DIE AND PROCESS FOR STAMPING PLASTIC MATERIAL

Abstract: A device for stamping a detergent bar, i.e. a bar comprising soap, a synthetic detergent active or a mixture thereof, includes a die. The die (11) has at least one bar stamping surface provided with at least one elastomeric coating (16) and includes a rim (30) having a rigidity greater than that of the elastomeric coating (16). Good release of the detergent bars from the die is possible without chilling of the dies. Use of the rigid rim increases the useful life of the coating.

Description: DIE AND PROCESS FOR STAMPING PLASTIC MATERIAL Field of the Invention The present invention relates to a device, process and apparatus for stamping a plastic material using a die to form a shaped article. In particular, it relates to process for stamping a detergent bar. By "detergent bar" is meant a tablet, cake or bar in which the level of surface active agent, which comprises soap, synthetic detergent active or a mixture thereof, is at least 20 wt. % based on the bar. Background of the Invention In the manufacture of detergent bars, a preformed composition comprising all components of the bar is typically extruded from a nozzle to form a continuous "rod" which is cut into smaller pieces of predetermined length, commonly referred to as "billets." These "billets" are then fed to a stamper or, alternatively, are given an imprint on one or more surfaces using, for example, a die of the same dimensions as the bar surface. The bar surface is hit with force by, eg., a mallet or a die in the shape of a roller. Stampers typically have a die formed in two halves each with a surface which contacts the billet during the stamping operation. These surfaces are adapted to close to a preset separation distance, thereby compressing the billet between the die halves to give the bar its final shape and appearance, and then separate. Excess composition is squeezed out from the die halves as they close. This is commonly referred to as "flash." The flash is then separated from the soap bar by transferring the bar through holes in a "deflashing plate. " Conventional die stamping machines include "pin die" shaped machines in which a pair of opposing die members or die halves meet during a compaction step and a "box die" machine in which a pair of opposing die members stamp a bar held within a through-opening in a box frame but do not meet during compaction, the peripheral face of the bar being restrained by the box frame. The die halves are often each provided with a die or ejector insert. These are normally held closed within the die halve by springs but can be pushed open by compressed air or mechanical means to assist in the release of the bar from the die. During closing of the die halves a vacuum can be applied to remove air trapped in the die cavity between the detergent bar and die surface and, in the case of rotary dies, this vacuum assists in retaining the bars in place during rotation. Stamping of detergent bars using a die is carried out to give the bars a reproducible shape, smooth surface and/or to imprint a design such as a logo, trade mark or the like onto at least part of a surface of the bar. However, as a result of die-blocking, i.e. amounts of residual detergent left on die halves which builds up during continued use of the dies, bars are often formed with visible imperfections on their surfaces or they may not release from the die surface. Numerous solutions to these problems have been proposed. One solution involves chilling the die halves during the stamping operation. Chiling uses valuable resources, particularly energy sources. Another solution is described in GB-A-746 769 which discloses a die set which includes a die box and a pair of companion die members made of plastic materials comprising polymers with a specified modulus of elasticity. A disadvantage of this system is that a die release agent is necessary to prevent detergent from adhering to and building up on the dies and marring the surfaces of subsequently pressed bars. Luckow, U.S. Patent No. 1,817,157 discloses a soap frame. Pinto, U.S. Patent No. 3,127,457 discloses a fluorocarbon film used as a release agent for molding polyurethane articles . Krugg, U.S. Patent No. 3,539,144 discloses a mold useful for producing polyurethane foams, which includes a structural mold shape and a coating of hot vulcanized silicon rubber disposed on the structural mold as well as a cold vulcanized silicone rubber deposited on the hot vulcanized silicon rubber . adney, U.S. Patent No. 3,889,919 discloses a plastic vent insert for a molding chamber for molding plastic parts. Molding cavity 26 includes a pheripheral lip 28. Cavanugh, U.S. Patent No. 4,035,122 discloses a soap saving device for compressing remnants of soap bars. Austin, U.S. Patent No. 4,076,207 discloses a food mold comprising a liner and a rigid holder wherein the liner has a rim for securement to the holder. Roussel, U.S. Patent No. 4,809,945 discloses a mold cavity for use in molding articles such as soap bars. atanabe, U.S. Patent No. 5,332,190 discloses an elastic molding die composed of an elastomer laminate film, the innermost layer of which does not contain fillers. In US Patent No. 5,269,997 it is proposed to provide each of two dies of a soap mold with an elastomeric septum stretched across their surfaces. Such a system would be complex to use at the speed required for commercial manufacture and a thin covering would be prone to tearing and logo reproduction would be expected to be poor. WO 96/00278 discloses a device for stamping a detergent bar comprising a die, the die having at least one bar stamping surface wherein the bar stamping surface is provided with an elastomeric coating, the total thickness of the elastomeric coating being less than 200 microns In a preferred embodiment, the elastomeric coating is the sole elastomeric material of the bar stamping surface. Another solution is proposed in EP 276 971 and U.S. Patent Nos. 4,793,959 and 4,822,273, which involve the use of two die members, each comprising a non-elastomeric and an elastomeric part. The elastomeric part, which contacts the soap bar during the stamping process, comprises an elastomer coating of at least 200 microns and having a modulus of elasticity within a specified range. The manufacture of thick coated dies is a complex and expensive process. Therefore, it is highly desirable to maximize the working life of such coatings. A difficulty which has been encountered with elastomeric coatings is that they tend to have short useful lives, on the order of a few days or more. The problems which this causes will be apparent. Summary of the Invention It has been found that short life spans of elastomeric die coatings result from the high stress experienced by the edge of the die during the soap pressing operation. This results from the high soap stamping shear forces which tend to tear the elastomer coating away from the metal base of the dye. This problem tends to negate the advantages of use of the elastomeric coated dies, e.g. the ability to avoid the use of expensive cooled metal dies. It has been discovered that the stress to which the elastomeric coatings are subjected can be reduced by use of a rim or flange having a rigidity greater than that of the elastomeric coating of the die, especially one which is itself similarly coated with elastomer. Preferably the edge or flange is made of a metal or other hard or rigid material. Use of the metal edge results in diminished tearing of the elastomer by lowering the stress to which it is subjected. Preferably, the edge extends over the leading edge of the elastomer and thereby protects the elastomer from the applied stressed during stamping. This appreciably improves the useful life of the elastomer coated die and permits the advantages of the coated dies to be enjoyed without the disadvantage of an extremely short useful life. Although it may be easier to make the edge of the same material as the die, this is not a necessity. The edge or flange may be made of a multitude of materials, including plastics urethane and composites. In a preferred embodiment, the edge is provided by a "T" or umbrella shaped structure wherein the elastomer is protected beneath the top of the umbrella. The invention is particularly useful for dies used to make detergent bars which have sidewalls which extend generally perpendicularly to the longitudinal axis of the bar, that is, "banded bars." Elastomeric dies for banded bars are believed to experience even higher stress than elastomeric dies for bars lacking a vertical sidewall or for which the vertical sidewall extends only a short percentage of the height of the bar. It is contemplated that the edge or flange of the invention may also be used on a die which itself is made of an elastomeric material, rather than just coated with one. The edge will generally be made of a harder material than the die. For a more complete understanding of the above and other features and advantages of the invention, reference should be made to the following detailed description of preferred embodiments and the accompanying drawings. Brief Description of the Drawings Fig. 1 is a perspective view of dies in accordance with the invention; Fig. 2 is a cross-sectional side view of a die according to the invention; Fig. 3 is a cross-sectional front view of a die according to the invention; and Fig. 4 is a cross-section of the die according to Fig. 3 wherein the die has been coated with the elastomeric coating. Detailed Description of the Invention The use of thick elastomeric coatings in soap bar stamping is described in EP 276971 and U.S. Patent Nos. 4,793,959 and 4,822,273, the disclosures of which are hereby incorporated by reference. Hereinafter by "surface decoration" is meant a uniform shape, smooth surface, a design such as a logo, trademark or the like. By "elastomeric" according to the invention is meant a material as defined in ISO (International Standard Organization) 1382 as an "elastomer," or a "rubber." Also included in the definition of "elastomeric" materials according to the invention are thermoplastic elastomers and copolymers and blends of elastomers, thermoplastic elastomers and rubbers . Elastomers are defined as polymers with long flexible chains, independent in the raw material and transformed via vulcanizing or crosslinking agents which introduce crosslinks and form a crosslinked network structure. The network structure retains the movement of the macro- molecular chain molecules and as a result returns rapidly to approximately its initial dimension and shape after deformation by a force and release of the force. With increasing temperature an elastomer goes through a rubbery phase after softening and retains its elasticity and elastic modulus until its decomposition temperature is reached. Thermoplastic elastomers comprise amorphous and crystalline phases. The amorphous phase has a softening range below ambient temperature and thus acts as an elastic spring while the crystalline segments whose softening range is above ambient temperature, act as crosslinking sites. Preferably the elastomeric material according to the invention is selected from those classes described in American Society for Testing and Materials D1418 which include: 1. Unsaturated carbon chain elastomers (R class) including natural rubbers e.g. Standard Malaysian Rubber; butadiene e.g. "BUNA' type ex Bunaweke Huls; and butadiene acrylonitrile copolymer e.g. "Perbunan" ex Bayer. 2. Saturated carbon chain elastomers (M Class) including ethylene-propylene types e.g. "Nordel" ex DuPont and fluorine-containing types e.g. "Viton" DuPont . 3. Substituted silicone elastomers (Q Class) including liquid silicone rubbers e.g. Silastic 9050/50 P (A + B) ex Dow Corning. 4. Elastomers containing carbon, nitrogen and oxygen in the polymer chain (U Class) including polyurethanes e.g. polyurethanes ex Belzona. The "elastomeric" material, as hereinbefore defined, may be pretreated, such as by forming a solution of a commercially available elastomer, prior to it being applied as a coating on the die surface. The elastomers, rubbers, and copolymers and blends thereof are generally cured or crosslinked, in- situ on the die surface. For example, the components including the base elastomeric material, cross-linking agents and other materials such as accelerators may be mixed prior to application as a coating. Once applied to the die the coatings are cured in-situ. This may be aided by the application of heat or other accelerating processes, for example pressure; radiation or UV light. The elastomeric material may be applied as either a liquid or a semi-solid. For instance, when applied as a liquid, the two halves of the die are held apart at a predetermined spacing to permit the elastomer to occupy the space between the two parts. Preferably, the die is filled with elastomer under pressure. In some cases, materials may be dissolved with an appropriate solvent, applied to the die and the solvent subsequently driven off. In the case of thermoplastic materials, they can be heated to melt condition applied to the die, cooled and resolidified. Materials suitable as elastomeric coatings in the present invention will preferably have a modulus of elasticity in the range 0.1 to 50 MPa, most preferably 1 to 35 MPa . The modulus of elasticity of an elastomeric coating may be measured by recording the force required to indent the coating as a function of the indentation depth. Typically an indentor with a spherical tip may be employed and the slope, s, of the force as a function of the indentation depth to the power 3/2 is determined. The indentation depth is the movement of an indentor into the coating after it first contacts the surface of the coating. In general, it is necessary to correct the measured indentation depth for the compliance of the measurement apparatus. That is, the actual indentation depth, d, is related to the measured apparent value d' by the following expression d = d 1 (F.C) where F is the indentation force. The compliance C is determined by compressing the indentor against a rigid surface and recording the apparent displacement as a function of the applied force which had a slope equal to C. The modulus of elasticity E is calculated from the following expression 1/2 2 E = 3/4: s 1/ (R) (1 b ) 3/2 where s = F / d , R is the radius of the spherical tip of the indentor and b is the Poisson's ratio of the coating which is equal to about 0.5 for elastomers. Under certain conditions to be described hereafter, the above indentation method may give falsely large values of the elastic modulus due to the influence of the rigid material onto which the coating is applied. In order safely to avoid this problem it is necessary to ensure that the contact radius of the indentor with the coating does not exceed about 1/10 of the thickness of the coating. The contact radius, a, is related to the indentation depth by the following expression 1/2 a = (dR) For coatings less than 200 :m, it is recommended that a nanoindentor is used which is capable of measuring indentation forces at small indentation depths using indentors with tips having a small radius. An example of such equipment is the "Nanolndenter II" (Nano-instruments) . The alternative is to make thick (greater than 200 :m) test coatings so that more conventional measurement equipment such as an Instron tester, (eg Model 5566) may be employed. Preferably the die comprises a rigid material selected from metals and their alloys, for example brass and other copper alloys, aluminum, and steels including carbon and stainless steel; and other non-elastomeric materials such as thermosetting and thermoplastic resins, for example polyester, epoxy resins, furan resins; hard cast polyurethanes; ceramics; composites and laminates. Additional materials, for example fillers, can be added to the elastomeric material to modify its mechanical and processing properties. The effects of filler addition depends on the mechanical and chemical interaction between the elastomeric material and the filler. Fillers can be used to modify the elastomeric material such that desirable properties, for example tear resistance, is achieved. Suitable fillers include carbon blacks; silicas; silicates; and organic fillers such as styrene or phenolic resins . Other optional additives include friction modifiers and antioxidants . The edge or flange should extend from the die wall to cover the thickness of the cavity coating and preferably the thickness of the coating outside the cavity. The dimensions of the edge for optimal performance are therefore determined by the desired coating thickness. The thickness and hardness of the elastomeric coating can be varied according to the detergent bar composition, processing temperature and/or process parameters such as the shape of the cavity in the die halves, speed of the stamping equipment and separation distance of the die halves, in order to achieve the desired result, for example, good release of the detergent bar from the die. For more complex logo bearing die or more complex die shapes, acceptable die release is favored by the use of thicker coatings and a lower modulus. Similarly, for a bar composition which is inherently more difficult to stamp, acceptable die release may be achieved with a thicker elastomeric coating and/or one having a lower modulus of elasticity. The device according to the invention can be used to stamp a detergent bar comprising a surface active agent which comprises substantially soap or a synthetic detergent or a mixture of soap and synthetic detergent. It finds particular application in the stamping of soft and/or tacky detergent and/or mild bars which contain synthetic surfactants, translucent and transparent soap bars having a reduced fatty matter content, for example, in the range 63- 78% wt . with respect to the total bar weight, and those bars containing skin beneficial agents such as humectants, polyols, oils, fatty acids and fatty alcohols. According to a further aspect of the invention there is provided a process for stamping a detergent bar comprising i) forming an elastomeric coating on a die having an edge for delaying tearing of the elastomeric coating; ii) feeding a detergent bar composition to the die of step i; iii) stamping the composition in the die to form a stamped bar; and iv) releasing the bar from the die, preferably in a manner such that a surface decoration is applied to the bar in an easily reproducible manner. Preferably, the elastomeric coating is bonded to the die stamping surface by mechanical and/or chemical means to increase the adhesion between the die and the coating. It is particularly preferred that the edge used to relieve the stress on the elastomeric coating is itself also partly coated with the elastomeric coating. Advantageously, the coating on the edge is thinner than that present on the bar stamping surface of the die. For example, it may range from 1 to 200 microns, especially from 10 to 50 microns. Referring to the figures in detail. Figure 1 shows die halves 10 comprised of individual dies 12. Each die half is provided, on the bar stamping surface 14, with an elastomeric coating 16 respectively (Fig. 4). Elastomeric coating is also provided on the non-stamping surface 18 of the die halves. One die half is provided with a logo 20 on the bar stamping surface. (In some cases both die halves will incorporate a logo) . This is also coated with an elastomeric coating. Die 12 includes metal edge 30 which projects inwardly from the upper pheriphery of the detergent bar cavity 32. As seen in Fig. 4, edge 30 covers the upper edge of the generally vertically ascending section of the coating 16. Preferably, surface 34 of edge 30 which faces the cavity 32 is at least flush with the surface 36 of elastomer 16 which likewise faces cavity 32. Still more preferred is an arrangement wherein edge 34 of metal edge 30 extends slightly beyond the surface 36 of coating 16. Using the edge or flange of the invention, the die is constructed so that the soft elastomer near the cutting edge is not damaged when the die cuts into the soap. This is accomplished by shielding the soft elastomer near the die edge with a rigid and stronger material. The shielding acts like an umbrella around the die edge slightly over lapping the soft elastomer preferably by approximately .001" to .015". This protects the soft material from excess forces in shear and tension which would accelerate damage to the soft material and render the die inoperative. While the stress relieving edge of the die has been described as being metal, this will generally depend on the material from which the die is made. Typically, the edge will be same as the material from which the die is made. However the edge will typically be fairly rigid to protect the elastomer. The invention may be used with conventional stamping equipment, such as Binacchi USN 100. Preferablly the edge is undercut giving a greater mechanical key. EXAMPLE A range of die halves are manufactured in carbon steel and spark eroded to a range of surface roughness values (Ra) degreased with acetone, treated with a primer and then coated with a range of elastomeric materials. A series of brass die halves are also used in the examples. Similarly, these are degreased with acetone, treated with a primer and then coated. Elastomeric coatings are formed from polyurethane. The polyurethane is prepared starting with a) Andur 80-5AP - Polyether based liquid isocyanated-terminated prepolymer. Supplied by: Anderson Development Co. 1415 E Michigan Street Adrian, MI 49221-3499; and is cured using b) Voronal 234-630 curative - Triol Supplied by: Dow Chemical Co 2040 Dow Center Midland, MI 48674 The bar compositions used in the examples are as follows Formulation A % wt . Anhydrous tallow soap 52.3 Anhydrous coconut soap 29.9 Coconut fatty acid 5.2 Water and minors to 100 Formulation B % wt . Sodium cocyl isethionate 27.00 Cocoamidopropyl betaine 5.00 Polyethylene glycol, M.Wt. 33.12 Fatty acid 11.00 Sodium stearate 5.00 Water + minors to 100 Formulation C % wt . Sodium cocyl isethionate 49.78 82/18 Soap 8.31 Sodium Stearate 2.98 Alkyl Benzene sulphonate 2.02 Stearic acid 20.15 Coco fatty acid 3.08 Sodium Isethionate 4.68 Water + minors to 100 The dies contain metal edges as illustrated at 34 in Fig 4. It is found that the metal edges relieve the stress on the upper pheriphery of the cavity during stamping and result in a longer useful life for the coating. It should be understood, of course, that the specific forms of the invention herein illustrated and described are intended to be representative only as certain changes may be made therein without departing from the clear teachings of the disclosure. Accordingly, reference should be made to the following appended claims in determining the full scope of the invention.

Claims: CLAIMS 1. A device for stamping a substrate, the device being of the type comprising a die having at least one substrate stamping surface covered with an elastomeric coating, the device being characterised in that the substrate stamping surface is provided with a rim having a rigidity greater than that of the elastomeric coating. 2. A device as claimed in claim 1 wherein the rim is metal . 3. A device as claimed in claims 1 or 2 wherein the rim is formed from the material of the die. 4. A device as claimed in claim 1, 2 or 3 in which the rim comprises a elastomeric coating thinner than that coating the die. 5. A device as claimed in any preceding claim in which the rim forms a flange which lies flush with or extends beyond, a leading edge of the elastomeric coating of the die. 6. The device as claimed in any of claims 1 to 5 in which the elastomeric coating is selected from elastomers, rubbers, thermoplastic elastomers and copolymers and blends thereof. 7. The device as claimed in any preceding claim in which the die comprises a rigid material selected from metals and their alloys, thermoplastic and thermosetting resins, hard cast polyurethanes, ceramics, composites and laminates. 8. A process for forming an elastomeric coating on a device for stamping a substrate, comprising the steps of :- i) pretreating the device by chemical and/or mechanical means to form a bonding surface for said elastomeric coating; and ii) applying said elastomeric coating to said device, characterised in that the device is provided with a rim having a rigidity greater than that of the elastomeric coating. 9. A process for stamping a detergent bar comprising the steps of:- i) forming an elastomeric coating on at least one bar stamping surface of a die having a metal rim for reducing stress on said coating; ii) feeding a detergent bar composition to the die of step; iii) stamping the composition in the die to form a stamped bar; and iv) releasing the bar from the die. 10. Use of a device according to any of claims 1 to 7 for stamping a detergent bar.

Inventors: BUZZEO PASQUALE MICHAEL, EDMONDSON BRIAN, HEINZ DANIEL JOHN, STORY EDWARD ROSS, BUZZEO, PASQUALE, MICHAEL, EDMONDSON, BRIAN, HEINZ, DANIEL, JOHN, STORY, EDWARD, ROSS
Applicants: UNILEVER NV, UNILEVER PLC, UNILEVER N.V.
Application year: 1997
Main classification: B30B15
IPC codes: B29C33, B30B15, C08L21, C11D13