Anti-Static Control Problems in The Plastics Industry
|Problems with static electricity in the plastics industries are numerous. Processes where static charge can be an issue include injection moulding, blow moulding, thermoforming, rotational moulding, parts conveying and collection and assembly processes.
The primary problems resulting from high levels of static charge are:
- Dust Attraction & Contamination
- Process Control & Quality Problems
- Operator Shocks
Dust Attraction & Contamination
The attraction of airborne contaminates as a result of static charge on a substrate is becoming more and more of an issue as the quality standards of companies continue to be raised.
Dust and particlulates attracted by the static charge on the part can result in high and expensive scrap rates. This is especially the case if the material is to be used in the medical or food packaging industries, or in the painting of automotive or white goods.
A recent stipulation of food packaging companies is also that static control equipment cannot use air to assist long range ionisation, for fear of blowing additional contaminants on to the products to be neutralised.
Process Control & Quality Problems
The cost penalties associated with uncontrolled static charge in manufacturing processes are many and varied. Static can force companies to run their machines at much slower speeds than might otherwise be the case. This is because static charge can be a direct cause of production problems such as parts sticking to each other in the conveying process, resulting in jams or product misbehaviour
This is becoming increasingly significant as companies strive to improve health and safety standards.
While static-related shocks can be painful, the effects are usually quite safe and short lived. However, the cost implications lie in the “recoil” reaction that is associated with the initial shock. When an operator receives a shock, there can be a moment of disorientation, bringing with it subsequent hazards
such as collision with other operators and/or machinery
Operator shocks are typically the result of an accumulated charge or “battery effect” occurring during the collection of parts in a bin or assembly area.
Small, light mouldings can stick to the face of the tool due to static charge. This can result in:
1. Mould damage
2. Crushed parts
3. Slower production speeds
A ionising air curtain positioned either above or to the side of the problem area will neutralise the static charge and help remove the parts. Air may be
pulsed through the air curtain only when the tool is open. Alternatively, a ionising blower can be installed as shown
Parison Blow Moulding
As molten plastic parisons drop toward an open tool, high static charges cause the following problems:
1. Mulitple parisons will repel each other
2. Single parisons will attract to the tool
The use of a long range Pulsed DC system delivers widespread ionisation without the requirement for an air delivery system. This is ideal for very thin gauge parisons which are prone to being prematurely cooled or misdirected by the slightest of airflows.
Large mouldings such as rubbish bins, plastic lawn furniture, toys, plastic pallets and containers can develop very large static charges. When the tool is removed from the part, the static charge can cause severe operator shocks and can attract dust from long distances.
The normal solution is the installation of a ionising blower of sufficient size to neutralise the charge on the part as the tool is taken away. A ionising air curtain or ionising gun are alternative solutions.
Vacuum and Thermoforming
A complete thermoform line has several static related problem areas:
1. The unwinding of the material causes dust attraction which is later impregnated into the thermoform.
2. As the forms exit the tool, the static charge can cause additional dust attraction or stacking problems.
Installing a ionising blower or long range Pulsed DC bar in Position [A] will prevent dust attraction. A ionising air curtain at Position [B] will ensure that the film is clean before entering the machine. A long range Pulsed DC bar or blower at Position [C] will neutralise the forms so that they will stack evenly.
Conveyor & Bin Collection
The cooling and conveying of plastic parts will accumulate a very high static charge. This results in:
1. Dust attraction on to parts on the conveyor and in the collection bin.
2. Severe shocks to personnel as a result of a “battery effect” in the collection bin.
3. Parts sticking to the conveyor belt
A Pulsed DC bar positioned over the conveyor and collection bin will neutralise the static charges. Mouldings release readily from the conveyor into the
collection bin without further dust contamination. Productivity is improved and operator shocks are eliminated. A ionising blower system can also be
used as shown.
Bottle & Parts Conveying
Blow molded plastic containers such as bottles can experience problems on conveyors and at packing stations such as:
1. Contamination due to dust attraction
2. Products falling down and disrupting flow
3. Operator shocks
A Pulsed DC bar mounted over the bottle line will create a constant flow of ions to neutralise the moulded parts, the conveyor and the operators’ bodies via their hands and arms, which are regularly placed within the ionisation field.
Small Parts Assembly & Conveying
Constant movement of parts during the manual assembly or conveying process can generate significant levels of static charge. This can result in:
1. Dust attraction
2. Operator shocks
3. Misbehaviour of very small parts so that they will not assemble properly.
Installation of a long range 976 Pulsed DC bar over the workbench or conveyor will alleviate the problem. The bar is typically mounted overhead, out of the way of the operator, and neutralises the charge on the part as well as the operator without the use of air.
Vibratory Bowl Feeders:
The constant friction generated by vibratory bowls can cause the following problems:
1. Opposite charged materials will cause “clumping” and not allow proper feeding.
2. Same polarity materials will cause small parts to be ejected from the bowl
The continuous ionisation created by either a Pulsed DC bar or ionising blower will neutralise the charges as soon as they are generated. Choice
of a bar or blower depends on whether an airflow is acceptable or not.
In - Mould Labelling
Plastic labels do not adhere properly to the inside of the tool during the moulding process. This can occur due to static charge or the ineffective use of
expensive air vacuum systems to hold the label in place.
The label is picked up by the robot arm and is then either:
1. Passed over a static charging bar and placed into the tool.
2. Moved into the tool and “pinned” directly to the surface of the tool.
Dust & Contaminant Removal
Warm and highly charged extruded profiles become heavily contaminated by swarf or dust after sawing or cutting processes. This can lead to:
1. Operator shocks
2. Charged swarf contaminating the machine and environment.
3. Statically attracted dust being carried to other parts of the process.
Carefully positioned ionising air curtains can both release and neutralise swarf and also neutralise the profile, assuring a clean and static free section at the end of the extrusion line.
Plastic Waste Recycling & Parts Conveying
The movement of material through ducting can result in static charges being generated due to friction between the material being conveyed, and also between the material and ducting walls. This can result in bridging and blockage of restricted openings.
The installation of a Ionising Flow Tube just prior to the area where blockages occur will neutralise the static charge and allow the material to flow freely through the ductwork.
Entrapment of particles in paint finishes or laminates can lead to expensive rejection rates. For very small items, compressed air driven cleaning systems can be economic and effective. For larger items this is often no longer the case.
The JetStream system provides an effective and economic solution for the removal of contaminants from product surfaces without the use of costly compressed air. This is achieved by directing filtered, dry, high velocity, ionised air to the contaminated surfaces via an external blower.