AUTOMATIC STRIPPING OF GLOVES
- A TECHNICAL PRESENTATION -
By: William L. Howe
Probably no innovation in the 1990's for automation in the glove manufacturing sector has impacted productivity of manufacturing plants like that of technology for automatic stripping of gloves. The purpose of this paper is to inform and prepare the reader for the following;
In general, this paper will address techniques for unsupported thin gauge gloves, with brief mention of techniques for household gloves and supported industrial work gloves.
The writer and his company have identified and/or designed for five (5) different techniques for automatic stripping. These techniques can also be considered for dipped products other than gloves, such as bags, condoms, catheters, balloons, etc. Selection technique for each application will depend upon many factors, which will be identified and briefly described in Section 3.0 of this presentation.
2.1 Pressure Pad / Rotating Brushes
This technique is used primarily for a "straight-off " stripping of a dipped product. For the glove industry, one will find this technique utilized for stripping of supported industrial work gloves.
Typically, a pair of cushioned pads (to protect ceramic formers) would encase the glove former as it passes, using a two axis motion (squeeze and drop), which through pressure, will enable the glove to loosen and free from the mold, for deposition onto a conveyor or tote bin.
A similar technique uses a pair of rotating brushes, which encase the former. The brush technique typically involves the use of one axis motion only (brush rotation) to accomplish the strip. However, a second axis can be added to accommodate different size formers entering the same brush system. This second axis is often accomplished with the use of compressed air or mechanical spring, to enable the brush to adjust to the differing former diameters.
The rotating brush technique is seldom employed with glove stripping, and is more conducive to "straight-off" stripping of symmetrical dipped products, such as condoms and toy balloons (See Figure 2. 1. 1).
2.2 Water Jet
Though not a popular choice for glove stripping, the use of water jets can enable gloves to be automatically removed from molds, if the "straight-off" method is desired. The primary disadvantage of this approach is that the glove becomes wet, which necessitates more attention to glove collection and downstream drying.
The writer has specified the use of water jet systems for back-up stripping, when requested. In this format, the actual automatic stripping is first conducted by "dry" means in the main stripping station. If gloves are missed (which is a normal occurrence), the water jet(s), located in the former washing station, would eject gloves from the mold onto a screen inside a containment tank. Normally, the gloves removed using this back up technique, are considered as Scrap.
The use of water stripping is more common for use with symmetrical products such as condoms and toy balloons.
2.3 100% Compressed Air
If removing unsupported gloves via a "straight-off" technique, the most reliable method is by compressed air, which typically requires significant volume and pressure to accomplish the strip. If the manufacturer is chlorinating both glove sides downstream, this method can be employed successfully. Otherwise, the texture of the mold is transferred over to the inside of the glove, which is typically not the preferred result. Furthermore, the outside of the glove when used, would represent the side of the glove having seen most effects of the protein wash station. This means that the inside of the glove, which is next to the user's skin, would be the side not seeing the effects of the protein wash.
The use of "straight-off" technique via air has the advantage of being a low cost in capital investment. However, operational costs are typically considered as high.
2.4 Combination Air/Mechanical
The technique patented by the writer's company, combines the use of compressed air and mechanical grasp, which has been designed for reversing the glove during the pick to minimize downstream product handling.
An artist rendering of the concept can be seen in Figure 2.4. 1. The technique is most commonly employed with continuous chain lines. A key consideration is that of line speed and the ability to synchronize the apparatus with the movement of the line. The system shown accomplishes that with mechanical gearing in conjunction with the conveyor chain. A second method for synchronization would be to accomplish this electronically by communicating pulses to the conveyor drive motor.
This technique (first proven in production in the late 1980's), uses a three (3) step approach as follows;
This technique has been considered an effective approach in stripping natural rubber latex gloves. I believe it fair and accurate to say that this specific technology has had limited success in conjunction with other polymers, such as nitrile and neoprene.
Investment capital cost and operational cost for this technology are considered in the moderate range.
2.5 Full Mechanical Pick Technique
Common sense would inform us that the best motion to simulate for automatic stripping would be that of the human motion. This technique involves the automation simulation of that thought. An example of such a technique can be seen in Figures 2.5. 1. The device normally employs the use of 3 axes for both batch machine and chain machine applications.
The key to its success is that of accurately engaging the "finger grasp mechanism" to the bead on the glove or inside the cuff of the glove. This accuracy is often accomplished by a roll down brush followed by mechanical finger engagement in the cuff area, followed by a roll up brush back over the fingers. The two mechanical fingers are now positioned between the glove film and the former. After this first step is accomplished, the mechanism can be moved vertically in a downward stroke, to effectively reverse the glove and remove it from the mold.
This technique has more universal appeal to different types of polymers, including natural rubber latex, nitrile, neoprene, and PVC.
However, the primary disadvantage to this technology is that capital acquisition cost is typically high. Furthermore, on going maintenance costs make for moderate to high operational cost as well.
This technique is adaptable to batch dipping systems or chain dipping systems.
3.0 CRITICAL FACTORS FOR SUCCESS
The following factors must be evaluated before advancing into the design and implementation phase of automatic stripping. A brief commentary on each factor will assist the reader in evaluation of his or her own factory situation.
3.1 Type of Machine
Two primary types of dipping units are employed in production manufacturing - batch and continuous chain. The technique used for automatic stripping will differ in accordance with the general overall type or machine employed. General access into the former rack or individual former is a consideration and must be evaluated.
A batch machine often employs the use of pallets measuring 1.5 by 3.0 meters, containing a dense former pack. The key to consider automatic stripping in this environment is accessibility to all formers. The best condition for batch machines are those whereby individual former "strips" (containing several formers) separate from the pallet, which enables free and clear access for the automation.
The key consideration for chain lines is that of former orientation and chain speed. For non-rotating former chain lines, orientation of the molds are already accomplished, making for an ideal auto stripping condition. However, rotating former lines, which are the most common type used in Asian factories, require the adaptation of a former orientation system when entering into .the automatic device. This can be accomplished with a "carrier spoke" or "D" cam device (machined flat surface on a round bar), both of which contact and slide across a frictionless surface to stabilize the mold.
3.2 Type of Glove and Sizing
In general, supported gloves utilize the "straight-off" techniques and unsupported gloves necessitate the "glove reversal" techniques available. However, there are some exceptions to unsupported gloves, which can mandate "straight-off" approach.
In general, ambidextrous gloves are easier to strip than hand specific ones, considering the reversal technique. The protruding thumb on a surgical glove former can make for a stubborn strip, unless employing the proper technique. Another consideration for hand specific gloves is that of straight finger versus curved finger design. The most challenging combination would be that of a curved finger surgical-mold, produced on a continuous chain conveyor. It can be accomplished but generally by using the full mechanical pick technique described in Section 2.5, which can involve a sizable investment in capital.
3.3 Type of Polymer
The type of polymer employed will also greatly sway the selection technique. A general order of automatic stripping complexity by glove polymer list for unsupported gloves, in the opinion of the writer, is as follows, listed from easiest to hardest;
3.4 Former Shape and Texture
Mention has already been made for the consideration of ambidextrous gloves (formers) versus hand specific gloves (formers). However, another key to auto stripping successfully, lies in the former shape and texture.
For natural rubber exam gloves, a more "tapered" mold shank from cuff to wrist area, functions better for certain strip techniques, particularly the combination air and mechanical approach. The other key area of the mold is that of the thumb orientation or protrusion. A more gradual 1, sloping" thumb allows the glove to work its way over the mold more easily, versus a sharp bend at this area.
A lesser consideration, at least for natural rubber products, is that of glove texture. In general, all former surfaces can adapt well for unsupported natural rubber glove former textures employed, including unglazed, spray bisque, and glazed. However, for synthetic polymers such as silicone and polyurethane, a glazed former surface will perform more consistently for removal techniques, both manual and automatic.
3.5 Glove Sizing Management
This factor may not affect many of the participants of this conference. In
general, most current chain lines in
However, larger volume machines (the writer's company has designed machines with volumes up to 60,000 pieces per hour) typically contain several gloves sizes on the same system. Therefore, two further considerations must be given to this condition;
3.6 Polymer Formulation
One word of caution to anyone considering implementation of automatic stripping technology to their plant - be prepared to alter your latex and coagulant formulations, if necessary. The writer is not qualified to comment on specifies of formulation adjustment. However, we have more often than not, seen our customer base require some modification to their formulation to avoid glove tearing (if using compressed air source as part of the technique) and ease of release from the mold. The amount of mold release in the coagulant may require adjustment.
3.7 Current Level of Formulation Reliability and Equipment Reliability
This is key - key - key. The writer cannot emphasize enough the importance of consistent glove production in a manual stripping environment, before investing in automatic technology. Inconsistent formulation management in film properties from day to day, machine to machine, etc. can allow the auto strip technology to work some days, and falter on other days. For example, if the level of calcium carbonate in the coagulant fluctuates, auto strip effectiveness can plummet.
On the machine side, one important performance statistic is that of "good beads (rolled cuffs)". If the bead roller unit on the machine misses beads from time to time, you can expect the auto strip device to do the same. If the system oven performance fluctuates thus causing the general state of curing to decrease, auto stripping performance will suffer. In general, auto stripping works best with a more highly cured glove.
You should consider yourself a candidate for automatic stripping technology only if your day to day machine and formulation performance is consistent and reliable.
4.0 IMPLEMENTATION OF TECHNOLOGY
The candidate for automatic stripping technology, after determining that they meet all prerequisites for institution of the automation, must be prepared to exercise patience during implementation.
Initially, the first phase of the evaluation, which is proper identification of technology, should occur by an on site study on the part of the automation provider. After thorough assessment of the application and other factors, expect a design phase to ensue, even in the event the automation provider has already supplied technology to other firms. As insinuated throughout this paper, every plant can differ in machine conveyance, type, speed, and especially formulation. The state of glove cure at the strip station is crucial for reliable performance.
After this phase, the automation equipment is fabricated and assembled for installation at the user's plant. Installation of the technology normally would require from 3 to 7 days to complete.
The commissioning phase of the technology is the area whereby the user needs to exercise patience. Several adjustments to the technology are typically necessary. As previously mentioned, it may by imperative for the formulation to change to assist reliable take off of the glove.
Itemized below for the reader's review is a representation of a typical schedule for an automation program;
5.0 A CHALLENGE FROM THE AUTHOR
The writer encourages the reader to not limit the prospects of their plant to automatic stripping alone. Without question, the implementation of auto strip technology will make for significant productivity improvements in your operation.
However, my challenging question to the reader is this - why limit your
thinking to that of glove removal only? Today, in the
The plants that remain active and successful in the
The writer has not only seen the advent of this technology, but has been intimate with it in concept and design. My challenge to you is to have this vision for your factory. This is not"drawing board fluff" -it is reality for the 21st century and beyond.