METHOD FOR RAPID PRODUCTION OF OBJECTS ANYHOW SHAPED

The present invention relates to a method for rapidly manufacturing items and/or mechanical parts of whatever shape, either in small quantities or as single prototypes. The method is based on a technique known as

"rapid prototyping" and allows a mechanical item to be formed with suitable mechanical features for direct delivery to its end application. The method is equally applicable to the manufacture of prototype articles and articles in small to very small mass productions, advantageously as small as a hundred pieces.

Prior Art

Rapid prototyping encompasses a number of technologies allowing shapes with any degree of complexity to be duplicated within a short time. Of these the SLS and SLA techniques are best known and widely used, whereby items are formed from powder materials and liquid resins, respectively.

After initially receiving widespread acceptance, these techniques have been losing ground on account of the unsatisfactory mechanical standard of the product articles obtained through their use. In fact, conventional materials, as supplied by the manufacturers of equipment for implementing said specific techniques, yield articles having low mechanical characteristics, such that only test models or mock-ups of limited practical usefulness can be provided for just design and composition testing purposes on the host machinery.

Aimed at ameliorating the above unsatisfactory mechanical performance, research work has been focused on the prime material aspect to evaluate compositions (all having the same polymeric base) from a variety of

different sources in an effort to lower the costs for prime materials as well as to improve the mechanical performance.

In connection with the SLS technique, materials filled with aluminium and carbon fibres are currently available which effectively provide both improved ultimate strength and elastic modulus; as for the SLA technique, new resins filled with a ceramic material have seen employment.

State of the Art

SLS is now an adult technique that has been in actual use for nearly 10 years and is still preferred by reason of its versatility and high output, as well as its suitability to yield articles that are more satisfactory from the points of view of accuracy, cost, mechanical properties, and processing time.

This technique provides for the use of a laser source to selectively sinter a polymer matrix, in the form of a powder polymer (Nylon™) , such that unaffected areas can be left out and re-cycling allowed of any uncured (or un-sintered) powder in a given pattern.

Basically, the file that represents three- dimensionally the item to be made is divided into a succession of thin layers parallel to one another, each as the outcome of the intersection of a pair of planes lying a predetermined distance apart, the 3D model and outcome of the intersections being then sintered. Thereafter, a previous layer is covered with another layer of "virgin" powder to be partially sintered at a new intersect region presented by a preset shift along the generation axis (usually the Z-axis) between two successive layers defined in the 3D model (file) .

Thus, the item is formed by lying in a stack all the layers that are to make up the target 3D model.

Recently, in order to raise the mechanical characteristics of items manufactured with the rapid prototyping technique to a more satisfactory level, fillers of various nature have been tried which can improve the elastic modulus and the properties of the resultant (sintered) polymeric matrix, such as micro- balls of aluminium and carbon fibres. Thus, a starting elastic modulus of the polymer up to three times higher could be achieved and the static mechanical characteristics improved significantly.

There remain to be overcome such problems as a depressed ability to be processed compared to pure polymer, uneven distribution of the characteristics, and reduced recycle ability of excess powders, which are typically encountered with materials so filled.

The state of the art provides no other methods or materials than those mentioned above, wherein the rapid prototyping technique can be employed to turn out a product article which is not only useful for the purpose of ascertaining aesthetic and functional suitability, pattern-making for vacuum forming, providing prototypes for assembly and snap-engagement trials, preparing masters for silicone replication, etc., but also to make items having high mechanical characteristics, such as a high elastic modulus, high shock resistance, and dimensional stability.

Brief Description

The state of the art admits of major improvements as to the possibility of providing a novel method of manufacturing three-dimensional items with suitable

mechanical characteristics for their intended use; that is, the possibility of improving on the mechanical characteristics currently achieved through the use of powder polymeric materials incorporating fillers in a conventional rapid prototyping process, in particular by the method of sintering (curing) powder layers (known as SLS) .

An additional object is to provide a novel way of constructing and stiffening three-dimensional items to improve their mechanical characteristics from the prime material.

Another object is to provide a stiffening structure which is easily applied and at once affords short- and long-term effectiveness, i.e. would not have its mechanical characteristics lessened at some future time and would be immune to environmental downgrading.

The technical problem that rises from the foregoing is to provide a method for improving the mechanical characteristics of items produced by the rapid prototyping technique, which can be easily implemented, and is effectual and cost-effective.

A further aspect of the invention is defining the shape of the surface coating to be applied so as to meet the requirements of an economical manufacture and of target mechanical characteristics.

Another technical problem is to provide a coating structure for the item obtained by rapid prototyping while retaining the shape of the item even in the presence of a coating designed to improve its mechanical characteristics; that is, to provide for a structural firmness and a thickness that may be large, without altering the shape of the item.

The invention solves the above technical problem by providing a method for rapidly manufacturing items and/or mechanical parts, including items yielded by a conventional rapid prototyping process, from a powder polymeric material, possibly incorporating fillers, by a process of sintering (curing) powder layers, characterized in that it comprises a first step of smoothing the item surface to a predetermined degree of surface roughness, and a second step of applying, to the item surface, a coating which will bond to the item surface irrespective of the item shape.

In a first embodiment of the method, the first step comprises smoothing the item surface with abrasive paper of grade 180, and the second step comprises applying a coating layer of a ceramic material, as by hot spray application with a torch, to provide an overall thickness of 0.10 to 0.50 mm.

In a second embodiment, the first step comprises smoothing the item surface with fine abrasive paper of grade 240-400, and the second step comprises applying a coating layer of a metal material by electroplating of one or more metals in succession; before the second step is carried out, the item surface is metallized using a metallic paint; and the electroplating step is _^ started at a low current intensity and followed by a settling step at medium current intensity before the final coating step is carried out at standard rate; the thickness of the electroplated layer being in the range of 0.10 to 0.45 mm. In a third embodiment, the first step comprises smoothing the item surface with fine abrasive paper of grade 240-400, and the second step comprises applying a coating layer of fibres which have been pre-impregnated with a resin or are impregnated with a resin during the layer coating; additionally to the fibre/resin layer,

applying a release agent over the whole item; a further resin curing step being carried out with the item held in a bag communicated with a vacuum source; and the bag with the item therein being placed into a resin curing oven.

Detailed description of the invention

Further features of the inventive method for rapidly manufacturing a single item or several items in small or very small mass productions, will be more clearly appreciated by having reference to the following description and the appended claims.

A way of implementing the invention is illustrated by way of example in the accompanying drawings, where: Figure 1 shows an item issuing from a rapid prototyping process of the SLS type and being surface strengthened according to a first embodiment of the invention; Figure 2 shows a product item surface strengthened according to a second embodiment of the invention; Figure 3 shows a product item surface strengthened according to a third embodiment of the invention; and Figure 4 illustrates the step of curing the resin of the vacuum-formed coating in the third embodiment.

Figure 1 shows an item 1 as this exits a rapid prototyping process, lines 2 being used to highlight layers that have been sintered in succession during the rapid prototyping process. In a first embodiment, the outer surface of the item is smoothed at 3, advantageously with abrasive paper of tapering grade to 180. A surface coating is then hot sprayed S with a torch 4. To provide support for the item sections of smaller thickness, one or more fixtures 5 are arranged to

hold up thin regions T of the item 1 on the remote side thereof from the jet S.

Figure 2 shows, additionally to the layers 2 forming the item 1, also the item surface 11 after smoothing with abrasive paper of tapering grade to 240-400. A metallic paint 12 is spray applied on the smoothed surface, being careful to spread it evenly in order to prevent metal particles from penetrating the underlying material of the item being processed. The outer layer 13 is applied by electroplating using an anode rod 14 of electrodes 15 to deliver the current as uniformly as possible across the item surface 16, with the treatment regions distributed according to the spread of the surface area and the presence of any rough spots thereon. This will urge the applied layer to level itself, as is typical in galvanizing processes.

Figure 3 shows, additionally to the item 1 and the layers 2 forming it, the item surface 11 after smoothing with abrasive paper of tapering grade to 240-400. The smoothed surface is coated a resin 17 for impregnating reinforcement fibres 18, where the latter have not been pre-impregnated with the resin. Alternatively, an impregnation resin 19 may be applied later to the fibre layer 18. Finally, a thin release layer 20 is applied.

The impregnation resin is cured in an oven under a vacuum, as shown in Figure 4 by way of example. The work-piece 21, as prepared with the resin/fibre reinforcement layers shown in the preceding Figure, is inserted into a bag 22 equipped with a vent 23 for drawing a vacuum V therein. On completion of the vacuum- drawing step, the vacuum is advantageously retained by a cut-off valve (not shown) on the bag.

Thus, the resin will be cured leaving no voids between the reinforcing fibres 18 and between the fibres and the smoothed surface 11, thereby to establish the firmest bond possible.

The first embodiment of this invention optionally includes applying a coating by a thermal spray process effective to coat high-modulus materials such as ceramic materials (e.g. alumina) onto the surface of an item produced by the rapid prototyping technique.

The minimum thickness of the coating is on the order of 0.10 to 0.5 mm, advantageously of 0.15 to 0.20 mm, the coating being applied with a handheld torch at the melting temperature of the material, so that the coating can be properly distributed across all the surface features. A thicker coating would also be practicable, within the time limits for subsequent applications of thin layers and if the attendant added cost is acceptable. The sintered material from the SLS process is a substrate forming a good steadfast base, and no peeling or crumbling problems are to be expected in use.

The manual application should, however, be carried out accurately, being careful to produce a substantially even distribution and thickness throughout the surface.

An optimum for the coating is preparing a substrate with a fair amount of residual roughness, i.e. not green from the sintering process, and smoothing it with abrasive paper of grade 180. This initial condition further enhances the bond of the surface coating layer.

Where the coating layer is applied by rapid prototyping on complex item geometries, heat build-ups may occur on the item, especially at the thin portion thereof. When the heat released from the hot spray material is too high, the work-piece may significantly

distort out of its original shape. This is more likely to occur on geometries having very thin regions and is overcome by utilizing a property of the rapid prototyping

(RP) process, and along with the item, fixtures are provided to keep the hind surface true of the side to be coated, thus supporting the surface to be coated throughout; optionally, a variety of fixtures may be arranged to hold up those portions of the item which are most exposed to the heat from the coating application. In this way, i.e. by providing enhanced structural support for the area to be coated, process-induced distortion can be minimized, while by suitably controlling the amount of energy delivered during the coating process, i.e. the amount of heat released from the spray material, an adequate bond can be ensured for the coating.

In addition, said fixtures are, by a specific feature of the rapid prototyping method, made simultaneously with the item to fit close against the item to be supported; such fixtures being provided on either sides of the item, i.e. either sides of the thin wall processed.

The net outcome of this technique is a significant increase in the elastic modulus of the substrate that comprises the rapid prototyping work-piece, especially at the thinner areas thereof, which are likely to develop significant distortion under loading in use.

A second embodiment of this invention provides an alternative method to the first embodiment just described, which results in the formation of a electroplating about 0.1-0.45 mm thick, preferably 0.15-

0.3 mm thick. By this method, even the finest of features on highly complex shapes can be given a coating of a uniform thickness, by virtue of a well-recognized

self-levelling effect of the copper bath. The electroplating may be applied after the matrix is rendered conductive, since the base material comprised of the powder polymer and any filler is insulating in nature and must be metallized before the metal material can be coated by electroplating.

The surface metallization may be carried out using a number of methods, including activation with palladium and chemical copper or nickel plating, silver plating, use of a conductive paint, etc.. In view of the item roughness, which clearly is dependent on the particle size of the powder, where a sintered powder is used, being not inconsiderable, the activation with palladium looks less suitable because any hard-to-reach spots by the mechanical smoothing process may result in activation differences and an incompletely coated shape. An activation with palladium can be employed, however, in the instance of items which are perfectly smooth.

The use of conductive paint is the most practical and reliable way. The items are first smoothed using abrasive paper of tapering grades to 240-400, and the conductive paint is then sprayed on with an airbrush. Heavy paint coating spots should be avoided, for otherwise the suspended metal particles may penetrate through the material at those spots and the organic part build up on the surface, creating a condition of local insulation that is apt to interfere with the electroplating. Advantageously, copper-, nickel- and silver-based paints are used. After the item is given an even coat of the paint, the electroplate metallization is carried out being careful to first have the work-piece activated by dipping it in an acid bath to remove any oxide film that may have formed on its surface.

The coating materials may vary and their thickness is easily controlled.

The coating is to provide structural strength for the item, and accordingly, a minimum thickness advantageously in the range of 0.15 to 0.25 mm will be found appropriate.

The coating may comprise copper (from an acidic copper-plating bath) , which is advantageous because only slightly stressed and easy to grow to 0.2 mm and above; the coating application is self-levelling and therefore inherently adapted to spread uniformly over the item. Highly advantageous is the construction of holding fixtures which can deliver a sufficient amount of electric current to the regions that are most deeply screened off. This will avoid insufficiently coated spots. Care should be taken to prevent such conductive holding fixtures from interacting with the item to tension load it. The electroplating bath will not stress the coated item significantly if the electroplating bath is applied at room temperature.

The coating may also be a nickel plating. This type of coating grants high mechanical strength for the work- piece, definitely higher than that afforded by copper plating, but is attended by fairly high residual stress (despite the bath being sulphamate-based, and having therefore lower residual stresses than other baths) . Furthermore, the coating temperature of about 65°C undermines its integrity where the coating thickness exceeds 50 μm. To overcome this constraint on thickness, a lower coating temperature in the range of 50° to 30 °C, advantageously of 35 °C, may be used in order to preserve the integrity of the layer.

Alternatively, the coating may comprise a copper/nickel plating. This combination allows large coating thicknesses to be obtained at no trade-off for

its ease of application (meaning ease of coating and its integrity) . Then, after an activation step by painting, a first layer of copper is coated to a thickness of 20 to 150 μm, advantageously of 70 to 100 μm, followed by a nickel layer 50 to 150 μm, advantageously 80-100 μm, thick.

These basic layers may then be overlaid with additional layers, e.g. a chrome plating of well recognized hardness. Thus, the coating provides the item with a mechanical protection (due to a boxing-in effect) that will outperform in use any weakening of the bond from working stresses. In this way, "fracture failure" of the part formed by rapid prototyping is also avoided, since before a sudden failure may occur, an affected section of the item is bound to have turned plastic to a large extent.

In addition, the coated item is quite effective to dissipate the heat transferred to the item in use, given the excellent thermal conductivity of the metal layer.

It has been found experimentally that the application of the first coating layer of copper can be carried out by dipping the item as treated with metallic paint in the electroplating bath for 10 minutes at a current density of 0.5 A/dm2 as initial value, followed by a settling step of 5 minutes at 1 A/dm2. Subsequently to this, the rate of electroplating may be in the range of 2.5 A/dm2 up to the moment when the target thickness of the copper layer is attained. Since the "polyamide", i.e. the plastics material serving as prime material in the rapid prototyping process, is recognized to be a somewhat difficult material to have coated, special care must be devoted to the emplacement of the electric supply electrodes 15 to the surface 12 treated with metallic paint. The shape of

the item may include sharp corners or thin edges whereat the coating would be enhanced by an "antenna" effect, resulting in locally augmented thickness of the coat and a distorted item. The uneven coating can be obviated by so placing the electrodes 15 in the specific areas 16 of the surface 12 as to deliver the amount of current from each electrode evenly over any sharp features of the kind of corners or thin edges, thereby to provide a self-levelling coating which is as uniform as possible, with each sharp spot having an electrode placed as close to it as possible. In addition, the electrodes usually are not insulated, and by the end of the treatment, they too will be covered with electroplate coating. The second metal layer of nickel is coated by electroplating much the same way as the copper layer, but with suitably modified process parameters. Thus, the initial stage should have here a duration of 10 minutes at a current density of 0.5 A/dm2, and the following settling stage take 5 minutes at 1 A/dm2. Subsequent to this, the coating rate may be in the range of 2 A/dm2, because it will coat at a faster rate than copper, until the thickness of the nickel layer attains a desired value, which rate may be as high as 4 A/dm2.

A third embodiment of this invention includes, as mentioned above, covering the work-piece with pre- impregnated or non-impregnated carbon fibres, or alternatively, with fibres of another kind to be impregnated with resin at a later time.

The items are first smoothed using abrasive paper of tapering grade to 240-400; then the item is coated on all sides, this being important especially with thin sections because significant distortion may be introduced by the coating resin curing process. That is to say, the curing

stresses in the resin might distort the item if the latter is coated unevenly.

Once the item is coated with fibre and resin, a release film is applied over the coated regions, and the coated item is sealed inside a valve-fitted bag.

The polymeric matrix of the coating is cured in an oven at a suitable temperature for the resin employed, and with the bag under vacuum state. Using an autoclave for the purpose is not recommended because the substrate material employed in the rapid prototyping process would not withstand the pressure therein.

A variety of resins can be used, and their application on the work-piece may be more or less easy. The minimum thickness of the pre-impregnated fibre coating, film, is about 0.2 mm, which is adequate to conform to all the features of the item being coated, the rapid prototyping technique being known for its suitability to form features of highly complex shapes.

The residence time in the oven and curing temperature will depend on the resin selected and the capability of the substrate or starting material of the rapid prototyping process to withstand the process temperature.

This coating grants the item remarkable mechanical strength and stiffness; accordingly, using the item formed by rapid prototyping and coating it to a thickness that may exceed the aforementioned 0.2 mm, since thickness can be increased as desired to suit the geometry and target mechanical characteristics, it being possible to differentiate them as specific functional areas by the application of successive layers. In this way, the item is not tied to the availability of a mould, and carbon fibre or glass fibre items can be made in a short time to highly complex geometries that would add considerably to the cost of a mould.

After the resin of the item coating has cured, excess resin can be removed being careful not to damage the fibres. Finally, the item is varnished with a clear lacquer if the fibre is to be left in view, or painted with colour paints.

Thus, the teachings of this invention enable items to be manufactured by rapid prototyping without the items being under the constraint of the characteristics of the prime material employed, the coating provided contributing enhanced mechanical characteristics as regards ultimate strength and stiffness of the end product. The resultant item can be utilized for better than a "model" or mock-up, become a definite component part of the machinery or application for which it is intended, and have adequate mechanical characteristics.

Thus, the first embodiment of a surface coating of the rapidly prototyped item, thermal spray, provides a high resistance to contact with heat sources that will be retained through a 10-minute time at temperatures as high as 300°-350°C. In addition, its coating is easily carried out both in the respect of thickness and conformability to the item. Finally, by reason of the high surface hardness of the coated material, the applied coating can be further smoothed to a desired roughness without significantly altering the thickness of the finished coating.

Also, the second embodiment of the rapidly prototyped item coating allows duplication of minute details, including undercuts and tight portions. The metal coating thus applyed exhibits high thermal and electrical conductivity and performs same as an item made of metal throughout. It is, moreover, resistant to chemicals likely to damage the base polymer, and this by

virtue of the sealing properties of the electroplate metal coating.

Thus, the third embodiment of the surface coating of the rapidly prototyped item allows the thickness of the coating layer to be controlled accurately by the application of successive fibre layers and the associated curable resin. Each layer can be laid with its fibres oriented for best mechanical strength in specific areas. The outer surface can be easily polished to remove excess resin from the outermost fibre layer. During this operation, care should be exerted not to damage the fibres in the outermost layer.

Advantageously, each of the above-described embodiments of the coating can be varnished for appearance or as required for the end use of the product item.

In practicing the invention, the materials, dimensions, and construction details may depart from those specified hereabove, and still be technical equivalents thereof within the juridical domain of this invention. Thus, although less conveniently, the electric supply electrodes to the electroplating according to the second embodiment, may be insulated to avoid the metal coating, if increased manufacturing and preparation costs associated with a small volume output, and hence an inconsiderable amount of metal coating on the electrodes, can be accepted.

In addition, the fibres and impregnation resins employed in the third embodiment may be variously selected to match target mechanical properties and the cost ceiling for the coating process.

Finally, the rapid manufacturing method as represented by its three embodiments, may be used for larger production volumes than a small series or single items . Rapid prototyping equipment is known in the art which, dependent on item size, can simultaneously produce more than one piece, within the limits of the rapid prototyping equipment own dimensional and curing

(sintering) capabilities. According as the numbers of items to be manufactured increases, the steps of spray coating, by electroplating, or fibre coating the item may be automated for improved economy of the rapid manufacture of more than small quantities of the items, of course without losing sight of the complexity of their shape. The steps of the above-described method are in fact carried out manually, for single or small quantity pieces which often make investing in special manufacturing equipment an unwise choice, to make for lower coating costs.