretching cylinder is pushed through the directional control valve to drive the stretching rod and the sealing cylinder to push up on the preform that has been closed. The sealing cylinder seals the mouth of the preform, and the stretching rod is heated and elastic to the preform. At the same time, the gas stored in the gas storage cylinder is stretched longitudinally, and the preform is bottom blown by the high-pressure blowing valve through the sealing cylinder, and then the preform is subjected to high-pressure inflation molding.

1.3. The heating chain plate passes through the first station of the stepping cylinder, is detected and positioned by the photoelectric switch, and then blown into the product by the molding mechanism. The stepping cylinder is pushing a station to detect and position, and continuous production is repeated.

1.44. The self-rotating chain is driven by the motor to work continuously, so that the preform in the drying tunnel is heated quickly and evenly.

1.5. The preform is conveyed and sorted through the feeding system, and the preform is installed on the bottle blowing machine base by the conveying manipulator and enters the drying tunnel

1.6. The heating consists of two relatively independent far-infrared lamp ovens. Each far-infrared lamp of each oven can be adjusted longitudinally according to different preforms.

1.7. Preform heating. While the preform rotates to make it evenly heated, the bottle mouth is cooled, and then hot air is blown to the preform by a fan to make the inner and outer walls of the preform evenly heated.

1.8. After the preform enters the blowing mold, the pre-blowing air enters to stretch the blown preform in the ring direction; when the stretching rod reaches the bottom of the mold, high-pressure air enters the cavity to further stretch the preform, making the bottle wall tight Paste mold bi.

1.9. The high pressure gas is kept in the mold for a certain period of time, on the one hand, it eliminates the internal pressure caused by the stretching of the preform.

On the other hand, make the bottle wall close to the mold wall to improve the crystallinity of the bottle plastic.

1.10. After the high-pressure gas is over, the venting and demolding will begin.

1.11, the blowing process is over.

1.12. The bottle is conveyed to the bottle-loading station through the chain plate, which is picked up by the bottle-loading cylinder, and then blown out by the airflow.

2. Blowing process adjustment

2.1. Installation and operation (notes)

1. Promote standardized operation actions;

2. When blowing the thermos bottle, pay attention to the damage to the body caused by the high temperature of hot kerosene. The mould is 125&#;-135&#;, and the mould temperature machine is 140&#;-150&#;.

3. Please turn off the control power when testing the mold temperature;

4. Pay attention to the damage to the body when the high pressure gas joint is loosened;

2.2. Blow molding conditions

1. Temperature: including:

A. Setting of lamp power;

B. The temperature setting of the exhaust fan;

C. The influence of ambient temperature on the heating box

D. The temperature of the hot mold: 125°C&#;135°C, water bottle mold 10&#;15°C;

2. Pressure: including:

A. Pre-blowing pressure is 5-15 kg/cm², the actual pressure is 9-12 kg/cm²;

B. Bottle blowing pressure: cold bottle 25-30 kg/cm²;

C. Cyclic blowing: 15 kg/cm²;

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3. Flow rate: the speed when the pre-blowing pressure is constant.

4. The speed of the tie rod: The speed at which the tie rod rises is directly related to the stretching of the preform.

5. Forming settings:

A. Pre-blowing delay.

B. Blowing delay.

C. Blow time, exhaust time, etc.

What Is Blow Molding?

Blow molding is a molding process used in the manufacturing industry to create hollow objects made of plastic. Like other molding processes, it involves the use of heated, liquid material that&#;s forced into a mold cavity under pressure. Blow molding is a special type of molding process, however, that leverages the properties of traditional glassblowing.

Overview of Blow Molding

Blow molding, also known as blow moulding in the United Kingdom, is a molding process in which heated plastic is blown into a mold cavity to create a hollow object. The defining characteristic of a blow molding is that it&#;s used to create hollow objects. Raw plastic is first heated, after which it&#;s formed into a parison. Next, the plastic parison is secured to the top of the mold. Finally, air blown down onto the plastic parison, thereby stretching it across the interior walls of the mold cavity.

Blow molding follows the same principle as glassblowing. With glassblowing, a glassblower blows air across heated glass, thereby creating a hollow glass object. With blow molding, a machine blows air across heated plastic that&#;s placed on top of a mold cavity. The air forces the heated plastic to expand across the interior walls of the mold cavity.

The Different Types of Blow Molding

There are several different types of blow molding, one of which is extrusion. Known as extrusion blow molding (EBM), it lives up to its namesake by extruding heated plastic into a parison. It&#;s a common molding process used in the manufacturing industry because of its ability to mass-produce a large volume of objects in the same size and shape.

Another common type of blow molding process is injection stretch blow molding. Using either one or two stages, injection stretch blow molding is typically used to create plastic bottles. It&#;s specifically effective for creating preforms of plastic bottles, which are then either sold to bottling companies or used to manufacture a bottle.

Blow Molding vs Injection Molding: What&#;s the Difference?

The terms &#;blow molding&#; and &#;injection molding&#; are often interchgeably when referring to molding processes. While similar, though, they aren&#;t necessarily the same. Both blow molding and injection molding involve the use of liquid material &#; typically plastic &#; that&#;s forced into a mold cavity. The difference is that blow molding is used to create hollow objects, whereas injection molding is used to create solid objects. For hollow objects, only blow molding offers a fast and effective solution for manufacturing companies.

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Blow molding

Manufacturing process for forming and joining together hollow plastic parts

The blow molding process

Blow molding (or moulding) is a manufacturing process for forming hollow plastic parts. It is also used for forming glass bottles or other hollow shapes.

In general, there are three main types of blow molding: extrusion blow molding, injection blow molding, and injection stretch blow molding.

The blow molding process begins with softening plastic by heating a preform or parison. The parison is a tube-like piece of plastic with a hole in one end through which compressed air can enter.

The plastic workpiece is then clamped into a mold and air is blown into it. The air pressure inflates the plastic which conforms to the mold. Once the plastic has cooled and hardened the mold opens and the part is ejected. Water channels within the mold assist cooling.

History

The process principle comes from the idea of glassblowing. Enoch Ferngren and William Kopitke produced a blow molding machine and sold it to Hartford Empire Company in . This was the beginning of the commercial blow molding process. During the s the variety and number of products were still very limited and therefore blow molding did not take off until later. Once the variety and production rates went up the number of products created soon followed.

The technical mechanisms needed to produce hollow-bodied workpieces using the blowing technique were established very early on. Because glass is very breakable, after the introduction of plastic, plastic was used to replace glass in some cases. The first mass production of plastic bottles was done in America in . Germany started using this technology a little bit later but is currently one of the leading manufacturers of blow molding machines.

In the United States soft drink industry, the number of plastic containers went from zero in to ten billion pieces in . Today, an even greater number of products are blown and it is expected to keep increasing.

For amorphous metals, also known as bulk metallic glasses, blow molding has been recently demonstrated under pressures and temperatures comparable to plastic blow molding.[1]

Typologies

Extrusion blow molding

Extrusion blow molding

In extrusion blow molding, plastic is melted and extruded into a hollow tube forming a tube like piece of plastic with a hole in one end for compressed gas - known as a parison. The parison is captured by closing it into a cooled metal mold. Air is blown into the parison, inflating it into the shape of the hollow bottle, container, or part. After the plastic has cooled, the mold is opened and the part is ejected.[2]

"Straight extrusion blow molding is a way of propelling material forward similar to injection molding whereby an Archimedean screw turns, feeding plastic material down a heated tube. Once the plastic is meleted the screw stops rotating and linearly moves to push the melt out. With the accumulator method, an accumulator gathers melted plastic and after the previous mold has cooled and enough plastic has accumulated, a rod pushes the melted plastic and forms the parison. In this case the screw may turn continuously or intermittently.[3] With continuous extrusion the weight of the parison drags the parison and makes calibrating the wall thickness difficult. The accumulator head or reciprocating screw methods use hydraulic systems to push the parison out quickly reducing the effect of the weight and allowing precise control over the wall thickness by adjusting the die gap with a parison programming device.

Continuous extrusion equipment includes rotary wheel blow molding systems and shuttle machinery, while intermittent extrusion machinery includes reciprocating screw machinery and accumulator head machinery.

Spin trimming

Containers such as jars often have an excess of material due to the molding process. This is trimmed off by spinning a cutting blade around the container which separates the material. The excess plastic is then recycled to create new moldings. Spin Trimmers are used on a number of materials, such as PVC, HDPE and PE+LDPE. Different types of the materials have their own physical characteristics affecting trimming. For example, moldings produced from amorphous materials are much more difficult to trim than crystalline materials. Titanium nitride-coated blades are often used rather than standard steel to increase life by a factor of 30 times.

Injection blow molding

Injection blow molding a plastic bottle

The process of injection blow molding (IBM) is used for the production of hollow glass and plastic 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 bottles. The process is divided into three steps: injection, blowing and ejection.

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 hot runner manifold where it is injected through nozzles into a heated cavity and core pin. The cavity mold forms the external shape and is clamped around a 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. similar in appearance to a test tube with a threaded neck.

The preform mold opens and the core rod is rotated and clamped into the hollow, chilled blow mold. The end of 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 as an option can be 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.

Injection stretch blow molding

Injection Stretch Blow Molding has two main different methods, namely Single-stage and Double-stage process. The Single-stage process is then again broken down into 3-station and 4-station machines.

In the single-stage process, both preform manufacture and bottle blowing is performed in the same machine. The older 4-station method of injection, reheat, stretch blow and ejection is more costly than the 3-station machine which eliminates the reheat stage and uses latent heat in the preform, thus saving costs of energy to reheat and 25% reduction in tooling. The process explained: Imagine the molecules are small round balls, when together they have large air gaps and small surface contact, by first stretching the molecules vertically then blowing to stretch horizontally the biaxial stretching makes the molecules a cross shape. These "crosses" fit together leaving little space as more surface area is contacted thus making the material less porous and increasing barrier strength against permeation. This process also increases the strength to be ideal for filling with carbonated drinks.

In the two-stage injection stretch blow molding process, the plastic is first molded into a "preform" using the injection molding 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 a reheat stretch blow molding machine. In the ISBM 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. The preform is always stretched with a core rod as part of the process.

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