AU567444B2 - Polyurethane foam products moulding system - Google Patents

Description

POLYURETHANEFOAMPRODUCTSMOULDINGSYSTEM

THIS ^* INVENTION relates to the moulding of articles from poiyurethane foam. It is particularly applicable to arti¬ cles in which the weight is extremely critical, consistent with an acceptable strength, such as in the surfboard and allied industries. It is equally applicable in other fields demanding similar criteria.

It has been the practice where lightweight, poiyurethane products are required to have acceptable strength and impact resistance, to either coat them with fibreglass resins as with existing surfboards, or to manufacture a variety of surface skins and then inject a light density poiyurethane into the cavity so formed.

The disadvantages of fibreglass coated poiyurethane are that it is extremely labour intensive, it is extremely rigid and hence has less ability to absorb impact. It tends to fracture easily, and separates, and it is a fairly unpleasant and hazardous material to handle.

Methods by which the skin is constructed prior to filling with a poiyurethane core material, have the large disad¬ vantage that due to the natural shrinkage of poiyurethane, separation of the skin from the core occurs, and- hence eventual breakdown.

These problems have been overcome by the present invention which involves the manufacture of the core first, with a very low density, lightweight material, then covering it with a thin medium density poiyurethane skin. Several types of core material may be used, including poiyurethane and styrene. Thus the present invention resides in a method of produc¬ ing articles from poiyurethane foam which comprises form¬ ing «-a core of low density material, placing the core within a mould so that at least part of the core is spaced apart from the wall of the mould and injecting materials to form a skin of poiyurethane foam into the space between the core and the wall of the mould, the skin having a density greater than the density of the core.

Best results have been obtained using a poiyurethane core over which is moulded a rigid, compatible, higher density poiyurethane skin. Flexible poiyurethane skins may be formed, which give a softer surface as an alternative to the hard, rigid skins. The same method of construction is used.

The most critical factor in surfboard construction is the weight, strength being sacrificed in many instances in favour of a lower weight. It is apparent that the lighter the board, the weaker it is, as the only ways to lighten it are to either reduce the density, or reduce its size.

Buoyancy is a major significant factor in that it relates directly to performance. The more buoyant a surfboard is, the less energy that is required to utilize it.

By utilizing a lower density core, and a medium density skin, both of which are extremely buoyant, a large range of combination can be achieved, allowing for varying balances between light weight, strength and buoyancy.

There are many advantages in such a buoyant board. A surfboard constructed with this method may be significan¬ tly reduced in size compared with a similar surfboard constructed with conventional methods, resulting in in¬ creased performance, yet carrying the same person. A major factor of this invention is the high volume pro¬ duction that can be achieved with relatively few rejects, resulting in extremely low cost per unit.

This invention preferably uses the R.I.M. method (Reaction Injection Moulding) of poiyurethane construction for the formation of both the core and skin.

Moulds may be constructed from several types of material, however, aluminium moulds were found to be the most suc¬ cessful. The mould described in this document was highly polished on the surface to be reproduced, with a minimum of 25 mm. aluminium thickness. The mould was made in two halves, a top and bottom, joining mid way around the edge, or rail, of the surfboard.

Correct clamping of the mould is essential for the produc¬ tion of high quality skins. Heating of the mould is also essential-

It has been found that a skin thickness of between 6mm. and 10 mm. with an average density of between 200-400 kilograms per cubic metre, moulded around a lightweight core of density between 28-50 kilograms per cubic metre gives the most satisfactory balance between weight and strength. Where weight is not as critical, the density of the skin may be increased to obtain more surface hardness and strength. An alternative method is to increase the density of the core, to increase the strength, and corres¬ pondingly reduce the skin thickness, thus realising the same overall weight, and strength. With strict control, skin thickness may be reduced to 2 mm., however, at this thickness, a minimum density of 250 kg./c . was most satisfactory.

OMPI For the purpose of this description, skin thickness is 8 mm. with a density of 250 kg./cm. moulded around a core of density 30 kg./cm.

Two inserts are constructed, in this case of fibreglass, however any material that can be moulded to the shape of the mould may be used. These inserts are 8 mm. in thick¬ ness, and made so that they fit exactly inside both the top and bottom of the mould (halves). When the mould is closed, the void inside with the inserts is 8 mm. thinner all over than the closed mould without the inserts.

To construct the cores from poiyurethane, the best method was found to be using the mould with the inserts inside. By initial trial and error, it can be calculated how much poiyurethane must be placed inside the mould (with in¬ serts), to exactly fill it at a density of 30 kg/cm.

When the poiyurethane has cured, it may be removed from the mould. The core is now 8 mm. thinner all over than the finished article is to be. The cores must be stored to minimise distortion and to cure properly. Alterna¬ tively, another mould may be made up that is exactly 8 mm. thinner all over than the master mould, and the cores may be manufactured separately.

The demould time of the cores is dependent upon the thick¬ ness.

The Skinning Process.

Moulding a skin around the core may be done by either of two methods?

Method 1. The aluminium mould is prepared in the usual manner. Spacers are either glued or placed on both sides of the core so that when it is placed inside the mould and it is closed, an even void of 8 mm. exists all around the core. It is into this void that the higher density skin¬ ning poiyurethane is to be injected and react. The spacers are positioned so that when the mould is closed, they hold the core tightly and evenly, not allowing any movement of the core when the skin material is reacting. The length of the spacers is 9 mm. so that when the mould is closed, they push into the core slightly, holding it tightly.

The spacers maybe of any suitable configuration. A pre¬ ferred form of spacers is formed of nylon or other suit¬ able plastic and comprises a disc having a central pig projecting from each face. One peg is pushed into the core until the disc bears against the surface of the core. This leaves the other pig projecting outwardly so that in position the outer end bears against the wall of the mould to hold the core spaced therefrom.

A predetermined amount of poiyurethane is injected into the void so that the required density is reached. The entry gates where the poiyurethane enters the mould must be constructed carefully to enable it to react evenly along both sides of the core. Bad design will result in more material forcing its way to one side of the core, with little or none of the other side. Each item to be moulded requires its own gating system. This method becomes extremely difficult where one side of the core is shaped in such a way as to allow for easier flow of the skinning material than the other. In such a case, Method 2 may be used.

Method 2. The mould is prepared as usual, and the bottom insert is positioned in the bottom of the mould. The core is then placed into the insert, fitting exactly as it was moulded in it beforehand. Spacers are then placed on the top of the core, so that when the mould is closed, there will'be a void of exactly 8 mm. around the top of the core only. The closure of the mould will force the spacers into the core, pushing it hard against the bottom insert, so that no material can find its way around the bottom half of the core.

A predetermined amount of poiyurethane is injected into the mould, which finds its way into the only void i.e. the top of the core. After the demould time, which is rela¬ tively short, the mould is opened. The skin around the top of the core adheres to the aluminium, and the whole thing comes away from the bottom of the mould. The bottom insert can now be removed. After cleaning down the mould, when it is closed there is now an 8 mm. void on the bottom of the core. Poiyurethane may now be injected into the mould, filling the only void, the bottom half. Adhesion to the top half that has already been skinned is excel¬ lent. After the demould time, the mould may be opened and the product removed.

While Method 2 involves extra time, for more complicated mouldings, it is preferable to Method 1.

The amounts of material to be injected into the mould can be calculated from test shots by trial and error.

Strengthening of the product may be carried out by the insertion of various strengthening rods or by cutting grooves in the core prior to inserting it in the mould, these grooves are filled during the skinning process. These grooves are an ideal method of reinforcing specific sections of the product with little effort. While spacers are specifically mentioned in this description, it was found that plastic straps, equidistant from each other. and fastened each side of the bottom of the mould so that they span the cavity, were satisfactory. When the mould τ was closed the edges of the upper part of the mould bear against the ends of the stoop so that they tightened across the core and hold it sufficiently for the skinning process.

Claims (9)

THE CLAIMS defining the invention are as follows:-

1. Α method of producing articles from poiyurethane foam which comprises forming a core of low density material, placing the core within a mould so that at least part of the core is spaced apart from the wall of the mould and injecting materials to form a skin of poiyurethane foam into the space between the core and the wall of the mould, the skin having a density greater than the density of the core.

2. A method as claimed in claim 1 wherein the core is formed of poiyurethane.

3. A method as claimed in claim 1 wherein the core is formed of polystyrene.

4. A method as claimed in claim 2 wherein the core has a density of between 28-50 kgs/cm.

5. A method as claimed in any one of claims 1 to 4 wherein the skin has a density of 200-400 kgs/cm.

6. A method as claimed in claim 1 wherein the core is formed by placing two or more removable inserts into the mould and injecting poiyurethane foam materials into the mould.

7. A method as claimed in claim 6 wherein the core is held in place by one of the removable inserts, part of the skin is formed and the removable insert is then removed and the remainder of the skin formed.

8. A method as claimed in any one of claims 1 to 5 wherein the core is held in place by a series of spacers bearing against the wall of the mould.

9. A method as claimed in claim 1 wherein the core is held in place by straps positioned across the cavity of the mould.