CA2187123A1

CA2187123A1 - Mold made of porous concrete

Info

Publication number: CA2187123A1
Application number: CA 2187123
Authority: CA
Inventors: Pierre-Claude Aitcin; Eric Dallaire; Claude Poulin
Original assignee: Universite de Sherbrooke
Current assignee: Universite de Sherbrooke
Priority date: 1996-10-04
Filing date: 1996-10-04
Publication date: 1998-04-04

Abstract

For thermoforming a sheet of plastic material, a mold is made of porous concrete. More specifically, the mold comprises a body of porous concrete having a mold surface section and a second surface section. An impervious wall made of high performance concrete is applied to the second surface section of the body of porous concrete for sealing this second surface section. A duct is formed through the high performance concrete of the impervious wall and is connectable to a vacuum equipment for sucking air through the mold surface section, the porous concrete of the body and the duct in view of applying a heated sheet of plastic material to the mold surface section of the porous concrete body. Preferably, the body of porous concrete comprises a core of concrete having a higher porosity, and an outer layer of concrete having a lower porosity in view of providing a smoother mold surface section.

Description

21~7123 MOLD MADE OF POROUS CONCRETE

BACKGROUND OF THE INVENTION

1. Field of the invention:

The present invention relates to a mold made of porous concrete, in particular but not exclusively for thermoforming a heated sheet of plastic material through a "vacuum forming" process.

2. Brief description of the prior art:

nVacuum forming" molds are usually made of epoxy or metal, generally aluminium, steel or stainless steel.

A drawback of metal mold is their high cost. Indeed, fabrication of such a mold requires time-consuming and expensive complex machining of a piece of metal.

21~7123 A drawback of less costly epoxy mold is their thermal instability when they are operated at high temperature. Depending on the temperature of operation, they cannot be used more than 10-50 times.

OBJECTS OF THF INVFNTION

An object of the present invention is to provide a mold made of a material capable of being molded to greatly simplify the fabrication of the mold while eliminating the drawback of epoxy.

Another object of the present invention is to provide a mold made of porous material and suitable for thermoforming a heated sheet of plastic material through a "vacuum forming" process.

A further object of the present invention is to provide a mold made of porous concrete.

SUMMARY OF THE INVFNTION

More particularly, in accordance with the present invention, there is provided a porous concrete mold, comprising (a) a body of porous concrete having 2l87l23 a mold surface section and a second surface section, (b) an impervious wall means mounted to the body of porous concrete for sealing the second surface section, the impervious wall means comprising an outer surface, and (c) a duct means extending through the impervious wall means from the outer surface of the impervious wall means to the porous concrete of the body, the duct means being connectable to a vacuum equipment for sucking air through the mold surface section, the porous concrete of the body and the duct means.

When the porous concrete mold is used in a "vacuum forming/' process, sucking of air through the mold surface section will draw and apply the heated sheet of plastic material to the mold surface section to give to the plastic material the shape of that mold surface.

In accordance with a first preferred embodiment of the subject invention, the impervious wall means is made of high performance impervious concrete, and has an inner surface applied to the second surface section of the body of porous concrete.
In accordance with another preferred embodiment of the subject invention, the mold can 21~7123 further comprise electric heating elements and/or cooling elements embedded into the porous concrete of the body for heating and/or cooling that body of porous concrete.

In accordance with a third preferred embodiment, the porous concrete of the body has a percentage of voids situated within a range extending between 15~ and 50~.

In accordance with a further preferred embodiment, the porous concrete of the body comprises aggregates having a particle-size distribution selected from the group consisting of the following 15particle-size distributions: 20/35, 20/40 and 40/70.
Aggregates can be of mineral or metallic origin.

In accordance with a fifth preferred embodiment of the subject invention, the porous concrete of the body comprises aggregates, and wherein a first half of the aggregates has a particle-size distribution of 20/40, and a second half of the aggregates has a particle-size distribution of 40/70.

25The body of porous concrete may comprise a core of concrete having a first higher porosity, and an outer layer of concrete having a second porosity 2l87l23 lower than the first porosity in order to form a smoother mold surface section. For example, the concrete of first higher porosity comprises aggregates having a particle-size distribution of 40/70, and the concrete of second lower porosity comprises aggregates having a particle-size distribution of 20/40.

The objects, advantages and other features of the present invention will become more apparent upon reading of the following non restrictive description of a preferred embodiment thereof, given by way of example only with reference to the accompanylng drawings.

BRIEF D~SCRIPTION OF THE DRAWINGS

In the appended drawings:
Figure 1 is a cross sectional, side elevational view of a porous concrete mold in accordance with the present invention;

25Figure 2 is a cross sectional, side elevational view of a porous concrete mold in -~ ' 21871~3 accordance with the present invention being fabricated by means of a concrete-receiving mold; and Figure 3 is a perspective view of a hand tamper usable for compacting the porous concrete when fabricating a mold in accordance with the present invention.

DETAILED DESCRIPTION OF THE pREFER~Rn EMBODIMENT

In the appended drawings, the porous concrete mold in accordance with the present invention is generally identified by the reference 1.

Referring to Figure 1, the mold 10 comprises an impervious base 11 made of non porous concrete. To reinforce the mold 10 that can be subjected to rough and intensive handling, the base 11 of the mold 10 is constituted by a slab of high performance impervious concrete in which reinforcing rods or a reinforcement wire mesh (see 12 in Figure 1) are/is embedded.
The porous concrete mold 10 further comprises a core 13 made of a concrete having a first ~ 2187123 higher porosity. The core 13 has a surface section 23 thereof bound and sealed by a top inner surface 24 of the base 11. Reinforcing rods 14 extend from the rods or mesh 12 through the base 11 and the core 13 to reinforce the mold 10. The reinforcing rods 14 also increase the binding force between the base 11 and the core 13.

The mold 10 further comprises a relatively thin topping 15 made of concrete having a second porosity lower than the first porosity of the core 13 to provide a mold surface section 16 having a smoother finish. Those of ordinary skill in the art will appreciate that the mold surface 16 must present a smooth finish to provide the thermoformed plastic sheet with a corresponding smooth finish.

Finally, the mold 10 comprises an air sucking duct 17 extending from the outer bottom surface 18 of the base 11 to reach the inside of the core 13. Preferably, the duct 17 is cylindrical to enable connection thereof to a conventional vacuum equipment (not shown).

In operation, the plastic sheet to be thermoformed is heated to a temperature sufficiently high to soften the plastic material. Conventional ~ 2187123 heating equipments or the type currently used in the thermoforming processes can be used for that purpose.
In thermoforming processing, it is also a usual practice to maintain the mold at a relatively constant temperature. To control the temperature of the mold 10, electric heating elements and/or cooling elements such as 25 can be embedded in the concrete of the base 11, the core 13 and/or topping 15.

To carry out the "vacuum forming" process, air is sucked by the vacuum equipment (not shown) connected to the cylindrical duct 17. More specifically, air is sucked by the vacuum equipment through the mold surface section 16, the concrete of the topping 15, the concrete of the core 13, and the duct 17. As the high performance concrete of the base 11 is impervious, no sucking of air is enabled through that base 11. Therefore, the suction is concentrated through the mold surface section 16 to apply the heated plastic sheet (not shown) to be vacuum formed to that surface 16.

Of course, the level of air aspiration through the mold surface section 16 must be sufficient to draw the heat-softened sheet of plastic material (not shown) and apply it to the surface 16 in view of thermoforming this sheet of plastic material to the , 2l~7l23 shape of the mold surface 16. For that purpose, the degree of porosity of the concrete of the core 13 and topping 15 and the level of air aspiration through the cylindrical duct 17 must be sufficient.

Fahrication of the mold 10:

1. Fabrication of a concrete-receiving mold:

To fabricate the porous concrete mold 10 a concrete-receiving mold 20 is used (see Figure 2).
That concrete-receiving mold 20 can be made of, for example, fiberglass. The use of other materials that can be easily formed to the desired shape may also be contemplated. However, it is a requirement that the surface 21 of the concrete-receiving mold 20 be as smooth as possible to give to the mold surface section 16 the desired smooth finish in view of eliminating from the mold surface section 16 any irregularity susceptible to be transferred and appear onto the molded plastic sheet material.

As mentioned in the preceding paragraph, the surface 21 of the concrete-receiving mold 20 must be as smooth as possible. It is recommended to coat the surface 21 with varnish or other product to ~1~37123 provide that mold surface with a smooth and glossy finish.

2. Fabrication of the porous concrete:

Simple and reliable methods have been developed for calculating the proportions of the different constituents (cement, aggregates, water and superplasticizer) in view of obtaining a concrete having a given percentage of voids. The results obtained with these methods are very reproducible. It is believed that these methods are well known to those of ordinary skill in the art, and since they form no part of the subject invention, they will not be presented in this specification.

A percentage of voids situated between 15 and 50~ in the concrete gives an adequate porosity.
However, the design of the mold 10 must also take into consideration that the mechanical resistance of the porous concrete mold is, in general, inversely proportional to the percentage of voids. Therefore, the porosity of the mold 10 should be fixed not only in relation to the required permeability but also in relation to other considerations such as the production rate, the production volume, the shape of 2 1 ~7 1 23 the pieces to be molded, the resistance, the smoothness of the surface, etc.

Mixing of the porous concrete should be carried out as follows:

1) the proportions of the different constituents are calculated, the constituents are weighted, and the dry aggregates are introduced into the concrete mixer; use of a mixer of the "Hobart"
type is recommended;

2) the concrete mixer is started, a small amount of water is added to moisten the aggregates (the quantity of water which is absorbed varies in relation to the type of aggregates; for metallic aggregates, the amount of water which is absorbed is about 0.5~ of the weight of the aggregates), and then the moistened aggregates are mixed for 1 minute;

3) all the cement is progressively added in the mixer and the resulting product is mixed for three minutes in view of coating the aggregates with no surplus of cement;

- ' 21~7123 4) the water and the plasticizer, having been mixed together beforehand, are progressively added in the mixer and then mixing is carried out for three minutes; and 5) the concrete obtained is ready to be poured into the concrete-receiving mold 20.

3. Preparation of the surface of the concrete-receiving mold 20:

Advantageously, a thin layer of de-molding agent is applied to the surface 21 of the mold 20 just before pouring the concrete thereon.
4. Pouring of the concrete:

The porous concrete of the relatively thin topping 15 is first poured into the mold 21. Then the porous concrete of the core 13 is poured. Finally, the high performance impervious concrete of the base 11 is poured. Obviously, the reinforcement wire mesh 12 and the reinforcing rods 14 are placed before pouring the different types of concrete. The duct 17 is also formed upon pouring the concrete.

_ 2187~23 As porous concrete has a relatively dry consistency, consolidation thereof should be made by compaction. The following procedure should be followed upon pouring of porous concrete:

1) the concrete is placed and compacted in the mold 20 by successive layers of about 20 mm thick;

2) each layer of concrete is compacted by means of a hand tamper, an electric tamper or a pneumatic tamper. Figure 3 shows the shape and dimensions of an example of suitable hand tamper 22.
Compaction should be aggressive and as uniform as possible;
3) the exposed surface of each layer is scarified after compaction thereof;

4) the steps 1-3 are repeated until the mold 20 (Figure 2) is filled with porous concrete (topping 15 and core 13); and 5) when the concrete-receiving mold 20 is full, the exposed surface of porous concrete is quickly covered with a wet fabric, itself covered with a plastic sheet in order to prevent any evaporation.

~ 2187123 Pouring of the porous concrete should be made within an area in which relative humidity is high enough to prevent the concrete to loose water during or after compaction. Also, in order to promote as much as possible the process of hydration of the concrete, it is very important to maintain the porous concrete wet, after hardening, until the end of the curing period. This can be done, as mentioned in the foregoing description, by covering the exposed surface of the concrete with a wet fabric, for example wet burlap.

The high performance impervious concrete of the base 11 will normally be poured 24 hours after pouring of the porous concrete of the topping 15 and core 13. It can also be envisaged to reduce this period of 24 hours by using a concrete setting accelerator in the fabrication of the porous concrete.
The base 11 is poured before de-molding of the porous concrete mold 10. Before pouring the high performance impervious concrete of the base 11, a corresponding form (not shown) is placed in view of producing the cylindrical air-sucking duct 17. A
suitable process for fabricating the base 11 of the porous concrete mold 10 comprises the following steps:

15 21~7123 1) Deep cleaning of the exposed surface of the porous concrete;

2) Applying an adhesion grout to the exposed surface of the microsporous or porous concrete (an example of composition is given for the binding grout and the high performance concrete in the following Table 1);

TAhle 1 High performance Binding grout impervious concrete Cement type III 370 g/L 1 480 g/L
Water 125 g/L495 g/L
Superplasticizer 6.7 g/L 40 g/L
Eucon 37 *
Sand 330 g/L
Gravel 10 mm 330 g/L

* The superplasticizer Eucon 37 is a naphthalene-base superplasticizer available from Adjuvants Euclid Canada Inc. of St-Hubert (Québec) Canada.

~5 3) Pouring the high performance impervious concrete of the base 11; and 2 1 ~7 1 23 4) Covering the base 11 and the mold 10 with a plastic sheet for a period of 14 to 18 hours.

5. De-molding:

When the shape of the concrete-receiving mold 20 is complex, de-molding will be carried out with the help of a water pressure applied to the summit of the porous concrete mold 10 through a hole (not shown) in the concrete-receiving mold 20. If the shape of the concrete-receiving mold 20 is simple, de-molding can be performed by simply turning the assembly concrete-receiving mold 20 - porous concrete mold 10 upside down and by tapping onto the outer wall surface of the concrete-receiving mold 20.

6. Hot water curing of the porou~ concrete:

After the porous concrete mold 10 has been withdrawn from the concrete-receiving mold 20, hot water curing is carried out. For that purpose, the porous concrete mold 10 is placed into a reservoir containing water at ambient temperature. Then, the temperature of the water is raised to 80~C. These conditions are maintained for a period of 6 to 16 hours. Thereafter, the water is cooled and when it reaches a temperature of 50~C the mold 10 is withdrawn - 21~7123 from the reservoir. The mold 10 is finally placed into a ventilated heat chamber, at a temperature of 100~C + 5~C for a period varying between 24 hours and a few days depending on the dimensions of the mold 10.
This treatment dries the mold 10 to remove the water from the pores of the porous concrete.

Examples Examples of porous concrete molds that have been constructed and experimented will now be described.

Regarding selection of the aggregates, it should be mentioned that any aggregates permitting to obtain the desired porosity and/or permeability could be used. However, since the porous concrete mold 10 is used into a thermoforming process, it would be advantageous to use thermally conductive aggregates.
For example, the use of metallic aggregates will result into a porous concrete mold 10 having a high thermal conductivity. However, it is within the scope of the present invention to use other types of aggregates such as mineral aggregates and the like, including sand, to produce porous concrete mold still having a high performance in specific applications.

~ 21~7123 To fabricate the molds, the following metallic aggregates have been used:

- metallic aggregates " 20/35- Anchorsteel W-428"
from the company Hoeganaen of New-Jersey, United States; and - metallic aggregates "AT-110 et AT-50"
commercialized by Les poudre~ métalliques du Québec Ltée of Sorel (Québec) Canada, from which metallic powders having particle-size distributions of 20/40 and 40/70, respectively have been obtained by sieving. The "AT-110"
metallic aggregate has also been used directly without sieving.

~ x~m; n~tion of these metallic aggregates with a scanning electron microscope has revealed that the particles are relatively spherical and that the surface of the particles is very rugose and slightly porous. These characteristics will help to create an adequate bonding between the metallic particles and the cement paste.

To fabricate the molds, Portland cement of Type III and the above mentioned superplasticizer Eucon 37 have been used. The use of Portland cement - ' 2187123 of Type III increases the short term mechanical resistance (when compared to Portland cement of Type I) and reduces the time required to fabricate the molds. The superplasticizer reduces the quantity of water required to fabricate the particle-coating grout to thereby obtain higher resistances to compression and traction for a same porosity.

The following porous concrete molds have been fabricated with the same concrete-receiving mold.
The particle-size distribution and, therefore, the porosity of the concrete are the only parameter varying from one mold to the other.

Mold # 1:
This mold is composed of an aggregate 20/35 and has a porosity of 50~. Its permeability is sufficient to enable adequate vacuum forming of plastic sheets. However, irregularities of the mold surface are transferred to the apparent face of the molded plastic sheet. The resistance of the mold to traction is also a little too low; this could reduce the mold's lifetime.

Mold #2:
The concrete of this mold comprises an aggregate 40/70 and has a porosity of 25~. This 21~7123 porous concrete will be adequate for smoothing the mold surface and to improve the resistance of the mold to traction. As the porosity of mold #2 is lower than that of mold #1, the permeability thereof is also lower.

Mold #3:
The concrete of mold #3 is made of 50~ of aggregate 20/40 and of 50~ of aggregate 40/70. The porosity thereof is 25~. The texture of the surface of mold #3 is a little less smooth than the texture of mold #2. The resistance or mold #3 to the traction is substantially the same as that of mold #2.

Mold #4:
Two different porous concretes have been used to fabricate mold #4.

The first concrete has a first lower porosity (25~) and includes aggregates having a particle-size distribution of 40/70. It is placed at the surface of the mold and has a thickness of about 20 mm. This first concrete gives to the mold surface section a smooth texture.
The second porous concrete comprises aggregates 20/40 and has a second higher porosity of 2~7~23 38~. It forms the core of the mold in order to increase the overall permeability of the mold.

The composition and porosity of mold #1 through #4 is summarized in the following Table 2:

Table 2 Mold Aggregates Porosity 2 40/70 25~
3 50~ of 20/40 25%
50~ of 40/70 4 Topping 40/70 Topping 25%
Core 20/40 Core 38~

All the molds #1 through #4 have proved to have a sufficient permeability to enable efficient vacuum forming of plastic sheets. This proves that efficient molds according to the invention can be fabricated with one concrete or a composition of two concretes having different porosities.

The following Tables 3a, 3b and 4 present the results of experiments conducted to evaluate the characteristics (permeability, resistance to compression, texture of the concrete surface, etc.) of different compositions of concrete that can be used in the present invention and that can be selected in relation to particular applications.

More specifically, Tables 3a and 3b present the different compositions (in g/L) of concrete:

Table 3a M-l P-l P-2 P-3 P-4 P-5 Nold #1 - - - #3, #5 #5 Cem-nt Port- 430 435 435 645 675 535 land Type III
Water 75 70 75 105 110 90 Sup-r-plactl- 20 26 27 39 40 32 clzer Aggre-gate~

AT-llO - - - - -Th-ore-tlcal 55 50 50 25 25 38 % of voldfl , _ Table 3b Mold - 3 - ~ ~ ~
C~m-nt Port- 560 635 755 785 780 690 0 land Type III
Wat-r 95 105 125 130 130 115 Super-pl~ti- 33 37 45 47 46 41 cizer Aggre-gate~

Theore-tical 38 38 15 15 15 38 % of void~

Table 4 presents the permeability, desired percentage of voids, real percentage of voids, the particle-size distribution of the aggregates, the resistance to compression, and the texture of the surface of the different porous concretes of Tables 3a and 3b:

Table 4 Con- P~ --hil~ty De~i- R-al % Aggre- Resis- Textu-cr-te test red % of gates tance re of of voids to com mold Read- % of voids pre~- sur-ding r-f-- sion f-c-rence~ (Mpa) M-2 11.3 100 55 2920/35 - rugose P-1 13.2 117 50 2320/40 27 rugose 0 P-2 12.1 107 50 3440/70 21 smooth P-3 12.7 112 25 2520j40 41 mean P-4 8.2 73 25 2740/70 42 smooth P-5 12.3 109 38 2620/40 51 mean P-6 8.4 74 38 2940/70 39 smooth P-7 8.6 76 38 2320/40 40 smooth and P-8 0.5 4 15 1020/40 89 mean and P-9 2.9 26 15 2240/70 64 smooth P-10 0.5 4 15 1420/40 91 mean P-11 9.2 81 38 28AT-110 42 very smooth * % of the reading of the permeability test for the concrete M-1.

2l87l23 Although the present invention has been described hereinabove with reference to a preferred embodiment thereof, this embodiment can be modified at will, within the scope of the appended claims, without departing from the spirit and nature of the subject invention.

Claims

1. A porous concrete mold, comprising:
a body of porous concrete having a mold surface section and a second surface section;
an impervious wall means mounted to said body of porous concrete for sealing said second surface section, said impervious wall means comprising an outer surface; and a duct means extending through the impervious wall means from the outer surface of said impervious wall means to the porous concrete of said body, said duct means being connectable to a vacuum equipment for sucking air through said mold surface section, the porous concrete of said body and said duct means.

2. A porous concrete mold as recited in claim 1, wherein the impervious wall means has an inner surface applied to the second surface section of said body of porous concrete.

3. A porous concrete mold as recited in claim 1, further comprising means for heating said body of porous concrete.

4. A porous concrete mold as recited in claim 3, wherein said heating means comprise electric heating elements embedded into the porous concrete of said body.

5. A porous concrete mold as recited in claim 4, further comprising cooling elements embedded into the porous concrete of said body to control the temperature of said body.

6. A porous concrete mold as recited in claim 1, wherein said body of porous concrete comprises a core of concrete having a first higher porosity, and an outer layer of concrete having a second porosity lower than said first porosity in order to form a smoother mold surface section.

7. A porous concrete mold as recited in claim 1, wherein the porous concrete of said body comprises aggregates having a particle-size distribution selected from the group consisting of the following particle-size distributions: 20/35, 20/40 and 40/70.

8. A porous concrete mold as recited in claim 6, wherein said concrete of first higher porosity comprises aggregates having a particle-size distribution of 40/70, and said concrete of second lower porosity comprises aggregates having a particle-size distribution of 20/40.

9. A porous concrete mold as recited in claim 1, wherein the porous concrete of said body comprises aggregates, and wherein a first half of said aggregates has a particle-size distribution of 20/40, and a second half of said aggregates has a particle-size distribution of 40/70.

10. A porous concrete mold as recited in claim 1, wherein the porous concrete of said body has a percentage of voids situated within a range extending between 15% and 50%.

11. A porous concrete mold as recited in claim 2, wherein the impervious wall means is made of high performance impervious concrete.

12. A porous concrete mold as recited in claim 1, wherein the porous concrete of said body comprises metallic aggregates.

13. A porous concrete mold as recited in claim 1, wherein the porous concrete of said body comprises mineral aggregates.