Production of PET bottles and containers is done by injection molding of preforms, stretching and blowing of preforms into bottles. Both steps can be combined in one machine (one step process) or in two (two step 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 One Step Process :
&#; 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.
The Advantages of the two-Stage Process:
&#; Scalable from to 72,000 bottles/hr;
&#; Fast cycle times;
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&#; 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.
PET bottle blowing machine is a PET (full name: polyethylene terephthalate) material blowing equipment, mainly used for carbonated beverage bottles, mineral water bottles, cosmetic bottles, oil bottles, medicine bottles and other plastic hollow containers blowing Bottle making. At present, the PET bottle blowing machine on the market are mainly divided into two categories. One is the one-step blowing machine, which directly uses PET raw materials for drying, injection molding, temperature adjustment, stretching, and blowing. (Abbreviation: injection stretch blowing) At present, the one-step machine is mainly used in the cosmetics industry. It is compatible with various bottle diameters, thicknesses and shapes. The disadvantage is that it is limited by the injection time, its production efficiency is low, and the machine price expensive. The second type is a two-step blow molding machine. When using a two-step blow molding machine, PET raw materials must be made into preforms before blowing. The bottle production process is completed by preforming, heating, and stretching and blowing. Compared with the one-step blow molding machine, the two-step blow molding machine is not only more efficient and more energy-saving, but its price is more favorable than the one-step blow molding machine.
The two-step blow molding machine is divided into semi-automatic blow molding machine, automatic linear blow molding machine, rotary blow molding machine.
Semi-automatic bottle blowing machine: single-cavity and two-cavity blowing machines are common, and their efficiency is low. Fully automatic linear blowing machine: common models are 2 cavities, 4 cavities, 6 cavities, 8 cavities, 9 cavities, 10 cavities, etc. Most of the linear machines on the market are still flip-in embryo and flip-out bottle, and cannot be directly connected to the air duct. The handover of bottle blowing machine and filling machine requires the help of a bottle unscrambler to complete the handover. The output is generally not too high due to model restrictions, suitable for small and medium-sized enterprises.
Rotary bottle blowing machine: The bottle blowing workstation rotates, and the heating furnace still moves linearly. The internal structure and control mode of the machine are more complicated. Common models have 8 cavities, 10 cavities, and 16 cavities. 20 cavities, 24 cavities, 30 cavities, etc. Suitable for large factories, expensive, complex structure, and higher maintenance costs.
The two-step blow molding machine is divided into semi-automatic blow molding machine, automatic linear blow molding machine, rotary blow molding machine.
Semi-automatic bottle blowing machine: single-cavity and two-cavity blowing machines are common, and their efficiency is low. Fully automatic linear blowing machine: common models are 2 cavities, 4 cavities, 6 cavities, 8 cavities, 9 cavities, 10 cavities, etc. Most of the linear machines on the market are still flip-in embryo and flip-out bottle, and cannot be directly connected to the air duct. The handover of bottle blowing machine and filling machine requires the help of a bottle unscrambler to complete the handover. The output is generally not too high due to model restrictions, suitable for small and medium-sized enterprises.
Rotary bottle blowing machine: The bottle blowing workstation rotates, and the heating furnace still moves linearly. The internal structure and control mode of the machine are more complicated. Common models have 8 cavities, 10 cavities, and 16 cavities. 20 cavities, 24 cavities, 30 cavities, etc. Suitable for large factories, expensive, complex structure, and higher maintenance costs.
Faygo FG series linear blow molding machine combines the advantages of rotary blow molding machine and linear blow molding machine. It uses cam linkage to integrate mold clamping, mold clamping, sealing lowering and bottom mold raising. Driven by a servo electric cylinder, the blowing cycle is greatly shortened, which not only improves production efficiency, but also makes the movement of the entire clamping mechanism softer. The machine runs more stable and efficient. The whole machine adopts a modular design concept, and the part card slot installation does not require position adjustment, which is convenient for customers&#; daily maintenance and mold replacement. The entire mold change time is basically about 25 minutes.
Heating method: The embryo feeding and heating method of FG series blow molding machine is the same as that of a rotary blow molding machine. The heating adopts a small pitch arrangement of 38mm to maximize the utilization rate of the heating furnace. So as to achieve a more energy-saving effect. Compared with the traditional linear blow molding machine, heating method, energy saving is about 40%. The inside of the light box is equipped with a bottle cooling system, a heating equalization fan and a residual temperature exhaust system. The constant temperature inside the light box makes the preform heating effect more stable. The heating furnaces of FG4 and FG6 models are heated by 9-layer infrared lamps, and bottles of 200ml~ml are compatible. The temperature of the light box can be controlled by adjusting the intensity of the output voltage through the display screen. Each layer of light tubes is individually adjustable, which can achieve more precise temperature control.
Variable pitch and preform take-up and bottle delivery: The heated preform is controlled by a servo motor to move the preform pitch to the blowing pitch through variable pitch control. The gripper for taking the embryo and the bottle is transferred to the mold for blowing. The gripper for taking the embryo and the bottle is a mechanical gripper. The cylinder is not used to control the opening and closing, and the operation is faster and more stable. The variable pitch and embryo fetching and bottle feeding mechanism are all servo controlled, which not only runs fast, but also runs very smoothly. The error per cycle will not exceed 0.01 second. Ensure the perfect transfer of the preform from the heating furnace to the mold.
High-pressure blowing system: FG series blow molding machine only requires the customer to provide 3.0MPa high-pressure gas, the internal pressure of the machine is reduced by a pressure reducing valve, and a precision filter is installed at the air inlet to ensure access The cleanliness of the machine air source. The machine is equipped with 3 stainless steel gas storage tanks to stabilize the pressure, respectively, the main gas tank, the pre-blowing gas tank and the second blowing gas tank to ensure the stability of the blowing pressure. The machine adopts the diaphragm valve of American Parker. This valve not only has a fast response speed, but also has a large flow rate. It can maximize the effect of blowing in the shortest time and greatly save the time of blowing. The high-pressure gas pipeline connection of the whole machine has been strictly measured and calculated to minimize the dead zone of high-pressure blowing, so as to achieve more energy-saving purposes.
Lifting mechanism embryo machine: The preforms are manually poured into the hopper, and the elevator detects whether there is a preform in the embryo falling track through the induction monitoring system, and then starts automatic feeding. During the feeding process, it can be adjusted according to the production speed of the machine. The lifting speed of the hoist is adjusted. If the embryo guide, signal shows that the embryo is full, the lifting machine will automatically stop the embryo. We use a preform recovery device for the unscrambler. If the elevator speed is too fast or there are excess preforms in the unscrambler, it will return to the hopper through the preform recovery device to ensure that the preforms can be safely and orderly Enter the embryo guide.
Bottle-out device: The bottle-out rail of the FG series bottle blowing machine can be directly connected with the air duct of the filling machine, and the bottle-out height is basically the same as that of a conventional filling machine. There is a set of bottle dropping cylinders on the bottle outlet guide. Its function is to detect a broken bottle and it will open to make the broken bottle fall down, so as to ensure that all the qualified bottles enter the air duct. It has another function: To prevent bottle blocking, if the air duct jams the bottle, it will open and close with the blowing rhythm of the machine, so that the blown bottle will fall into the bottling bucket below, and the machine will not stop because of the air duct jam. This makes the machine more automated.
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