by Jay Perry
With the increasing popularity of the in-mold labeling (IML) process for injection molding, electrostatics has emerged as an excellent option for IML. Applying the label during the molding process eliminates secondary operations, more importantly, the end result is permanent, which makes it especially attractive for product liability and instructional information, as well as for UPC codes or RFID tags, logos, and decoration. IML also is cleaner and more sanitary because there is less product handling. Plus, recyclability is enhanced if the label is the same material as the molded part.
Perhaps the most complex feature of IML is the way the label is held in place in the injection molding tool. Electrostatics offers a reliable and cost-effective alternative to the vacuum method in holding the label in its proper location in the die. This approach provides distinct benefits to the molder and the end user.
Securing with Vacuum
Specially designed and machined vacuum ports can secure the label in the desired location in the mold. The process sequence is as follows: the robot picks the label from the label magazine, places it in the proper position in the die, the vacuum is turned on, and the mold is shot.
There are cost and performance issues that must be considered when using vacuum for IML applications. Tooling costs can be significant for the design and machining requirements necessary to add the vacuum ports in the die. Next, the label must be strong enough to support the suction and avoid being sucked into the vacuum port, which will cause defects or “pimples” on the surface of the finished product. Also, vacuum passages in the molding die may cause non-uniform die temperatures that affect the efficiency of the process.
A cautionary note about utilizing the vacuum method is that it is very important that the robot does not miss a label. Injecting the mold without a label in place will result in very time-consuming and costly downtime to remove the die and clean out the vacuum ports and passageways. In order to avoid this condition, a means of detecting the vacuum must be employed to stop the injection of the polymer into the die if there is no label in place.
However, there are benefits to using the vacuum method. The vacuum is advantageous when the shape of the molded product requires complex preformed labels or when the molded part and/or label is required to have a textured finish.
Securing with Electrostatics
Using an electrostatic IML process offers cost and reliability benefits by eliminating the need for vacuum in the die. When a label of suitable material and construction is electrostatically charged, it will have an affinity to stick to the grounded metal surface of the die with sufficient adhesion to hold the label in place for up to several minutes.
In the electrostatic process, the robot picks up the label from the label magazine with suction. A high static charge is placed on the label either as the end-of-arm tool (EOAT) with the label approaches the press or as the label approaches the die surface (See Figure #1A). The robot positions the label, releases the vacuum, and the label is transferred to the surface of the die. No vacuum in the die or adhesive on the label is necessary in the electrostatic process (See Figure #1B).
|Figure 1A – Mandrel with suction cups embedded in antistatic foam rotates to pick up label from magazine, while static charge is applied to the label.|
|Figure 1B – Mandrel inserts label into mold (top), where label is attracted to grounded tool. Vacuum is shut off and mandrel retracts.|
Some injection molders have attempted to charge the label and place it in the die manually; however, experience has shown this to be a labor-intensive and unreliable approach that also slows down production time.
To recognize the full advantage of the electrostatic process, the following is required:
- a robot with a suitably designed EOAT
- a label magazine
- a high-voltage DC charging power supply with 30kV adjustable output capability
- a label of proper material and construction to accept and maintain the static charge
Standard Charging Method
Incorporating the charging applicator(s) on the EOAT offers a high degree of reliability and repeatability. Unfortunately, it also presents design complexity for the EOAT. The charging applicator may be a straight specific length static-charging bar with a row of emitter pins, or it may be a series of individual emitter modules. The design and quantity of applicators depends on the size and shape of the label and the contour of the die surface on which the label will be placed. Therefore, each EOAT will require its own unique charging applicator set-up.
As the suction cups on the EOAT are holding the label, the charging applicators are located directly behind the label (Figure #2). The emitter pins typically face the back of the label from a distance of about 1 inch. Once the robot places the label against the die surface, the charging power supply is turned on for a period of approximately 0.5 to 2 seconds. This process deposits a static charge on the label and the label instantly adheres to the grounded metal die. The vacuum is then turned off and the robot extracts the EOAT from the press.
|Figure 2 – The conventional IML method is to mount the label charging applicator on the robot end-of-arm tool.|
There are a few design guidelines that must be considered when designing the EOAT to incorporate the charging applicator. As an example, if the emitters are to be placed 1 inch behind the label, any metal part of the EOAT should be grounded and at least 1 ½ inches away from the emitter pins. Metal any closer than this will attract some of the electric field resulting in less charge being deposited on the label.
When using individual emitters on a plate that also hold the vacuum suckers, the plate must be made of a nonconductive material such as polyethylene, PTFE, PVC, UHMW-PE, or acrylic. Any component of the EOAT close to the charging applicator also should be made of a nonconductive material, if that can be accomplished without sacrificing strength and structural integrity. Any component that is electrically conductive (metal) must be grounded.
There are two types of charging applicators available for IML: current-limited and non-current-limited. Current-limited units are generally a straight static-bar style or individual emitter modules. They contain a resistor that is in series with the high-voltage supply. The advantage is that these units will not “hard arc” if it gets too close to the metal and offers a higher degree of safety if personnel should accidentally touch the unit while it is energized.
A hard arc is a high-current arc-over that is usually seen as a distinct bright white or yellow spark from the high-voltage emitter of the charging applicator to a conductive surface, such as the metal mold cavity. This can occur when a non-current-limited applicator is positioned too close to the mold or when the operating voltage is set too high. The high-voltage energy from the applicator’s emitter breaks down the insulation properties of the air between the emitter and the grounded metal surface of the mold, causing the arc. Such arcs can cause pits in the die surface and radio frequency interference (RFI) that may affect microprocessor controls of the robot or the press.
On the other hand, the current-limited applicator’s resistor limits the amount of current that can be drawn from the emitter, thus preventing a high-voltage arc.
It is recommended to use an electrostatic power supply that contains arc-sensing circuitry designed to protect the solid-state components of the power supply when it is used with a non-current-limited applicator. If excessive current draw is sensed by the power supply, its control circuitry immediately goes into arc-protection shutdown mode, which turns off the high-voltage output in order to protect the electronics. When this occurs, static charging is interrupted. This is a common occurrence when using non-current-limited applicators.
As another safety precaution, support must be provided for the high-voltage cable as it runs along the robot arm to allow for sufficient slack for unrestricted movement of the robot arm and to minimize the physical stress on the cable. These cables should be inspected weekly and replaced if any fracture, abrasion, or weakness is detected in the cable.
Simple Charging Method
Using a remote-mounted charging applicator is a simplified way to charge a label. Little modification is required of the EOAT; it is relatively easy to set up; and it can satisfy the requirements of many different sizes and shapes of labels with the same charging applicator. This method works for most film labels that are applied to relatively flat mold surfaces.
In this process, the robot picks up the label at the magazine, orients it, and passes it by the charging bar. The ground reference surface behind the label attracts the electric field from the charging applicator, causing the label to become electrostatically charged. The robot then places the label in position against the surface of the molding die, releases the vacuum to the suction cups, and the label stays in position on the surface of the die.
The applicator, in this approach, is mounted on a permanent fixture between the molding press and the label magazine. The charging power supply can be turned on manually and left on for the duration of the run, or it can be turned on and off remotely by the robots PLC.
The label may not always release readily from the vacuum suction cups and might skew slightly due to mutual electrostatic attraction caused by static charges building on the surfaces of the suction cups. If this happens, a static neutralizing bar can be mounted in the robot’s path between the charging bar and the label magazine. As the robot returns to pick up a new label, the suction cups will be neutralized. Using smaller diameter suction cups in the design will reduce the charged surface area, which may help minimize the problem.
The fixture on the EOAT must have a grounded conductor like a metal plate. The conductive surface should be at least as large as the label and mounted one quarter to a half inch directly behind the label. The suction cups should have the minimum diameter necessary to provide sufficient holding power to prevent the label from slipping and attracting to the grounded conductive surface of the fixture. All conductive components of the robot must be grounded and should have rounded edges and corners. There should be no sharp edges or corners within one (1) inch of the label.
Another version of the simplified method provides even better physical support and uniform transfer of the label from the EOAT to the die. This process works the same as described above but requires the addition of a piece of antistatic foam mat bonded to the metal ground reference plate on the EOAT. The antistatic material should have a thickness of approximately 0.375 inches and an electrical surface and volume resistivity of 10 to the ninth power/10 to the tenth power ohms per square inch. This material is commercially available from most static control distributors who service the circuit-board assembly industry. Most of these materials are manufactured with an embossed textured surface and may require sanding to achieve a smooth, uniform surface. Sanding the surface also enhances charging of the label.
The suction cups are built into the foam material and should be flush with the surface (See Figure #3). Since the antistatic foam has a high electrical resistance compared with the grounded metal mold surface, the charged label has a greater affinity for the mold and transfers from the foam pad to the mold when the vacuum is switched off. Figure 4 is an example of how this can be accomplished for 360 degree labels on round containers.
|Figure 3 – Simplified electrostatic IML approach for flat containers places the remote charging applicator in a fixed position where the robot passes it on the way to the mold.
|Figure 4 – Simplified electrostatic IML for round containers requires rotating the mandrel holding the label to get overall charge coverage. Antistatic foam provides good support and easy release of label.|
Key Label Properties
The physical and electrical characteristics of the label are extremely important to the reliability of using static charges to adhere the label to the mold. The surface of the label that is to contact the mold cavity must be a good insulator to accept and maintain the static charge. Ideally, this surface should have a resistivity of 10 to the power of twelve ohms per square inch or greater. The higher the resistivity, the better the label will accept the charge without bleeding the charge to ground when it contacts the metal mold cavity. If the charge is not maintained when in contact with the die, the adhesion is lost and the label slips from the intended position. The label’s resistivity can be measured by using a commercially available surface resistivity meter.
If conductive inks, coatings, or foil laminations are used, they must be on the backside of the label, opposite the surface that is to contact the mold. In this instance, the best method for charging the label is with the charging applicator external to the press (simplified charging method) and the conductive surface of the label against the vacuum ports on the EOAT. If the charging applicator is mounted on the EOAT behind the label, the high-voltage field cannot penetrate the conductive layer and sufficiently charge the surface that will contact the mold cavity.
Be aware, however, that a charged foil or conductive layer will most likely discharge in the form of an arc as it approaches the mold surface. The RFI that results from this arc may cause problems with microprocessor controls, especially if unshielded sensors or cables are close by. Continuous arc over a long period of time will produce pits on the surface of the die cavity.
Label thickness, curl, and surface texture affect label adhesion. As an example, a thick label that may have some curl due to an asymmetrical laminated structure or improper storage may break loose from a flat die surface if the electrostatic forces are unable to overcome the physical forces that cause the label to curl. Similarly, preformed labels may be required for compatibility with contoured die surfaces.
Also, textured label or die surfaces can cause poor adhesion due to the reduction in contact surface between the label and the die surface. A relatively thin, non-textured label with good dielectric properties on a non-textured die surface will produce the best electrostatic adhesion. Keep in mind, there are still other important adhesion factors to consider, such as molding temperature, polymer compatibility with the label, gate location, and material flow rate when the die is injected. For these reasons, consulting a label supplier with IML experience can eliminate a lot of time and frustration.
Jay Perry is the marketing manager for Simco Industrial Static Control. For more information on Simco’s elecrostatics in-mold labeling process, call (215) 822-6401 or visit www.simco-static.com.