AU601791B2 - Moulding method and apparatus - Google Patents

Description

2 Application Number:

AUSTRALIA

Patents Act 0 a

WIN

0 1 vIPLETE SPECIFICATION (ORIGI1NAL) Class Int. Class Lodged: Complete Specification Lodged* Accepted: Published: Priority Related Art: IThis document contains -t he lanendments made und~r_ Stion 49 and is correct for printing APPLICANT'S REF.: C.A of PH1 9721 Name(s) of Applicatnt(s): Address(es) of Applicant(s): Adi!uaI InvenlorWs) KIWOISLEY NOMINEES PTY. LTD.

(formerly Peter Bayly Asociates Australia Pty. Ltd.) 73 Whitehorsie Road, 3alwyn, Ilictoria, 310:3, Australia Dennis Samuel Stones Peter Kingsley Bayly Address for Service is:.

PH-ILLIPS, ORMONDE AND FITZPATRICK Patent and Trade Mark Attorneys 367 Collins Street Melbourne, Austrdlt, 3000 Compige, Speclfipatiott for the LO11D E Q AT' LI30 pp "NUDN METHtOD AND APARAT, 3 FE~1~ description of this. Invcntloia, including townl to The' Nl)owing statement 4 full applicants): P19/3/84 4

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BACKGROUND OF THE INVENTION This invention relates to the moulding of products by the introduction of heated material under pressure into a mould cavity. The invention is particularly concerned with moulding products from crystalline plastics materials such as those classified as polyolefins, polyamides and polyacetals, by way of example, but it may be applied to amorphous plastifs also.

Products made from polyolefins such as polyethylene and polypropylene are commonly manufactured by means of a hot injection moulding process. That is, the plastics material is e c heated to a flowable state and while in that state is injected under pressure into a mould cavity. Polyolefin materials, however, experience a substantial degree of shrinkage on cooling so that the final product generaly has a volume le6s than that of the mould cavity in which it is formed.

One consequence of the post-moulding shrinkage is that a surface or surfaces of the final product can be distorted so as tO not follow the configuration intended. For example, a flat surface may be depressed by the shrinkage to have a shallow concavity. In the case of polypropylene, which is a relatively rigid material, voids may be created within the body of the product thereby disturbing its integrity.

The distortion problems resulting from shrinkage are particularly troublesome in, products having relatively thick and thin sections because of the different coolinq rates of those sections. In that regard, polyolefins are poor conductors of heat so that the cooling rate is in part dependent on conduction through contact with the body of the mould. Thicker sections shrink more than thinner sections and K)ton n are therefore more likely to lose contact with the mould body so that an air gap is formed and that reduces the cooling rate of the associated material section.

Various solutions have been proposed for the foregoing problem, but none have been entirely successful. One proposed solution involves including f ino talc in the plastics material, but that has the disadvantage of reducing the ability to achieve a smooth surface finish on the product.

The use of blowing agents has the same result. Another *~proposal involves the use of very high injection pressures so

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as to over-pack the mould cavity, but that does not entirely solve the problem since p. amature orystallinity can result.

Cc CC It is an object of the present invention to provide an cc improved method and apparatus for forming products from plastics materials.

SUMMARY OF THE INVENTION According to one aspect the invention provides a method of forming products from plastics material including the steps of, heating a quantity of plastics material to bring that material to a flowable state., introdijcing the heated material into a, mould cavity and continuing that introduction to at cco least: substanti ally, fill said cavity with a body of said material, allowing s;aid body to cool and solidify while ,contained in, said cavity, reducing, the volumetric size of said cavity by a predetermined extent while said cooling is occuring so as to at least substantially compensate for shrinkage of said body, and. removing the solidified body from said cavity. It is preferred, that the, afc~rementioned reduction in volumetric size is effected in a progressive and continuous manner.

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IL _V M1 According to another aspect of the invention there is provided a method of forming products from plastics material including the steps of, substantially filling both parts of a two-part mould cavity with a body of hot flowable plastics material, allowing said body to cool while contained in said cavity, transferring said material between said mould cavity parts while it is ll in a flowable state so that substantially the entire ,aid body of material is contained in one of said cavity parts, and allowing said body to solidify in said one cavity part. The aforementioned transfer is

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preferably achieved by positively displacing material from one part of the cavity and simultaneously transferring it to the other part of the cavity. The overall volumetric size of the Scavity is thereby reduced and the displacement preferably continues in a progressive manner until the cavity size and shape is complementary to that of the required product.

According to yet another aspect of the invention there is provided apparatus for moulding products from plastics material including, a mould having a cavity formed therein, said cavity comprising a shrinkage compensating part and a C product forming part, an opening through which said cavity parts communitate, an entrance port through which material is introduceG into said cavity, and means operable to tranfer material through said opening from said shrinkage compensating part to said product forming part.

An embodiment of the invention is described in detail in the following passages of the spedificaton which refer to the accompanying drawings. The drawings, however, are merely illustrative of how the invention might be put into effect, so that the specific form AT -4and arrangement of the various features as shown is not to be understood as limiting on the invention.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a semi-diagrammatic sectional view of one form of apparatus suitable for carrying out the method according to the invention.

Figure 2 is a cross-sectional view taken along line II-'I1 Figure 1.

Figure 3 is an eniarged view of portion of the apparatus shown In figure and showing material injected into the mould cavity.

Figure 4 is a view similar to Figure 3 but showing the mould cavity reduced in size to a further extent.

Figure 5 is a view similar to Figure 4 but showing the mould cavity reduced to its final size and shape.

Figure 6 is a diagrammatic view showing the mould separated to permit removal of the formed product, DESCRIPTION OF A PREFERRED EMBODIM04NT It will be convenient to hereinafter~ describe the invention with reference to a particular form of apparatus for carrying out the foregoing method,. The invention is not limited, however, to that form of apparatus.

The apparatus, which is shown in part in Figure 1, includes a mould I having a cavity 2 shaped as required to produce a particular produc. A conventional injection device (not shown) can be arranged to inject flowable plastics material into that cavity through a gate 3, which may also be conventional in terms of shape, size and location. The mould 1 is characterised, however in that the cavity 2 is composed of two parts a product forming part 4 and a shrinkage 1 cc cc e

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compensating part 5 which communicates with the cavity part 4 through an opening 6. Means is provided whereby the size of the cavity part 5 can be varied in the construction shown and that is achieved by forming the base 7 of the cavity part 5 as a movable surface. In the example shown, that base 7 is formed by the end face of a plunger 8 which is movable towards and away from the c~vity part 4.

It is preferred, as shown, that the cavity part 5 opens into the cavity part 4 through a substantially flat surface 9 of that cavity part 4. Such an arrangement is not essential, but is generally more conveniant than forming the opening 6 in a contoured surface of the cavity 2. Apart from the foregoing, the location of the cavity part 5 can be selected to suit the form of the cavity part 4 and the nature of the product to be formed in the cavity 2. For example, if the product is to have a handle, it may be convenient to locate the cavity part 5 at that part of the cavity part 4 which is to form an outer end portion of the handle, Yn the, arrangement shown, the plunger 8 is preferably slidably mounted in the cavity part 5 so that the effective size of that cavity part 5 is reduced as a consequence of movement of the plunger 8 towards the cavity part 4 (Figures 3 and For that purpose, it is generally convenient to form the cavity part 5 as a cylinder having its axis extending through the general plane of the cavity 9. The opening 6 is therefore generally circular and of a relatively large size as compared with the kntrance part 10 through which the mould gate 3 communicates ;vith the cavity 2. It is not necessary, however, that the ctvity opening 6 be of the same size and/or shape as tha cylindrical body of the cavity part 5 within e 00 -6-

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which the plunger 8 slides. For example, both the pl.unger 8 and the cavity part 5 may be stepped so as to provide a pair of opposed abutment surfaces which engage to positively limit movement of the plunger 8 towards the cavity part 4.

The plunger 8 is movable between retracted position and an extended position. The volumetric size of the cavity part 5 is at its maximum and minimum v >en the plunger 8 is at its retracted (Figure 1) and extendeO, (F0igure 5) positions respectively. It is preferred, but not necessary, that the effective size of the cavity part 5 is substantially zero in the extended position of the plunger 8 and that the end f acl 7 of the plunger 8 forms a segment of the surface 9 of the cavity part 4.

c r It will be understood from the foregoing that when the plunger 8 is in its retracted position (Figure 1) the uavity part 4 of the mould 1 has -Q lateral protrusion or sink hole formed by the cavity part 5 an~d as a result the cavity 2 is unable to form a product of the intended shape. That is, material injected into the mould cavity 2 wiJJ. define a product having the desired shape apart from a Laterally protruding segment having a size and shape cowisterrt with to that of the cavilty part 5. When the plunger 8 is in the extended position (Figure however, the aforementioned protrusion disappears and the cavity part 4. encloses a space having the configuration of the intended product.

The size of the cavity part 5 can be determined to suit particular circumstances and requirements. It is generally found, however, that satisfactory results are achieved by having the volume of the. cavity part 5 when at it,% maximum effective size, in the range of 4 to 6 percent of the volume AT-7 of the cavity part 4. That is, for crystalline plastics, and a volume difference of approximately 1 to 2% might be adopted for amorphous plastics. A volumetric relationship above or below either of the stated ranges may be adopted in some circumstances. It has been calculated that a volumetric shrinkage of approximately 3.8 percent occurs in injected moulded products formed of polyolefins, so that a make-up volume of material in the hot state of 4 to 6 percent will 'usually adequately compensate for the aforementioned shrinkage in such materials.

In the arrangement shown, the mould gate .ay open into the cavity part 5 but such an arrangement is not essential once the gate 3 could open direct into the cavity part 4. One advantage of the arrangement as shown, is that the gate 3 is positively closed by the plunger 8 during movement of the plunger towards the extended position (see Figure 3) and backward flow of material out of the mould cavity 2 is thereby positively prevented. Under normal circumstances, however, such backward flow is resisted to a sufficient extent by the relatively small size of the entrance part 10 and the S relatively rapid solidification of material at the gate 3.

S" When apparatus of the foregoing kind is in use, hot plastics material is introduced into the mould 1 through the gate 3 while the plunger 8 is in its retracted position as shown in Figure 1. Material entering the mould 1 passes through the cavity part 5 and into the cavity part 4 to fill both of those cavity parts 4 and 5. The injection of material is stopped at a time which is predetermined to provide complete filling of the cavity 2 while subjected to a particular internal pressure or pressure range AT 8 I, Aj Filling of the connected cavity parts 4 and 5 by the hot flowable material constitutes the first stage of the process.

The second stage commences with initiation of movement of the plunger 8 towards its extended position. That initiation may take place at the time of *a-ing the first stage, or slightly in advance of or slightly after that time. The second stage is completed when the plunger 8 reaches its extended position as shown in Figure 5, and the plunger 8 is held at that position for the necessary cooling phase of the mould 1 and may not be retracted -until after the product 12 (Figure 6) is expelled from the mould 1.

The working pressure for the plunger mar be greater or less than the pressure under which material is injected into the mould 1, and the selection will depend at least to some extent o the configuration of the mould cavity. In one example arrangement the working pressure is approximately to 20% of the injection pressure. The end face 7 of the plunger 8 may be flat, contoured, or patterned, as required to blend into or form a continuation of the a~jacent surface 9 of the cavity part 4 so as not to disturb the final appearance of the moulded product 12.

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Go it is tound that a skin 13 (Figures 4 and 5) forms around the body 14 of material contained in the mould cavity 2 before and/or during movement of the plur.Uer 8 towards its extended position. That skin 13 is formed at the surfaces of the cavity parts 4 4nd 5 because of the higher cooling rate at he interface between the mould I and the contained material.

That is, solidification of the material commences at the inti.rface between the body 14 of injected material and the 6urfaces of the mould cavity 2. The resuliing cote of

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V relatively hot and less solidified material therefore extends across the imaginary (at that time) boundary between the two cavity parts 4 and As the plunger 8 moves towards its extended position, it pushes material from the cavity part 5 intu the cavity part 4 so as to progressively increase the amount of material contained in the latter cavity part. That increase serves to at least subs tant ial ly make up for losses in material volume within the cavity part 4 resulting from shrinkage during cooling and in some cases may exceed the expected locses due to shrinkage, In any event, the resulting progressive addition to the relatively hot core of material in the ca~vity part 4 holds the material skin 13 in contact with the sur'rounding surface of the mould 1 and thereby promotes rapid cooling of the material. As will be seen from Figures 4 and -portion of the skin 13 which is displaced from the cavity ~part 5 is caused to punctuate into the material body 14 within the cavity part 4, Also, the progressive addition of material to the cavity part 4 counters the problems normall. associated with uneven cooling of thick and thin produci sectionst since it ensures Sthat art expansion effect is maintained in the thicker sections during their relatively slow cooling process. Distortion due to shrinkage of such -,actlQns is thereby avoided or at least substantially alleviated, The: second, stage of the process is completed when the cavityr plunger 8 reaches the end of 'ts stroke into the extendod position. At, that time, the outer end of the plunge, 8 is ptefertibly located so that its end face 7 establishes the form, and relative disposition of at least part of, a sUrface of AT-0 the mole rdc 2. The mould 1 may be retained in a closd psitin fr asuitable cooling period after completion of that second stage, The moulded product 12 may be removed from the mould I in any suitable fashion, It appears that the second stage of the process causes an inward folding of part of the skin 13 as referred to above, In the example described above, movement of the plunger 8 towards its extended position progressively forces that part of the skin 13 which forms around the cylindrical surf ace of the cavity part 4, to move into the body 14 of m~aterial within 'the cavity part 4 *Some folding of that skin part occurs during that process, but such folding does not disturb the integrity of the product 12. The pressure applied by the plunger 8 ensures that th~t core material is closely packed against the folded skin part. Furthermore, the appearance of the product. will not; be disturbed, at least substantially, by that folding of the skin 13, it iN iormally found that the cleavage line which is formed at the outside surf ace of the product 12 because of that folding,. cannot be detected without the aid of magnification. As a precaution, however, the product may be designed so that the cleavage line i's located at a step, groove, ridge, or other discontinuity in the product surface and is thereby hidden from detection under P 16normal circumstances.

it is found that the method described results in well firished products of predictable and accurate form. Surface distortions due to uneven cooling rates and high shrinkage rates are virtually eliminated. so that. it is poss~ble to form quality products of complex shape which could not be formed by, conventional injection moulding techniques, -41- Various alterations, modifications and/or, additions may be introduced into the constructions and arrangements of parts previously described without departing from the spirit Q7 ambit of the invention as defined by the appended claims.

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Claims (4)

1. A method of forming products from a plastics material including the steps of: introducing hot flowable plastics material into a mould cavity by way of an entrance port so as to substantially fill said cavity with said material, said cavity comprising a product forming part which has the shape of the product to be formed and a shrinkage compensating part which communicates with said product forming part through an opening substantially larger than said entrance port, said entrance port communicating with said shrinkage compensating part so that said material is introduced into said product forming part by way of said shrinkage compensating part; rr allowing said body of material to cool by contact with the surrounding surfaces which define said cavity and thereby producing a relatively denue skin around a relatively hot core or 1 of the material, commencing to displace the material from said Sshrinkage compensating part at substantially the time said substantial filling is completed; continuing said displacement so as to progressively move the material contained in said shrinkage compensating part through said opening and into said product forming part to thereby add to the material contained in said product forming part and substantially maintain said skin in contact with said surrounding surfaces as said material body continues to cool and shrink, said displacement causing a portion of said skin to be projected into the portion of said material body contained in said product forming part; automatically closing said entrance port after commencement and before completion of said displacement; controlling said displacement so that the pressure within said cavity does not rise above the pressure therein at the time said substantial filling is completed; stopping said displacement; allowing said material body to cool to a satisfactory extent, and then removing the cooled body from said cavity.

2. A method according to clai 1, wherein said displacement results in said material being substf Itially completely 39 exhaused from said shrinkage compensating part. KA I 13- VA* ci l:ne ilowing statement is a full description of this inventioh, including t I it kjowr to applicant(s): P19/3/84 //l

3. A method according to claim 1 or 2, wherein said material is introduced into said product forming part by way of said shrinkage compensating part.

4. A method accordiing to any preceding claim, wherein said displacement of material from said shrinkage compensating part occurs at a pressure substantially less than the pressure at which said material is introduced into said cavity through said entrance part. A method according to any preceding claim, wherein a plunger is slideably mounted in said shrinkage conrpensating part for movement towards and away from said product forming part, an end surface of said plunger forms a moveable surface of said shrinkage compensating part, and said displacement is terminated when said end surface is positioned substantially at the boundary between said two cavity parts. A method of forming products substantially as herein S particularly described with reference to what is shown in the S accompanying drawings. DATED 1 June, 1990 PHILLIPS ORMONDE FITZPATRICK Attorneys fort KINGSLEY NOMINEES PTY. L c c e 4546k S LS KA 14 'j