GB2089731A - Platen assembly - Google Patents

Abstract

A platen assembly for impact printing apparatus comprises a hollow cylindrical core member 12, coated with a relatively thin elastomeric layer 14. The inside of the core member may be filled with a plastic foam material 22 which reduces the noise generated during impact printing. The relatively thin elastomeric layer requires less printing energy than a conventional platen and prolongs the life of the printing element. <IMAGE>

Description

SPECIFICATION Platen assembly The present invention relates to impact printing apparatus e.g. a typewriter and more specifically to a platen assembly for such apparatus.

A recurring problem in the development of typewriter impact printing apparatus e.g. a typewriter is the provision of a suitable platen member. With the present trend toward ever increasing printing speeds, the platen member has become a vital element of the printing assembly. The platen member must combine the proper resistance to the impact forces of the type elements with resistance to abrasion, durability and producibility. Excessive compliance or softness of the surface of the platen member results in rapid wear of the platen as well as excessive energy dissipation during the printing stroke. When used with a daisy wheel printing member, in which individual print characters are mounted on flexible cantilever members, excessive deflection of the platen results in excessive deflection of the cantilever members causing fatigue and premature failure.An excessively stiff platen surface is also undesirable in that it results in rapid wear of printing elements especially when made of plastics and a loss of printing quality.

The problem of noise generation during impact printing presents a special difficulty for platen design. The increased use of high speed impact printing equipment in office areas has resulted in an increased emphasise on noise control in the light of recent requirements placed on machinery and equipment by regulatory agencies such as OSHA in the United States and to ensure user acceptance and comfort.

Increasing the compliance or softness of the platen generally tends to decrease the noise generated during impact printing, however this results in excessively rapid platen wear and excessive energy dissipation as indicated previously.

Examples of previously proposed typewriter platens which attempt to utilize various combinations of materials in order to provide an effective platen structure are disclosed in U.S. Patent Number 1,801,396 to L.A. Thatcher and U.S. Patent Number, 2,182,141 to A.E. Vickery which show typewriter platens having a relatively thin layer of elastomeric material covering a relatively thick layer of rubber. The use of the elastomeric layer is an attempt to improve impact and abrasion resistance and to prolong the useful life of the platen as compared with a rubber platen. When used with high speed impact printing equipment the relatively thick rubber layer results in excessive surface compliance and excessive printing energy dissipation, as has been previously indicated. U.S.Patent Number 960,679 to W.P. McLaughlin shows a hollow typewriter platen, the inside of which is filled with sawdust for sound deadening. The deficiencies of this design include variations in sound deadening performance which result from non-uniformity in manufacture caused both by variation in the particle size of the sawdust and variation in the initial compaction of the sawdust filling. The assembly depends on intimate contact between the inside surface of the platen and the sawdust filling for absorption of the acoustic energy of the platen. During use, additional compaction and settling of the filling tends to decrease the sound deadening capability of the assembly.

The present invention provides a platen assembly for impact printing apparatus comprising a rigid hollow cylindrical core member having an outer surface, and a thin elastomeric layer formed on said outer surface of said outer surface of said core member.

In accordance with a preferred embodiment of the present invention of the present invention of a platen assembly is provided which comprises a hollow cylindrical core which is filled with a foam plastics material such as urethane foam. The outer surface of the core has a relatively thin layer of a plastics material such as polyurethane. The layer of polyurethane is uniform in thickness and the thickness of this layer may range from a minimum of 0.005 inches to a maximum of 0.015 inches. A preferred thickness for this layer has been determined to be in the order of 0.010 inches. The polyurethane layer is formed using a spray technique. The relatively thin polyurethane layer requires approximately 60 per cent less printing energy than that required when using the relatively thick layer of rubber found on conventional typewriter platens.This reduction in printing energy also reduces fatigue due to a reduction in noise generated during printing, and increases the life of the printing element especially daisy wheel type printing members. Further, the platen assembly has an increased resistance to abrasion and can be manufactured in high volume at a relatively low unit cost.

In order that the present invention be more readily understood, an embodiment thereof will now be described by way of example with reference to the accompanying drawing, in which Figure 1 is a fragmentary cross-sectional perspective view of a platen assembly; Figure 2 is a transverse cross sectional view of the platen assembly of Figure 1.

There is shown in Figures 1 and 2 a platen assembly 10 which comprises a hollow cylindrical core member 12 of circular cross-section and a relatively thin coating or layer 14 on the outer surface 16 of the core member 12. A pair of platen knob members are rigidly mounted on the ends of the core members using conventional attachment means. One of the platen knobs is shown in the drawings and is indicated by the reference numeral 18. A shaft member 20, which is conventional in nature projects between the end of the core member 12 and the platen knob 18 to facilitate supporting the platen assembly 10. The interior of the core member 12 is filled with a plastics foam material 22 to reduce the noise generated during impact printing in a manner which will be presently described.

The layer or coating 14 which forms a major feature of the assembly comprises a plastics material such as polyurethane having a uniform thickness which may range from a minimum of 0.005 inches to a maximum of 0.015 inches. A preferred thickness for this coating 14 has been determined to be in the order of 0.010 inches. Although the coating material has been indicated as polyurethane it is understood that this has been done by way of example only and that other elastomeric materials having similar properties may be substituted such as styrene butadiene.

The core member is made of a rigid material e.g. metal such as aluminium and has a wall thickness which ranges from a minimum of 0.035 inches to a maximum of 0.045 inches. A preferred wall thickness has been determined to be in the order of 0.040 inches.

A preferred method for fabricating the coating 14 has been found to be spray coating of the core member 12. This technique is described in Ames Technical Report No.77-2 titled "Elastomeric Spray Technology" published by the Ames Rubber Corporation, Hamburg, N.J. The polyurethane materials utilized for spray coating have the following typical properties: Hardness Shore A Shore D 80 90 50 60 75 Tensil Strength, psi 3000 4500 5000 8000 10,000 Ultimate Elongation, % 800 400 400 300 250 Modulus at 100% Elongation, psi 400 1100 1700 2900 4,000 Tear Strength (Die C), psi 300 450 580 620 700 These properties are nominal values for the polurethane material if it were moulded rather than sprayed because measurements of these properties cannot be made on sprayed coatings.

The coating 14 may be fabricated using either an airless spray process or an electrostatic spray process. In the airless spray process two high pressure pumps meter the components of the coating to a two component spray gun. The two materials mix at the spray gun and the coating is applied through a nozzle. The sprayed parts are then batch cured in a hot air oven. In the electrostatic spray process, the core member 12 is conductive and is electrically grounded and then conveyed past a rotating disc which has a high positive electrical charge. A two-component elastomeric solution is passed through a static mixer and onto the disc where it picks up the electrical charge. Centrifugal force from the rotation of the disc causes the solution to travel outward from the disc.The electrically charged spray solution becomes atomized and is attracted toward the electrically grounded core member 12 thereby causing the core member to become coated.

Other methods for fabricating the coating 14 are dip coating or extruding a cylindrical shell.

As indicated previously the core member 12 is filled with a plastics foam material 22 such as urethane foam or polyester syntactic foam. The foam plastic material may be fabricated and inserted into the core member 12 or alternatively, formed in place in the core member using any one of a number of conventional techniques. The foam material 22 is in intimate contact with the inner surface 24 of the core member 12 and vibration of the core member 12, resulting from impact forces caused by printing elements striking the coating 14 during impact printing, are transmitted to the foam material 22 where they are damped. This damping action absorbs a portion of the acoustic energy generated during impact printing and reduces the noise which is generated.

Claims (11)

1. A platen assembly for impact printing apparatus comprising a rigid hollow cylindrical core member having an outer surface, and a thin elastomeric layer formed on said outer surface of said core member.

2. A platen assembly according to claim 1 in which said layer has a uniform thickness of 0.005 inches to a maximum thickness of 0.015 inches.

3. A platen assembly according to claim 2 in which said layer has a preferred thickness in the order of 0.010 inches.

4. A platen assembly according to claim 1,2 or 3, in which said core member has a uniform wall thickness, measured between said inner surface and said outer surface which ranges from a minimum thickness of 0.035 inches to a maximum thickness of 0.045 inches.

5. A platen assembly according to claim 4 in which said core member has a uniform wall thickness, measured between said inner surface and said outer surface, having a preferred value of 0.040 inches.

6. A platen assembly according to any one of the preceding claims in which said layer comprises polyurethane.

7. A platen assembly according to any one of the preceding claims in which said core member is filled with a plastics foam material.

8. A platen assembly according to claim 7 in which said core member is filled with urethane foam.

9. A platen assembly according to any one of the preceding claims in which said core member is made of aluminium.

10. A platen assembly according to any one of the preceding claims further comprising a knob member attached to one of said end portions of said core member.

11. A platen assembly substantially as hereinbefore described with reference to the accompanying drawings.