GB2321424A

GB2321424A - A moulding apparatus with means to stir flow fronts meeting in use

Info

Publication number: GB2321424A
Application number: GB9624574A
Authority: GB
Inventors: Paul Stidworthy
Original assignee: University of Warwick
Current assignee: University of Warwick
Priority date: 1996-11-27
Filing date: 1996-11-27
Publication date: 1998-07-29
Also published as: GB9624574D0

Abstract

Injection moulding apparatus comprising a mould cavity (34) having an inlet (26) for injection of mouldable material, and a region (14, 16) in which at least two flow fronts of mouldable material meet in use due to the disposition of an insertion 30 to form a hole 12 in the article 10 to be made by the apparatus, wherein means (40) is provided for effecting by physical contact therewith localised movement of the mouldable material in said region in use. The means 40 is a rotatable member which is flush with surface 28 of a plate 22 defining mould cavity 34 with plate 24 so that member 40 does not protrude into the cavity 34.

Description

IMPROVED INJECTION MOULDING APPARATUS AND METHOD The invention relates to an apparatus and method of injection moulding materials such as thermo-plastics like polymers and polymer matrix composites. In particular, the invention relates to an apparatus and method which mitigates against the formation of weld lines at converging the flow front of molten materials in a mould.

In known injection moulding apparatus, it is common to obtain internal and surface defects in the moulded product. Such defects reduce mechanical properties and aesthetic qualities of the moulded products. These defects are known as weld lines and form when two or more converging flow fronts meet. This may occur when multiple injection points are used or flow around on insert happens. For example, in forming a wheel trim for a car wheel, it is known to injection mould the wheel trim in a mould having a centrally located injection sprue through which the molten plastics is injected. Inserts in the moulding cavity are used to create features such as apertures within the wheel trim to enhance the appearance thereof. However, these inserts create a divergence in the radial flow of the molten material thereby causing two flow fronts eventually to abut on the outermost side of the mould insert.

Several techniques are known for trying to prevent the formation of deformations at such abutting flow fronts, including for example the use of ultrasonic vibrations as described in Japanese patent JP 6262648. Also, it is known to use complex pumping systems to effect oscillations at flow fronts as described by P.S. Allan and M.J. Bevis in "Polymer Processing" of January 1994 at pages 7 to 9. However, these techniques require sophisticated and relatively expensive apparatus.

An object ofthe invention is to avoid or at least mitigate the problems ofthe prior art.

According to one aspect of the invention there is provided injection moulding apparatus comprising a mould cavity having an inlet for injection of mouldable material, and a region in which at least two flow fronts of mouldable material meet in use, wherein means is provided for effecting by physical contact therewith localised movement ofthe mouldable material in said region in use. Beneficially, the movement can enable enhanced diffusion of molecules of the mouldable material at the interface of the two flow fronts thereby enhancing mechanical properties of the material at the flow front and also the surface structure thus improving the aesthetic qualities of the moulded product. Hence surface contact eliminates weld lines by mechanical action.

Preferably, the movement means comprises a member which physically contacts the surface ofthe mouldable material in use and more preferably is substantially flush with the mould cavity. Beneficially therefore the member does not protrude into the mould cavity thereby to influence the overall shape of the moulded product.

Preferably, the movement means comprises a rotatable member operably positioned adjacent said flow front region. The rotating member can comprise a finger having an end shape to be flush with the mould cavity. The finger can for example be substantially cylindrical in shape. The rotatable member can further comprise a driven member such as a sprocket comprising a series of teeth. Accordingly, the rotatable member can be driven for example using a chain and a variable speed motor to drive the sprocket and hence effect localised surface movement at the flow front region.

According to another aspect of the invention there is provided injection moulding apparatus comprising a mould cavity having an inlet for injection of mouldable material, and a region in which at least two flow fronts of mouldable material meet in use, wherein surface stirring means is provided for effecting movement of the plastic in said region in use.

An embodiment of the invention will now be described by way of example only, with reference to the accompanying drawings in which; Figure 1 is schematic front elevation view of a known wheel trim; Figure 2 is a schematic side elevation view of an injection moulding apparatus according to the invention; Figure 3 is a schematic front elevation view of part of the apparatus shown in figure 2; and Figure 4 is a schematic perspective view of a rotatable member according to the invention.

Referring to figure 1 there is shown a wheel trim 10 which has been formed using a mould having a single injection sprue which allows molten mouldable material such as plastics to be injected into a moulding cavity.

The material is injected approximately centrally ofthe trim 10 thereby effecting radial flow of molten materials throughout the cavity. Inserts in the cavity create apertures 12 since no plastic material passes into these regions during the moulding process. The inserts of the moulding cavity, however, cause two flow fronts as indicated by arrows F about an insert to form during the moulding process. The flow fronts abut one another on the radially outermost side of an insert, thereby causing weld lines 14 and 16 on the surface of the moulding 10, as well as internal structural irregularities beneath the surface of the moulding.

Figure 2 shows a schematic side elevation view of a moulding apparatus 20 according to the invention comprising a first plate 22 and a second plate 24. First plate 22 comprises a central sprue 26 shown in dotted lines, a mould surface 28 and an insert 30 for forming an aperture such as 12 shown in figure 1. Plate 24 comprises an end-most section 32 having a mould recess or cavity 34.

Plate 22 further comprises a recess 36 housing a rotatable member 40. Referring to figure 4, it can be seen that rotatable member 40 comprising a finger 42 having an end surface 44. The opposite end of finger 42 is joined to a sprocket 46 comprising a series of teeth which can engage a chain for example thereby to enable member 40 to be rotated within recess 36 of plate 22. In this example, finger 42 is substantially cylindrical and surface 44 is substantially flat.

Recess 36 and member 40 are adapted so that surface 44 lies flush with surface 28 so that member 40 does not protrude into cavity 34 when plates 24 and 22 are brought together. Additionally, recess 36 acts as a bearing for member 40. For example, plates 22 can comprise metal or other material having a melting temperature substantially above that of injection moulded plastics. Similarly, rotatable member 40 can be made of metal. A form of bearing can be provided between finger 42 and recess 36, other than a metal interface such as a ball bearing mechanism.

Figure 3 shows a front elevation the position of surface 44 of member 40, insert 30 of plate 22, sprue 26, and the active surface 'moulding' area 28 of plate 22 is outlined schematically by a circle.

In use, plates 22 and 24 are brought into abutment so that surface 28 encloses recess 34. For example, plate 24 might be moveable axially in and out with respect to plate 22.

Accordingly, a moulding cavity is defined between the plates so that mouldable material injected through sprue 26 flows throughout the cavity and around insert 30.

However, in use, rotatable member 40 is driven thereby to effect localised surface movement at the flow front region at a position radially away from central sprue 26 and beyond insert 30.

Since surface 44 does not protrude into cavity 34, it does not affect the final shape of the moulded products. However, rotation of member 40 is found to provide a particularly beneficial influence or stirring at the flow front interface region, reducing the surface deformation which otherwise results in unsightly weld lines such as 14 and 16 shown in figure 1. Indeed, dramatic improvement in the mechanical properties of the product at the weld line region is also obtained using the above apparatus and method of operation.

Claims

1. Injection moulding apparatus comprising a mould cavity having an inlet for injection of mouldable material, and a region in which at least two flow fronts of mouldable material meet in use, wherein means is provided for effecting by physical contact therewith localised movement of the mouldable material in said region in use.

2. Apparatus according to claim 1 wherein, the movement means comprises a member which physically contacts the surface of the mouldable material in use.

3. Apparatus according to claim 2 wherein the member is substantially flush with the mould cavity.

4. Apparatus according to claims 1, 2 or 3 wherein the movement means comprises a rotatable member operably positioned adjacent said flow front region.

5. Apparatus according to claim 4 wherein the rotatable member comprises a finger having an end shaped to be flush with the mould cavity.

6. Apparatus according to claim 5 wherein the finger is substantially cylindrical.

7. Apparatus according to claim 4, 5 or 6 wherein the rotatable member further comprises a driven member such as a sprocket comprising a series of teeth.

8. Apparatus according to any of claims 4 to 7 comprising means for rotating the rotatable member.

9. Apparatus according to claim 7 wherein the rotating means comprising a chain and a variable speed motor.

10. Injection moulding apparatus comprising a mould cavity having an inlet for injection of mouldable material, and a region in which at least two flow fronts of mouldable material meet in use, wherein stirring means is provided for effecting movement of the plastic in said region in use.