Plastic and nonmetallic article shaping or treating: processes – Mechanical shaping or molding to form or reform shaped article – To produce composite – plural part or multilayered article
Patent
1995-03-01
1997-11-25
Ortiz, Angela Y.
Plastic and nonmetallic article shaping or treating: processes
Mechanical shaping or molding to form or reform shaped article
To produce composite, plural part or multilayered article
264266, 264322, 264325, B29C 5102, B29C 5112
Patent
active
056908806
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a process for the production by controlled molding-thermoforming of parts of plastic material resistent to friction and to flow.
A certain number of plastic material parts such as prosthetic joints or mechanical parts such as pinion, guide plate, roller, bearing, and screw, etc. have a limited lifetime because of the phenomena of material flow and wear connected with friction arising during use of said parts. This problem is particularly crucial in the case of orthopaedic implants, principally hip and knee prostheses which are made of metal with the interposition of a wear member. High molecular weight polyethylene (HMWPE) is particularly used for this production. However, this material, although having exceptional friction and anti-wear properties, cannot be used in conventional techniques applied to polymers, such as injection. This is why most of the implants are produced by machining. Unfortunately, these machined productions have the great drawback of not having an excellent surface, as they have machining ridges. Moreover, the poor coefficient and roughness (Ra) give rise to wear debris which the body throws off by the generation of macrophages which give rise to enzymes. These latter then attack the prosthesis by dissolving in particular the sealing materials, thereby giving rise to destruction of the connection of this latter and requiring a new surgical intervention.
As a result, to improve the mechanical properties of these parts subjected to large mechanical stresses, there has already been proposed a certain number of solutions. There is thus known, from FR-A-2.578.780, a process which permits producing plastic material parts by forging, orienting the chains of molecules of said used material, which is generally a high molecular weight material, and thus improving significantly the mechanical properties of said parts, particularly the resistance to flow. However, the process described in this patent does not permit improving substantially the resistance to flow, to wear and the obtention of elastic memory of the homogeneous material guaranteeing dimensional stability. The lack of resistance to wear and flow therefore does not permit increasing substantially the lifetime of the implants by this process.
A process for the production of parts of plastic material by controlled molding-thermoforming is also described in U.S. Pat. No. 4,459,092. This patent comprises the introduction into a mold cavity of a blank of plastic material P1 and of a plastic material P2 at a temperature near or equal to the melting temperature of P2, the part being formed in the mold by means of a punch movable in said cavity and by the injection pressure of the plastic material P2. The blank P1 is subjected to no thermal treatment.
The object of the present invention is therefore to provide a process for the production of parts such as prosthetic articulations, various mechanical parts, etc., having substantially improved mechanical properties particularly as to resistance to flow, to tension, to wear and to fatigue without having substantial shrinkage during production.
The present invention thus relates to a process for the production of plastic material parts by controlled molding-thermoforming, characterized in that there is introduced into the interior of a mold cavity at least one blank P1 of plastic material first heated so as to have different temperature regions, forming the part in the mold by means of a punch movable in said mold cavity and/or by the injection pressure of a plastic material P2 introduced into said cavity at a temperature near or equal to the melting temperature.
According to a preferred embodiment of the invention, the plastic material P2 which has a molecular weight equal to or less than P1 is introduced by injection and the punch is displaced so that it comes preferably into contact with the blank of material thereby causing the blank and the injected plastic material to match the shape of the closed matrix constituted by the walls of the mold cavity forming t
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