GB2177964A - Cooling moulds - Google Patents

Abstract

In an injection moulding apparatus the mould cavity is defined by a thin metal shell which is arranged to be heated prior to insertion of the molten plastics material, the shell being arranged to be surrounded during moulding by a cooled metal enclosure which, upon insertion of the melt into the shell, comes into intimate contact with the shell whereby rapidly to cool the shell and solidify the melt. The shell is defined by two, relatively displaceable, metal shell members (16, 32) which can be separated as shown for the introduction of a suitable heating means (not shown), such as a gas burner, therebetween. The cooled enclosure comprises two metal platens (10, 12) upon which the members (16, 32) are resiliently and displaceably mounted and which, when they are brought into mutual abutment, bring the members (16, 32) together to define the mould cavity therebetween, the members (16, 32) then lying in respective recesses (14, 30) in the platens. <IMAGE>

Description

SPECIFICATION Improvements in and relating to injection moulding The present invention is concerned with injection moulding.

The moulding conditions chosen for injection moulding by traditional techniques are a compromise between two conflicting requirements. On the one handthe mould temperature needsto be high in orderto facilitate mould filling and to obtain a high integrity product and on the other hand the mould needs to be cold in order to reduce the product cooling timeto a minimum.The usual practice isto have the cooling fluid at orjust below the maximumtem- perature at which the product can be ejected from the mouldwithoutwarping,andtoensureadequateflow by adjusting the temperature ofthe molten polymer in the barrel of the machine. The resulting cooling time is normally about 75% ofthe total cycle time.

It is an object of the present invention to provide a means whereby the latterfigure can be reduced significantly.

In accordance with the present invention, the mould cavity is defined by a thin metal shell which is arranged to be heated priorto insertion of the molten plastics material, the shell being arranged to be surrounded during moulding by a cooled metal enclosure which, upon insertion ofthe melt intotheshell, comes into intimate contact with the shell whereby rapidly to cool the shell and solidify the melt.

Preferably, the shell is defined by two, relatively displaceable, metal shell members which can be separated for the introduction of a suitable heating means, such as a gasburner,therebetween.

Preferably, the cooled enclosure comprises two, relatively displaceable, metal platens which, when they are brought into mutual abutment, are arranged to bring the two metal shell memberstogetherto define said mould cavitytherebetween.

Advantageously, each platen has a respective surface recess shaped to receive one of said metal shells whereby, when the platens are brought in abutting engagement, the shell members lie in the respective recesses in the platens and define the mould cavity therebetween.

The shell members may be spring loaded relative tothe platensso thatwhenthe platens are moved aparttheshell members are automatically displaced out of said recesses. Alternatively, this displacement out of the recesses can be achieved by respective plungers.

By arranging forthe molten plastics material to be injected into a hot mould, easierfilling ofthe mould is obtained and one can use a lower barrel temperature for the molten plastics material. Rapid cooling ofthe mould immediately after injection as a result ofthe then intimate contact with the cooled platens results in a lowering ofthetime requiredforsetting ofthe moulded product and hence of the time at which the mould can be opened and the product ejected.

By use ofthe invention, therefore, a lower cycle time can be achieved and a greater number of products produced per unit time.

Afurther advantage is that by injecting into a hot mould, there is less tendency for condensation to form between the mould and the moulded product than when injection takes place into a cool mould.

The invention is described further hereinafter, by way of example only, with reference to the accompanying drawing which is a diagrammatic sectional view of one embodiment of an injection moulding tool in accordance with the present invention.

The illustrated injection moulding tool comprises a pairof platens 10,12 made of a metal such as steel or aluminium having a high coefficient of heat conductivity. The platens 10, 12 are adapted to be cooled by means of internal and/or external passages or tubes carrying a cooling liquid orfluid. Inthesimplestand preferred arrangementthe platens 10, would be water-cooled.

The platen 10 is formed with a rectangular recess 14 which can receivetherewithin a correspondingly shaped metal plate 16, preferably made of steel. The depth ofthe recess 14 corresponds substantially identically to the thickness of the plate 16so that when the plate lies in the recess 14 its right-hand surface, as viewed in the drawing, lies flush with the right-hand surface 18 of the platen 10.Rigidlycon- nected to the plate 16 and extending perpendicularly thereto are a number of rods 20 (two visible in the drawing) which pass through respective bores 22 in the platen 10. The extent of possible displacement of the plate 16 to the right, as viewed in the drawing, is limited by enlarged end portions 24 on the rods 20.As shown in the drawing, the plate 16 is biassed to the right by means of respective helical coil springs 25 disposed around said rod 20, the left-hand ends ofthe springs engaging against respective shoulders formed in the bores 22. The plate 16 has a central aperture 26 in alignementwith afurther bore 28 in the platen 10 for supplying materialtothemouldasde- scribed further below.

The platen 12 is formed with a stepped recess 30 for receiving therewithin a plate 32 having a recess 34 of a shape corresponding to the shape of the part to be moulded. In this case the recess 34 is cylindrical for forming a circular disc. The flange portion 36 around the shaped recess 34 is flat and its left-hand surface, when the plate 32 is received in the stepped recess 30, lies flush with the left-hand surface 38 of the platen 12. In a similar mannerto the plate 16, the plate 32 is mounted on rods 44 extending into bores 40 in the platen 12, respective helical coil springs 41 being mounted around these rods for biassing the plate 32 to the left as viewed in the drawing. Disposed in the central region ofthe plate 32 is a hole 33 through which a plunger42 can be passed for use in releasing the moulded partfrom the recess 34 in the plate 32 as described below.

The two platens 10,12 are adapted to be relatively displaceabletowards and away from each other (by means not shown) in a direction perpendicular to the plates 16,32. It will be appreciated that when the faces 18 and 38 of the platens are brought into abutment the plates 16,32 are received snugly in the recesses 14 and 30, respectively.

The apparatus also includes a means (notshown) for heating the plates 16,32 when they are in the condition shown in the drawing inwhichtheyare separated from the cooled platens 10, 12. One way of heating the plates is simply to subject them to the flame(s) of a suitable gasd burner,for example a matrix or linear burner of known type. A matrix burner, for example, supplies arrays of small flame jets which cna be suitably directed against the plates 16, 32. The burner(s) can be selectively displaceable into the space between the plates 16,32 when the latter plates are to be heated.

The aforegoing injection moulding apparatus operates as follows.

With the parts in their positions shown in the draw ings,the burner(s) (not shown) is introduced between the plates 16,32 and these plates are heated to an elevated temperature, e.g. 20000. The platens 10,12 arethen movedtowards each otherand into abut ment whereby the plates 16,32 enterthe recesses 14, 30 respectively. However, due to the lack of an intimate contact between the plates 16,32 and the platens 10, 12, the plates 16,32 are not subjected to immediate rapid cooling at this stage. Immediately afterthe platens 10, 12 are brought together, molten plastics material is injected via the bore 28 and hole 26 into the mould cavity defined between the plate 16 and the recess 34 in the plate 32.Because the plates 16,32 are still at a relatively high temperature, the flow of plastics material into the mould cavity is en hanced whereby the mould can be filled completely to obtain a high integrity product. However, upon the mould cavity becoming pressurised by the injection pressure, the plates 16,32 are urged outwards and into more intimate contact with the permanently cooled platens 10, 12 whereby the plates 16,32 become rapidly cooled. This in turn aids the rapid cool ing ofthe moulded product so that the necessary cooling time before extraction from the mould is cor respondingly reduced. After a few seconds, the platens 10,12 can be moved apart, thereby enabling the springs 25,41 to eject the plates 16,32 from the platens.The product itself can be ejected from the recess 34 by means ofthe plunger 42.

The cycle is then repeated. The fast cooling time of the moulded product enables a higher cycle repetition rate and hence a greater number of products per unit time compared to conventional injection mould ing techniques.

The invention is not limited to the particular means of heating described above nor, or course, is the mould shape restricted to the disc shape described.

One alternative means of heating the plates 16,32 is to form them as resistive elements through which an electric current can be passed at low voltage. In this event, the plates 16,32 have to be mounted in an electrically insulated manner. Other means of achiev ing the spring biassed mounting for the plates 16,32 can be used. For example, the springs could be achieved byforming two or more edge regions ofthe plates 16,32with integral resilient portions attached to the platens.

In other embodiments, the springs may be omitted altogether and replaced by other means of causing the relative displacement ofthe plates 16,32 from the platens 10, 12. For example, each plate 16,34 can be urged away from its associated platen 10, 12 by means of a respective plunger, such as the plunger42 so as to bring the parts into their relative positions shown in the drawing.

Claims (9)

1. An injection moulding apparatus having a mould cavity defined by a thin metal shell which is arranged to be heated priorto insertion of the molten plastics material, the shell being arranged to be surrounded during moulding by a cooled metal enclosure which, upon insertion ofthe melt into the shell, comes into intimate contact with the shell whereby rapidly to cool the shell and solidify the melt.

2. An injection moulding apparatus as claimed in claim 1, wherein the shell is defined by two, relatively displaceable, metal shell members which can be separated for the introduction of a suitable heating means, such as a gas burner, therebetween.

3. An injection moulding apparatus as claimed in claim 2, wherein the cooled enclosure comprises two, relatively displaceable metal platens which, when they are brought into mutual abutment, are arranged to bring the two metal shell memberstogetherto define said mould cavitytherebetween.

4. An injection moulding apparatus as claimed in claim 3, wherein each platen has a respective surface recess shaped to receive one of said metal shells whereby, when the platens are brought in abutting engagement, the shell members lie in the respective recesses in the platens and define the mould cavity therebetween.

5. An injection moulding apparatus as claimed in claim 4, wherein the shell members are spring loaded relative to the platens so thatwhen the platens are moved apart the shell members are automatically displaced out of said recesses.

6. An injection moulding apparatus as claimed in claim 4, including respective plungers by which, when the platens are moved apart, the shell members may be displaced automatically out of said recesses.

7. An injection moulding apparatus as claimed in any of claims 2to 6, including a plunger which can be projected through a hole in one of the shell members for assisting the ejection of a moulded parttherefrom.

8. An injection moulding apparatus as claimed in any of claims 2 to 7, wherein one of said shell members contain an aperture for enabling the insertion of the molten plastics material to the mould cavity.

9. An injection moulding apparatus, substantially as hereinbefore described with reference to and as illustrated in the accompanying drawing.