The production of PET containers requires injection molding of preforms, which are then stretched and blown into bottles. These two operations can be combined in one machine (single-stage process) or two (two-stage processes). Both methods have their apparent advantages and disadvantages. It is recommended that bottle manufacturers and brand owners understand these to make the right choice for their products.
The advantages of single-stage are:
• Flawless bottles;
• No need for adapter ring;
• Control the production of preforms;
• Possibility of good conditioning for rectangular bottles
• The starting point of the thread can be selected to be consistent with the bottle shape;
• Compact and flexible.
In my opinion, these advantages make this process a breeze for all non-beverage containers. However, there are some disadvantages, including:
• Long cycle time;
• Long conversion time;
• Uneven distribution on the wall;
• The quality problem of a hot runner (valve gate)
Available and recommended);
• Need to keep the machine running 24 hours a day to avoid a higher scrap rate;
• Operators need a longer learning curve because they must master the two processes and PET drying;
• The blow molding station is inefficient because the injection station always precedes the cycle time.
The latter problem leads to a subcategory of single-level machines, which I call integrated two-level machines. These machines are characterized by two or three injection cavities per blow molding cavity, and their blow moulding part circulates two to three times in each injection cycle. This saves blow moulding cavity, reducing tool costs, a vital issue for small and medium-sized applications.
The uneven wall distribution results from the viscous heating of the melt. When the melt passes through the barrel and hot runner channel, it heats up unevenly. When divided into two streams (usually a left stream and a right stream), the hotter material flows behind the new channel. If you stand in front of the machine, you may notice that the back of the finished bottles are usually thinner because they are blown by the hotter material in the back panel.
Although PET is very good at self-levelling (the strain hardening effect forces the initially hotter area to blow out after the colder area is blown), this effect is not enough to cover up uneven preform heat. Over the years, various measures have been implemented, with varying degrees of success.
Most hot runners are not naturally balanced. Natural balance means that the path from the melt to each cavity has the same length and number of turns. This is usually impossible due to the geometry (all the preforms are arranged in a row); therefore, the preforms are not filled at the same speed, which exacerbates the problem. Changing the nozzle diameter to allow the slower-moving melt to pass through a larger opening is helpful, but it can usually only be optimized for a very narrow weight range. Another solution is to add obstacles to the flow path of the cavity that moves faster.
In my opinion, hot-gate hot runners are not as good as valve-gate hot runners, but most machines still use the former.
Consider the benefits of the two-stage procedure. right now:
• Can be expanded from 1000 bottles/hour to 72,000 bottles/hour;
• Fast cycle time;
• Fast conversion;
• Flexibility (prefabs can be made and stored elsewhere);
• Excellent wall distribution
• On average, the weight of a round bottle may be lower;
• At any point during the process, you can halt it.
The main disadvantage of the two-stage type is that when the preform rolls onto the conveyor belt and enters the storage container, then is poured into the hopper of the blow molding machine again; the preform may be damaged. Many minor nicks and scratches can be stretched when using a high stretch ratio. However, this isn’t always the case, especially when the supplier selects the preform and is not entirely suitable for the bottle to be blown. Wrap tags or sleeves are an excellent way to hide these marks, which is one of the reasons they are so popular today.
A little-known problem with low-cavitation machines is that when using indexing machines, the heat of each preform can be quite different. Two, four, or six preforms are loaded and blown together in these machines.A chain is used to index them around the oven system.
Due to the difference in lamp output and exposure to cooling air, the preforms are exposed to different temperatures in the oven. This means that the preform temperature is different from cavity to cavity, and adjusting the process to suit them can be a considerable challenge. There are many linear machines that can continuously move preforms and avoid most of these problems. Rotary machines certainly do not have this problem at all, because each preform is subjected to the same heat.
Today, beverage bottles account for more than 80% of all bottles made, and the vast majority of them are manufactured using two-stage technology. The remaining 15% to 20% can choose any process. Many decisions are influenced by the cost of tools. For a production volume of less than 2 million per year, even the purchase of a four-cavity single-stage tool is difficult to justify, as is the case with many custom applications. The capital expenditure required to purchase a preform and run it on a two-cavity reheater is less. In the past 10 years, with suppliers all over the world, the number of available preforms has increased dramatically, because preforms are different from bottles and transportation is cost-effective.
To make an educated decision about which process to use, you need to understand the details of the application and make a decision based on the characteristics or economy of the bottle. If your bottle must be flawless, oval, and have a fixed thread, a single stage is the best choice. If none of these apply, then economics must be scrutinized. Single-stage preforms are always customized (unless you plan to use the same preform to make various shapes), so the fit can be guaranteed. However, if you buy a preform, you will start looking for a suitable preform. Not all preforms with the correct neck finish and weight are suitable for a particular bottle. The expert should evaluate the available preforms and select the most suitable one.
Let us look at a hypothetical application that involves a 1 liter round bottle with a 33 mm bottleneck and a weight of 42 grams. The annual output in this case is 750,000 bottles. In two stages, this will require one or two-chamber machines, which may run at a speed of about 600 to 800 bottles/chamber/hour. Although the numbers published by machine manufacturers are much higher, most custom applications run slower for a variety of reasons. This will keep the single-chamber machine busy for 1,152 hours, while the double-chamber machine is half as busy. The investment cost is very low because only one or two blow molding cavities need to be purchased.
In a single-stage, the cycle time is approximately 13 to 16 seconds—for example, 250 cycles/hour. A two-chamber system will produce the required bottles in 1500 hours, and a four-chamber system will produce the required bottles in 750 hours. The capital cost of each cavity is high, of course, most machines will be able to run four or even six cavities. The decision will be made based on the cost of each item in the various cavitations as well as the job’s future prospects.