WO1994022653A1

WO1994022653A1 - Apparatus for and method of injection moulding

Info

Publication number: WO1994022653A1
Application number: PCT/GB1994/000614
Authority: WO
Inventors: Terence Colwyn Pearson
Original assignee: Gas Injection Limited
Priority date: 1993-03-27
Filing date: 1994-03-24
Publication date: 1994-10-13
Also published as: AU6287494A; GB9306425D0

Abstract

Apparatus for injection moulding comprises a mould having two mould parts (2 and 3) defining a mould cavity (I). A plastics material feed to the mould cavity comprises a sprue (5) and a runner (6). The mould cavity comprises a restriction (7) disposed between the feed and a portion (8) of the mould. The restriction (7) is intended to form a hinge in the moulded product. A gas channel (10) formed in mould part (2) leads to a sintered porous metal insert (II) disposed at the surface of the mould cavity. An annulus (12) of V-shaped cross section also formed in mould part (2) surrounds the insert (II) and forms, in operation, a seal around it which confines gas supplied through the channel (10) to a small area around the insert thus pressurising the plastics material in that area and helping to avoid sink marks.

Description

APPARATUS FOR AND A METHOD OF INJECTION MOULDING

The present invention relates to apparatus for and a method of

injection moulding.

It is a well established practice to use the excellent fatigue

resistance to repeated bending by using the plastic material

polypropylene and its various compounds. In order to mould in an

integral hinge between two portions of a moulding it is well known to include in the flow path of the plastic, during filling of the mould, a thinned out section. When the portions of the moulding each side of the thinned out section are flexed relative to each other a bend occurs at the thinned out section, and if flexing is repeated molecular orientation occurs at the thin section, producing an integral hinge which will withstand many more such bending movements. However as a result of the thin section, the material in it cools more rapidly after the flow of plastic has completed the filling of the mould. The cooled material in the thin section then forms a barrier and it becomes impossible to further pressurise plastic material in the portion of the mould on the side of the barrier remote from the feed point. It therefore becomes impossible to compensate for the material volumetric shrinkage by forcing more material into the portion remote from the feed point. This results in sinkage occurring in this portion of the moulding. Attempts have been made to overcome this problem by injecting

material at feed points on both sides of the hinged section. However,

this frequently is not successful because the two material flows meet at

the hinge, causing a weld between the two flows at the hinged section

which produces a weakness at the proposed hinge position.

Problems of this nature may also be encountered in other

applications. For example it is common practice in conventional injection

moulding to inject the plastics into a mould cavity through a relatively

small feed gate aperture, in some cases less than Imm in diameter. This

method of injecting plastic is often used for mouldings of relatively thin

general wall section of between 2.5mm and 0.5mm thickness. In such

cases it is often a problem to compensate for the material volumetric

shrinkage by forcing more plastic into the mould cavity from the

moulding machine because the feed gate freezes off before the moulded

article. This results in sink marks occurring where the material shrinks

from the mould cavity surface.

Other examples of restrictions of flow of plastic within a mould

cavity occur when protrusions, cored-out holes, or locally thinner

sections are situated between the feed gat or gages and thicker sections

of the moulding. As a result it is difficult to apply pressure during

cooling of the plastic by the moulding machine plastic injection unit

through the feed gate, and to force more material into the mould cavity to compensate for volumetric shrinkage of the material as it cools. This occurs when the thinner section freezes off before the thicker section down stream of the plastic flow.

It is an object of the present invention to overcome or mitigate the problems.

According to one aspect of the present invention, there is

provided apparatus for injection moulding comprising a mould having two parts which may be placed together to form a mould cavity, a feed enabling material to be moulded to be fed to the mould cavity, a restriction in the mould cavity either at the feed or disposed between the feed and a portion of the mould cavity and means for feeding gas under pressure to one surface of the portion of the mould cavity whereby to

enable material in that mould portion to be pressurised independently of material fed through the feed.

According to a second aspect of the invention, there is provided

a method of injection moulding comprising the steps of feeding synthetic plastics material to a mould cavity defined between first and second

mould parts and having a restriction either at the point at which material is fed to the mould cavity or between that point and a portion of the mould cavity, allowing the material to cool and applying gas pressure after cooling and freezing of the plastic at the restriction to the material in that portion and removing the gas pressure before opening the mould.

The invention also covers moulded articles made by the above

apparatus or method.

In a preferred embodiment of the invention, the restriction is

disposed intermediate the feed and a portion of the mould cavity and is

intended to produce a hinge in the moulded article between two different

parts of the article. In another arrangement, the restriction is in the feed

itself, the feed being the "pin point" type. The means for feeding gas

comprises a gas channel in one mould part leading to a sintered porous metal insert disposed at the surface of the mould part. An integral seal

formed in the mould part surrounds the insert to prevent the leakage of

gas thereby enabling control of the gas pressure. This may be annular

or any other suitable shape. Advantageously, the seal has a V-shaped

cross-sectional shape but other cross-sections may be employed.

In order that the invention may be more clearly understood,

embodiments thereof will now be described by way of example with

reference to the accompanying drawings, in which:-

Figure I shows a diagrammatic partial cross-sectional view through

a prior art mould. Figure 2 shows a diagrammatic plan view of the mould of Figure

I,

Figure 3 shows a diagrammatic partial cross-sectional view

corresponding to Figure I of a mould according to the invention.

Figure 4 shows a diagrammatic underplan view in partial section

of the mould of Figure I,

Figure 5 shows a diagrammatic cross-sectional view of another prior art mould.

Figure 6 shows a diagrammatic plan view of the mould of Figure I, and

Figure 7 shows a diagrammatic cross sectional view of the mould of Figure 6 modified in accordance with the invention.

Referring to figure I a mould cavity I is defined between mould parts 2 and 3. The mould split line between the two parts is referenced

4. The mould cavity I synthetic plastics material feed comprises a sprue 5 and a runner 6. The mould cavity I comprises a restriction 7 disposed

intermediate the feed and a portion 8 of the mould. The restriction is intended to form a hinge in the moulded article. This may, for example. be between a main body and an integral cap of a single moulded article

of a closure for bottle and other containers. When made of

polypropylene or other like synthetic plastics material, molecular

orientation at the hinge occurs after flexing and the hinge will then

withstand many more such bending movements.

The restriction 7 which enables the hinge to be produced

produces problems of its own however. Because the restriction is so

thin, the material there cools and solidifies much more rapidly than in

other parts of the mould. This then forms a barrier between the plastics

material feed and the portion 8 of the mould on the side of the

restriction 7 remote from the feed. This in turn means that that portion

8 can no longer be pressurised from the feed which in turn makes it

difficult to compensate for volumetric shrinkage in this portion 8 and

that can result in sink marks 9 in the surface of the moulded articles

which spoils the appearance of the moulded article.

In the modified mould shown in Figures 3 and 4, in order to

overcome the above problems provision is made for the injection of gas

to the surface of the plastic in portion 8 of the mould. (In Figure 3 and

4 parts equivalent to the parts of the mould of Figures I and 2 bear the

same reference numerals). The gas is injected via a gas channel 10

formed in mould part 2 and a sintered porous metal insert II disposed in

mould part 2 at the surface of the mould cavity. The porous metal insert permits passage of gas but prevents ingress of plastic. An annulus 12 of V-shaped cross-section also formed in mould part 2 surrounds the insert II and forms an integral seal around the insert. This formation 12 provides an integral seal which confines the gas to a small area thereby

enabling control of the gas pressure. This in turn pressurises plastics

material in that area forcing it against the mould cavity surface during cooling and solidification of the plastic thereby preventing shrinkage

away from the surface and the consequent formation of sink marks. This

is further assisted by local displacement of plastic under pressure of the gas from within the sealed area to compensate for volumetric shrinkage outside that area but within close proximity to it. The formations where sink marks would normally appear comprise two downwardly dependent pillars 8a and a downwardly dependent rib 8b.

Referring to Figure 5 and 6, a mould for a closure comprising mould parts 21 and 22 is shown. The mould split line between the parts

is referenced 23. The mould comprises a formation 24 for a strengthening rib. The feed to the mould is from a sprue 25 via a "pin point" feed gate 26. In operation the dimension of the pin point feed gate is so small that the material solidifies there first causing a barrier to

pressurisation of the mould via the plastics feed. Volumetric shrinkage cannot therefore be catered for and a sink mark therefore appears above the formation 24. In the modification in Figure 7 (in which equivalent parts bear the same references as in Figures 5 and 6) volumetric shrinkage is

compensated for feeding gas via gas channel 28 in the mould part 22 and a sintered porous metal insert 29 disposed at the surface of the mould cavity. This formation 24 provides an integral seal which confines the gas to a small area. This in turn pressurises plastics material

in that area forcing it against the mould cavity surface during cooling and solidification of the plastic thereby preventing shrinkage away from

the surface and the consequent formation of sink marks. This is further assisted by local displacement of plastic under pressure of the gas from within the sealed area to compensate for volumetric shrinkage outside that area but within close proximity to it.

It will be appreciated that the above embodiments have been described by way of example only and that many variations are possible without departing from the scope of the invention.

Claims

1. Apparatus for injection moulding comprising a mould having two parts (2 and 3) which may be placed together to form a mould cavity,

a feed (5,6) enabling material to be moulded to be fed to the mould cavity, a restriction (7) in the mould cavity (1 ) either at the feed or

disposed between the feed and a portion (8) of the mould cavity and

means for feeding gas (I0) under pressure to one surface of the portion

(8) of the mould cavity whereby to enable material in that mould portion to be pressurised independently of material fed through the feed.

2. Apparatus as claimed in claim I, in which the means for feeding gas (I0) comprises a gas channel (I0) in one mould part (2) leading to a sintered porous metal insert (II) disposed at the surface of the mould part.

3. Apparatus as claimed in claim 2, in which the mould part (2) is shaped to form an integral seal in use around the insert (II) to prevent

leakage of gas thereby enabling control of the gas pressure.

4. Apparatus as claimed in claim 3, in which the mould is shaped to form an annular seal.

5. Apparatus as claimed in claim 3, in which the mould is shaped to form a seal of V-shaped cross section.

6. A method of injection moulding comprising the steps of feeding

synthetic plastics material to a mould cavity defined between first and

second mould parts and having a restriction either at the point at which

material is fed to the mould cavity or between that point and a portion

of the mould cavity, allowing the material to cool and applying gas

pressure after cooling and freezing of the plastic at the restriction to the

material in that portion and removing the gas pressure before opening

the mould.

7. A moulded product made by the method of claim 6.