GB2175533A

GB2175533A - Injection moulding nozzle filter

Info

Publication number: GB2175533A
Application number: GB08609657A
Authority: GB
Inventors: Terry Griffiths
Original assignee: C R M DESIGN SERVICES Ltd
Current assignee: C R M DESIGN SERVICES Ltd
Priority date: 1985-04-22
Filing date: 1986-04-21
Publication date: 1986-12-03
Also published as: GB8609657D0; GB8510185D0

Abstract

An improved nozzle for a machine for injection moulding very thin rubber articles having a thickness of less than 20 thousandths of an inch, e.g 8 to 12 thousandths of an inch, said nozzle having at least two replaceable screen filters (18, 19) sandwiched between a pair of perforated walls (14, 16) therein, the coarser filter (19) being located downstream of the fine filter (18). As a support for the fine filter (18) against moulding pressure. Conveniently a medium screen filter can be located upstream of the fine filter (18). <IMAGE>

Description

SPECIFICATION Improved machine and method for injection moulding very thin rubber articles Description This invention relates to an machine and method for injection moulding very thin rubber articles, that is articles having a thickness of no more than 20 thousandths of an inch. The invention also relates to an improved nozzle for use with such a machine and method.

It is currently extremely difficult to effectively injection mould very thin rubber articles because of the problem of keeping particulate matter out of the base rubber mix entering the mould. It will be appreciated that if a very thin rubber article is to be moulded, for instance having a thickness in the range of between 8 and 12 thousandths of an inch, substantial flaws will be present in the moulded article if any particulate matter is contained in it. One way of overcoming the problem is to thoroughly clean the base rubber material to be moulded before it is put into the extruder which feeds the mix to the injection nozzle.

For mouldings over 20 thousandths of an inch thick pre- cleaning the rubber mix provides an acceptable solution because although the mix still often contains some particulate contaminant, it is generally much smaller in size than the thickness of the moulding itself so it does not affect its quality.

For very thin mouldings as defined, pre-cleaning the base mix, however carefully it is carried out or however successfully, still does not overcome the problem because the moulding machine itself produces particulate contaminant as a result of the mix being fed from the extruder to the moulding tool. The reason for this is that the feed ram which produces the enormous pressure needed to push the rubber mix into the mould moves in a steel barrel which has a small clearance between the piston and the barrel to permit rubber material to pass therebetween to lubricate the piston as it moves. Owing to the enormous pressure and heat generated during the feed step, small pieces of rubber mix can remain in the barrel and become vulcanised due to the residual heat.The nett result of this is that by the time the material is fed to the injection nozzle and into the moulding tool, what started out as clean rubber material now contains cured particulate contaminate material.

It is an object of the invention therefore to endeavour to ensure that the rubber material actually entering the mould is as free from particulate contaminant as possible.

According to one aspect of the invention there is provided a method of moulding very thin rubber articles comprising the steps of supplying a rubber mix ready for moulding from an extruder to an injection head provided with an outlet nozzle and forcing said mix through a fine mesh filter which will entrap particles having a size greater than 20 thousandths of an inch and subsequently forcing the mix after it leaves the fine mesh filter through a coarser mesh filter and then out of the outlet nozzle and into a mould.

According to a preferred method, the rubber mix.

is first forced through a medium mesh filter before it reaches the fine and coarser filters.

According to a further aspect of the invention there is provided a nozzle assembly for an injection moulding machine comprising a barrel with an inlet at one end and an outlet nozzle at the other end, a pair of spaced perforated walls in the barrel between which a fine and a coarser mesh filter are sandwiched, the coarser mesh filter being juxtaposed the perforated wall nearest the outlet nozzle and the fine mesh filter having a screen size which will not permit particulate material of a size greater than 20 thousandths of an inch to pass therethrough.

Preferably a medium mesh filter is additionally provided, this being located between the fine mesh filter and the perforated wall furthest from the outlet nozzle.

Conveniently the filters are tightly sandwiched against each other between the pair of spaced perforated walls. In a preferred embodiment, the perforated wall furthest from the injection nozzle forms the end wall of a cylindrical plug insert which is screw fitted into the barrel, the filters being removably mounted in the barrel and access thereto being gained by unscrewing the plug insert.

According to the preferred embodiment, the fine mesh filter has a screen size of 100 x 100 (aperture size 140y) the coarse mesh filter has a screen size of 20 x 20 (aperture size 900tri) and the medium mesh filter has a screen size of 40 x 40 (aperture 400#). The dimensions of 20 x 20, 40 x 40 and 100 x 100 refer to the number and arrangement of openings per square 2.54 cms (per square inch) in each of the filter elements. Other combinations of screen filter sizes can however be used e.g. 80 x 80 or 60 x 60. The actual combination of filter sizes will depend on the thickness of the article being moulded and the required flow rate of the rubber mix through the nozzle.

Further features and advantage of the invention will become clear from the following description of a preferred embodiment thereof, by way of example only, with reference to the accompanying diagrammatic drawings in which: Figure 1 shows an injection moulding machine fitted with a nozzle assembly of the invention; and Figure 2 is a cross section through the injection nozzle assembly fitted to the machine shown in Figure 1.

Figure 1 shows an injection moulding machine comprising an extruder 1 fitted with a feed 2 for the introduction of the rubber material to the extruder. The extruder heats and processes the basic rubber mix into a form in which it will flow but not cure and this processed mix is supplied to an injection chamber 3. An injection ram 4 moved by piston 5 cyclically forces the rubber mix in the chamber 3 out of nozzle 6 into moulding tool 8.

The construction of the moulding machine is well known so it will not be described in any further detail here.

The crucial part of the machine illustrated in Figure 1 is the injection nozzle assembly 6 shown in more detail in Figure 2. This nozzle assembly comprises a barrel 10 having an outlet nozzle 11 at one end thereof. The other end is threaded at 12 to receive a cylindrical plug insert 13 whose end wall 14 is perforated with holes 15. A front wall 16 with holes 17 in it is fitted in the barrel 10 adjacent the outlet nozzle 11. A fine mesh filter 18 is sandwiched between a coarser mesh filter 19 and a medium mesh filter 20, all three filters being tightly held together against the perforated wall 16 by means of the end wall 14 of the plug insert 13. It is most important that the filters cannot move in the barrel 10 otherwise they may burst due to the enormous pressure exerted on them by the rubber mix as it is forced through them by the ram 4.

Preferably, the filters are made of a staidless steel wire mesh.

It is important feature of the injection nozzle described and illustrated that the coarser mesh filter 19 is located downstream of the fine screen 18.

The purpose of the coarser screen 19 is not so much to filter out particulate material but more to support the fine screen 18 and thereby stop it from bursting under the extreme pressure of the rubber mix passing through it. By way of example, if the holes in the walls 14 and 16 were 1.5 mm in diameter, the coarse screen 19 would have a mesh size of 20 x 20, which would be sufficient to prevent the fine mesh screen which has a mesh size of 100 x 100 from being forced into the relatively large holes 17 in the wall 16 under the enormous pressure of the rubber mix and it would burst.

Medium mesh filter 20 would have a mesh size of 40 x 40. This additional filter is not essential but it is highly desirable as it removes larger particles of contaminant before the rubber mix reaches the fine filter 18.

The moulding tool illustrated is of known type and is made up in two parts comprising an upper part 8a and a lower 8b. The lower part is separable from the upper part by means of piston 9. Depending on how dirty the original rubber mix is or how much partly vulcanised particulate material is generated by the ram 4 during its operation, the or each filter 18, 19, 20 will require changing and replacement on a regular basis.

Claims

1. A method of moulding thin rubber articles comprising the steps of supplying a rubber mix ready for moulding from an extruder to an injection head provided with an outlet nozzle and forcing said mix through a fine mesh filter which will entrap particles having a size greater than 20 thousandths of an inch, subsequently forcing the mix after it leaves the fine filter through a coarser mesh filter and then out of the outlet nozzle and into a mould.

2. A method as claimed in claim 1 wherein the mix is first forced through a medium mesh filter before it reaches the fine and coarser filters.

3. A nozzle assembly for an injection moulding machine comprising a barrel with an inlet at one and an outlet nozzle at the other end, a pair of spaced perforated walls in the barrel between which a fine and coarser mesh filter are sandwiched, the coarser mesh filter being juxtaposed the perforated wall nearest the outlet nozzle, the fine mesh filter having a screen size which will not permit particulate material of a size greater than 20 thousandths of an inch to pass therethrough.

4. A nozzle as claimed in claim 3 wherein a medium filter mesh is located between the fine mesh filter and the perforated wall furthest from the outlet nozzle.

5. A nozzle assembly as claimed in claim 3 or 4 wherein the filters are tightly sandwiched against each other between the pair of spaced perforated walls.

6. A nozzle as claimed in any one of claims 3 to 5 wherein the perforated wall furthest from the injection nozzle forms the end wall of a cylindrical plug insert which is screw fitted into the barrel.

7. A nozzle assembly as claimed in claim 6 wherein the filters are removably mounted in the barrel, access thereto being gained by unscrewing the plug insert.

8. A nozzle assembly as claimed in any one of claims 3 to 7 wherein the fine mesh filter has a screen size of 100 x 100 as hereinbefore defined.

9. A nozzle assembly as claimed in any one of claims 3 to 8 wherein the coarse mesh filter has a screen size of 20 x 20 as hereinbefore defined.

10. A nozzle assembly as claimed in any one of claims 4 to 9 wherein the medium mesh filter has a screen size of 40 x 40 as hereinbefore defined.

11. A injection moulding machine fitted with a nozzle assembly as claimed in any one of claims 3 to 10.

12. A injection moulding machine substantially as herein described with reference to the accompanying drawings.

13. An injection moulding nozzle assembly substantially as herein described with reference to the accompanying drawings.