METHOD OF AND SYSTEM FOR PRODUCING FOAMED RESIN MOLDING

Abstract: A method of producing a foamed resin molding by filling prefoamed resin beads, a mold and foam-molding them by heating, characterized in that, in order to reduce variations in weight between products, the apparent density of the prefoamed beads is measured, and the cracking width of the mold is adjusted to a predetermined value determined by the apparent density at the time of filling of on the basis of the results of measurement, so that the amount of the beads to be filled into the mold is made constant. A system for embodying said method comprises an automatic foam-molding machine wherein the raw material is removed from a material hopper (1) before the molding, devices (2, 3, 4) for measuring the apparent density and a current-mold-closing-position-measuring device (8) operationally associated with a mold opening-closing positioner (9) are provided, and further there is provided a computer automatic cracking-width-control device (10) for receiving inputs of a measured apparent density value and a current-mold-closing-position, calculating an optimal cracking width determined by the apparent density in accordance with a prepared program, closing hydraulic supply lines for mold-closing in a mold opening-closing hydraulic unit (11-16) and stopping the mold closing at a position that gives said optimal cracking width.

Description: Technical Field This invention relates to a method and an apparatus for manufacturing a molded resin foam, wherein pre-expanded resin pellets or granules are filled in a mold cavity and subjected to expansion molding under heating to produce energy absorbers such as automobile bumper core materials, planks, containers, shock-absorbing packaging materials, e.g. side pads, etc. More particularly, the invention is concerned with the production of a molded resin foam, in which the weight of the pre-expanded resin pellets to be charged to the mold cavity is made constant whereby foam products are yielded with minimized scattering in weight. Background Art As represented by beads molding technique of expandable styrene beads in a mold cavity, the expansion molding method of pre-expanded pellets of a resin such as styrene resins, olefine resins, e.g. polyethylene, polypropylene, etc. or the like has hitherto been known widely, which comprises charging pre-expanded resin pellets into a mold cavity and meld-bonding them by steam heating followed by cooling to yield a molded product conforming to the mold contour. With this method, the charging procedure is generally conducted in such a manner that when the pre-expanded pellets are discharged from a feedstock silo attached to a molding machine through a charging equipment into the cavity of mold, the mold is not completely closed, but the mold sections are spaced apart a clearance or gap, which is called a cracking (hereinafter the term "cracking" will be used), on the order of 0 - 20 mm, usually 2 - 8 mm and thereafter, the pre-expanded pellets are charged through the charging equipment into the mold cavity by blowing compressed air into it, thus taking advantage of suction and entrainment actions of the pre-expanded pellets. In this case, spacing the cracking is aimed at filling the prefoamed pellets equally and homogeneously in the mold cavity by releasing the air blown therein. The mold is constructed so that even if the cracking width is larger than the diameter of the prefoamed pellets, the prefoamed pellets, when charged, may not leak out of the mold. In spite of the fact that if the apparent density of prefoamed pellets disperses, the weight of the resulting molded products will naturally disperse, the method usually carried out does not take account of controlling the dispersion in apparent density thereby to decrease the dispersion in weight of the molded products. Hence, it is a common practice that the dispersion in weight of molded products is taken for granted. As a consequence, by taking account of the dispersion in weight of the molded products when produced according to the current molding method in advance, the standard in weight or expansion ratio of the products is set broad to some degree, whereby quality control (in weight) is performed. In situation where the weight of molded foam is measured and found to be off the standard, a worker on-site knows that the manipulation of the cracking width of mold enables to rectify the weight of product (expansion ratio) and he may cope with the situation on the basis of his skill by manual operation of the cracking. However, it is virtually impossible to make such adjustment frequently even if a few times might be possible. Furthermore, in case where prefoamed pellets disperse more than usual, the frequency of manual operation to the cracking width is increased, and consequently, in the status quo, there is practically no way of taking countermeasures. No matter how preferable it is to enhance the quality of products by narrowing the current prevailing range of standard of product weight, the manufacturing method now in practice has still limitations as it is. For instance, in the application of a foam material to an automobile bumper core material or the like in which the foam serves to absorb the energy upon collision, the dispersion in expansion ratio of the foam directly leads to dispersion in energy absorption magnitude and causes significant differences in that function with the result that excessive safeguard design against collision is necessitated. Minimizing the dispersion in expansion ratio of a bumper core material to the utmost is therefore desired by all means. Also in another examples, it goes without saying that a small dispersion in expansion ratio of foam products is advantageous in quality or economic aspect. In the manufacturing method of a foamed product by expansion molding of prefoamed resin pellets in a mold cavity as described above, such an approach to expansion molding has not yet been found that the quality of products can be enhanced by making the range of standard in expansion ratio of the products narrower than the current method and nevertheless, the yield of products can be maintained at the same level as the current method. Neither has been found such an expansion molding technique that even if the apparent density of prefoamed resin pellets is more or less dispersed more than usual, molding is nonetheless enabled. US-A-4 615 849 shows the features of the preamble of claim 1 and shows closing the mold to a predetermined gap width. In view of the forgoing problems the prior art method poses, the present inventors have intensively researched for a molding method capable of solving the problems and come to the conclusion that in the current method, weight dispersion of foamed products is impossible to reduce insofar as the dispersion in expansion ratio of prefoamed resin pellets as a feedstock for expansion molding is not reduced. According to the invention, there is provided a method of manufacturing a molded resin foam comprising charging pellets of a prefoamed resin into a cavity of a mold and subjecting them to expansion molding under heating, which method is characterised in that prior to said charging, the apparent density of the prefoamed pellets is measured; upon charging, the cracking width of the mold is regulated, on the basis of the measurement result, to a definite value determined depending in the apparent density, thereby making the weight of the pellets charged in the mold cavity constant; and thereaffer, the mold is closed to perform the molding. The invention is capable if minimising the weight dispersion of molded products. That is, with a view toward solving the foregoing problems, the present inventors have intensively investigated into the causes why the expansion ratio of molded foams scatters. As a result, we have found out that the prefoamed pellets as a feedstock are difficult to expand in the same expansion ratio from pellet to pellet because of their manufactural limitations. They area mixture of pellets having an expansion ratio distribution which is centered on a target expansion ratio and ranges on both sides of it, and even to expand the mixture strictly in an average expansion ratio of the target value is generally difficult. Accordingly, it is difficult to diminish the weight dispersion of molded products in the current molding method, which neglects the dispersion in expansion ratio of their feedstock, insofar as the prefoamed pellets are supplied to foaming molding. From the sampling and weighing of a definite volume of prefoamed resin pellets to be charged in mold cavity, it has also been proved that the prefoamed pellets as stated above cause classification when delivered into a stock silo and transferred from the silo to a raw material hopper for a molding machine, and consequently, they produce a wide dispersion in weight thereof though it depends on the kind of prefoamed pellets. This invention has been discovered on the basis of the foregoing findings and is aimed at making the weight of prefoamed pellets constant thereby to manufacture a molded resin foam with a minimized dispersion in weight. In the present manufacturing method, the measurement of the apparent density followed by setting of the cracking width occurs preferably every molding shot, but may be every several shots provided that the apparent density of prefoamed pellets is stabilized. It is advantageous that the measurement of apparent density is made with a batch of prefoamed pellets immediately before charging into a mold cavity or its adjacent batch of ones and automatic regulation of the cracking width is conducted. It may be occasionally necessary to set upper and lower limits of the cracking width, judging from the molding situations. In that case, the lower limit value is such that escape of air upon charging is not impeded and non-uniformity of charging doesn't take place, for example, 1 mm whereas the upper limit value is such that no excessive charging is made to a degree that prevents the passage of steam at the subsequent heating step and causes insufficient inner fusion of molded product, for example, 20 mm. In molding, after the cracking width is regulated and prefoamed pellets are charged into mold cavity, the cracking width is brought into closing state, and heating and cooling are performed, as is the case with customary molding procedure. Accoring to another aspect of the invention, there is provided an apparatus for carrying the aforesaid method into effect which is constructed of an automatic expansion molding machine including a feedstock hopper, a feedstock charger, a mold, a hydraulic mold opening and closing unit, a mold opening or closing positioner and a mold heating or cooling system. The apparatus is characterized by further provision of an arrangement for sampling a portion of feedstock from the feedstock hopper and measuring its apparent density before molding; an equipment for measuring a current position of mold closing which is provided at the mold opening or closing positioner to interlock therewith; and a computer automatic cracking width control device for obtaining the inputs of measured apparent density and current position of mold closing from the former two, computing an optimal cracking width determined depending on the apparent density according to a program prepared beforehand, and closing a hydraulic feed line for mold closing of the hydraulic mold opening or closing unit to stop the mold closing at a position of the optimal cracking width. In the aforementioned apparatus, the position of the optimal cracking width may be somewhat deviated owing to change in oil temperature of the hydraulic unit and dispersion in response time of the control system. Consequently, it is effective to measure the deviation and conduct automatic operation and correction in the next molding. This invention is also characterized by controlling to that end by means of a computer automatic cracking width control device. The resin to be employed in this invention includes polystyrene, acrylonitrile-styrene copolymer, polymethylmethacrylate, polyethylene/polystyrene composite polymer, polypropylene, propylene-ethylene random or block copolymer, propylene-ethylene-butene terpolymer, low-density polyethylene, low-density linear polyethylene, medium-density polyethylene, high-density polyethylene, ethylene-vinyl acetate copolymer, ethylene-acrylic acid or metal acrylate copolymer, polybutene-1, poly-4-methylpentene-1, etc., but should not be construed as limiting to these examples. In particular, propylene-ethylene copolymer or polyethylene series resins are preferred. The prefoamed pellets of the resin to be employed in this invention may be prepared by any methods which are not limited. There are enumerated, for example, prefoamed pellets prepared by placing resin pellets impregnated with a volatile foaming agent or inorganic gas in a prefoaming machine followed by heating with steam; prefoamed pellets prepared by dispersing and suspending a volatile foaming agent, olefinic resin pellets and dispersing agent in water in a pressure-resistant sealed vessel, elevating the temperature to a required temperature under a required pressure and releasing the olefinic resin pellets and water into a reduced pressure area; prefoamed pellets prepared by heating and kneading an olefinic resin and foaming agent in an extruder, cooling, extruding the mixture through a multihole die equipped at the top of the extruder as foamed strands and cutting the strands with a cutter in an appropriate length; or any other pellets prepared in any methods. For bumper core materials or vibration-absorbing packages, at the outset, expansion ratio of a molding material is determined to accommodate the performances or quality required for the final product and such prefoamed pellets that have an apparent density adapted to the expansion ratio are selected. At this time, the prefoamed pellets have not an identical expansion ratio from pellet to pellet, but are a mixture of foam pellets having a distribution in expansion ratio. Moreover, it is difficult to expect that prefoamed pellets having a strictly constant apparent density be always supplied for expansion molding. Therefore, generally, variation in apparent density of foaming materials to some degree is conceded and concomitant variation in weight of molded products is considered to be unavoidable. Also in this invention, such usual prefoamed pellets having a distribution in expansion ratio and an allowance in apparent density are used for the prefoamed resin pellets material. When a molded foam is manufactured by the application of the manufacturing apparatus and method as described above, the optimal cracking width is obtained, for example, by the following formulae: 1) Theoretical formula with a rectangular parallelepiped shape (square bar, plank, etc.) Let the meanings of symbols be: the longitudinal dimension of mold = A (cm) the transverse dimension of mold = B (cm) the dimension in thickness direction (opening or closing direction) of mold = C (cm) the cracking depth = D (mm), and the mold charging volume E at a cracking width of 0 mm is represented by : and the increment F of mold charging volume at a cracking width of D (mm) is represented by : On the other hand, assuming that the measured apparent density of the prefoamed pellets is ρ(g/l) and median value of standard product weight is X (g), the required charging volume G to the mold is : G = X ρ (l) . Since there is a relation of G = E + F , the optimal cracking width D is represented by : 2) Correction of the theoretical formula with a rectangular parallele piped shape When the molding is actually performed, deviation from the theoretical formula may be caused. Accordingly, in order to correct the theoretical formula, the median standard value of product weight is multiplied by a deflection coefficient α and the following formula is used, instead: 3) Formula with a profile shape In the case of molding of a profile shape, the mold charging volume E (g/l) at a cracking depth of 0 (mm) and the area S (cm²) of the matched portions of male and female mold sections are measured, and the following formula is adopted: Thus, when a molded foam is manufactured according to this invention by determining an optimal cracking width value according to the program formulae indicated above and regulating the cracking width of mold to the value obtained, the cracking width is made small when the apparent density is large whereas conversely, the cracking width is made large when the apparent density is small. Thereby the weight of prefoamed pellets charged in the mold is made constant, and as a result, the weight (the same meaning as the expansion ratio) of the molded foam thus obtained is made constant. In this way, dispersion in expansion ratio of the resulting molded foams is minimized and product quality is enhanced. Brief Description of the Drawings Fig. 1 is a schematic illustration showing one example of an apparatus for carrying the production method according to this invention into effect. Best Mode for Carrying Out the Invention Examples of this invention will be further described hereinbelow. In the single figure, the respective reference numerals designate: 1 a feedstock hopper, 1A its feed portion, 2 to 4 a measurement arrangement for apparent density of prefoamed resin pellets according to this invention, 5 a pair of male and female mold sections, 6 a feedstock charger, 7 a cracking width (clearance between male and female mold sections) to be automatically controlled according to this invention, 8 a measurement device for current mold-closing positioner, 10 a computer automatic control device of cracking width. The computer automatic control device of cracking width 10 serves to input the measurement values of apparent density of a feedstock immediately before molding from a weighing equipment 3, input the current mold closing position from the measurement device for current mold-closing position 8, compute an optimal cracking width according to a program beforehand prepared and close a hydraulic feed line for mold closing of a hydraulic mold opening or closing unit described below thus stopping the mold closing at a position of the aforesaid optimal cracking width. The computer-automatic control device of cracking width 10 includes a mechanism for measuring a deviation between the actual mold-closing stop position and the computed setting position of mold-closing stopping and conducting automatic operation and correction for that in the next cycle of molding. The reference numerals 11 to 16 designate a hydraulic unit; 17 designates a supply line of steam for heat molding or cooling water and 18, a drain pipeline. The apparent density measurement arrangement includes a constant volume sampler 2, a weighing equipment 3 and a receiver tank 4. The prefoamed pellets after weighing are returned to the feedstock hopper 1. In the hydraulic unit, the reference numeral 11 designates a hydraulic cylinder and 12, an electromagnetic valve for mold opening or closing which has positions of mold closing, stop, and mold opening from the left side. The reference numeral 13 is an electromagnetic valve for hydraulic pressure supplying which has positions of stop supplying on the left and supplying on the right. The reference numerals 14, 15, 16 are an oil sump, a motor and a hydraulic pump, respectively. The figure as viewed shows the state of cracking stop. Now experimental examples utilizing the apparatus shown in the figure will be described. Experimental Example 1 A square bar made of polypropylene expanded in an expansion ratio of 15 times was formed by beads molding technique in a mold cavity. Pre-expanded beads used in this example are ones expanded from propylene random copolymer having a basic density of 0.9 g/cc and vary from 49 g/l (18.4 times) to 55 g/l (16.4 times) in apparent density (apparent expansion ratio). One evaluation unit was prepared by mixing five flexible containers (200 kg in total) of prefoamed beads with different apparent densities, each flexible container having a 40 kg capacity. In order to impart expansion power to the pre-expanded beads during molding, pressure treatment was applied, before molding, to the pre-expanded beads under heating by compressed air so that the air pressure inside the beads may be ca. 2 atm. The configuration of mold used for the square bar in this example (here, it means a configuration of their space to be charged) is a rectangular parallelepiped having dimensions of 1000 mm in longitudinal length, and 160 mm in transverse length and thickness. Being expected to have a dimensional shrinkage of 2.5% in respective directions, the resulting molded product has a size and volume approximately of 975 mm in length and 156 mm in transverse length and thickness, and 23.7 liters. Assuming that final products of this square bar should have an expansion ratio standard of 15 ± 2 times, such products that have the following weight range in dried state after molding are acceptable: Weight standard of products (unit;gr.) = 1255(min. value)∼1422(median value)∼1641(max. value) Here, the weight can be computed according to the formula: Likewise in the case of a narrowed expansion ratio standard of 15 ± 1 times, acceptable are: Weight standard of products (unit;gr.) = 1333(min. value)∼1422(median value)∼1641(max. value) The program for computing cracking width control values from measurements of apparent density was based on the theoretical formula regarding the computing formula of cracking width with rectangular parallelepiped shape as stated above, and the formula given below was used except for the cases below where the computed cracking widths are not less than 20 mm and not more than 1 mm: 1) Where the computed cracking width is in the range of 1 - 20 mm: (α: a coefficient of deviation between charged weight and molded weight and here, assumed to be 0.1) 2) Where the computed cracking width is not less than 20 mm: Cracking width = 20 mm 3) Where the computed cracking width is not more than 1 mm: Cracking width = 1 mm Evaluation was made of expansion molding in this way and the results obtained are shown in Table 1 together with those of comparative examples. Results of Evaluation of Expansion Molding Evaluation Item Comparative Ex. This Experimental Ex. Apparent density of prefoamed beads (g/l) (flexible container unit) 49.2 49.1 50.3 50.3 52.6 52.1 53.3 53.4 54.8 55.0 Cracking width fixed to 8 mm Automatically controlled: average = 6.3 mm max = 18.7 mm min = 1.0 mm Number of moldings 142 147 Average weight(gr) 1431 1417 Maximum weight(gr) 1822 1583 Minimum weight(gr) 1232 1345 Max. - Min.(gr) 590 238 Standard deviation (gr) 105 41 Product yield(%) (15±2 times) 97.2 100 Product yield(%) (15±1 times) 66.9 97.3 As seen from Table 1 above, the present experimental example of this invention yielded clearly superior results as compared with the comparative example according to conventional molding. The product yield was more than 95 % though the product standard was altered from 15±2 times to 15±1 times, and so, commercial production is feasible. Experimental Example 2 A plank of expanded polyethylene with a factor of 27 was formed by molding of its beads in mold cavity. Pre-expanded beads used in this example were expanded from low-density cross-linked polyethylene having a basic density of 0.92 g/cc to vary from 23 g/l (39.1 times) to 24 g/l (37.5 times) in apparent density (apparent expansion ratio). Three flexible containers (90 kg in total) of pre-expanded beads with different apparent densities were mixed to make one evaluation unit, each flexible container having a 30 kg capacity. The mold (configuration of their space to be charged) for the plank in this example is fashioned in the form of a rectangular plate having dimensions of 1288 mm in longitudinal length, 975 mm in transverse length and 42.0 mm in thickness. The resulting moldings turned out, from the past empirical data, to have an approximate size and volume of 1230 mm in longitudinal length, 930 mm in transverse length and 40.5 mm in thickness and 46.3 l, on the average. Provided that the expansion ratio standard with final products of the plank is 27±3 times and when their weights are computed according to the formula of: products having the following range of weights in dried state after molding are rendered acceptable: Product weight standard (unit;gr) = 1496(min.value)∼1579(median value)∼1672(max. value) The following formula was used as a program for computing regulated values of cracking width from measurements to apparent density : 1) Where the cracking width computed is 1 - 20 mm, (α: coefficient of deviation between charged weight and molded weight, and in this case, chosen to be 1.03). 2) Where the computed cracking width is not less than 18 mm, Cracking width = 18 mm. 3) Where the computed cracking width is not more than 1 mm, Cracking width = 1 mm. Evaluation was made to expansion molding of this example, and results obtained are shown in Table 2 given below together with those of Comparative example. Results of Evaluation to Expansion Molding Evaluation Item Comparative Ex. This Experimental Ex. Apparent density of prefoamed beads used (g/l) (flexible container unit) 23.0 23.0 23.6 23.4 24.1 24.0 Cracking width fixed to 12 mm Automatically controlled: average = 11.9 mm max. = 15.2 mm min. = 9.6 mm Number of moldings 55 56 Average weight (gr) 1583 1575 Max. weight (gr) 1685 1622 Min. weight (gr) 1426 1528 Max. - Min. (gr) 259 94 Standard deviation (gr) 42.3 15.6 Product yield (weight)(27±3 times) 100 % 100 % Product yield (weight)(27±1.5 times) 81.8 % 100 % Rejected number due to excessive shrinkage 9 (lightweight) 1 Final product yield (27±3 times) 83.6 % 98.2 % Final product yield(27±1.5 times) 65.5 % 98.2 % As will be seen from Table 2 above, the present experimental example yielded apparently good results as compared with the comparative example according to conventional molding. The product yield was 98.2% even though the product standard was changed from 27±3 times to 27±1.5 times, which enables commercial production. The more lightweight the products are, the more susceptible they are to influences of heating conditions upon molding and drying conditions after molding even if within the weight standard, and as a result, the molded products have a tendency to shrink. Such molded products that are short of the dimensional standard are therefore treated as rejected products. In this experimental example, it is apparent that the reduction of dispersion in product weight diminishes lightweight products and accordingly, reduces rejection due to molding shrinkage. As described above, this invention provides a method of manufacturing a resin molded foam in mold cavity by charging resin prefoamed pellets in the mold cavity and subjecting them to foaming molding under heating and is designed to make the charging weight in the mold cavity constant by measuring the apparent density of the prefoamed pellets prior to charging and, on the basis of the results, regulating the cracking width of the mold. In no case have prior art molding methods taken account of the dispersion in apparent density of prefoamed pellets as a feedstock and hence, it has been deemed difficult to decrease the dispersion in expansion ratio (weight) of products and to enhance the quality more than the current situation. However, the experimental results of this invention show that the present manufacturing method tends to reduce significantly the dispersion in expansion ratio of products and not only to enhance the quality, but to bring economical benefit. The methods as set forth in claims 2 and 3 serve to enhance commercial practice of the aforesaid method. Further,the apparatuses claimed in claims 4 and 5 are suitable for the industrial practice of the methods stated above and permit to advantageously produce molded foams according to the methods above. Industrial Applicability The method and apparatus for manufacturing a molded resin foam pertaining to this invention are thus suited to obtain products with low dispersion in weight, and the products are useful for energy absorbers such as automotive bumper core material, planks, containers, shock-absorbing packages such as side pads, or the like.

Claims: A method of manufacturing a molded resin foam comprising charging pellets of a prefoamed resin into a cavity of a mold (5) and subjecting them to expansion molding under heating, which method is characterized in that prior to said charging, the apparent density of the prefoamed pellets is measured; upon charging, the cracking width (7) of the mold is regulated, on the basis of the measurement result, to a definite value determined depending on the apparent density, thereby making the weight of the pellets charged in the mold cavity constant; and thereafter, the mold is closed to perform the molding. The method of manufacturing a mold resin foam, according to claim 1 wherein the measuring of the apparent density is conducted with a portion of the prefoamed resin pellets ready to be charged in the mold cavity or its adjacent portion of the pellets, whereby dispersion in weight of the molded foam is minimized. The method of manufacturing a molded resin foam as set forth in claim 1, wherein the regulating of the cracking width is automatized, whereby dispersion in weight of the molded foam is minimized. An apparatus for manufacturing a molded resin foam comprising an automatic expansion molding machine including a feedstock hopper (1), a feedstock charging equipment (6), a mold (5), a hydraulic mold opening or closing unit (11 - 16), a positioner (9) of opening or closing the mold and a system for heating or cooling the mold, which apparatus is characterized by providing said automatic expansion molding machine with an arrangement (2 - 4) for sampling a portion of a feedstock from the feedstock hopper and measuring its apparent density prior to molding; an equipment (8) for measuring a current mold-closing position which is provided on the positioner of mold opening or closing so as to interlock therewith; and a computer automatic cracking width control device (10) for obtaining the inputs of measured apparent density value and current mold closing position from the foregoing two, computing an optimal cracking width determined depending on apparent density according to a program prepared beforehand and closing a hydraulic feed line for mold-closing of the hydraulic mold opening or closing unit to stop the mold-closing at a position of said optimal cracking width. The apparatus for manufacturing a molded resin foam as set forth in claim 4, wherein said computer/automatic cracking width control device is such a computer/automatic cracking width control device that includes measuring the deviation between an actual mold-closing stop position and a computed setting position of mold-closing stop and, in the next molding cycle, conducting automatic computing and correction.

Inventors: FUKASAWA YUKIO, FUKUI HISATOSHI, TOJO SHIRO, FUKASAWA, YUKIO, FUKUI, HISATOSHI, TOJO, SHIRO
Applicants: KANEGAFUCHI CHEMICAL IND, KANEGAFUCHI CHEMICAL INDUSTRY CO., LTD.
Application year: 1990
Main classification: B29C44
IPC codes: B29C44, B29K105, B29K25