Static Control in the Plastic Bottling Industry

Transcription

1 Page 1 Static Control in the Plastic Bottling Industry Meech Static Eliminators USA Inc 2915 Newpark Drive Norton, OH USA ph: fx:

2 Page 2 Static Control in the Bottling Industry Contamination and product misbehavior is a major problem for bottle producers, especially where highly insulated materials are used. Until recently, the only way to provide effective long-range static neutralization was to use conventional AC powered static eliminators in conjunction with air provided by fan or compressed air systems. However, such systems cause contaminants to be blown back onto the neutralized substrate. There has been a recent trend in end users of the bottles specifying that air cannot be used on the bottling lines. This presents the bottle processor with a dilemma; the need to neutralize the static charges that a) attract airborne contaminants b) cause bottle conveying and production problems, but using conventional static elimination systems that can themselves create contamination. Recent developments in AC and Pulsed DC static elimination systems can present a solution to this dilemma. Static Electricity Defined Simply defined, static electricity is the excess or deficit of electrons on a material. It is typically only found on the surfaces of non-conductive materials or insulators. In normal atoms there is a balance of positive and negative charges. This balance is achieved through the presence of protons (+ charge) in the nucleus of the atom and electrons (- charge) orbiting the nucleus. When an external force is applied to these atoms, electrons from one atom are drawn or torn away by the electrons of the other atom. This process is known as ionization. It leaves the atom with the surplus of electrons a negative charge and the atom with the deficit of electrons a positive charge. s with static electricity in the bottling industries are numerous. Processes where static can be an issue including parison blow molding, injection molding of performs, parts conveying, bin collection, bottle cleaning and bottle palletizing. The primary problems resulting from high static electrical charges would be 1) Dust attraction and contamination. 2) Operator/personnel shocks. 3) Process control and quality problems. Why long-range ionization is needed Static charges are generated on the packaging materials and products during the production process. This results in the attraction of air-borne particle contamination. Advances in machinery design have resulted in smaller more compact systems. Because of this, the use of conventional AC static control systems, which are effective only at short distances, is difficult. Also, since conventional AC systems have a limited range (approx 1 ) they are not effective for use with 3- dimensional parts.

3 Page 3 Conventional Static Elimination Systems AC powered static eliminators operate at supply frequency (50/60HZ). The mains voltage (110v to 240v) is increased through a transformer to generate voltage between 4.5kV and 7kV, depending on the equipment in use. This high voltage is carried to the ionizing pins through the HT cable. During the positive cycle of the AC current the electrode pin will be at a possible voltage compared to the grounded casing. This generates a strong electric field between the two, which is highly concentrated at the sharp point of the electrode pin. Positive ions are generated at the pinpoint, which are then repelled from the pin due to there like charge. On the negative half of the cycle, the opposite occurs and negative ions are created at the electrode pin. A very intense, compact ion cloud is produced around the electrode pin. Anything with a static charge passing near the proximity of this cloud will attract ions of the opposite polarity, therefore rendering that material neutral. In the absence of any statically charged material these ions will either neutralize each other or dissipate to ground. However, because of this intense compact ionization corona, the range of conventional AC ionizers is very limited (typically about 1 for a standard shockless unit). Recent developments in AC ionizers have significantly extended this range and new resistively coupled AC ionizers now have effective static neutralization ranges of 6 or more. This allows them to fit in areas that were originally too space restrictive for conventional AC bars as well as being used on small three dimensional parts. Air-Assisted Long Range AC Ionization To obtain a longer range with conventional AC controlled antistatic equipment the ions have to be air assisted. This can be done with either electrical fan or compressed air systems. The electrical fan system, typically known as an ionized air blower, usually comprises ionizing bars housed within an assembly that contains one or more small electrical fans. The fans blow air across the ionizing bars to help propel the ions towards their target, thereby extending their range. Compressed air systems are most commonly found as ionized air curtains/knives or small nozzles. Air assisted systems are most commonly used over collection bins and conveyor belts.

4 Page 4 Both types of systems may be effective at neutralizing static charges but can blow contaminants, either from the devices themselves or from the surrounding air, onto the web. Dust can build up within fan units, and even if equipped filters they can pull dust from the air and blow it towards the target substrate. Compressed air devices will blow oil or water from the air supply on the products, even when supplied with filters. Most compressed air systems also act as air amplifiers. This means that they are designed to draw ambient air into their airflow. Consequently any airborne dust will also be drawn into the ionized airflow and be transported to the substrate. Compressed air systems are also known to have very high running costs. A typical manufacturing company can spend about 10% of its electricity bill on producing compressed air. New Pulsed DC Controlled Systems Similar to AC equipment, Pulsed DC also operates by using high voltage to produce ionized air. However, Pulsed DC operates at a much lower supply frequency and can be adjustable (.1-20Hz) to optimize performance for specific applications. This means the distance/time between the ve and +ve pulsed can be changed. This gives you the ability to increase or decrease the distance between the ions and therefore change the ion recombination distances. I.e. 1Hz. Low pulsed rate increases effective ion travel of 24 to 36 (depending on the application) without the use of air. A 20Hz High pulsed rate decreases the effective ion travel but increases concentration. Minimum ionization distance would be 6. This low frequency of operation lends Pulsed DC equipment to long range neutralization without the use of air. Pulsed DC Bars are composed of a series of ionizing pins with alternating polarities. During the positive AC cycle of current, a large positive field of ions is produced at the positive emitters only. Since the bar itself has no proximity to earth, as in the AC bar, the similar charge of the ion and emitter drives the ions away from the bar. On the negative half of the cycle the power supply delivers a high negative voltage to the alternate set of emitters. Again in similar fashion to the AC eliminators, negative ions are produced at the emitter point. A statically charged object in the vicinity of the ionizing bar will attract or repel the ions, dependent upon their relative polarities. A negatively charged surface such as a plastic web will attract the positive ions produced by the positive emitters. A positively charged surface will attract the negative ions. The low frequency of operation lends Pulsed DC equipment to long range neutralization. The relatively long duration of each half of the cycle cause large clouds of ions of alternating polarity to be emitted from the bar. This distance between the positive and negative ions close to the bar greatly reduces the rate of re-combination, (positive and negative ions coming together and canceling each other out). An additional feature of the Pulsed DC system is that the output waveform can be altered and the duration of the negative and positive section of the waveform can be increased or decreased. For instance if the charge to be neutralized is known to be positive the duration of the negative part of the output can be increased and conversely the positive part of the waveform reduced. This will increase the production of negative ions and decrease the production of positive ions, making the system more efficient at neutralizing the positive charge. In similar fashion for a known negative charge the output can be biased toward positive ion production.

5 Page 5 Typical applications for the Pulsed DC systems would be as direct replacements for air driven AC ionizers. They are ideal for use on bottle palletizing lines, bottle ganging areas prior to palletizing, parison blow molding and conveyor belts, or wherever long-range ionization is required without the use of air. Conclusions AC ionizers have traditionally been the mainstay of static control and are the technology most widely accepted/ known in the industrial market. AC ionizers are characterized by: 1) Fixed ve and +ve output (non adjustable) 2) Ionizing range limited to approximately 1 3) Ions recombine very quickly 4) Air assisted units can extend the range from 24 to 36. Pulsed DC systems on the other hand offer adjustable ve and +ve ion balance to suit the application and a similar ionization range as AC air assisted ionizers without the use of air. They are particularly relevant where bottle contamination is an issue or concern. Static control equipment will continue to develop based on industry requirements. The competitive nature of the industry it is more important than ever to offer customers a clean, high quality product with a low scrap rate. To achieve this end, bottlers will need to look at more modern technologies for controlling their dust attraction and static control needs. On the following pages you will find some sample solutions for static control equipment for the unique needs of the plastic bottling industry. Potential problems are NOT limited to what is shown. Please contact Meech if you have any additional questions or problems you would like to discuss.

6 Page 6 Parison Blow Molding As molten plastic parisons drop towards an open tool, high static charges cause the following problems:- 1. Where there is more than one parison per tool, charges are always of the same polarity. This causes the parisons to repel each other. This often results in a failed delivery into the tool and quality failures due to stress lines in the finished molding. 2. With a single parison similar problems can occur when the lone parison is attracted to one of the many metal machine parts. The Meech 977v3/976 Pulsed DC ionization system delivers widespread long range ionization without the requirement for an air delivery system. The spread of ions from this system ensures total neutralization from the parison extrusion head to the top of the mold tool. This is ideal for very thin gauge parisons which are prone to being prematurely cooled or misdirected by the slightest of air flows. Larger applications may require more than one bar. The 977v3 power supply is capable of powering up to four bars simultaneously.

7 Page 7 Bottle Conveying Prior to Palletizer Blow molded plastic containers such as beverage bottles can experience problems at the conveying station such as: 1. The highly charged bottles attract dust, leading to unacceptable contamination of what is expected to be clean product often used in the food/beverage industry. 2. The charged bottles repel each other causing them to misbehave which results in them falling over, jumping off the conveyor belt or being repelled off the final palletized stack. 3. Very unpleasant operator shocks (which result in a recoil reaction that can cause more damage than the initial shock). Operator shocks also lead to a high level of operator discontent. 4. The use of ionized air blowers (typical solution) now being discouraged due to the desire to limit the possibility of blowing additional contaminants on to the bottles. Bottles on a conveyor The positioning of long range Model 976 Pulsed DC Ionizing Bars as shown above resolves these problems without the use of air. One or more ionizing bars are built to cover the entire width of the conveyor belt. Ionizing bars are then connected to a single Model 977v3 PDC Controller. Ionizing bars are typically mounted 6-18 away from the top surface of the bottles. Constant streams of ions neutralize the blow molded bottles, the conveyor and the operators bodies via their hands and arms which are placed within the ionization field.

8 Page 8 Bottle Palletizing Stacker Blow molded plastic containers such as beverage bottles can experience problems along the moving conveyor belt such as: 1. As the charges are of the same polarity the bottles will repel from each other causing them to jump from the finished pallet and fall to the floor resulting in in creased rejection rate costs. 2. Very unpleasant operator shocks (which reusult in a recoil reaction that can cause more damage than the initial shock). Operator shocks also lead to a high level of operator discontent. 3. Using other alternatives, such as blowers or airknives, may blow airborn particles on to or inside the bottles. Pallet Direction The positioning of long range Model 976 Pulsed DC Bars as shown resolves all the issues mentioned above without the use of air. The first bar is mounted across the width of the stack pointing down at a 45 degree angle. This bar will ionize the front of every layer as the stack begins to grow in a downward progression. As the stack is fully built, the stack moves 2 feet down the conveyor past two ionizing bars mounted vertically. These bars saturate the front, sides and back of the stack with ions. Ionizing bars are typicallymounted 6-18 away from the surface of the bottles. The constant flow of ions emitted for the pulse DC bars neutralize the whole stack along with anything else that is placed within the ionization field.

9 Page 9 Single File Bottle Conveying Blow molded plastic containers such as shampoo bottles can experience problems at the packing station such as: 1. The highly charged bottles attract dust, leading to unacceptable contamination of what is expected to be a clean product. 2. Increased production costs, due to additional cleaning at the pre-filling station. 3. Very unpleasant operator shocks (leading to operator discontent). The positioning of a long range Model 976 Pulsed DC ionizing bar as shown above resolves the problem. Constant streams of ions neutralize the blow molded bottles, the conveyor and the operators bodies via their hands and arms which are regularly placed within the ionization field. The bars are typically mounted above the conveyor on a raised strip light gantry.

10 Page 10 Static on Palletizing Slip Sheets The use of platic slip sheets, instead of cardboard, for interleaving of the bottles on the pallet, has resulted in it own unique static control problems such as: 1. When the suction cups pick up the top sheet, the second sheet adheres to the underside of the top sheet due to static problems. This often will result in the bottom sheet falling off prematurely and knocking down or disrupting the palletized stack. 2. Because the sheets are statically charged, they will often repel the plastic bottles as they are pushed on to the sheet. This results in the bottles jumping off the stack and falling to the floor, resulting in high scrap levels. Top View Side View The careful placement of four ionized air curtains at the sheet sides maintains a constant stream of ionized air between the top sheet and the stack, preventing sheet adhesion. The 957s inject a blade of ionized air between the top sheet and the stack. The continious blast of ionized air hleps float the sheet and removes the static charge.

11 Page 11 Dust on Bottle Conveying Line Static charges are generated on plastic bottles as a result of the molding process and friction from movement of the bottles on the conveyor. Consequently, bottles misbehave (e.g. fall over) and attract dust contaminants. A Model 957 Ionizing Air Curtain positioned above the conveying line will neutralize the static charges, and also remove any contaminants that have already adhered statically to the bottle. Through the use of compressed air, the 957 Ionized Air Curtain produces a narrow, high velocity beam of ionized air operating at up to 100 psi. It can remove dust from any 3-dimensional object. The air curtain has an ionizing range of up to 20 feet.

12 Page 12 Conveyor and Bin Ionization 1. Static charges on moldings may cause them to stick to the conveyor. 2. As moldings fall into the collection bin a battery effect can result in a huge charge being accumulated. This causes a high level of dust attraction and operator shocks. A Model 976 Pulsed DC bar positioned over the conveyor and collection bin will neutralize the static charges. Moldings release readily from the conveyor into the collection bin without further dust contamination. Productivity is improved and operator shocks eliminated. A pulsed DC system is used to alleviate the possibility of blowing airborne dust and contaminants on to the parts. An ionizing air blower can also be used if airborn contamination is not a concern.