Method and apparatus for providing antistatic protection to plastic lenses

A process and apparatus are disclosed for providing antistatic protection to a plastic lens, the apparatus being adapted to carry out the process and comprising: PA1 (A) deionizing means for providing a flow of ionized air to at least a major surface of the lens; PA1 (B) an airless spray system for applying a thin, substantially uniform coating of liquid antistatic material onto the lens surface following its deionization. A decay chamber may be provided for at least partially isolating the lens from airborne contaminants as the antistatic material dries.

INTRODUCTION 
The present invention relates generally to providing anti-static protection 
to the surface of plastic lenses. More particularly, the invention relates 
to a method and apparatus for decontaminating the surface of the plastic 
lens and applying a coating of antistatic agent to reduce recontamination. 
BACKGROUND OF THE INVENTION 
In the manufacture and assembly of articles incorporating plastic lenses, 
for example lenses made of acrylic, polycarbonate, etc., such as in the 
manufacture and assembly of motor vehicle instrument panels and the like, 
the lenses often are assembled with other components such that at least 
one surface of the lens is not readily accessible for cleaning. Attempts 
generally are made, therefore, to remove lint, dust and like contamination 
from the surface of the lens before its incorporation into the assembly. 
Such decontamination of the lens surface can be done by any of several 
known means including, for example, wiping with a cloth. The surface may 
become again contaminated, however, as the result of any static charge on 
the surface of the part. Such static charge will develop over time under 
normal conditions. The static charge will attract contaminants, 
particularly airborne contaminants, to the lens surface. 
Various processes and equipment are known and used to neutralize static 
charge at specific points on the surface of a work piece, such as 
ionizers, static bars, grounding brushes, etc. However, non-conducting 
materials, such as plastic, which typically accumulate large static 
charges quite readily, often cannot be effectively point discharged. Also, 
as the part moves through a production or assembly process, it may quickly 
pick up a static charge again, often only a short time after it has been 
neutralized. It often is impractical to place numerous grounding or 
ionizing devices throughout a production process. Also, antistatic 
protection for a material may be important to the product after it is 
assembled. None of the above-mentioned devices or methods provide 
significant antistatic protection to a part during end use of the 
assembled product. 
Antistatic agents have been used for many years in various industries to 
reduce static problems during processing and use of plastic parts. They 
reduce the attraction of dust and dirt to the surface of the part by 
reducing the static charge on the surface of the part. Antistatic agents 
may be applied to the surface of the finished article or incorporated into 
the bulk polymer during processing. They are believed to function 
generally by decreasing the rate of static charge generation and/or by 
increasing the rate of charge dissipation. For plastic lenses, the 
incorporation of an antistatic agent into the bulk polymer of the part has 
been found in many cases, however, to decrease the transparency or 
otherwise to adversely affect the optical quality of the lens and for this 
reason is often an unsuitable approach. The application of an antistatic 
agent to the surface of a plastic article by dipping, wiping or spraying 
has been used effectively. For cost effective high volume manufacturing 
and assembling processes, however, dipping or wiping an antistatic agent 
onto the plastic part presents several difficulties. These processes tend 
to be time and cost inefficient and an open bath of antistatic agent can 
become contaminated over time. 
An antistatic agent can be sprayed from a reservoir in a closed container, 
which will reduce contamination, but significant processing problems are 
presented. Typically, local plant air is used in the spray system and 
airborne contaminants may be entrained with the antistatic material and 
deposited onto the surface of the lens. Also, difficulties can be 
encountered in controlling the amount of material deposited on the surface 
of the part and the distribution of the material over the surface of the 
part. Typically, spray systems have been found unable to produce 
sufficiently fine atomization of liquid antistatic agent required for a 
uniform thin film coverage, which is important for preserving the optical 
qualities of a plastic lens. When an excessively heavy coating is applied, 
the coating will be slow to dry, may smear upon contact with another 
surface and may otherwise impair the optical quality of the lens. In 
addition, it has long been a problem associated with the spraying of 
antistatic material that the material, even if a suitable amount is 
applied, does not distribute itself evenly over the surface of the part, 
being heavily concentrated in certain areas and sparse in others. 
Insufficient coverage can even result in bare spots, i.e., areas on the 
lens surface not coated with the antistatic material. Sparsely coated and 
uncoated areas are found to have unacceptably high static charges. This 
results in uneven antistatic protection, poor appearance and poor lens 
optics. 
One approach which has been tried for removing the static charge from a 
plastic lens is the use of a flow of ionized air. Commercial units are 
available which generate a flow of ionized air which can be used to blow 
contamination from the surface of a plastic lens and simultaneously 
deionize the surface. While this method has proven effective, its effects 
are not long lasting. That is, plastic lenses so treated have been found 
to have unacceptably high static charge levels within as little as several 
hours from the time of treatment. Hence, dust, lint and other 
contamination is attracted to the surface of the lenses during its 
assembly with other components and during shipment and use of the final 
product. 
Accordingly, there is a need for a method and apparatus for providing a 
durable antistatic protection to the lens. Preferably, the method and 
apparatus would be suitable for high volume commercial production 
applications. These and other objects met by the invention, or in certain 
cases met by preferred embodiments of the invention, will be better 
understood in the light of the following disclosure and discussion of the 
invention. 
SUMMARY OF THE INVENTION 
According to an apparatus aspect of the invention, an antistatic treatment 
apparatus for providing antistatic protection to a plastic lens comprises 
deionizing means for directing a flow of ionized air at the lens and 
coating means comprising an airless spray system for then applying a 
coating of liquid antistatic material. In a typical commercial embodiment 
the apparatus comprises: 
(A) deionizing means at a first location in the apparatus for providing a 
flow of ionized air to at least first major surface of the lens; 
(B) coating means at a second location in the apparatus for applying a 
coating of liquid antistatic material to the part after it leaves the 
first location, the coating means comprising an airless spray system for 
spraying the liquid antistatic material onto the lens; 
(C) optionally, decay chamber means for at least partially isolating the 
lens from airborne contaminants for a time period sufficient for the 
coating to at least substantially dry; and 
(D) conveyor means for conveying the lens through the apparatus, from the 
first location to the second location and from there to the decay chamber 
means (if any). 
The decontamination and antistatic treatment for plastic lenses provided by 
the present invention provides certain innovations and also combines 
certain features which, for reasons discussed further below, would have 
been thought redundant and for that reasons incompatible in an efficient 
and cost effective system by those skilled in the art. The antistatic 
treatment apparatus in accordance with preferred embodiments of the 
invention can be arranged as a treatment line, optionally as a mobile 
unit, positionable adjacent an injection molding apparatus in which the 
plastic lenses are being made, for immediate processing of the lenses. The 
invention is found to provide exceptionally good antistatic protection to 
plastic lenses, such that they remain substantially contamination free for 
extended periods under normal conditions of product assembly and use. 
In accordance with a process aspect of the invention, antistatic protection 
is provided by first exposing the lens to a directed flow of ionized air 
followed by applying a coating of liquid antistatic material by an airless 
spray system. In a typical commercial embodiment the process provides 
antistatic protection for each in turn of a series of such lenses by: 
(A) conveying a first lens to a first location and there exposing at least 
one major surface of the lens to a flow of ionized air; 
(B) subsequently conveying the first lens to a second location and there 
applying a coating of liquid antistatic material to the first lens by an 
airless spray system; 
(C) in substantial synchrony with step (B) conveying a second lens to the 
first location and there exposing at least one major surface thereof to 
the flow of ionized air; and, optionally, 
(D) conveying each of the lenses in turn into a decay chamber in which it 
is at least partially isolated from airborne contaminants for a time 
period sufficient for the coating to at least substantially dry. 
As noted above, the present invention combines certain features which would 
have been thought redundant and hence inappropriate by those skilled in 
the art for providing antistatic protection to plastic lenses. In 
particular, the present invention first exposes the plastic lenses to a 
flow of ionized air. While not wishing to be bound by theory, it presently 
is understood that a flow of ionized air will deionize the surface of a 
plastic part and will blow away most or all existing surface contaminants, 
e.g. dust, lint, etc. Under normal conditions such treatment is capable of 
leaving the surface of a plastic part substantially free of static charge 
and for this reason is used by some manufacturers and assemblers of 
plastic lenses. This treatment provides, however, no protection against 
the subsequent buildup of a static charge on the surface of a plastic part 
and, as static charge builds, airborne contaminants can be attracted to 
the surface. While others have instead used the application of an 
antistatic coating material to both decharge and also provide continuing 
antistatic protection, thus overcomming the aforesaid deficiency of 
deionizing a plastic surface, it has heretofore been unknown to precede an 
application of liquid antistatic material by treatment with ionized air 
flow. 
Surprisingly, however, these treatments are now found not to be merely 
redundant in decharging a plastic surface. Rather, it is now found that 
deionizing the surface of the plastic part prior to application of a 
coating of liquid antistatic material results in more even distribution of 
the antistatic material, that is, a more uniform coating is achieved. In 
this way, there is greater assurance that portions of the surface are not 
left uncoated or inadequately coated and, in addition, there is reduced 
tendency of the coating to be excessively heavy over certain areas of the 
surface. As noted above, a coating which is heavier than necessary is 
undesirable, since it can affect the optical quality of the lens, requires 
longer drying time and is more likely to smear. 
While not wishing to be bound by theory, it presently is believed that an 
existing antistatic charge on the surface of the plastic lens may cause, 
in part, the uneven distribution of liquid antistatic material by 
attracting and/or repelling the material at certain areas of the surface. 
In any event, the combined pretreatment with ionized air flow followed by 
spray application of liquid antistatic material is found to provide 
adequate antistatic protection without smearing, etc. by applying a very 
thin and uniform film of antistatic material over substantially the entire 
surface of the plastic lens. In fact, under optimal conditions a highly 
desirable "mono-layer" of antistatic material can be achieved in 
accordance with preferred embodiments of the invention. A so-called 
"mono-layer" is a surface layer without discontinuities or gaps and which 
is nominally a single molecule deep after drying. 
An additional unique feature of the invention involves the use of an 
airless spray system to apply the liquid antistatic material. It has been 
known previously to use spray systems in which air is used as a carrier 
for a liquid antistatic material. Such air, however, typically is drawn 
from the surrounding atmosphere and, hence, usually carries airborne 
contaminants. These contaminants can deposit and remain on the surface of 
a part being treated. In the present invention, in contrast, pressurized 
air is not used as a carrier for the liquid anistatic material and, hence, 
airborne contamination in the surrounding atmosphere is not sent through 
the spray system to deposit on the lens. 
In addition, the use of an airless spray system, various suitable models of 
which are commercially available and well known for other applications, 
can be operated at very high pressure, thereby providing better control 
over both delivery rate and distribution of the spray plus finer particle 
size and increased delivery rates. This, in turn, allows the use of lower 
concentrations of liquid antistatic material, which typically comprises an 
aqueous solution, thereby further facilitating a thinner and more uniform 
film. A not insignificant additional benefit is the reduction in material 
waste achieved by the better controlled spray application of the 
antistatic material. 
These and additional features and advantages of the invention will be 
better understood from the following discussion and detailed description 
of certain preferred embodiments thereof.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
The apparatus now described is referred to in some instances as an 
antistatic treatment line. It should be understood that this line can be 
incorporated into a larger processing line, for example, a manufacturing 
line including injection molding or other molding equipment for 
fabrication of the plastic lens. The overall manufacturing line may 
further include other treatment steps and the assembly of the lens with 
other components. 
Referring now to the drawing, antistatic treatment apparatus 10 is seen to 
be a substantially self-contained mobile unit mounted on wheels 12. As 
such, it can be rolled into position as an integrated part of a larger 
manufacturing or assembly line. For purposes of the following discussion, 
it will be assumed that apparatus 10 is positioned adjacent an injection 
molding machine (not shown) in which plastic lenses are being made, and 
that automatic transfer means, such as any suitable robot transfer arm, is 
transferring the lenses from the injection molding machine as they are 
made to the beginning location 14 of conveyor 16 described further below. 
Such robot transfer arm (not shown) serves merely as an automatic part 
unloader from the injection molding tool and may be any of numerous 
commercially available models such as, for example, those available from 
Martin Industries. The robot arm could employ suction cups attached to a 
source of vacuum to pick up the plastic lens, optionally picking up 
multiple Plastic lenses made simultaneously in the molding tool and 
depositing them at location 14 for simultaneous treatment by apparatus 10. 
The robot could be positioned, at least in part, in safety cage 18 
positioned over beginning location 14 of the conveyor 16. Safety cage 18 
would have an opening at bottom right area 20 through which the plastic 
lenses would pass on the conveyor for antistatic treatment. 
Conveyor 16 comprises a conveyor belt which is not separately shown in 
drawing for purposes of simplifying the illustration. The belt may be any 
of numerous commercially available types, preferably of open construction 
through which liquid antistatic material can be sprayed. Preferably the 
belt is a wire mesh type such as that available from Wiremation Company, 
Ohio, but numerous other conveyor belts on which the plastic lenses can 
ride without scratching or other damage will be apparent to the skilled of 
the art in view of this disclosure. The upper portion of the conveyor belt 
travels horizontally from left to right in the drawing. 
Lenses passing out of protective cage 18 travel on the conveyor belt to a 
location 22 directly under deionizing fans 24 mounted on bracket 26. As 
noted above, the deionizing fans blow ionized air at the plastic lenses to 
deionize the surface of the lenses. This eliminates static charge and also 
removes at least most of the contaminants, if any, on the surface of the 
lenses. Suitable deionizing fans are commercially available and will be 
apparent to the skilled of the art in view of the present invention. Such 
fans are available, for example, from Simco Company, Hatfield, Penna. 
Preferably the conveyor belt moves lenses in step-wise fashion from 
station to station along the antistatic treatment apparatus 10. Thus, it 
would move an incremental distance from location 14 to location 22 and 
then stop a period of time during which the lens or lenses would be 
exposed to the flow of ionized air from deionizing fans 24. It has been 
found that a sufficient flow of ionized air against a major surface of a 
plastic lens frequently is adequate to neutralize static charge on the 
entire surface of the lens. Nevertheless, it will be readily understood by 
those skilled in the art in view of this disclosure that additional 
deionizing fans may be used to create a flow of ionized air against 
additional surfaces of the lens. Following the predetermined dwell time, 
the conveyor belt is reactuated, preferably automatically, to move the 
plastic lenses to location 28 (distances along the conveyor line not 
necessarily being to scale). As the lenses pass sensor 30, preferably an 
electric eye of which suitable versions are commercially available, a 
predetermined delay time is started after which an airless spray system is 
actuated to coat substantially the entire surface of the lenses with a 
thin uniform coating of liquid antistatic material, as discussed further 
below. 
Control panel 32, the exact positioning of which is a matter of design 
choice well within the ability of those skilled in the art in view of the 
present disclosure, contains several control mechanisms. Included among 
the control mechanisms should be a photo sensor relay to set and control 
the sensitivity of electric eye 30. Also included in the control panel 
should be a delay timer to set and control the time delay between the time 
that a lens passes sensor 30 and the time that the airless spray system is 
actuated. Typically, such delay would be on the order of one second or 
less, although this will depend on the conveyor speed and the distance 
between the sensor 30 and location 28. Also included in the control panel 
should be a second delay timer to control the duration of spray system 
actuation, that is, the time period during which the liquid antistatic 
material is being sprayed onto the plastic lenses. As discussed above, an 
airless spray system is found to provide a more uniform coating and, in 
addition, a less heavy coating is required in view of the better 
distribution of liquid antistatic material achieved on the surface of the 
plastic lens in view of the prior deionization step. Accordingly, such 
spray duration typically can be approximately one second or less. Suitable 
commercially available models for each of the aforesaid control devices 
will be readily apparent to the skilled of the art in view of the present 
disclosure. 
The airless spray system comprises an enclosed reservoir 34 of liquid 
antistatic material. One suitable material is Staticide (trademark) 
available from ACL Corp., Elkgrove, Ill. Typically, this material is 
diluted with deionized water at a ratio of from about 150:01 to about 
300:1. The solution is fed by feed line 36 to airless pump means 38. 
Various suitable pumps are commercially available and will be apparent to 
the skilled of the art in view of the present disclosure. One such pump is 
model 64-B Stainless Steel Hydraulic Pump available from Nordson Corp., 
Amhurst, Ohio. This is a single piston, reciprocating, air driven pump 
with a 15 to 1 liquid to air pressure ratio. It is designed to supply 
atomizing pressure to multiple airless spray guns. It can be operated 
indefinitely in a stall condition without damage and automatically 
compensates for varying fluid demands and intermittent spray operation. 
The liquid antistatic solution is pumped from reservoir 34 through a 
filter (not separately shown) and supply line 42 to fluid Pressure 
regulator 40. The fluid pressure regulators preferably have dial pressure 
indicators for purposes of monitoring the operation of the system. 
Preferably all fluid communication lines, valving, fittings, etc. are 
corrosion resistant stainless steel or other suitable material. As a 
general matter, pressure regulating devices, valving, conduit and like 
components of the system and also wiring and electronic devices described 
herein are readily commercially available and selection of suitable such 
devices will be within the ability of those skilled in the art with the 
aid of the present disclosure. 
The liquid antistatic material flows then through supply line 44 to the top 
and bottom airless spray guns 46 and 48, respectively. Good results can be 
achieved at fluid pressures of approximately 300 to 325 psi. Frequently, 
upwardly directed spray guns positioned below the lens will require closer 
proximity to the lens than upper spray guns, since the spray from the 
lower guns must overcome gravitational effects. One suitable, commercially 
available spray gun is Model A7A available from Nordson Corporation. Fluid 
circulation kit Model 64B and 180C, available from Nordson Corporation, 
provides other suitable components for the system. Preferably each spray 
gun is equipped with a cross-cut nozzle having an extremely small orifice 
diameter. When functioning properly under optimal conditions, these 
nozzles can disperse a fine mist to coat each lens with an even 
"mono-layer" of antistatic solution. 
When a conveyed lens passes by sensor 30, a signal is sent by the sensor to 
the trigger delay mechanism mentioned above. After the pre-set delay, the 
trigger activates a pneumatic valve which then initiates gun spraying. 
Preferably the lenses are moved continuously past location 28 during 
actual spraying, coming to rest at location 50, since this is found to 
substantially improve the uniformity of spray distribution over the 
surface of the lenses. 
It will be understood from this disclosure that any number of spray guns 
may be used both above and below the lenses. The appropriate number will 
depend upon whether multiple lenses are being treated simultaneously, the 
size of the lenses, etc. Usually there will be nothing between the upper 
spray gun 46 and the lenses. In contrast, however, the upwardly directed 
spray from lower gun 48 must pass through the conveyor belt. To minimize 
the interference of the conveyor belt the return track of the belt 
preferably passes below lower gun 48. It is well within the skill of those 
in the art in view of the present disclosure to employ, e.g., idler 
sprockets or the like to arrange such routing of the conveyor belt. It 
should be understood that reference to applying a coating of liquid 
antistatic material to the lens means that preferably, but not 
necessarily, the entire surface of the lens is coated. According to the 
preferred embodiment of the invention illustrated in the drawing, a return 
line 52 is provided for circulating the liquid antistatic material back 
into reservoir 34. 
As stated above, while the spray system employed in the invention is 
airless, pump 38 may be pneumatically driven. Thus air pressure supply 
line 54 is shown along with air pressure regulators 56 for supplying 
pneumatic power to pump 38. Supply line 54 typically would be connected to 
a general in-plant supply of compressed air. Suitable pressure for pump 
operation will typically be in the range of 30 to 35 psi. 
According to the preferred embodiment of the invention illustrated in the 
drawing, spray guns 46 and 48 are housed within spray chambers 60 and 62, 
respectively. These chambers serve to at least partially isolate the 
lenses during and after spraying from airborne contaminants and thus aid 
in producing a higher quality product. Such chambers can be fashioned, for 
pxample, of 1/8 inch plexiglass or the like. Clear Plexiglass has the 
advantage of allowing visual observation of the spray operation, which can 
aid in the detection and correction of malfunctions, etc. Optional exhaust 
fan 64 serves to remove excess antistatic material from within the 
chambers. 
The next incremental actuation of the conveyor 16 moves the lenses from 
location 50 to a location within decay chamber 66. The "decay chamber" is 
an enclosed space in which the lens can be held in at least partial 
isolation from airborne contamination and the like while the liquid 
antistatic coating dries and becomes substantially fully effective. Thus, 
decay chamber 66 prefferably is sufficiently long to house a lens over 
several incremental advances of the conveyor. The determination of a 
sufficient time period for substantially complete drying of the antistatic 
solution will be within the skill of the art. This period is known to be a 
function of the type and amount of antistatic material used, the relative 
humidity of ambient air, temperature, grounding and other factors. Decay 
chamber 66 can be constructed in much the same fashion as the spray 
chambers 60 and 62 and, hence, preferably comprises thin plexiglass walls 
or the like. Lenses pass from decay chamber 66 to the end of convey 16 and 
there are unloaded, either manually or automatically, for storage or 
further processing, assembly, etc. Reference above to movement of the 
lenses along the apparatus by conveyor 16 "in substantial synchrony" is 
intended to mean that parts move through the apparatus one after another 
at the same pace. This would be the case, for example, where all lenses 
are conveyed on a common conveyor through the apparatus, as is the case in 
the preferred embodiment illustrated in the drawing, but a lens may move 
from one location to another in synchrony (as that term is used herein) 
with another lens even if one or both move to intermediate locations 
before reaching the location at which the prior lens was treated. 
Emergency switch 70 is conveniently located to allow an operator of 
apparatus 10 to shut down the conveyor and other operating components of 
the line when needed. Electrical panel 72 and auxiliary electrical outlet 
74 provide electrical power to conveyor 16 and various other components of 
the line. It will be within the skill of the art to design and position 
these devices appropriately. 
In addition to concentration, other factors, which it will be within the 
skill of the art to optimize with the aid of this disclosure, include the 
flow rate at the spray gun, the spray pattern, dispensing pressure, 
distance between the spray gun and the part, amount of liquid dispensed, 
dryness of the part surface prior to spraying and the temperature of the 
lenses being treated. 
Plastic lenses treated by the apparatus and process of the present 
invention are found to be reliably and consistently of high quality, as 
measured for example by the decay time of the antistatic protection. A 
suitable test procedure is described in Federal Test Method, Standard No. 
101, Method 4046 (1969). Also, the use of static charge meters to measure 
the surface voltage on plastic lens treated by the invention demonstrates 
the improved results achieved. Thus, a static charge meter such as the 
Simco Electrostatic Locator, e.g. models Type SS-2X and Type SS-2, 
available from Simco Company, Hatfield, Penna., can be used to measure the 
"electric field" or "electric potential" produced by any static charge on 
a surface area of the lens. 
The foregoing description and discussion of preferred embodiments of the 
invention is intended to modifications of and additions to the invention 
will be apparent to the skilled of the art in view of this disclosure and 
all such modifications and additions are intended to be within the scope 
of the appended claims.