US 5,324,473 AGranted

Method for molding stress free amorphous and crystalline thermoplastic resins

OwnerFrank J. Baresichinventor

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Abstract

Methods and apparatus for molding a thermoplastic article bring the surface of the mold cavity above the flow stress relaxation temperature of the molten plastic during molding of a part sufficiently long enough to relax the resin at the surface of the molded part and produce negligible residual stress. The surface of the mold cavity is maintained above the flow stress relaxation temperature of the molten plastic as the center of molten plastic in the mold cavity cools from a temperature above the solidification temperature toward the solidification temperature, thereby reducing flow-induced and temperature gradient induced stresses in the solidified mold material of the molded part. The heat of the molten plastic introduced into the mold is used as the source of the heat for heating the surface of the mold cavity. A layer of thermal flow control material about the mold cavity has a mathematical product of thermal conductivity, specific heat and density and appropriate thickness, so that heat from the molten plastic introduced into the mold cavity raises the temperature of the surface of the mold cavity above the flow stress relaxation temperature of the plastic being molded. The methods and apparatus are particularly useful for the molding of amorphous and crystalline resin parts and for rewritable optical data storage discs and optical lenses with minimized birefringence.

Claims10

Dependent claims

A method of molding a thermoplastic article that has very low residual stress comprising the steps of:

(a) providing a mold containing first and second mold portions which are joined together to form a mold cavity in the shape of an article to be molded, each respective mold portion comprising:
(1) one or more non-metallic thermal flow control insulator elements of predetermined thicknesses that form cavity surfaces of a mold, the insulator elements being made of materials that have much smaller mathematical products of thermal conductivity, density, and specific heat than tool steel, preferably one-tenth or less that of tool steel, and durability for use as the cavity surfaces of a product mold, preferably the materials are selected from the group consisting of quartz glass, pyrex glass, sapphire, and polyimide thermoplastic, whereby upon direct contact by molten plastic the insulator elements cause temperatures at cavity surfaces to immediately increase several times as much as they would if a metal mold is used;
(2) dies adjacent to and in thermal communications with the insulator elements, said dies providing structural support to the insulator elements;
(3) thermal means for applying substantially constant temperature control stimuli to said dies;
(b) applying substantially constant temperature control stimuli to the dies throughout the molding cycle, the control stimuli being adjusted until, after many molding cycles the cavity surfaces are at predetermined temperatures that are below the solidifying temperature of the thermoplastic just prior to contact by molten thermoplastic;
(c) increasing temperatures of the cavity surfaces by introducing molten thermoplastic into direct contact with the cavity surfaces of the insulator elements and using heat flowing from the molten plastic into the insulator elements in cooperation with said predetermined temperatures to immediately heat the cavity surfaces to temperatures that relax substantially all flow stresses from the molten thermoplastic in a predetermined short time, preferably temperatures are chosen that relax flow stresses in 3 to 5 percent of the time elapsed from introduction of molten thermoplastic until the mold is opened;
and
(d) after the cavity is filled, using heat flowing from the molten thermoplastic in cooperation with thermal resistance of the insulator elements to hold the cavity surfaces for said predetermined short time at or above temperatures that relax substantially all flow stresses, whereby flow stresses are virtually eliminated throughout a molded thermoplastic article, particularly at and near its surfaces.

5 more elements

A method of molding a crystalline thermoplastic article that has low residual stress and more uniform crystallization comprising the steps of:

(a) providing a mold containing first and second mold portions which are joined together to form a mold cavity in the shape of an article to be molded, each respective mold portion comprising:
(1) one or more non-metallic thermal flow control insulator elements of predetermined thicknesses that form cavity surfaces of a mold, the insulator elements being made of materials that have much smaller mathematical products of thermal conductivity, density, and specific heat than tool steel, preferably one-tenth or less that of tool steel, and durability for use as the cavity surfaces of a production mold, preferably the materials are selected from the group consisting of quartz glass, pyrex glass, sapphire, and polyimide thermoplastic, whereby upon direct contact by molten plastic the insulator elements cause temperatures at cavity surfaces to immediately increase several times as much as they would if a metal mold is used;
(2) dies adjacent to and in thermal communications with the insulator elements, said dies providing structural support to the insulator elements;
(3) thermal means for applying substantially constant temperature control stimuli to said dies;
(b) applying substantially constant temperature control stimuli to the dies throughout the molding cycle, the control stimuli being adjusted until, after many molding cycles the cavity surfaces are at predetermined temperatures that are below the solidifying temperature of the crystalline thermoplastic just prior to contact by molten crystalline thermoplastic;
(c) increasing temperatures of the cavity surfaces of the mold by introducing molten crystalline thermoplastic into direct contact with the cavity surfaces of the insulator elements and using heat flowing from the molten plastic into the insulator elements in cooperation with said predetermined temperature to immediately heat said cavity surfaces of the mold above the crystallization temperatures of the molten crystalline thermoplastic;
(d) after the cavity is filled, using heat flowing from the molten crystalline thermoplastic in cooperation with thermal resistance of the insulator elements to hold the cavity surfaces above the crystallization temperature of said crystalline thermoplastic for a period of time sufficient to relax substantially all flow stresses, whereby flow stresses are virtually eliminated throughout a molded crystalline thermoplastic article, particularly at and near the surface;
and
(e) after the flow stresses are relaxes, using thermal resistances of the insulator elements of said predetermined thicknesses in cooperation with heat flow from the molten crystalline thermoplastic to cause the cavity surfaces to cool slowly while the molten crystalline thermoplastic that is not in contact with the cavity surfaces cools faster thereby greatly reducing temperature gradients through the molten crystalline thermoplastic and also causing it to gradually cool down through the crystallization temperature range of said crystalline thermoplastic, whereby more uniform crystallization is achieved throughout a molded crystalline thermoplastic article particularly at and near the surface.

6 more elements

Dependencies are inferred from the claim text.

Facts

Application: 07/709,639Filed 3 Jun 1991 · continuation in part
Priority: 6 May 1988
Granted: 28 Jun 1994
Term ends: Not estimatedThe source records don’t support an estimate.

Classification8 codes

B29C 45/00
Injection moulding, i.e. forcing the required volume of moulding material through a
B29C 45/2642
With mould wall parts provided with fine grooves or impressions, e.g. for record discs: heating or cooling means therefor
B29C 45/37
Moulds: mould cavity walls , i.e. the inner surface forming the mould cavity, e.g. linings
B29C 45/73
Heating or cooling: of the mould