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The advantages of single stage are:
In my view, thisadvantage makes this process a no-brainer for all non-beverage containers.However, there are some dis advantages as well, including:
Long changeover times;
Un even wall distribution;
Quality problems with thermal-gated hot runners
Need to run machines run 24 hr/day to avoid higher scrap percentage;
Longer learning curve for operators, as two processes and PET drying mustbe mastered
The latterissue has led to a subcategory of single-stage machines that I call integratedtwo-stage machines.
These machines feature a multiple of two or three injectioncavities per blow cavity, and the blow sections of them cycle two or threetimes for every injection cycle.
This saves blow cavities and thereby reducestooling costs, a critical issue for small- and medium-volume applications.
The unevenwall distribution is a result of viscous heating of the melt. As the melt movesthrough the barrel and hot-runner channels it heats up unevenly.
When it isthen divided into two streams (left and right, typically), warmer materialflows to the back of the new channel.
If you were to stand in front of themachine you might notice that finished bottles are often thinner at the backbecause they were blown with warmer material in the back panel.
While PET isvery good at self-leveling (the strain-hardening effect that compels initiallywarmer areas to blow out after cooler areas have blown), this effect is notsignificant enough to mask the uneven preform heat. A variety of measures havebeen employed over the years with varying success.
Most hotrunners are also not naturally balanced. Natural balance means that the path ofthe melt to each cavity has the same length and number of turns.
Because ofgeometry 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.
Changingnozzle diameters to allow slower-moving melt through larger openings ishelpful, 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-movingcavities.
Thermal-gatedhot runners are inferior to valve-gated ones, in my opinion, but most machinesstill run with the former.
Now letslook at the advantages of the two-stage process:
Scalable from 1000 to 72,000 bottles/hr;
Fast cycle times;
Flexibility (preforms can be made elsewhere and stored);
Very good wall distribution for
On average, lower gram weights possible for round bottles;
Process can be stopped at any time.
Withtwo-stage, the main disadvantage is the potential damage to the preforms thatoccurs when they tumble onto conveyor belts and into storage containers, andthen again when they are dumped into the hoppers of blow molding machines.
Manyof the little nicks and scratches can be stretched out when high stretch ratiosare used. But this is not always the case, particularly when a preform ischosen 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 onereason they are so popular nowadays.
A lesserknown problem with low-cavitations machines is that heat to each preform can bequite different when indexing machines are used. In these machines, two, four,or six preforms are loaded and blown together. A chain indexes them around theoven system.
Preforms areexposed to varying temperatures inside the ovens because of differences in lampoutput and exposure to cooling air. This translates into different preformtemperatures from cavity to cavity, and adjusting a process to suit them allcan be quite a challenge.
There are now a number of linear machines availablethat move preforms continuously and avoid most of these issues. Rotary machinesof course do not have this problem at all, as each preform gets identical heat.
Today, morethan 80% of all bottles produced are for beverages, and the vast majority ofthese are made using two-stage technology. It is the remaining 15% to 20% whereeither 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 volumesof fewer than 2 million/year, which many of the custom applications are.
Thereis less capital expense involved to buy preforms and run them on a two-cavityreheat machine.
The number of available preforms has increased dramaticallyover the last 10 years with suppliers spanning the globe, because preforms,unlike bottles, are cost-effective to ship.
In order tomake an informed choice on which process to use, you need to understand theparticulars of the application and make your decision either on the basis ofbottle features or economics.
If your bottle must be blemish-free, oblong inshape, and with a fixed thread, single-stage is the way to go. If none of thoseapplies, the economics must be scrutinized.
The preforms for single-stage arealways custom-made (unless you plan to make a variety of shapes out of the samepreform) so the fit is guaranteed. If preforms are bought however, the questfor the right one starts.
Not every preform that has the right neck finish andweight is suitable for a particular bottle. An expert should evaluate the available preforms and choose the best fit.
Let look at a hypothetical application involving a round 1-liter bottle with a 33-mmneck finish and a weight of 42 g. The yearly volume in this scenario is 750,000bottles.
In two-stage, this will require a one- or two-cavity machine that willprobably run around 600 to 800 bottles/cavity/hr.
While machine builderspublish much higher numbers, most custom applications run slower for a numberof reasons.
This will keep the one-cavity machine busy for 1152 hr, and thetwo-cavity for half of that. Capital cost is quite low as only one or two blowcavities have to be purchased.
Insingle-stage, cycle time will be about 13 to 16 sec say 250 cycles/hr. Atwo-cavity system would then make the required bottles in 1500 hr, afour-cavity system in 750 hr.
Capital cost is significant for each cavity, andof course, most machines will be able to run four or even six cavities.
Herethe decision would be based on cost per piece in different cavitations, andwhat future outlook the job holds.