GB2320455A - Heat transfer control in moulding plastics material - Google Patents

Abstract

The appearance of witness marks in vehicle air bag covers or other articles of plastics material in which two regions are connected by an integral film forming a tear strip or film hinge, is reduced by using a mould with a flush surface 15 on one side and rib 23 or 23' on the other that form opposite sides of the film and providing heat transfer control means in the form of insulation blocks 18 or 26' or air gaps 26 to prevent premature cooling of the film by reducing the rate of flow of heat from molten plastics material in the mould in the region of the film.

Description

TITLE Mould for manufacturing articles of plastics material DESCRIPTION This invention relates to moulds from producing articles of plastics material, in particular articles having a region such as a film hinge or a tear strip which is thinner than adjacent regions.

One example of such an article is a cover for a vehicle air bag restraint system. Air bag restrain systems comprise a bag which can be rapidly inflated to protect the occupants of the vehicle in case of an impact. The air bag and inflation device are mounted in a housing, which can be installed in an appropriate position on the vehicle, such as in the centre of the steering wheel or in the dashboard, to protect the occupant from front impacts, or in a door panel, to protect the occupant from side impacts. The housing is closed by a cover which is designed to break open when the air bag is inflated. Where the cover is made of plastics material, the cover is provided with a tear strip, ie an integral film of plastics material in the form of a strip connecting adjacent regions of the cover that ruptures on expansion of the air bag. The tear strip is usually formed by moulding a recess into the inner surface of the cover, so that the outer surface has a flush finish. However, one problem that arises with the manufacture of such articles is that the finish of the outer surface is impaired by socalled "witness" marks, which reveal the presence of the recess on the inside of the cover.

The problem of surface defects in plastics mouldings is well known in the art. For example in products of uniform wall thickness, stress marks can appear over the outer surface as the plastics material cools in the mould. It is known from Japanese patent publication JA-002186 and US Patent specification US-A-5362226 to provide a continuous layer of insulating material on the inner surface of the mould cavity to reduce the rate of cooling and thereby reduce the appearance of stress marks. In articles having a rib or boss on one surface, imperfections in the form of "sink" marks appear on the opposite surface. In order to reduce the appearance of sink marks, European patent specification EP-A-0006289 discloses the incorporation of insulating inserts in the mould in the region of the rib or boss so as to keep the surface of the rib or boss molten whilst the remainder solidifies.

The present invention is based on investigations into the appearance of witness marks on a flush outer surface of an article over an integral film such as a tear strip or film hinge formed by a recess in the inner surface of the article.

According to the present invention, there is provided a mould for manufacturing an article of plastics material in which two regions are connected by an integral film comprising a first mould surface having a flush region forming one side of the film, a second mould surface forming the other side of the film that carries a rib projecting towards the flush region of the first mould surface, and heat transfer control means for reducing the rate of flow of heat from molten plastics material in the mould in the region of the film.

The invention is based on the observation that heat is dissipated more quickly from molten plastics material in thinner regions of the moulded article than in thicker regions, and that this difference in heat loss can lead to the appearance of witness marks. The appearance of witness marks in the area of an integral film can therefore be reduced by controlling the flow of heat through the mould in the region of the film so as to reduce premature cooing of the plastics material in the region of the film, and to provide a more uniform rate of cooling in the region of the tear strip in comparison with adjacent areas of the article.

The control of heat transfer in the region of the integral film may be through the positive application of heat, eg by positioning heating elements in the mould in the region of the film. Preferably however, heat transfer is controlled by reducing the rate at which heat flows from the mould by providing thermal insulation in the mould in the region of the film. This is simpler and cheaper than providing means for heating the mould in the region of the film, and avoids the risk of over-heating the plastics material.

The thermal insulation may be in the form of an insert of insulating material or it may be in the form of an air gap.

If the insulation is placed on the inside of the mould it should comprise a durable material such as a ceramic or heat-resistant glass. The thermal insulation is preferably provided on the exterior surface of the mould so that the internal surface of the mould is uninterrupted. In this case the material may be formed from fibreglass wool or an asbestos material or, generally from any material with a low thermal conductivity and a low heat capacity. Where the mould comprises two separable mould parts, the heat transfer control means may be provided on one mould half, but is preferably provided on both, on opposite sides of the tear strip.

A preferred embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 is a plan of an air bag cover of plastics material produced in a mould of the present invention; Figure 2 is a cross-section through the air bag cover of Figure 1 taken along line II-II; Figure 3 is a schematic drawing illustrating a mould in accordance with the present invention for producing the air bag cover of Figures 1 and 2.

Figure 4 is a schematic drawing illustrating an alternative mould in accordance with the invention.

Referring to the drawings, Figures 1 and 2 illustrate a cover 1 for a housing (not shown) of an air bag device intended to be installed in the hub of a vehicle steering wheel. The housing is a 1-piece moulding of a plastics material having appropriate tear characteristics for an air bag cover. Blends of EPDM rubber and polypropylene such as that sold under the trade mark VISTAFLEX 911B1 are typically used for this purpose. The cover has a dome-shaped outer surface 2 provided with a high-quality grained surface finish. The inner surface of the cover includes an integral wall 5 by which the cover may be mounted on a container for an air bag and trigger mechanism (not shown) Across the centre of the cover is a V-shaped groove 8 in the form of the letter H. The base of the groove lies close to the outer surface of the cover, leaving an integral film of plastics material between the two regions of the cover on each side of the groove, forming an H-shaped tear strip 10.

When the cover is in use in an air bag system and the system is triggered, the cover ruptures along the tear strip 10, allowing the air bag to inflate.

The cover 1 is manufactured in a 2-piece mould shown in Figure 3. The mould comprises an upper mould half 15 and a lower mould half 20. The upper mould half 15 has an inner surface 16 carrying the grained pattern of the cover. The surface in the region of the tear strip is even and generally flush with the rest of the mould surface. The outer surface of the upper mould half 15 is provided with an insert 18 in the form of a strip of a ceramic insulating material that overlies that part of the mould forming the tear strip. Since the insulating element 16 is in a recess in the rear face of the upper mould half 15, the internal face of the mould half 15 presents a continuous, even surface without seam lines which could mark the surface of the finished cover 1.

The lower mould half 20 conforms to the inner surface of the cover 1 and in the region of the tear strip includes a metal insert 22 forming a rib 23 of V-shaped cross-section that projects towards the flush surface 16 of the upper mould half 15 to define the tear strip . The material of the lower mould half behind the insert 22 is removed to leave an air gap 26 along the length of the H-shaped tear strip.

In use, molten plastics material is introduced into the cavity between the two mould halves 15, 20 and is allowed to cure and cool. In conventional moulds, witness marks tend to form on the outer surface of the cover 1 in the region of the tear strip 10. Without intending to limit this invention to any theories, one explanation for this phenomenon arises from the fact that the plastics material is introduced into the mould at a temperature typically of around 2200C and begins to solidify first on contact with the walls of the mould, as indicated at 17 and 19 in Figure 3, which are maintained at a lower temperature, eg 700C, leaving a central core of molten material, indicated at 21.

In a product such as an air bag cover, which has a wall thickness of no more than 5mm, usually 2-3mm, the layers of solid material constrict the space available for the central core to about 1/3 the thickness of the product, eg to about imam. In the region of the tear strip however, the wall thickness is no more that 0.5mm, usually 0.2-0.3mm. The space through which molten material can flow in this region is therefore much more constricted, eg 0.05mm or less, and, in a conventional mould become completely closed by solidified material before the rest of the mould has been filled. As a result, the plastic material is not urged into contact with the surfaces of the mould at the tear strip to the same extent as in adjacent areas where the solidified layers are still in contact with molten resin under pressure. Also, as the constriction at the tear strip decreases in size, the plastics material flowing through it is subjected to increasing shear forces, which affect the morphology of the plastics material in the region of the strip, elongating the molecules of resin to a greater extent than in neighbouring parts of the mould.

By providing heat transfer control means in the form of the insert 18 and the air gap 26, the plastic material in the region of the tear strip cools less rapidly than the adjacent areas of the cover. As a result, the layers of solidified plastics material 17a,19a on the internal walls of the mould build up more slowly in this region. The material is therefor urged into contact with the walls of the mould for longer, providing better definition on the outer surface of the moulding. Also, the molten material 21a flowing between the layers in the region of the tear strip is subjected to less stress during the moulding operation. Finally, internal stresses caused by rapid cooling of the resin in the region of the tear strip are reduced. All these factors can influence the physical appearance of the plastics material at the tear strip, and may contribute to the reduction in witness marks in this region.

It will be appreciated that the precise location size and nature of the insulating elements will vary according to the details of the design of the cover 1, the plastics material used for moulding and the heat transfer properties of the mould. The optimum design will be determined by the person skilled in the art by routine experimentation.

Figure 4 illustrates an alternative mould in accordance with the invention. In this mould, the lower mould half 20' comprises a rib 23' formed by a V-shaped indentation 24 in the lower mould half. The lower mould half 20' is provided with heat transfer control means in the form of a block of ceramic insulating material 26' that is positioned in the Vshaped indentation 24.

Claims (6)

1.A mould for manufacturing an article of plastics material in which two regions are connected by an integral film, the mould comprising a first mould surface having a flush region forming one side of the film, a second mould surface carrying a rib that projects towards the flush region of the first mould surface to form the other side of the film, and heat transfer control means for reducing the rate of flow of heat from molten plastics material in the mould in the region of the film.

2.A mould according to claim 1 wherein the heat transfer control means comprises thermal insulation.

3.A mould according to claim 2 wherein the thermal insulation comprises a element of insulating material mounted in the external surface of the mould.

4.A mould according to claim 2 or claim 3 where in the thermal insulation comprises an air gap.

5.A mould according to any one of the preceding claims comprising two mould halves, and heat transfer control means in each mould half on opposite sides of the said region.

6.A mould according to any one of the preceding claims wherein the integral film comprises a film hinge or tear strip.