GB2158002A

GB2158002A - Injection moulding

Info

Publication number: GB2158002A
Application number: GB08411554A
Authority: GB
Inventors: James Watson Hendry; Geoffrey David Gahan
Original assignee: PEERLESS FOAM MOULDING CO
Current assignee: PEERLESS FOAM MOULDING CO
Priority date: 1984-05-04
Filing date: 1984-05-04
Publication date: 1985-11-06
Also published as: GB2158002B; GB8411554D0

Abstract

An injection moulding of plastics material has at least one area 14 of increased thickness extending to a predetermined part 16 of the moulding. The thicker portion 14, e.g. a rib, contains an internal cavity 17 resultant from the transmission during the moulding process of an internal pressure through the thicker portion to the plastics material at the predetermined part without substantial pressure drop. The cavity in the or each thicker portion may be formed by injecting a relatively inert fluid, e.g. nitrogen, into the plastics material being moulded, the fluid subsequently being evacuated from the cavity. <IMAGE>

Description

SPECIFICATION Injection mouldings This invention relates to injection mouldings. More particularly, the invention concerns mouldings having at least one loc#ally thicker area, e.g. a rib, extending to a predetermined part of the moulding.

Our British Patent Application No. 8312933 describes a process of producing an injection moulding of plastics material in which the plastics material is introduced in the form of a molten stream into a mould space and a predetermined finite quantity of a non-plastics fluid, which is inert relative to the plastics material being moulded, is injected under pressure into the stream of plastics material at the controlled rate. The fluid is injected through a nozzle around which flows the stream of plastics material whereby the fluid is injected directly into the plastics material, the fluid thereafter flowing simultaneously with the plastics material between the mould surfaces to fill the mould space and exerting a pneumatic pressure on the surrounding plastics material to pressurise the plastics material outwardly towards the mould surfaces.

During filling of the mould the pressure of the fluid injected fluctuates with the pressure of the plastics material, whilst remaining higher than that pressure, to ensure a uniform injection of the fluid throughout the period of injection.

In one embodiment the fluid is a gas, e.g. nitrogen, which forms at least one mass within the plastics material which exerts said pneumatic pressure on the plastics material towards the mould surfaces, the pressure being maintained until the plastics material is self-supporting. The pneumatic pressure of the fluid compensates for the natural shrinkage of the material during cooling. Subsequently the pressure is relieved by causing the fluid to leave the mould, and the mould can then be opened. The evacuation of the fluid leaves a cavity within the resultant moulding. The injection moulding thereby produced employs less plastics material and is lighter than if the moulding is solid.

Also the creation of the internal pneumatic pressure results in a moulding having an external surface which corresponds closely to the internal surface of the mould and which, unlike conventional structural foam mouldings, has a swirl or "splay free" surface finish which may be acceptable without post moulding treatment such as painting, and which minimises the tendency for such marks associated with injection mouldings.

According to the invention there is provided an injection moulding of plastics material having at least one area of increased thickness extending to a predetermined part of the moulding, wherein the thicker portion contains an internal cavity resultant from the transmission during the moulding process of an internal pressure through the thicker portion to the plastics material at the predetermined part without substantial pressure drop.

The provision of the or each thicker portion through which the said internal pressure is transmitted to the associated predetermined part allows lower locking force presses to be used than heretofore to prevent undesirable shrinkage of the plastics material and sink marks in the predetermined part of the moulding.

Preferably the or each thicker portion is a rib.

In one embodiment of the invention the predetermined part associated withthe thicker portion or at least one of the thicker portions is a boss or foot.

Alternatively the predetermined part associated with the thicker portion or at least one of the thicker portions may be an edge or rim portion of the moulding.

Preferably the cavity within the or each thicker portion is produced by injecting a non-plastics fluid, which is inert relative to the plastics material being moulded, under pressure into the plastics material, exerting a pneumatic pressure on the surrounding plastics material, the pressure being maintained until the plastics material is self-supporting, whereafter the fluid is evacuated leaving the desired cavity extending through the thicker portion.

By way of example, a specific embodiment in accordance with the invention will be described with reference to the accompanying drawings in which: Figure 1 is an underplan view of an injection moulding; Figure 2 is a section along line A-A in Figure 1; Figure 3 is a section along both lines B-B and B' B' in Figure 1; Figure 4 is a section along line C-C in Figure 1.

Referring to the drawings, an injection moulding 10 of thermoplastics material has a generally rectangular flat top 11 and dependent walls 12. On the undersurface of the top is an array of ribs 13, 14 radiating from a central point. The rib 13 extends to a boss 15 and each of the three ribs 14 extends to a respective rib 16 on the inside of one of the walls 12 or in the internal corner between two walls 12. The arrangement and configuration of ribs may be changed as desired, but in each case the ribs 13, 14, 16 are hollow having a cavity extending longitudinally therethrough. The boss 15 is also hollow, but the top 11 and walls 12 of the moulding are otherwise solid.

The presence of the ribs 13, 14, 16 serves to strengthen the top 11 and the walls 12 of the moulding, and the existence of the cavities 17 means that the moulding is lighter and employs less plastics material than if the ribs were solid.

However, more importantly, the selected array of ribs is chosen in order that during the moulding process an internal pressure may be transmitted to the plastics material at predetermined parts of the moulding without substantial pressure drop.

In this embodiment the cavities 17 extending longitudinally of each rib are created by the use of an inert non-plastics fluid which is introduced into the mould space simultaneously with the plastics material at the meeting point of the ribs 13, 14, the flow of fluid restricting itself to the thicker areas defined by the ribs 13, 14, 16 and the boss 15 and urging the plastics material towards the mould surfaces and to fill the thinner areas between the ribs.

Subsequently the fluid is evacuated leaving the cavities 17. Thereby the moulding can be formed with the minimum of sink marks caused by shrinkage, by employing considerably lower pressures than in conventional injection moulding as a result of the effective transmission of pressure from the feed point at the centre of the moulding to the extremities of the moulding and to the thicker portions of the moulding. This has not been possible hitherto because of pressure drop in the solidifying plastics material as it becomes more viscous during cooling.

The particular moulding process employed is described in our British Patent Application No.

8312933. It is therefore sufficient to explain herein that the non-plastics fluid, e.g. nitrogen, which is inert relative to the thermoplastics material being moulded, is injected under pressure into the plastics material at a controlled rate simultaneously to the introduction of the plastics material in the form of a molten stream into the mould space. The feed position is at the meeting point of the ribs 13, 14.

As the plastics material flows throughout the mould space, the fluid constricts itself to the thicker areas defined by the ribs 13, 14, 16 and the boss 15, exerting a pneumatic pressure on the surrounding plastics material longitudinally of each rib thereby urging the plastics material to each side and against the mould surfaces. Moreover, the pressure applied by the fluid on the plastics material at the beginning of each rib 13, 14 is transmitted without substantial pressure drop to the boss 15 and the distant end of each rib 16.

The fluid is injected in a manner that its pressure fluctuates with the pressure of the plastics material, whilst remaining higher than that pressure, to ensure a uniform injection of the fluid throughout the period of injection. The pressure applied by the fluid is maintained until the plastics material is self-supporting, whereafter the fluid is evacuated leaving the cavities 17. The mould is then opened and the moulding removed.

It will be appreciated that the invention is not restricted to the specific article shown in the drawings. For example, the invention is equally applicable to any article having a thicker area, e.g.

a rib, extending from the feed point from which the plastics material injected with fluid flows throughout the mould space, wherever may be the chosen feed point for the plastics material.

Also, the invention is applicable where the feed point for the plastics material is different from the feed point for the fluid. For example, the feed point for the plastics material may be at any selected point within the confines of the mould space depending on the boss and/or rib construction of the article being moulded, but the feed point for the fluid would always be into or connected with the or one of the thicker areas.

In the case of our previous British Patent Application No. 8312933, as the fluid flows with the plastics material to fill the mould space, the fluid tends to spread outwardly in all directions thereby forming a pool of fluid within the plastics material.

After evacuation of the fluid the area containing the resultant cavity may then be relatively weak.

Should such a result be a disadvantage it can be overcome in accordance with the present invention in which the cavity or cavities are restricted to the thicker areas.

Moreover, as explained above, the formation of the cavities 17 in the thicker areas assists the passage of the plastics material to the extremities of the mould space without requiring excessive pressures, and allows the relatively light pressures employed to be transmitted to the extremities of the mould space efficiently substantially without pressure drop and then maintained so that the resultant moulding is substantially free of sink marks due to shrinkage.

In addition the present invention enables the achievement of a substantial reduction of the moulded-in stress which during conventional injection moulding results from the necessity to move the plastics material in laminar flow under pressure to compensate for the natural shrinkage during cooling.

Claims

1. An injection moulding of plastics material having at least one area of increased thickness extending to a predetermined part of the moulding, wherein the thicker portion contains an internal cavity resultant from the transmission during the moulding process of an internal pressure through the thicker portion to the plastics material at the predetermined part without substantial pressure drop.

2. A moulding as claimed in Claim 1, wherein the or each thicker portion is a rib.

3. A moulding as claimed in Claim 1 or Claim 2, wherein the predetermined part associated with the thicker portion or at least one of the thicker portions is a boss or foot.

4. A moulding as claimed in Claim 1 or Claim 2, wherein the predetermined part associated with the thicker portion or at least one of the thicker portions is an edge or rim portion of the moulding.

5. A moulding as claimed in any one of the preceding claims, wherein the remainder of the moulding not defined by the or each thicker area is solid.

6. A moulding as claimed in any one of the preceding claims, wherein the cavity within the or each thicker portion is produced by injecting a non-plastics fluid, which is inert relative to the plastics material being moulded, under pressure into the plastics material, the fluid exerting a pneumatic pressure on the surrounding plastics material, the pressure being maintained until the plastics material is self-supporting, whereafter the fluid is evacuated leaving the desired cavity extending through the thicker portion.

7. An injection moulding substantially as hereinbefore described with reference to and as shown in the accompanying drawings.