GB2197242A - Moulding from expandable beads - Google Patents

Abstract

A moulding system for manufacturing tubs and drinking cups is disclosed, this comprising a male member (44) including a core (50) and a shell (48). Steam, water and air enter the mould through an inlet (54) and exit through an outlet (56). Passages (58, 60) place the inlet (54) is communication with the mould cavity which lies externally of the shell (48). A plug (62) opens and closes the entrance to the passage (58). During moulding steam is fed in to raise mould pressure to a predetermined level. Thereafter steam supply is terminated so that pressure decays to a lower level and is thereafter recommenced so that the lower level of pressure is maintained for a predetermined period. This action eliminates water droplets in the steam which is subsequently fed into the mould cavity through the passages (58, 60). <IMAGE>

Description

SPECIFICATION Manufacture of articles from expandable beads of synthetic plastics material THIS INVENTION relates to the manufacture of articles from expandable beads of synthetic plastics material.

Tubs and drinking cups are manufactured by filling a moulding space with pre-expanded beads of plastics material. The most commonly used beads are of polystyrene. The mould is heated and steam is injected into the moulding space so as further to expand the beads so that they fill the moulding space.

The male and female mould parts have jackets through which heating steam and cooling water are cyclically fed.

According to one aspect of the present invention there is provided a method of moulding articles from expandable beads which comprises feeding steam to the jacket of a mould until the pressure rises to a predetermined higher pressure level, terminating steam supply at that pressure level so that the pressure in the mould jacket decays to a predetermined lower pressure level, and maintaining steam supply at the predetermined lower pressure level for a predetermined period.

The method can further include the step of commencing feed of cooling water to the jacket simultaneously with termination of the supply of steam at the lower pressure level.

Upon the supply of cooling water being terminated, purging air is fed through the jacket and thereafter supply of steam at said higher pressure level recommences.

A drain from said mould jacket is preferably closed shortly after supply of steam at said higher pressure level commences and only reopens upon supply of steam at said lower pressure level ceasing.

According to a further aspect of the present invention there is provided moulding apparatus for producing articles from expandable beads, the apparatus comprising a mould having a jacket, first means for supplying steam at a higher pressure level to the jacket, second means for supplying steam at a lower pressure level to the jacket, and control means for initiating supply of steam by said first means and for thereafter terminating such supply by the first means while maintaining supply of steam by the second means.

The apparatus can include first pressure regulator and a first steam valve for connecting said jacket# to a supply of steam, and a second pressure regulator and a second steam valve for connecting said jacket to a supply of steam, said first regulator being set to close at the higher pressure level and the second regulator being set to close at said lower pressure level, said control means closing said first steam valve to terminate supply of steam at the higher pressure level.

For a better understanding of the present invention reference will now be made, by way of example, to the accompanying drawings in which: Figure 1 is a diagrammatic representation of a moulding system, Figure 2 illustrates the male part of one of the moulds of the system; Figures 3 and 4 diagrammatically illustrate th#.pnmary primary and secondary functions of the moulding system of Fig. 1; and Figure 5 is a steam pressure curve.

Referring firstly to Fig. 1, the moulding system illustrated comprises six identical moulds 10. Each mould 10 is of conventional form and comprises two mould parts, the first mould part being a female and defining a cavity and the second mould part being in the form of a core which is received in the cavity.

The space between the core and the female mould part has the shape of the cup or tub which is to be moulded. Both the male and female parts of the moulds are formed with jackets, water, air and live steam being fed through the jackets in sequence as will be explained in more detail hereinafter. The con struction of the mould will also be explained in more detail hereinafter with reference to Fig. 2.

Steam is supplied to the jackets along two supply lines which have been designated 12 and 14 in Fig. 1. In the line 12 there is a steam pressure regulator 16 and a pneumatically operated steam supply valve 18. The valve 18 opens and closes as will be described to control flow of steam from the main steam line 20 to the regulator 16. The main steam line 20 is also connected via a further pneumatically operated steam valve 22 and pressure regulator 24 to the moulds 10.

A pressure of 700 Kpa in the line 20 is assumed in the following description. It is also assumed that the regulator 16 is set to close at 235 Kpa and the regulator 24 at 410 Kpa.

Reference numeral 26 designates a valve which, when opened, supplies compressed air to the steam valve 18 and, via a control valve 28, to the steam valve 22. The air supplied to the valves 18 and 22 is for control purposes only and is used for switching the valves between their open and closed positions.

Supply of water to the moulds 10 is by way of a pneumatically controlled water flow control valve 30. Su$1y of air for control purposes to the valve 30 is by way of an air valve 32. The downstream side of the valve 30 is connected to the mould system by way of the regulator 24.

A further control valve 34 is connected. via a one way valve 36 to the pipe joining the steam valve 22 to the pressure regulator 24 and serves to supply purging air to the moulds.

On the 'downstream' side of the moulds 10 there is a drain valve 38 which is opened and closed during production of cups or tubs. The drain valve 38 is under the control of two timers 40 and 42. The function of these timers will be described in more detail hereinafter but it is noted that two timers are necessary as the drain valve 38 is held open both before and after cycle commencement which begins with platen opening. The timer which is operative before platen opening is rendered inoperative at platen opening which is why the#second timer is needed.

Before describing the operation of the system illustrated, the secondary moulding functions will be explained with reference to Fig.

4. Normally, the male mould part is immovably mounted and it is the female mould part which moves up and down between the closed position of the mould and the open position of the mould. In Fig. 4, platen opening indicates the moment at which the female mould parts lifts off the male mould part to open the mould, and platen closing indicates the moment at which the female mould part moves downwardly to close the mould.

Once the mould parts have been moved together, pre-expanded polystyrene beads are injected into the moulding space. This is the portion of the cycle designed 'filling' Immediately following this, steam is ejected into the moulding space so as to further expand the polystyrene beads and create the cup or tub.

In Fig. 2, the male part 44 of a mould 10 is illustrated, this being secured to a back plate 46. The mould part 44 comprises a shell 48 and a core 50. Between the shell 48 and the core 50 is the space, designated 52, which forms the jacket of male part 44.

The inlet for steam, water and air is at 54 and the outlet at 56. The shell 48 has a short axial passage 58 therein and a plurality of radial passages 60 leading outwardly from the passage 58. A plug 62 normally closes the passage 58, the plug being moved by a cylinder 64 at the appropriate point in the cycle to a position in which it allows communication between the passage 58 and the space 52.

During the latter part of the period for which the platen IS opening, compressed air is blown through the mould parts so as to release the moulded cup or tub from the mould parts and lift it away from the male mould part. Immediately this has been accomplished, platen clos ng commences.

It will be understood that the commencement of the platen opening portion of the cycle means that the cup is ready for ejection and is thus completely formed. At this moment, heating of the moulds for the next cycle can commence. The control valve 28 is opened so that both steam valves 18 and 22 are themselves held open. Steam under pressure thus flows from the line 20 through the pressure regulators 16 and 24 to the moulds 10. In Figs. 3 and 5, 'steam I' means steam entering via the regulator 24 and 'steam II' means steam entering via the regulator 16.

For a very short while after the steam valves 18 and 22 are opened, the drain valve 38 is held open by the timer 40. As soon as the drain valve 38 closes, the pressure in the jackets of the male and female parts of the mould commences to rise to the maximum pressure of 410 Kpa set on the regulator 24.

The regulator 16 opens briefly at the commencement of steam supply but closes as soon as the pressure in the jackets rises to 235 Kpa. The valve 28 is then closed. Termination of the air supply to the steam valve 22 causes it to close and consequently steam can now only be supplied through the steam valve 18 and the regulator 16. What this means is that, immediately the steam supply valve 22 closes, steam pressure decays (as shown in Fig. 5) until the pressure set on the regulator 16 is reached. The regulator 16 then opens and steam supply continues at the pressure for which the regulator 16 is set.

When supply of air by the control valve 26 ceases the steam valve 18 also closes. Simultaneously, the drain valve 38 is opened by the timer 42 and the valve 32 opens. Opening of the valve 32 results in air being supplied to the valve 30 which itself opens so as to supply water under pressure to the jackets of the male and female mould parts for the purpose of cooling them.

Once the mould temperature has dropped to the desired level, supply of water is terminated and simultaneously supply of air through the valves 34 and 36 commences so that water is purged from the pipe system and the mould jackets. As the supply of purging air is switched off, steam supply recommences to initiate the beginning of another cycle.

Any moisture droplets in the mould jackets when these have a pressure of 410 Kpa has a sensible heat of 299 K/J. When the pressure decreases to 235 Kpa, the sensible heat content drops to 263 K/J and the difference in sensible heat becomes available as latent heat which serves to flash some of the water in the jacket back to steam.

The internal surfaces of the jackets acquire, when the pressure therein is 410 Kpa, temperatures above those which can be sustained at 235 Kpa. The surfaces therefore have a superheating action on the steam and on the moisture droplets remaining in the jacket. The fact that heat is being put into the steam contained in the jacket causes the remaining moisture droplets to flash back into steam.

Thus the desired mould wall temperatures can readily be achieved and the steam which is injected into the moulding spaces when the cylinder 64 displaces the plug 62 for the purpose of expanding the polystyrene beads is dry i.e. is water free. This is important as steam injection into the moulding spaces normally takes place (as seen in Fig. 2) in the regions where the side walls of the tub or cup join the base. If water droplets are injected at this region, then the cup or tub can have undesirable porosity and 'weeping' of the liquid contents at this region becomes possible.

The techniques described above are further improved by opening the drain valve 38 for a brief period (0.1 to 0.2 secs) immediately after the platen closing stage. This causes a rush of live steam through the system which displaces water droplets out of the jackets of the mould.

Claims (6)

1. A method of moulding articles from expandable beads which comprises feeding steam to the jacket of a mould until the pressure rises to a predetermined higher pressure level, terminating steam supply at that pressure level so that the pressure in the mould jacket decays to a predetermined lower pressure level, and maintaining steam supply at the predetermined lower pressure level for a predetermined period.

2. A method as claimed in claim 1 and which includes the step of commencing feed of cooling water to the jacket simultaneously with terminating of the supply of steam at the lower pressure level.

3. A method as claimed in claim 2, wherein, upon the supply of cooling water being terminated, purging air is fed through the jacket and thereafter supply of steam at said higher pressure level recommences.

4. A method as claimed in claim 1, 2 or 3, wherein a drain from said mould jacket is closed shortly after supply of steam at said higher pressure level commences and only reopens upon supply of steam at said lower pressure level ceasing.

5. Moulding apparatus for producing articles from expandable beads, the apparatus comprising a mould having a jacket, first means for supplying steam at a higher pressure level to the jacket, second means for supplying steam at a lower pressure level to the jacket, and control means for initiating supply of steam by said first means and for thereafter terminating such supply by the first means while maintaining supply of steam by the second means.

6. Moulding apparatus as claimed in claim 6 which includes a first pressure regulator and a first steam valve for connecting said jacket to a supply of steam, and a second pressure regulator and a second steam valve for connecting said jacket to a supply of steam, said first regulator being set to close at the higher pressure level and the second regulator being set to close at said lower pressure level, said control means closing said first steam valve to terminate supply of steam at the higher pressure level.