CA1133214A - Disposable face mask - Google Patents

Abstract

ABSTRACT
A method for forming a disposable face mask for medical purposes to carry out the administration of gases to a patient is disclosed and which provides a relatively soft, pliant surface that readily conforms to the patient's anatomical features in a gas-tight fit. The mask, as formed, comprises two parts; a semi-rigid cone backing to which there is affixed a cushion formed in accordance with this invention. The cushion is produced by in-jection molding a thermoplastic compound of a predetermined hard-ness. The thermoplastic compound is first combined with a blow-ing agent, and the mixed compound heated to break down the blow-ing agent to give off a gas, such as nitrogen. The heated, plasticized material is then injection molded into a mold of the desired configuration. The molding conditions, such as the vent-ing thereof, are controlled to cause formation of a small, rela-tively uniform cellular structure within the body of the finished product cushion. The control of the conditions involved in the injection molding process enables the final cell structure to be uniform and made up of cells which exhibit excellent pliant characteristics for conforming to the anatomical features of a patient.

Description

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Back~round of the Invention Basically, face masks are used for many different purposes in administering gases to a patient. Typically, such purposes include the administration of inhalant anesthetics to patients for anesthetizing the same during surgical procedures.
Face masks are also included as part of a patient breathing circuit wherein the inhalation of air or oxygen enriched air is assisted by the development of positive pressure within the breathing circuit.
The mask characteristics must be such as to be easily and comfortably conformed to the patient's facial anatomical features despite the obvious unlimited variety of such features.
Normally, such masks must provide a relatively gas-tight fit against the face of the patient to prevent escape of gases to the atmosphere that are intended for the patient, yet it is ad-vantageous that such seal be obtained with a minimum of pressure against the patient, to minimize facial trauma.
Many masks presently on the market are also readily disposable, that is, they are produced and marketed at a price that makes it advantageous for the hospital, or other user, to discard the mask after each use, rather than attempt to clean or sterilize the mask for use on subsequent patients. Disposabil-ity, of course, completely alleviates the risk of cross-infection between patients.
Mask producers, therefore, are faced with the produc-tion constraints that the eventual mask must be comfortable, pliable against the patient's face, and yet the production pro-cedures must be susceptible of inexpensive mass production capa-bilities to produce a mask competitive with other commercial dis-posable products.

- 2 -Present commercial masks are formed by a variety of - methods. One type includes a cushion with a flexible flap which contacts the patient's face, having no sponge or foam character-istics. Another type is a single piece molded flexible mask, again having no viable cushion that conforms to the patient's face.
One further commercial type is produced by an in situ foaming, on a cone backing, of a polyurethane foam that forms a soft cushion. Other masks are believed made with a "foam" type cushion, however, such are made by cutting a foam to the desired size and bonding the same to a cone backing.
The present masks still have some undesirable features, however, either in the facial compliance or as to the manufac-turing procedures which are expensive. Specifically, flexible flaps and single piece masks do not comfortably comply to a patient's anatomical features even though they can be produced very inexpensively, while the present formed in situ masks do have good compliance characteristics but have certain disadvan-tages in the manufacturing thereof, they require manual trimming and involve the use of chemicals that are undesirable in the environmental surroundings.

Summary of the Invention This invention relates to a method of producing dis-posable face masks used in administering various gases to patients for medical treatment. More specifically, the present invention comprises a method of forming a mask cushion by injection mold-ing of such cushion through the use of a thermoplastic molding compound (also referred to as a "dry blend") in which there is combined a blowing agent. The injection molding is carried out ~133Z14 under certain predetermined conditions, such that the thus formed cushion exhibits soft, pliable characteristics that enable it to be easily fitted to the face of a patient and a relatively gas-tight fit is possible.
The present invention overcomes certain difficulties of the aforedescribed present commercial masks by providing a method whereby the foam cushion is injection molded, thus the formed cushion is of the desired dimensions and the labor time and cost of trimming, cutting and the like are significantly reduced.
Also, the finished foam characteristics are such that a cellular foam consistency is covered by a resilient skin and which is soft and pliable and thus readily conforms to the in-dividual features of the patient's face without excessive pres-sure thereagainst to effect an essentially gas-tight seal.
The overall process is readily adapted to high quanti-ty mass production with a mlnimum of hand operations but with good consistency of product.
The process achieves these advantages over the various prior art processes and masks made by using prior art processes by the combining of a thermoplastic compound with a blowing agent and heating the mixed compound prior to the injection mold-ing step. The heated, plasticized material, including the re-leased gas, is thereafter injection molded with a mold having the desired cushion dimensions and configuration. The foam cushion is thus formed in the mold by the released gas from the foaming agent, such that the controlled conditions bring about a particu-lar foam characteristic of relatively uniform cellular structure having small cells. A resilient skin is likewise formed about the cellular structure and, acting in conjunction with the cellu-1~33Zi4 lar foam, presents a soft pliable surface to the patient's face for conformance thereto in a gas-tight relationship.
The improved process for producing a molded face mask in accordance with the present invention is illustrated in the accompanying drawings, which show how the preferred embodiment - of the invention is carried out.

Brief Description of the Drawin~s ; FIG. 1 is a block diagram showing the sequential steps taken to produce a mixed molding compound used in the present invention; and FIG. 2 is a schematic view of an injection molding machine used to carry out the steps of forming the face mask of this invention; and FIG. 3 is an isometric view of a mask cushion formed in the machine of FIG. 2, using a molding compound produced in accordance with FIG. l; and FIG. 4 is an isometric view of a completed mask pro-vided in accordance with the process of the present invention.
Description of the Preferred Embodiment In Fig. 1 there is shown a block diagram of the sequen-tial steps taken to produce the molding composition used to pro-duce a mask in accordance with the present invention. A source 10 of suitable molding compound is provided for producing the desired composition. Preferably, the molding compound is a thermoplastic material, and the more preferred of such materials is polyvinyl chloride (PVC) having a hardness of about 38 duro-meter on the Shore A scale. Such grade is commercially available and the further description will refer to PVC as the molding ` `~

compound for convenience, although it will be recognized that other thermoplastic molding materials may be utilized.
A chemical blowing agent source 12 is also provided and which supplies a chemical blowing agent for mixing in pre-determined properties with the molding compound to produce a composition for use with this invention. A7.odicarbonamide is the most widely used chemical blowing agent. One such suitable chemical blowing agent is Kempor 200, sold commercially by Stepan Chemical Company and which gives off nitrogen and other gases under certain molding conditions. That particular blow-ing agent is available in powder form and, therefore, is very convenient for mixing with the molding compound which is also preferably in powder form. It is possible to utilize a blowing agent in the form of liquids, also commercially available, how-ever, it is considerably more difficult to obtain a suitable uniform mixture of the same with a molding compound in a powder form.
With the preferred ingredients, therefore, the PVC
molding compound powder is mixed with the blowing agent powder in a commercially available ribbon blender 14 in predetermined proportions, with standard grade PVC powder and the aforemen-tioned Kempor 200 blowing agent, a mixing ratio in the range of one part blowing agent to 99 parts molding compound is utilized and which produces excellent results, however, on a weight per-cent basis, adequate results have been achieved when the blowing agent percent ranges from about .25% to about 1.5% by weight.
The mixed compound, of uniform consistency and mixing, is desired from the ribbon mixer 14 as shown at 16. The mixed COmpOUn(] iS l-OW ready for usc witll an injcction moldillg m~chine in a manner to be described.
TRADEMARK

Turning now to Fig. 2, there is illustrated an injec-tion molding machine 18 in schematic form. The injection mold-ing machine 18 comprises a main barrel 20 in which is positioned a screw 22 which is adapted to rotate by means of motor 24. Mold-ing compound is gravity fed into the barrel 20 from hopper 26 where a supply of such compound is retained. In the present in-vention, of course, the molding compound is the mixed compound from 16 (Fig. 1). A plurality of heating zones 1-4 and numbered, respectively, 28, 30, 32 and 34, are positioned between that por-tion of the main barrel 20 receiving mixed compound from hopper26 and the nozzle 36 from which the mixed compound is ultimately injected into a mold 38. The mold 38 has a suitably shaped cavity 40 in the form of the mask cushion which is desired to be formed. The mold 38 varies, of course, in accordance with the particular mask cushion being produced, i.e. adult size, child-ren's size, etc. A shut-off valve 42 is also included at the end of the heating zones and prior to the injection of the mixed compound from nozzle 36.
At the opposite end of the barrel 20 from nozzle 36, there is located a hydraulic cylinder 44 which is adapted to move the screw 22 when activated, as will be later explained.
In the operation of the injection molding machine 18, the mixed compound in the hopper 26 provides a continuous feed of the same into the barrel 20. The heating zones are initially brought up to temperature in progressively higher temperature to heat the mixed compound as it progresses towards nozzle 36. With PVC compound and the preferred blowing agent, the heating zones may be set such that zones 28, 30, 32 and 34 are respectively at temperatures of about 260F, 260F, 320F and 350F. A typi-cal injection cycle, to be described, takes about 20 seconds and the temperature of the mold 38 is about 60F to about 120F, al-; though the mold temperature is not a particularly critical value.
The cycle is initiated as mold 38 is closed. The hy-draulic cylinder 44 thereupon moves the screw 22 forward toward mold 38. Screw 22 acts like a plunger and forces the mixed com-pound within barrel 20 toward the mold 38. The valve 42 opens and the heated mixed compound is thus forced into the mold cavity 40 into the configuration of a mask cushion.
At the end of the forward stroke of hydraulic cylinder 44 and screw 22, the shut-off valve 42 again closes and the screw 22 rotates to force additional mixed compound from the hopper 26 in the direction of the mold 38 within the barrcl 20. As the barrel 20 fills with mixed compound, the screw 22 rotates itself backwards and causes an internal backpressure on the mixed com-pound as the screw 22 moves back to its original position. The controlled backpressure exerted by the screw 22 on the mixed com-pound serves to pack the compound tightly and also serves to pre-vent the escaE)e of gas that is being released from the blowing agent backward toward hopper 26. At the end toward the mold 38, ~he valve 42 is, of course, now sllut so that no gas or plasti-cized material is lost or leaked into the mold 38.
As the mixed compound is compacted in the end of the barrel toward the mold 38, the compound passes through the heat-ing zones 28, 30, 32 and 34. As the mixed compound approaches the final heating zone 34, it becomes plasticized and the par-ticular blowing agent breaks down as a result of the pressure and temperature conditions and gives off a gas (in the case of the preferred blowing agent, that gas is nitrogen). The gas goes into solution with the plasticized material and remains in such form until the next injection step.

11~3Z~4 As the cycle is completed, the plasticized material that has been injected into the mold 38 cools, the mold 38 is opened and the thus formed mask cushion is removed.
The mold cavity 40 is vented to allow the escape of a predetermined amount of "freed" gas to achieve the desired mask cushion consistency. The amount of controlled venting determines the cellular size and therefore determines the pliability of the cushion. Too much venting results in coarse cell structure re-sulting in loss of pliability, while too little venting creates very fine, almost non-existent cell size, again resulting in an absence of pliability. The venting is determined by testing the particular mold cavity and setting and adjusting the amount of venting such that the desired cellular structure is achieved which exhibits soft pliable characteristics.
The resulting cushion 46 removed from the mold 38 is shown in Fig. 3. In the preferred form, the cushion 46 has a specific gravity of between about .5 to about .8 and with a base compound of 38 durometer (Shore A). The pliability exhibits adequate compressibility and is readily adapted to conform to the facial anatomical features of a patient.
The final patient mask 48, Fig. 4, is produced by af-fixing the foam cushion 46 to a cone 50. The cone 50 is formed by conventional methods, such as injection molding and is prefer-ably of a thermoplastic material such as PVC. The cone 50 may be opaque, transparent or translucent, depending upon the material used. Preferably the cone 50 is made of such a material 'PVC) such that the cone 50 may, if desired, be solvent bonded to the foam cushion 46. Alternatively, the cushion 46 and cone 50 may be affixed together by mechanical means or through bonding by means of a suitable adhesive, or by inserting the cone into the mold and molding the cushion to it ~insert molding).

g 11;~3Z14 Thus, there is produced a face mask having particular-ly advantageous softness characteristics by a judicious selec-tion of materials and conditions, such that a foam cushion is injection molded and which is affixed to a cone backing to pro-duce a face mask that is inexpensive to produce, yet which is easily adaptable to conform to the facial anatomical features of a patient. The cushion support is kept to a minimum through the cone backing and which allows its soft characteristic to be fully utilized in molding itself to the patient features with a minimum of pressure on the face mask, such as to reduce the pos-sibility of patient facial trauma.
It will be understood that the scope of the method and product of this invention is not limited to the particular steps or materials disclosed herein, by way of example, but only by the scope of the appended claims.

Claims (5)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method for producing a patient face mask comprising the steps of:
a) combining and mixing uniformly together in a pre-determined ratio, polyvinyl chloride molding compound having a plasticizer and a blowing agent;
b) heating the combined mixed compound from step (a) to plasticize the compound and to cause the release of gas from the blowing agent;
c) injection molding the heated mixed compound into a mold having a cavity in the configuration of the desired mask cushion;
d) controlling the venting of mold during the injection step;
e) cooling the molded compound to cause solidification thereof;
f) removing the thus formed cushion from the mold; and g) affixing the cushion to a semi-rigid cone to produce a completed face mask.

2. A method as set forth in Claim 1 wherein said blow-agent used in step (a) is azodicarbonamide and is present in the resultant mixture in a range from about .25% to about 1.5% by weight

3. A method as set forth in Claim 2 wherein said heat-ing step takes place at a temperature in excess of about 300°F.

4. A method as set forth in Claim 1 wherein said affixing step comprises solvent bonding the cushion to a polyvinyl chloride cone.

5. A method set forth in Claim 1 wherein the cushion is insert molded to said cone.