GB2278308A

GB2278308A - Gas assisted injection moulding method and apparatus

Info

Publication number: GB2278308A
Application number: GB9309773A
Authority: GB
Inventors: Terence Colwyn Pearson
Original assignee: GAS INJECTION Ltd
Current assignee: GAS INJECTION Ltd
Priority date: 1993-05-11
Filing date: 1993-05-11
Publication date: 1994-11-30
Also published as: GB9309773D0; GB9314773D0

Abstract

Method and apparatus for the gas-assisted injection moulding of thermoplastic or thermosetting components so as to avoid the formation of sink marks and other surface imperfections opposite to bosses or pillars moulded integral with the main body of the components in which pressure is applied within a tubular section at the base of the pillar or boss, or in close proximity on the outside surface adjacent to a pillar or boss within a sealed area surrounding the pillar or boss created by moulding of integral seals to prevent the escape of gas, and to enable the pressure to be exerted on the surface of the plastic remote from the mould cavity surface to maintain the plastic material in good contact with the mould cavity surface. <IMAGE>

Description

GAS ASSISTED INJECTION MOULDING METHOD AND APPARATUS CASE 4 The invention relates to the injection moulding of thermoplastic and thermosetting materials, and in particular a method of injection moulding which involves the injection of gas into the mould cavity in order to create and control a pressure through the medium of the gas on the surface of the plastic in locally sealed areas and which are surrounded by a seal to prevent the escape of the gas.

The conventional well known, widely practiced process of injection moulding involves the heating of thermoplastic materials in granular powder form to a melt temperature at which they become molten and are then injected into a mould cavity under comparatively high pressure of up to 1400 kg per sq cm (20000 lb per sq inch or more). Moulds normally consist of two halves which are mounted on moulding machine presses in order that the molten plastic material may be injected into the mould cavities at high controlled pressures. The moulds normally consist of two halves to form the mould cavities into which the plastic is injected. Pressure is maintained on the plastic by the moulding machine whilst the plastic cools and solidifies. During the cooling cycle the molten plastic shrinks volumetrically, typically by up to 6%, and therefore it is necessary to compensate for this shrinkage by injecting additional material into the mould cavity under high pressure in order to overcome the resistance to flow caused by the plastic material during its transformation from a molten into a solid state.

One result of the natural volumetric shrinkage is for the plastic to move away from the mould cavity surfaces, particularly in positions opposite to ribs, bosses or other protrusions from the opposite face of the plastic section. As a result sink marks are caused on the outer surfaces which are often unacceptable when the visual appearance of the moulding is important.

During recent years there have been numerous attempts at developing versions of injection moulding which reduce the requirements for high pressures and improve the quality of the mouldings, with reduced tendency for the material to sink away from the mould cavity surfaces during cooling. These processes include a group known as "gas assisted or gas injection moulding" in which gas is injected into the molten material flow, either during the injection of the plastic into the mould or following initial filling of the mould cavity with molten plastic. In both cases the gas forms continuous channels or hollow sections within the plastic. Thereby it is possible to create and transmit a pressure through the gas within the plastic material, and for the gas to expand to compensate for shrinkage in the material. At the same time the plastic material continues to be forced against the mould cavity surfaces by the internal gas pressure, thereby making mouldings which replicate the mould cavity surface and are free of sink marks. However there are restrictions and limitations in what can be achieved by the use of internal gas processes.

In one group of processes continuous gas channels and hollow sections are formed with the objective of being able to transmit and control pressure within the moulding from an external gas source, and then at the end of the cooling cycle it is possible to exhaust the gas from the moulding thereby relieving the pressure before ejecting the moulded article from the mould. In this case the gas is totally enclosed within the molten material and the path of entry, and is used as a means of exerting pressure within the plastic material. These processes have limitations on their application by virtue of the moulding thickness and the impracticability of injecting gas within thin sections. It has also been found that it is virtually impossible to create surfaces on flat mouldings which do not bear evidence of bosses or pillars on the opposite side, and therefore do not fulfil the requirements of what are known as "class A finishes" by the automotive industry. Class A finishes require surfaces which have no visual optical changes or contour on the surface before or after paint finishing. In many cases it is impossible to design the moulding so that a thicker section can be included in which a gas channel can be formed in order to create internal pressure adjacent to parts of the moulding where sink marks occur, and in particular at the base of pillars or bosses which are often required as a means of fixing other parts or components to the moulding by the use of screw threaded inserts or with self-tapping screws.

One injection moulding method, which is the subject of patent application PcT/GB 93/00212 by the same inventor as this invention, provides a means of applying gas pressure to the surface of the plastic within the mould cavity by injecting gas so that it forms a film or layer of gas between the plastic material and the adjacent mould cavity surface in a restricted area which is totally surrounded by an integrally moulded seal.

An embodiment of this invention is the application of pressure to the base and internal surface of a hollow pillar or boss by feeding gas under pressure down a hollow tubular core which forms the internal hollow section of a boss or rib, so that the gas is injected onto the plastic surface at the end of the core thereby creating a pressure on the material on the opposite side of the plastic to the outer visible face on which a sink mark would otherwise occur. This enables pressure to be created and controlled on the plastic whilst the plastic cools and is transformed from a molten to a solid state during the moulding process. At the extremity of the core and the point at which the gas exits from the core, the core is constructed so that gas passes through a porous metal insert which has a porosity which enables the gas to pass through it but prevents the more viscous molten plastic entering into it, and possibly blocking the gas channel.

An alternative means of preventing the gas entering the tubular gas feed is to fit a solid pin or rod within the tubular core to enable the gas to pass between the solid pin or rod and the inner surface of the tubular core by allowing sufficient clearance between the two. This will enable the gas to pass through the space between the rod and the inner surface of the tube, but will prevent the ingress of the more viscous molten plastic. A typical clearance between the rod or pin and the tube is 0.003 mm (0.001").

Another embodiment of the invention enables the gas pressure to be applied to the surface of the plastic surrounding the pillar or boss, as illustrated in Figures 2A and 2B, by injecting gas from a gas channel in the mould and through a porous metal insert or inserts positioned within close proximity to the base of the pillar or boss.

The gas is prevented from escaping and leaking across the surface of the plastic between the plastic and the adjacent mould cavity surface by the inclusion of an integrally moulded seal, surrounding the gas entry points and the pillar or boss and in some cases strengthening ribs or webs which connect the pillar or boss to the body of the moulding.

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 1A is a section view through a hollow plastic boss which is integral with the body of the material and strengthened with the addition of four supporting ribs.

Figure 1B is a plan view of the tubular boss illustrated in Figure lA Figure 1C is another section view of the hollow boss showing the passage of gas and the creation of pressure within the plastic.

Figure 1D is a plan view of the boss illustrated in Figure 1C.

Figure 2A is a section through a hollow boss and illustrating an alternative position for the injection of gas which is totally surrounded by an integrally moulded seal.

Figure 2A is a section through AA of Figure 2B.

Figure 2B is a plan view of the boss shown in Figure 2A illustrating the position of the integrally moulded seal and four positions for gas injection outside the moulded boss.

Figure 2C illustrates the passage of gas through a channel within the mould and a porous metal insert creating pressure within the plastic on the outside of the boss but within the area which is surrounded by an integrally moulded seal, and is a section AA through Figure 2D.

Figure 2D is a plan view of the moulded boss and supporting ribs shown in Figure 2C.

Referring to Figure 1, the injection moulding apparatus comprises a mould having two parts 1 and 2. The mould parts define a mould cavity and in particular the moulded hollow boss or pillar 3. Typically but not essentially the boss is supported by, and made more rigid by, the inclusion of four triangular webs 4. The hollow tubular section of the boss is formed by a mould core 5 which is also hollow in order to provide a gas channel 6. This enables the gas to be fed through the tubular section of the core and then through a porous metal insert 7 in order that a layer or film of gas can be formed at the base of the core 8, thereby enabling pressure to be exerted on the plastic from the gas source 9. In Figure 1C it is illustrated that local expansion of the gas at the base of the boss will compensate for the local volumetric shrinkage of the plastic at the base of the boss by local movement within the plastic, thereby forcing the plastic on the opposite face of the body of the moulding against the mould cavity surface during cooling and preventing it sinking back from the surface.

As an alternative to the sintered metal 7 a pin or rod can be fitted within the core enabling gas to pass down the channel and between a clearance of the pin and rod and the tubular section 5. A typical clearance of 0.003 mm (0.001") will allow the passage of gas but prevent the ingress of plastic.

In the example shown in Figures 1A, B, C and D the gas is prevented from escaping across the surface of the plastic and between the plastic and the mould cavity surface by the walls of the boss 3 which create a seal around the core, thereby preventing the gas from leaking between the plastic and the core surface and escaping from the mould.

Referring to Figure 2A another embodiment of the invention is illustrated and shows a method of creating gas pressure on the outside of the boss or pillar by passing gas through a gas channel or channels 10, and then through porous metal inserts 14 positioned at the mould surface and adjacent to the base of the pillar or boss and within the area created by the integrally moulded seals 11.

The above described method and apparatus operates as follows :1. Molten plastic is injected from the moulding machine in the conventional manner to the mould cavity in the mould mounted on the moulding machine press platens.

2. The moulding cavity is filled or nearly filled with molten plastic including filling of pillars, bosses and other protrusions.

3. Gas is injected into the passages and channels within one half or both sides of the mould and injected onto the mould cavity surface, either through porous metal inserts or through a restricted space between the inner core section and a rod or pin as described.

4. The gas is injected at a controlled variable pressure and forms a layer between the point of gas injection and plastic surface. This layer or film of gas may expand in order to compensate for the local volumetric shrinkage of the plastic as it cools and is transformed from a molten into a solid state.

5. The gas pressure is adjusted so that what is referred to as "hold-on pressure" can be applied to the plastic as it solidifies, forcing the plastic onto the opposite mould cavity surface thereby preventing sink marks at the base of the pillar or boss 13.

6. After the plastic has solidified the pressure in the gas is reduced by exhausting through the path of entry of the gas. The mould is then opened in order that the finished moulded article can be ejected and removed.

Gas is not injected into the plastic material and gas does not penetrate the wall section as in other gas assisted processes.

The above described apparatus and process enables the manufacture of mouldings with improved quality, with particular reference to the avoidance of sink marks on outer visible surfaces of injection mouldings.

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 this invention, and in particular the size of bosses and pillars may vary in proportion to the main body of the moulding.

The above description of the invention relating to apparatus and method also applies to mouldings made thereby.

Claims

I. A method of injection moulding a moulded article comprising at least one protrusion including the steps of injecting molten material into the mould cavity to fill the cavity, allowing the molten material to set, and injecting a gas under pressure into the mould as the material sets to pressurise the setting material at or adjacent the base of the protrusion and thereby press the material against the mould surface remote from the protrusion thereby to inhibit the formation of sink marks in the moulded product on the surface adjacent the protrusion.
2. A method of injection moulding as claimed in claim I, in which the gas is injected through a porous insert which prevents molten material flowing into the channel of the gas.
3. A method of injection moulding as claimed in claim I, in which a restriction is formed in the incoming channel for the gas to inhibit the inflow of material into that channel whilst permitting the passage of gas.
4. A method of injection moulding as claimed in any preceding claim, in which pressure of the gas is varied whilst the material is setting.
5. A method of injection moulding substantially as hereinbefore described with reference to the accompanying drawings.
6. Apparatus for injection moulding comprising a mould having two parts defining a mould cavity, a mould core in the mould cavity shaped to provide a protrusion in the moulded article in use, the mould core being hollow to provide a gas channel for the injection of gas whereby in use gas may be applied under pressure at or adjacent the base of the protrusion during moulding in order to inhibit the formation of sink marks in that surface of the moulded article remote from the base of the protrusion.
7. Apparatus as claimed in claim 6, in which a member is disposed in the mould core to form a restriction to inhibit ingress of moulding material into the gas channel defined thereby whilst permitting passage of gas.
8. Apparatus as claimed in claim 7, in which the member is a rod.
9. Apparatus as claimed in claim 8, in which the member is a pin.
10. Apparatus as claimed in claim 7, 8 or 9, in which the member defines a gap of 0.003mm with the mould core.
II. Apparatus as claimed in claim 6 in which a porous insert is disposed in the mould core to inhibit ingress of moulding material into the gas channel defined thereby whilst permitting passage of gas.
12. Apparatus as claimed in claim II, in which the insert is metal.
13. Apparatus as claimed in any of claims 6 to 12, in which a gas channel is provided outside the mould core to enable gas to be supplied adjacent the base of the protrusion.
14. Apparatus as claimed in claim I in which a porous insert is disposed in the gas channel to inhibit ingress of moulding material whilst permitting passage of gas.
15. Apparatus as claimed in claim 14, in which the porous insert is metal.
16. Apparatus as claimed in any of claims 6 to 15 in which a seal is provided adjacent the mould core to inhibit the escape of gas in use along the surface of the protrusion.
17. Apparatus as claimed in claim 16, in which the seal is formed by a profiled extension on a sleeve ejector forming part of the mould.
18. Apparatus for injection moulding substantially as hereinbefore described with reference to the accompanying drawings.
19. A moulded article when made by the method of claims I to 5 or the apparatus of claims 6 to 18.