Sub-ambient pressure morphology control process for use in molding extruded polymer foams, and parts produced therefrom

Abstract

A method of sub-ambient pressure processing of blow-molded polymer foams and skin-over-foam sandwich panel configurations for lightweight components having improved structural properties. The method can create either skinned or un-skinned foams that offer smooth interior and exterior surfaces, zero or controlled surface porosity, skins of pre-defined thickness, and foam cells that are expanded and oriented normal to the material plane, effectively spherical or polyhedral in nature, and offering improved bending and compressive strength.

Claims

1. A method of sub-ambient pressure processing of blow-molding polymer foams and skin-over-foam sandwich panel like wall structures comprising the steps of: extruding a piece of parison from a polymer material into a mold; inflating the parison and offer a degree of internal cooling to begin forming an internal skin; forming an in-mold vacuum level and gently draw the polymer material tightly to a mold surface; applying an internally applied pressure relative to the in-mold vacuum level maintained at the surface of said mold; maintaining said mold for a pre-determined hold time under vent vacuum and pre-blow to establish a part having a definition and inner and outer skins; applying a cavity interior vacuum at a predefined level to a cavity interior sufficient to expand a warm foam core; regulating said cavity interior vacuum inside said cavity interior for a pre-determined period to achieve a desired foam expansion; venting said cavity interior to relieve said cavity interior vacuum; applying a positive pressure to said cavity interior; turning off both said positive pressure and said in-mold vent vacuum; extracting said part from said mold.

2. The method of sub-ambient pressure processing of blow-molding according to claim 1 wherein the step of maintaining said mold for a pre-determined hold time includes a delay of 0.1 to several seconds after venting to allow the outer skin to more fully form while not internally compressing foam cells.

3. The method of sub-ambient pressure processing of blow-molding according to claim 1 wherein said step of applying a vacuum at a predefined level sufficient to expand a warm foam core is about a 1-2 Hg differential of vacuum levels from inside to outside the part, such that there remains a sufficient net outward force on the plastic to keep it drawn to the mold surface.

4. The method of sub-ambient pressure processing of blow-molding according to claim 1 wherein said step of maintaining a vacuum inside the mold for a pre-determined period of about 10-30 seconds, with the hold time selected to sufficiently expand a given quantity of resin to a desired final thickness.

5. The method of sub-ambient pressure processing of blow-molding according to claim 1 wherein said step of applying positive pressure to the cavity interior is to establish contact with the mold wall for improved cooling until the part is sufficiently cooled to demold.

6. The method of sub-ambient pressure processing of blow-molding according to claim 1 wherein said part has a foam structure with auxetic properties.

7. The method of sub-ambient pressure processing of blow-molding according to claim 1 including the step of nucleating new cells from gas laden resin.

8. The method of sub-ambient pressure processing of blow-molding according to claim 1 wherein said step of regulating a vacuum inside the mold for a pre-determined period achieves a foam expansion of up to three times an original thickness.

9. The method of sub-ambient pressure processing of blow-molding according to claim 1 including the step of creating a foam structure with an open celled foam core covered by closed cell foam and skins on the inner and outer layers.

10. The method of sub-ambient pressure processing of blow-molding according to claim 1 including the step of creating a foam structure with a density gradient variation of 20 to 80 percent between adjoining cords.

11. The method of sub-ambient pressure processing of blow-molding according to claim 1 including the step of creating a part containing regions exhibiting expansion ratio differences from 20 percent to 100 percent.

12. The method of sub-ambient pressure processing of blow-molding according to claim 1 wherein said step of applying an internally applied vacuum relative to the vacuum level maintained at the surface of the mold expands flattened foam cells in the wall into generally spherical cells that are elongated normal to the material plane relative to their initial flattened state.

13. The method of sub-ambient pressure processing of blow-molding according to claim 1 wherein said step of applying an internally applied vacuum relative to the vacuum level maintained at the surface of the mold expands flattened foam cells in the wall into generally polyhedral cells that are elongated normal to the material plane relative to their initial flattened state.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is an enlarged photograph of a cross section of a flattened cell structure of a prior art product made from a blow molded foam technology featuring a proprietary, improved resin system, with an expansion ratio of 1.76 times and 2.1 mm thickness;

(2) FIG. 2 is an enlarged photograph of a cross section of a product made from current non-enhanced HDPE resin and commercially available chemical foaming system under conventional blow molding conditions, without sub-ambient pressure processing, and having an expansion ratio of 1.3 times and a 1.6 mm thickness;

(3) FIG. 3 is an enlarged photograph of a cross section of a product made from the instant invention blow molding foam technology using a thick internal film forming variant of the sub-ambient pressure process, yielding an expansion ratio of 1.68 times and at a 1.7 mm thickness;

(4) FIG. 4 is an enlarged photograph of a cross section of a product made from the instant invention blow molding foam technology using a minimal film forming variant of the sub-ambient pressure process yielding an expansion ratio of 2.67 times with a 2.5 mm thickness;

(5) FIG. 5 is a process flow diagram of the preferred embodiment with dual pin/needle flow-through cooling;

(6) FIG. 6 is a state chart of the process cycles and timing used to implement the basic sub-ambient pressure process;

(7) FIG. 7 is a schematic of the simplest and most basic form of sub-ambient pressure cycling apparatus annotated to refer to control elements in FIG. 6;

(8) FIG. 8 is a drawing of the right hand demister duct formed from the present blow mold;

(9) FIG. 9 is a drawing of the left hand demister duct formed from the present blow mold;

(10) FIG. 10 is a flow schematic of the sub-ambient pressure process steps;

(11) FIG. 11A is a pictorial of a parison formation;

(12) FIG. 11B is a pictorial of a parison formation;

(13) FIG. 12 is a picture of a cross section of cavity wall with a cryo-fractured sample showing sparse flattened cell structure for Example 1;

(14) FIG. 13 is an illustration of a flattened cell structure for Example 1;

(15) FIG. 14 is a picture of a cross section of cryo-fractured sample showing predominantly spherical or polyhedral cell structure for Example 2;

(16) FIG. 15 is an illustration of a cavity wall with a predominantly spherical or polyhedral cell structure for Example 2;

(17) FIG. 16 is a picture of a cross section of cryo-fractured sample showing spherical cell structure in the center, but with a solid internal skin formed as a result of process timing for Example 3;

(18) FIG. 17 is an illustration of a cavity wall with a spherical cell structure with a thick internal skin for Example 3;

(19) FIG. 18 is a picture of a cross section of a razor cut sample revealing dense, slightly foamed outer skins sandwiching a highly foamed, low density core for Example 4;

(20) FIG. 19 is a picture of a cross section of a razor cut natural colored foam product with dyed cells to reveal a skin over foam sandwich structure. This sample has a specific gravity of 0.34, an expansion ratio of about 2.8 and is about 3.5 mm thick for Example 4;

(21) FIG. 20 is an illustration of a cavity wall with a thick outer skin and thick internal skin for Example 4;

(22) FIG. 21 is a picture of a cross section of a razor cut duct wall section revealing a transition from foamed interior wall, to that of a solid skin at the location where local heat was applied on the mold surface for Example 5;

(23) FIG. 22 is an illustration of a cavity wall with a stiff skin-over foam region for Example 5;

(24) FIG. 23 is an illustration of a cavity wall with soft and compliant areas for Example 5;

(25) FIG. 24 is an illustration of a cavity wall with a local exterior area with fully expanded foam reinforced by a thick interior skin for Example 5;

(26) FIG. 25 is a picture of a cross section of a cryo-fractured sample revealing the presence of an open-cell network structure in the center of the wall, surrounded by closed cell structures with skins both above and below the centerline for Example 6;

(27) FIG. 26 is an illustration of a cavity wall with cells having auxetic properties for Example 6;

(28) FIG. 27 is an illustration of a cavity wall with an open cell network comprising both the core and innermost layer of the total wall structure for Example 6;

(29) FIG. 28 is a graph presenting currently known or experimentally derived capabilities for foamed thickness vs. nominal thickness without foam using chemical foaming agents.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

(30) A detailed embodiment of the instant invention is disclosed herein, however, it is to be understood that the disclosed embodiment is merely exemplary of the invention, which may be embodied in various forms. Therefore, specific functional and structural details disclosed herein are not to be interpreted as limiting, but merely as a basis for the claims and as a representation basis for teaching one skilled in the art to variously employ the present invention in virtually any appropriately detailed structure.

(31) Sub-ambient pressure processing of blow-molded polymer foams and skin-over-foam sandwich panel configurations for lightweight components with improved structural properties. A sub-ambient pressure processing method has been developed that provides a highly configurable method for creating blow-molded articles comprised of polymers and foamed by the action of either chemical or physical foaming methods. This process can create either skinned or un-skinned foams that offer smooth interior and exterior surfaces, zero or controlled surface porosity, skins of pre-defined thickness, and foam cells that are expanded and oriented normal to the material plane, effectively spherical in nature, and offering improved bending, and compressive strength. The current development is focused on hollow members such as automotive HVAC ducting, but could easily extend to air induction ducting, underbody shielding, and other blow-moldable polymer products.

(32) The inventive process proposed herein differs from conventional foam blow molding processes in that it employs a pressure-vacuum cycle to first form the basic shape of the part, then expanding the hitherto flattened foam cells in the still molten material wall into generally spherical or polyhedral cells or cells that are elongated normal to the material plane. As the foam solidifies under internally expansive forces, under tension, the normally disposed cell walls are pre-strained and solidified in an expanded, maximum volume state rather than buckled, offering significant stiffening of the structure as well as expansion of the collective cellular volume of the part, significantly lowering the density of the part. The process is variable and can be used to specifically tailor the properties of the cellular foam and can create a solid outer skin on both inner and outer surfaces of the part, thus providing a sandwich panel with smooth, nonporous walls, and offering the superior structural properties commonly associated with foam-cored sandwich panels.

(33) Specifically, this process offers the following advances over current technology:

(34) A) Sub-ambient pressure processing reverses the trend of foam cell collapse due to in-mold pressurization, intra-cellular pressure decay, and forming induced cell elongation. It enables the re-expansion of cells from partially to completely, with subsequent volume inflation of existing cells, and creation of new cells as remaining gas-laden polymer strives to reach equilibrium with the newly imposed environment; thus reducing part density far beyond conventional approaches. Expansion ratios of up to 3 times have been observed with this process employed on resin and chemical foaming systems known to achieve expansion ratios of barely 1.37 under standard HDPE blow molding practiceswith no externally applied internal blow pressures after the mold halves have closed. Positive internal blow pressures of any magnitude are known to further reduce achievable expansion ratios.

(35) B) With appropriate nucleating agents, this process has been observed to achieve excellent foam densities on prior foamed regrind with no added chemical foaming agents. Very small additional chemical or physical foaming agents in such systems yields impressive foam expansion with this system when appropriate cell nucleation conditions or additives are present.

(36) C) The system, developed initially for polyolefins such as HDPE and PP, can be used with many polymer types and blends, foaming agents, cell nucleators, and reinforcements; each offering specialized properties.

(37) D) Allows the minimal use of foaming additives since foam cells are maximized with the process rather than degraded. This can reduce foaming additive usage by 50% or more.

(38) E) Lower foaming additive amounts, resulting in lower initial extruded cell volume, improves the material properties and formability of the resin system, and preserving the predominantly closed-cell nature of the foam, thus allowing for more difficult geometries to be formed, since the foam will be expanded after the part is formed.

(39) F) Owing to the expanded processing window afforded by sub-ambient pressure processing of standard resins, the approach can offset, or eliminate entirely, the need for exotic, expensive, or problematic material enhancements, such as long chain branching, cross linking, co-polymers, rubber-phase additives, nanoclays, etc.

(40) G) Elongates and pre-stresses the polymer cell walls both generally and specifically normal to the material plane, thus imposing significant improvements in stiffness. This elongation and alignment of the polymer also preferentially orients mineral or other micro- or nano-reinforcements for optimal reinforcement.

(41) H) Many varying implementations of either locally applied or whole-part rapid mold heating and cooling technologies can be applied to this process to impose the thermal boundary conditions needed by this process for both foam structure manipulation and optimal cooling for reduced cycle times. Such methods include, but are not limited to induction, steam, oil, electric heater cartridges, infrared, internal hot gas injection, etc.

(42) I) Process temperatures, stage timing, parison thickness, and vacuum and pressure levels can be used to achieve both the ideal cellular structure and the presence and thickness of the skins on either or both material faces. Skin thicknesses from film thin to millimeter scale are possible. Open-celled and auxetic cellular structures are also possible with exposure to properly staged in-mold sub- and super-ambient pressure conditions.

(43) J) The process, owing to the possibility of forming skins, can produce interior surfaces of extremely smooth nature. This is important for efficient fluid handling.

(44) K) The process can be configured to produce a foam density gradient normal to the material plane, by virtue of temperature control, stage timing, and pressure-vacuum levels employed.

(45) L) The sandwich structure imposes varying speeds of sound from a skinned surface, through the foamed core, to the opposite skin face; creating a series of impedance mismatches. This is beneficial for acoustic and structural vibration performance.

(46) M) The skinned foam core comprises a sandwich panel which is known to offer superior weight-normalized structural performance vs. simply foamed or solid materials.

(47) N) The process can dramatically expand a polymer foam system, offering superior thermal insulation properties.

(48) O) The process allows for localized foam expansion with local application of heat through the mold, for example, an extra thick foam pad could be formed in place by spot-heating, or selectively insulating against polymer-mold heat transfer, a specific area of the mold during the foam expansion stage.

(49) P) Parting lines, frequently known as pinch-offs in blow-molding terminology, tend to be reinforced as the foam is expanded volumetrically into the interior of the part cavity. As the resin moves inward, it competes for space as the internal surface area decreases, causing increased polymer chain entanglement at the interface as the two fronts converge and intermingle.

(50) Q) This process, although developed initially on a single extruder, monolayer blow molding system, will work with co-extrusion blow molding systems as well as with single or multi-sheet thermoforming, direct extruded sheet forming, and compression molding.

(51) FIG. 1 is an enlarged photograph of a flattened cell structure using an existing blow molded foam technology referred to by U.S. Pat. Nos. 8,517,059 and 8,535,589 and which reveals an expansion ratio of 1.76 times and 2.1 mm thickness.

(52) FIG. 2 is an enlarged photograph of a product made from current resin and chemical foaming system under conventional blow molding, without sub-ambient pressure processing, and having an expansion ratio of 1.3 times and a 1.6 mm thickness. This is similar to what conventional blow molded foam structures would resemble, with flattened and largely collapsed cells.

(53) FIG. 3 is an enlarged photograph of a product made from the instant invention, blow molded foam technology using a thick internal film forming variant of the sub-ambient pressure process, yielding an expansion ratio of 1.68 times and at a 1.7 mm thickness. Notably, a thin skin is formed on one surface and a thick skin is formed on the other, with a steep foam density gradient being formed from each surface to center of the wall's thickness.

(54) FIG. 4 is an enlarged photograph of a product made from the instant invention blow molded foam technology using a minimal film forming variant of the sub-ambient pressure process yielding an expansion ratio of 2.67 times with a 2.5 mm thickness. Note the smooth skins on both top and bottom surfaces over a highly expanded foam core;

(55) FIG. 5 is a process flow diagram of the preferred embodiment with dual blow pin/needle arrangement allowing for regulated, flow-through cooling. Also depicted are the connections for computer programmable operation of valves and regulators to impose the necessary process conditions.

(56) FIG. 6 is a state chart of the process cycles and timing used to implement the simplest and most basic sub-ambient pressure process. Depicted is the pre-blow cycle, internal vacuum cycle, post blow cycle, mold vacuum cycle and blow pin pressure hold cycle;

(57) FIG. 7 is a schematic of the simplest and most basic form of the sub-ambient pressure cycling apparatus employing the logic/timing chart of FIG. 7;

(58) FIG. 8 is a drawing of the right hand demister duct formed from the present blow mold;

(59) FIG. 9 is a drawing of the left hand demister duct formed from the present blow mold;

(60) FIG. 10 is a flow schematic of the sub-ambient pressure process steps;

(61) The sub-ambient pressure process comprises the following steps:

(62) 1. Extruding a parison (10) via the best procedure for the equipment used for the specific resin, foaming agent, and nucleating agent employed. This foamed parison should not be over-foamed, but simply foamed with only lightly expanded cells. Over-foaming will induce cell coalescence, collapse, and weak parison. Thicker parisons, related to die size, die gap, and extrusion rate, will offer improved control over skin thickness and core foam properties.

(63) 2. Use suitable pre-blow, both top and bottom, to gently inflate the parison and offer some degree of internal cooling to begin forming the internal skin, if desired.

(64) 3. As the mold begins to close, turn on both in-mold vacuum at vents and manage top and bottom pre-blow at low pressures and proper flow rates to gently draw the material tightly to the mold surface. The vacuum at the vents can be zoned or moderated to control parison movement, but should otherwise be as complete a vacuum as possible for maximum part definition and retention to the mold surface.

(65) 4. The mold closes completely at a programmed rate that works in conjunction with the vacuum draw through the mold vents and internally applied pressure (relative to the vacuum level maintained at the surface of the mold) to provide for optimal drawing in of the parison onto the mold.

(66) 5. After a pre-determined hold time under vent vacuum and pre-blow to establish part definition and inner and outer skins, vent the pressure from the interior of the formed parts.

(67) 6. A delay of 0.1 to several seconds can be employed after venting to allow outer skin to more fully form while not internally compressing foam cells.

(68) 7. Apply vacuum at a predefined level, as much as needed to expand the warm foam core, being careful to moderate the internal vacuum to prevent it from pulling the plastic article away from the mold surface. A 1-2 Hg differential of vacuum levels from inside to outside the part is suggested as sufficient to both expand the foam and prevent part collapse, but can be varied to adjust to varying resin properties or to a specific mold's tendency to release a part from the surface while forming. Under proper temperature and material property conditions, the timing, ramp-rate and magnitude of internal vacuum exposure can be effectively used to create an open-celled structure inside the core while the inner and outer surfaces remain with solid films or closed-cell structure. If desired, an open-cell structure can also be imposed on the innermost layer.

(69) 8. Hold vacuum inside the mold for a pre-determined period of time to achieve the desired foam expansion. 10-30 seconds appears to be effective for the current product, depending upon desired thickness. Within this stage, all gas cells in the still molten resin will begin to grow at a rate generally in proportion to the temperature of the resin where the cells are located. Even invisible, microscopic cells will grow and become evident; possibly to the extent that new cells can be nucleated from gas-rich regions of un-foamed resin. The extreme sub-ambient pressure, or vacuum, within the part can be maintained either statically or while flowing a suitable cooling medium, such as air, through the interior of the part if internal cooling at this stage is desirable.

(70) 9. Vent the cavity interior to ambient conditions to relieve the vacuum.

(71) 10. Apply positive pressure to the interior of the cavity to help establish solid contact with the mold wall for improved cooling until the part is sufficiently cooled to de-mold. This can also be maintained while flowing a suitable cooling medium, such as air, through the interior to speed cooling, but simply maintained at a higher absolute pressure than that used in step 8 above. If sufficient positive pressure is applied at the right time, while the foam in the core remains in a softened, yet un-molten state, the foam within the core can be cooled in a compressed state, which can result in foam cells with an auxetic structure.

(72) 11. Turn off both internal pressure and vacuum at the in-mold vents and vent to ambient conditions to relieve all pressure forces on the part.

(73) 12. Open the mold and extract the formed and foamed part.

(74) Referring to FIG. 10, the sub-ambient pressure process is defined by the following steps:

(75) extruding a piece of parison from a polymer material (10);

(76) inflating the parison and offer some degree of internal cooling to begin forming an internal skin (12);

(77) forming an in-mold vacuum and gently draw the polymer material tightly to the mold surface (14);

(78) applying an internally applied pressure relative to the vacuum level maintained at the surface of the mold (16);

(79) maintaining said mold for a pre-determined hold time under vent vacuum and pre-blow to establish a part definition and inner and outer skins while venting the pressure from the interior of the formed parts (18); applying a vacuum at a predefined level sufficient to expand a warm foam core (20);

(80) regulating a vacuum inside the mold for a pre-determined period to achieve a desired foam expansion (22);

(81) venting the cavity interior to relieve the vacuum (24);

(82) applying positive pressure to the cavity interior (26);

(83) turning off both positive pressure and in-mold vacuum (28);

(84) extracting the formed and foamed part from the mold (30).

(85) In another embodiment of the invention, maximum possible foaming without rapid mold heating or secondary addition of heat to the parison during foam expansion, with flow-through cooling of hollow part interior comprising the steps of:

(86) a. Parison consisting of blow mold grade pre-foamed 82.5% HMW HDPE regrind, 15% virgin HMW HDPE, with 0.5% carbon black color concentrate and 2% endothermic chemical foaming agent. Specific gravity of base resin blend is 0.96. Parison optimized for an automotive climate control duct, extruded at 420 F sufficient to produce a 1.5 mm thick foamed part.

(87) b. Aluminum mold is configured for at least two blow pins, blow needles, or a combination. Mold is treated with a surface conductivity reducing coating, such as thermal barrier paint or hard-coat anodizing, in all areas where maximum foam expansion is desired. This will reduce heat loss from the parison to the mold before foam expansion can occur and to help balance internal and external cooling rates. Mold is maintained at 80 F. Mold cavity vent vacuum turned on prior to contact with parison. Vacuum set to maximum achievable setting or 29.5 Hg.

(88) c. Parison is extruded from the die tip. Pinch bar closes on bottom of parison to close the parison. Pre-blow through top blow pin at 2-5 PSIG at a sufficient flow rate to properly pre-inflate parison prior to contact with mold. Mold halves move toward close.

(89) d. Mold closes on parison. Hold pressure inside cavity for minimum possible time until parison makes full cavity contact, preferably less than 2 seconds.

(90) e. Turn off blow pin pressure and apply vacuum to cavity through blow pin at maximum achievable vacuum level without pulling the parison off of the cavity walls, usually about 27.5 Hg. Hold vacuum until foam expands and begins to cool and stabilize, usually 10-15 seconds.

(91) f. Pierce formed part with cylinder mounted blow needle or use second blow pin at a remote end of the part to begin flowing high volume/low pressure air through for internal cooling, maintaining at least 25 Hg vacuum inside part for until foam is completely solidified, around 15 seconds.

(92) g. Switch top blow pin from vacuum to freely venting to ambient pressure. Increase blow needle flow rate and pressure sufficient to balance cooling rates between inside and outside the part to reduce warping and shrinkage. Hold for 20 seconds.

(93) h. Turn off mold cavity vent vacuum and normalize pressures. Turn off secondary blow pin or needle airflow and normalize pressures.

(94) i. Extract part from mold. Typical cycle time should be ?60 seconds for 1.5 mm thick foam, which will be expanded such that the density of the part walls are reduced by >50% from non-foamed wall sections.

(95) j. Resulting cell structure is highly foamed with spherical or polyhedral cell shape with a slight gradient in foam expansion from lower on the mold side of the part wall, to higher foam expansion in the center, to slightly lower expansion on the inner surface of the wall. The skins on both inner and outer faces will be thin, yet continuous with exceptionally few to no open cells on the surface. The expansion ratio for the foam will typically be between 2? and 3.5? for such a part, depending on actual resin thickness and the ratio between skin and foam thicknesses in any particular location. As such, the specific gravity will generally fall between 0.48 and 0.27. With forced heating of the parison during foam expansion, or through the use of a highly efficient insulation on the mold surface to retain heat, it is projected that the expansion ratio in such areas will approach 4? with current chemical foaming agents; and perhaps more when using physical foaming methods. Higher foam expansion and resulting thickness will increase cycle times due to inefficient cooling as the foamed resin self-insulates and retains heat longer.

Example 1Minimal Foam in Standard Foam Blow MoldingLow Pressure Pre-Blow Followed by 0.0 PSI Gage Blow Pressure

(96) Referring to FIGS. 12 and 13.

(97) a. Parison consisting of blow mold grade pre-foamed 82.5% HMW HDPE regrind, 15% virgin HMW HDPE, with 0.5% carbon black color concentrate and 2% endothermic chemical foaming agent. Specific gravity of base resin blend is 0.96.

(98) b. Parison optimized for an automotive climate control duct, extruded at 420 F. Aluminum mold temperature is 80 F.

(99) c. Mold cavity vent vacuum turned on prior to contact with parison. Vacuum set to ?29.5 Hg.

(100) d. Pinch bar closes on bottom of parison to close the parison. Pre-blow through top blow pin at 2-5 PSI and 20 CFM to pre-inflate parison prior to contact with mold. Mold halves begin to move toward close.

(101) e. Mold closes on parison. Internal pressure/flow through top blow pin turned off.

(102) f. Turn off mold cavity vent vacuum and normalize pressures.

(103) g. Hold part in mold until cool enough to de-mold. Turn off mold cavity vent vacuum and normalize pressures. Cycle time is approximately 68 seconds total.

(104) h. Resulting cellular structure is sparse, with cells flattened and elongated along the plane perpendicular to the thickness direction to the wall.

(105) i. The average specific gravity of the resulting material is 0.72, for an expansion ratio of ?1.33.

Example 2Highly Expanded Foam

(106) Referring to FIGS. 14 and 15.

(107) a. Parison consisting of blow mold grade pre-foamed 82.5% HMW HDPE regrind, 15% virgin HMW HDPE, with 0.5% carbon black color concentrate and 2% endothermic chemical foaming agent. Specific gravity of base resin blend is 0.96.

(108) b. Parison optimized for an automotive climate control duct, extruded at 420 F. Aluminum mold temperature is 80 F.

(109) c. Mold cavity vent vacuum turned on prior to contact with parison. Vacuum set to ?29.5 Hg.

(110) d. Pinch bar closes on bottom of parison to close the parison. Pre-blow through top blow pin at 2-5 PSI and 20 CFM to pre-inflate parison prior to contact with mold. Mold halves begin to move toward close.

(111) e. Mold closes on parison. Internal pressure/flow through top blow pin turned off for 2 seconds.

(112) f. Vacuum at a level of ?20 Hg is applied through the top blow pin to the inside of the part for 30 seconds. Top blow pin vacuum is turned off and normalized.

(113) g. Hold part in mold until cool enough to de-mold. Turn off mold cavity vent vacuum and normalize pressure. Cycle time is approximately 72 seconds.

(114) h. Resulting cellular structure is densely foamed with cells generally spherical or polyhedral in nature, with a mild gradient to slightly flattened cells near the outer (mold-side) surface.

(115) i. The average specific gravity of the resulting material is 0.45, for an expansion ratio of ?2.13.

Example 3Structural Skin on Inside Surface Only

(116) Referring to FIGS. 16 and 17.

(117) a. Parison consisting of blow mold grade pre-foamed 82.5% HMW HDPE regrind, 15% virgin HMW HDPE, with 0.5% carbon black color concentrate and 2% endothermic chemical foaming agent. Specific gravity of base resin blend is 0.96.

(118) b. Parison optimized for an automotive climate control duct, extruded at 420 F. Aluminum mold temperature is 80 F.

(119) c. Mold cavity vent vacuum turned on prior to contact with parison. Vacuum set to ?29.5 Hg.

(120) d. Pinch bar closes on bottom of parison to close the parison. Pre-blow through top blow pin at 2-5 PSI and 20 CFM to pre-inflate parison prior to contact with mold. Mold halves begin to move toward close.

(121) e. Mold closes on parison. Internal pressure/flow through top blow pin turned off for 2 seconds.

(122) f. Vacuum at a level of ?20 Hg is applied through the top blow pin to the inside of the part for 20 seconds. Top blow pin vacuum is turned off and pressure of 30 PSI is applied while innermost layer of the parts inner wall is still molten.

(123) g. Hold part in mold until cool enough to de-mold. Turn off mold cavity vent vacuum and blow pin pressure; normalize pressure. Cycle time is approximately 70 seconds.

(124) h. Resulting cellular structure is densely foamed with cells generally spherical or polyhedral in nature, with a mild gradient to slightly flattened cells near the outer (mold-side) surface but with a thick skin on the inside surface of the part.

(125) i. The average specific gravity of the resulting material is 0.57, for an expansion ratio of ?1.68.

Example 4Skin Over Foam Structure

(126) Referring to FIGS. 18-20.

(127) a. Parison consisting of blow mold grade pre-foamed 82.5% HMW HDPE regrind, 15% virgin HMW HDPE, with 0.5% carbon black color concentrate and 2% endothermic chemical foaming agent. Specific gravity of base resin blend is 0.96.

(128) b. Parison optimized for an automotive climate control duct, extruded at 420 F. Aluminum mold temperature is 80 F.

(129) c. Mold cavity vent vacuum turned on prior to contact with parison. Vacuum set to ?29.5 Hg.

(130) d. Pinch bar closes on bottom of parison to close the parison. Pre-blow through top blow pin at 2-5 PSI and 20 CFM to pre-inflate parison prior to contact with mold. Mold halves begin to move toward close.

(131) e. Mold closes on parison. Internal pressure/flow through top blow pin turned off for 6 seconds.

(132) f. Vacuum at a level of ?20 Hg is applied through the top blow pin to the inside of the part for 20 seconds. Top blow pin vacuum is turned off and pressure of 30 PSI is applied while innermost layer of the parts inner wall is still molten.

(133) g. Hold part in mold until cool enough to de-mold. Turn off mold cavity vent vacuum and blow pin pressure; normalize pressure. Cycle time is approximately 72 seconds.

(134) h. Resulting cellular structure is densely foamed with cells generally spherical or polyhedral in nature, with a mild gradient to slightly flattened cells near the outer (mold-side) surface.

(135) i. The average specific gravity of the resulting material is 0.6, for an expansion ratio of ?1.6.

Example 5Local Skin

(136) Referring to FIGS. 21-24.

(137) a. Parison consisting of blow mold grade pre-foamed 82.5% HMW HDPE regrind, 15% virgin HMW HDPE, with 0.5% carbon black color concentrate and 2% endothermic chemical foaming agent. Specific gravity of base resin blend is 0.96.

(138) b. Parison optimized for an automotive climate control duct, extruded at 420 F. Aluminum mold temperature is 80 F.

(139) c. An insulator or heat source is placed in a local area on the mold surface to preserve resin heat.

(140) d. Mold cavity vent vacuum turned on prior to contact with parison. Vacuum set to ?29.5 Hg.

(141) e. Pinch bar closes on bottom of parison to close the parison. Pre-blow through top blow pin at 2-5 PSI and 20 CFM to pre-inflate parison prior to contact with mold. Mold halves begin to move toward close.

(142) f. Mold closes on parison. Internal pressure/flow through top blow pin turned off for 2 seconds. Local heat source is turned off, allowing cooling to begin.

(143) g. Vacuum at a level of ?20 Hg is applied through the top blow pin to the inside of the part for 20 seconds. Top blow pin vacuum is turned off and normalized. Hold part in mold until cool enough to de-mold. Turn off mold cavity vent vacuum and normalize pressure. Cycle time is approximately 74 seconds.

(144) h. Resulting cellular structure is reversed from typical outcomes, with highly expanded and generally spherical cells on the surface near the mold, instead of flattened, and near the center of the wall. A locally formed skin is formed on the internal surface, which offers a stiff reinforcement placed as needed in a part.

Example 6Open Cell Foam

(145) Referring to FIGS. 25-27.

(146) a. Parison consisting of blow mold grade pre-foamed 82.5% HMW HDPE regrind, 15% virgin HMW HDPE, with 0.5% carbon black color concentrate and 2% endothermic chemical foaming agent. Specific gravity of base resin blend is 0.96. Lower melt strength materials are preferred for generating open cell foams.

(147) b. Parison optimized for an automotive climate control duct, extruded at 420 F. Aluminum mold temperature is 80 F.

(148) c. The top blow pin should be extended and blowing air at 2-5 PSI and <20 CFM during parison extrusion to begin chilling the very inner surface of the parison.

(149) d. Mold cavity vent vacuum turned on prior to contact with parison. Vacuum set to ?29.5 Hg.

(150) e. Pinch bar closes on bottom of parison to close the parison. Pre-blow through top blow pin at 2-5 PSI and 20 CFM to pre-inflate parison prior to contact with mold. Mold halves begin to move toward close.

(151) f. Mold closes on parison. Low internal pressure of 5-20 PSI is maintained for 8-10 seconds to rapidly chill skin layers on inner and outer surfaces.

(152) g. Vacuum at 27.5 Hg is applied through the top blow pin to the inside of the part for 10 seconds to rapidly expand the still warm inner layer sufficient to rupture many of the cell walls in the center layer; creating a network of open cell passages throughout the core of the sample. Hold until the core is in a barely molten state.

(153) h. Top blow pin vacuum is turned and re-pressurized to 20-50 PSI to re-compress the open cell foam, creating a re-entrant, auxetic, structure to many of the cells in the core. Hold part in mold until cool enough to de-mold. Turn off mold cavity vent vacuum and normalize pressure. Cycle time is approximately 68 seconds.

(154) i. The resulting structure yields closed cell outer skins, each of approximately 30% of the wall thickness. The inner 40% of the wall thickness contains a high proportion, 40-80%, open cell network structure. If the part was re-pressurized prior to core solidification, the volumetric compression of the core creates re-entrant cells of an auxetic nature. The open celled structure in the core, especially if made semi-auxetic, can effectively create a semi-coupled, double-walled structure that has acoustic and vibration transmission benefits. The specific gravity of such structures typically range from 0.6 to 0.4, for an effective expansion ratio of 1.6? to 2.5? when producing walls 1.5-2.0 mm thick.

(155) FIG. 28 is a curve that represents the current and experimentally derived foaming capability of the sub-ambient pressure blow molding technique with CO2 bearing endothermic chemical foaming agents. The process is known to work similarly with physical foaming approaches, but the full capability range has not yet been developed.

(156) All patents and publications mentioned in this specification are indicative of the levels of those skilled in the art to which the invention pertains. It is to be understood that while a certain form of the invention is illustrated, it is not to be limited to the specific form or arrangement herein described and shown. It will be apparent to those skilled in the art that various changes may be made without departing from the scope of the invention and the invention is not to be considered limited to what is shown and described in the specification and any drawings/figures included herein.

(157) One skilled in the art will readily appreciate that the present invention is well adapted to carry out the objectives and obtain the ends and advantages mentioned, as well as those inherent therein. The embodiments, methods, procedures and techniques described herein are presently representative of the preferred embodiments, are intended to be exemplary and are not intended as limitations on the scope. Changes therein and other uses will occur to those skilled in the art which are encompassed within the spirit of the invention and are defined by the scope of the appended claims. Although the invention has been described in connection with specific preferred embodiments, it should be understood that the invention as claimed should not be unduly limited to such specific embodiments. Indeed, various modifications of the described modes for carrying out the invention which are obvious to those skilled in the art are intended to be within the scope of the following claims.