GB2151971A

GB2151971A - Two stage injection moulding of brushes

Info

Publication number: GB2151971A
Application number: GB08407217A
Authority: GB
Inventors: Robert Dietrich Schad
Original assignee: Husky Injection Molding Systems Ltd
Current assignee: Husky Injection Molding Systems Ltd
Priority date: 1983-12-30
Filing date: 1984-03-20
Publication date: 1985-07-31
Also published as: JPS60145818A; FR2557498A1; DE3408451A1; GB8407217D0

Abstract

A composite brush having a set of bristles 44 of a flexible modified rubber and a handle 28 of a rigid and more rapidly hardening plastics material, e.g. polyethylene or polystyrene is produced by injection molding. The handle 28 is produced in a first mold cavity (54), Fig. 1 (not shown), and is used as a boundary of another mold cavity 55a in which the bristles 44 are formed along with a backing layer 43 fused on to the handle 28. The separation of the mold portions 20'' and 27 defining the latter cavity 55a proceeds slowly in an initial phase and rapidly in a terminal phase of an opening stroke to protect the moulded bristles 44. <IMAGE>

Description

SPECIFICATION A method of operating a two-shot injection-molding machine and a method of making brushes The present invention relates to a method of operating an injection-molding machine of the two-shot type and to a method of making brushes. Composite brushes consist partly of a soft plastics material which tends to cling to a mold cavity wherein the article is formed and partly of a more rigid plastics material. In such brushes the bristles are formed of a modified rubber, known commercially as "Kraton" (a Trade Mark of the Shell Chemical Company), and the handle made from a relatively rigid and more rapidly hardening plastics material, e.g.

polyethylene or polystyrene.

The elasticity and flexibility of "Kraton" makes it an excellent material for the bristles of the brush but creates great difficulties in the molding thereof.

Thus, if "Kraton" were used to make an integral brush in which undercuts or side cores in the handle are intended to pull on the latter and thus release the bristles from the cells of their cavity, the stickiness of the bristles would cause them to remain stuck in these cells and would forcibly detach the handle from the mold portion on which it is meant to stay during the mold-opening stroke.

A method of so operating a machine for molding plastics two-component brushes of the type referred to which allows the use of a modified rubber of the "Kraton" type for the bristles while ensuring proper release of the molded bristles from the cavity cells in which they are formed will be described hereinafter.

According to one aspect of the invention the method requires separately molding the handle, on the one hand, and the bristles together with a common backing layer therefor, on the other hand, in different stages of operation from th rigid material and from the modified rubber, respectively, with adhesion of the handle to the backing layer so as to form an integral two-component brush. The extraction of the bristles from mold cells used for their formation proceeds, during an initial phase of a mold-opening stroke, at a controlled rate slow enough to maintain the continuity of the brush so molded and at a significantly faster rate during a terminal phase of such stroke.

According to one possibility of implementing the method of this invention, the modified rubber ("Kraton" TM) is injected into a first cavity formed at one station of the machine between a primary mold portion and a secondary mold portion juxtaposed with each other, the first cavity including a multiplicity of elongate cells in the primary mold portion and an adjoining clearance in the secondary mold portion to produce a set of bristles integral with their common backing layer. With the bristles retained in their cells on the primary mold portion, the secondary mold portion is removed therefrom to expose a major surface ofthe backing layer while letting same cool together with the bristles.Next, a tertiary mold portion is juxtaposed at another station of the machine with the primary mold portion to form a second cavity bounded by the major surface of the backing layer. A relatively rigid plastics material is then injected into the second cavity to produce a handle adhering to the backing layer along the exposed major surface thereof. After the handle has set during a period in which the bristles and the backing layer have undergone further cooling, the primary and tertiary mold portions are separated from each other while the handle is being gradually pulled away from the primary mold portion at the controlled rate referred to, followed by a faster rate of separation.

That procedure, accordingly, allows the bristles and their backing layer to cool for a considerably longer time than the handle so as to reduce the adhesion of the "Kraton" TM to the cavity walls. For an efficient operation of the molding machine, however, it is desirable to let the injection and hardening of the handle at the second station coincide with the molding of a new set of bristles and an associated backing layer at the first station.

This requires, of course, a duplication of the bristleforming primary mold portion so that one of these mold portions can be juxtaposed with the secondary mold portion at the first station while the other coacts with the tertiary mold portion at the second station. A machine used for this purpose may include a turret with four sides each carrying one such primary mold portion.

Since the bristle-forming mold portion with its multiplicity of narrow cells is considerably more expensive to manufacture than the relatively simple tertiary mold portion used for producing the handle, the need for providing two or more mold portions of the bristle-forming type for every mold portion of the handle-forming type for every mold portion of the handle-forming type is something of a drawback.

Another inconvenience is the establishment of an intermediate station in which the backing layer integral with the bristles is exposed to the atmosphere for cooling sufficiently to serve as a boundary for the handle cavity in the next station.

A preferred way of performing the method of the present invention is based on the discovery that a clean extraction of bristles of modified rubber such as "Kraton" TM from their cavity cells is possible even after an abbreviated cooling period, provided that there has already been formed a substantially solidified handle of more rigid thermoplastic material to which the backing layer integral with the bristles can adhere. The initial phase of a moldopening stroke, during which the mold portions defining the bristle cavity are slowly separated at a controlled rate as discussed above, may equal or slightly exceed the length of the bristles so that the latter are detached from the cell walls before the mold is completely opened, in order to allow for an ejection of the brush just produced.The exact point of detachment depends, interalia, on the taper of these cell walls; thus, a larger taper enables a foreshortening of the slow-moving phase of the stroke.

Thus, an advantageous sequence of operation comprises the steps of injecting the relatively rigid plastics material into a first cavity, formed at one station between a primary mold portion and a secondary mold portion juxtaposed with each other, to produce the handle, upon a hardening of the handle and separation of these two mold portions from each other, a tertiary mold portion is juxtaposed at another station with the primary mold portion which still contains the handle, the tertiary mold portion including a multiplicity of elongate cells and an adjoining clearance that faces the primary mold portion whereby the cells and the clearance form a second mold cavity bounded by the handle.Next, modified rubber- a term intended to mean a material substantially the same as "Kraton" TM --is injected into the second cavity to produce a set of bristles integral with their common backing layer fused to the handle, this being followed by a separation of the tertiary mold portion from the primary mold portion at the aforementioned rates.

The molded composite brush is retained on the primary mold portion from which the brush is subsequently ejected.

For best results, as has been found, the controlled rate of separation of the mold portions should amountto less than one millimetre per second.

Bristle cells somewhat shorter than 2 cm, for example, will then be cleared in about 18 seconds whereupon the mold-opening stroke can be completed at a greatly accelerated rate, preferably on the order of at least 10 cm per second. Thus, even an overall stroke of 20 to 30 cm can be completed in 20 seconds or less.

As described by the manufacturer, "Kraton" TM is a block copolymer of the form SBS (styrenebutadiene-styrene), SEBS (styrene-ethylenebutylene-styrene) or SIS (styrene-isoprene-styrene).

The first two of these are particularly suitable, since they bond well to polystyrene and polyethylene, respectively. A combination of a polystyrene handle with an SEBS bristle body, however, is facilitated by admixing a small quantity of polystyrene with the "Kraton"TM, e.g. upto about 5% by weight. Bythe same token, a similarly small quantity of polyethylene may be admixed with "Kraton" TM of the SBS type if the handle is made of polyethylene.

Thus, it is to be understood that the term "modified rubber" as used herein may allow for a small proportion of some other thermoplastics material blended with the basic block copolymer.

A two shot injection molding machine for performing the method embodying the invention will now be described by way of example with reference to the accompanying diagrammatic drawing in which: Figure 1 is a plan view, partly in section, of a mold assembly carried on platens of an otherwise conventional injection-molding machine; Figure 2 is an enlarged sectional view of a detail marked by a circle II in Figure 1; Figure 3 is a viewsimilarto that of Figure 2, drawn to a somewhat smaller scale and showing additional details; Figure4is a view similar to that of Figure 3, illustrating a modification; and Figure 5 is a set of graphs relating to the separation of the mold portions forming the bristle cavity.

The injection-molding machine partly shown in Figure 1 is similar to one disclosed in commonly owned US patent No. 3,482,284 in the name of Herbert Rees. The machine comprises a bed or frame 10 supporting a stationary platen 11 as well as the usual clamp drive of which only two reciprocating rams 14' and 14" have been illustrated. These rams are rigid with a movable platen 13 sliding on a set of tie bars 12.

A central shaft 15 is fixedly connected with a supporting plate 16 swingable through 1800 about its axis. Plate 16 carries two identical primary mold portions 20' and 20" which, in the illustrated closure position, respectively adjoin a secondary mold portion 26 and a tertiary mold portion 27 carried on platen 11 through the intermediary of a fixed backing and distributing plate 50. The latter has a recess 51 accommodating a flow divider 52 through which a quick-hardening thermoplastics material, e.g.

polyethylene, is admitted from an injection unit 53' to a pair of first cavities 54 defined between mold portions 20' and 26. Mold portions 20" and 27 concurrently define a pair of second cavities 55 each of which, as shown in Figure 2, includes a space 55a communicating with a multiplicity of narrow, outwardly converging cells 55b. Clearance 55a is bounded, on its side opposite cells 55b, by a handle 28 of thermoplastic material previously molded in a corresponding cavity 54. A flow of modified rubber ("Kraton" TM) is introduced into cavities 55 to form bristles 44 in tapered cells 55b integral with a common backing layer 43 filling the clearance 55a as seen in Figure 3, that layer bonding to the preformed handle 28. The ends of cells 55b of each cavity, narrow enough to prevent penetration by the "Kraton" TM, open into a venting space 55c.

The width of each cavity 55 in the region of clearance 55a slightly exceeds that of cavities 54 so as to leave a small annular space 29 about the preformed handle 28 into which the "Kraton", introducted through an orifice 58, can flow to form a rim embracing the handle. This space and an adjoining beveled surface of mold portion 27, defining a wedge-shaped gap 30 around the periphery of the handle, ensure that the latter is not damaged when the mold closes.

Swing plate 16 has two cutouts 59', 59" accommodating respective ejector plates 24', 24" having pins 124', 124" which pass through mold portions 20' and 20" and terminating in a retracted position adjoining the spaces occupied by handles 28. The ejector plates are biased into that retracted position by suitable means such as captive coil springs 60', 60".

A pushrod 23 traverses the platen 13 in the area of the station served by injection unit 53" and is advanced by a hydraulic cylinder 22 at the end of each molding cycle, upon a withdrawal of platen 13 from platen 12 in a mold-opening stroke, to dislodge a pair of finished brushes with handles 28, backing layers 43 and bristles 44 from the cavities 55. New handles concurrently formed in the station served by injetion unit 53' remain in position on mold portion 20' or 20", coacting with mold portion 26, since that station is not equipped with a pushrod such as the one sown at 23.It will be understood that, after each ejection of a pair of finished brushes and before the next mold-closing stroke, swing plate 16 is rotated through 180" (as more fully described in the aboveidentified Rees patent) to interchange the positions of mold portions 20" and 20" in advance of a new injection at both stations.

The fluidized "Kraton" TM is supplied to the two cavities 55 from unit 53" through a central tube 32 having heating channels 61 as well as an annular shoulder 62 bounding a peripheral gap 63 (Figure 2) into which some of the "Kraton" TM flows, as seen in Figure 3, during the injection stage. The "Kraton" TM enters through the central bore 64 of tube 32 which it leaves via a transverse slot 65 at the free end thereof on its way to the orifices 58 of the two cavities. The material remaining outside the cavities forms a hot runner, in a manner generally similar to that described in commonly owned US patent No.

4,184,836 in the name of Herbert Rees.

Figure 4 illustrates a modified injection system for cavities 55 according to which the central part of the otherwise unchanged mold portion 27 has a supply conduit 40 terminating at a gate 41 from which the molten "Kraton" flows into a passage 42, similar to slot 65 of Figure 2, leading to the two injection orifices 58 (only one shown in Figure 4). Passage 42, open towards mold portion 20" (or 20' in the alternate position of swing plate 16), communicates with a frustoconical undercut 46 in that mold portion to form a plug a boss integral with a cold runner which is detachedfrom the contents of cavities 55 at orifices 58 upon the separation of the two mold portions from each other.Slanting branches 42a of passage 42, lying wholly within mold portion 27, are cleared thereby of the extremities of the cold runner which thus remains attached to the receding mold portion 20" (or 20') until a stem 45 carried on the associated ejector plate 24" (or 24') dislodges it. The plastics material in the conduit 40 is kept fluid by heating channels 61 surrounding that conduit, thereby acting as a hot runner.

Each mold portion 20' and 20", as particularly illustrated for the latter in Figures 2-4, has similar frustoconical undercuts each adjoining the tip of an associated ejector pin 124' or 124" to form a boss 28a integral with handle 28 which ensures the retention of the handle in that mold portion during the open stroke. The bosses 28a remaining on the handle of the finished workpiece are non-objectionable and, in fact, are advantageous in the next operation where a sponge may be glued on to this handle surface.

Other ways of retaining the handle on the core are also possible, such as undercuts on the inner lateral handle surfaces.

There will now be described, with reference to Figure 5, the timing of the mold-opening stroke found necessary in order to combine a safe extraction of bristle 44 from cells 55b with the shortest possible machine cycle. The diagram of Figure 5 is based on a cell length of about 16 mm, designed for bristles of substantially the same length. The opening of the mold proceeds in two phases, namely an initial phase extending over 2 cm and a terminal phase encompassing 28 cm. Graph A represents a mode of operation in which, with "Kraton" TM as the injected material, the initial phase idealized as a linear motion lasts for slightly less than 3 seconds whereas the terminal phase takes place in approxi mately one second.This timing is perfectly satisfac tory for the usual thermoplastics materials such as polyethylene or polystyrene but is unsuitable for "Kraton" since the bristles cannot be detached from their cavities. According to graph B, the initial phase has been lengthened to 10 seconds (with no signifi cant modification of the terminal phase) which is found to allow a complete detachment of the bristles but leaves some of them objectionably deformed.

Only when the time allowed for the initial phase approaches 18 seconds, as indicated by graph C, will completely straight bristles result; this corresponds to an average separation speed of slightly less than 1 mm/sec.

The mold-closing stroke can be performed at about the same rate as the terminal phase of the mold-opening stroke. With an injection and cooling period on the order of 10 seconds, the overall cycle length will be approximately 30 seconds under the assumed conditions.

Claims

1. A method of operating a two-shot injectionmolding machine to produce composite brushes each having a set of bristles of a relatively flexible modified rubber with a tendency to cling to a mold cavity during an extended hardening period and further having a handle or a relatively rigid but more rapidly hardening plastics material joined to said set of bristles, the method comprising that the handle, on the one hand, and the bristles together with a common backing layertherefor, on the other hand, are separately molded during different stages of operation from the rigid plastics material and from the modified rubber, respectively, with adhesion of the handle to the backing layer to form an integral two-component brush and that the extraction of the bristles from mold cells used for their formation, proceeds during an initial phase of a mold-opening stroke at a controlled rate slow enough to maintain the continuity of the brush so molded and at a significantly faster rate during a terminal phase of such a stroke.

2. A method according to claim 1, wherein the initial phase of the mold-opening stroke does not substantially exceed the length of the bristles.

3. A method according to claim 1 or to claim 2, including the following sequence of steps.

(a) injecting the relatively rigid plastics material into a first cavity, formed at one station of the machine between facing primary and secondary mold portions to produce the handle; (b) upon a hardening of the handle and separation of the primary and secondary mold portions from each other, causing a tertiary mold portion at another station of the machine and the primary mold portion to face each other, the latter still containing the handle, the tertiary mold portion including a multiplicity of elongate cells and an adjoining clearance facing the primary mold portion whereby said cells and said clearance form a second cavity bounded by the handle;; (c) injecting the modified rubber into the second cavity to produce the set of bristles integral with the common backing layer fused to the handle (d) separating the tertiary mold portion from the primary mold portion to expose the molded bristles, with retention of the molded brush on the primary mold portion; and (e) ejecting the molded brush from the primary mold portion.

4. A method according to claim 3, wherein the controlled rate amounts to less than 1 mm per second.

5. A method according to claim 3 orto claim 4, wherein said substantially faster rate is at least 10 cm per second.

6. A method according to any preceding claim, wherein the relatively rigid plastics material is polystyrene or polyethylene.

7. A method according to claim 3, wherein during step (b), a duplicate of the primary mold portion is positioned to face the secondary mold portion to re-establish the first cavity, followed by a repetition of step (a) substantially concurrently with step (c).

8. A method according to claim 3, wherein during step (c), the modified rubber is injected past an edge of the handle into said clearance and from there into the bristle-forming cells.

9. A method of forming a brush using an injection molding machine comprising the steps of: (a) injecting the relatively rigid plastics material into a first cavity, formed at one station of the machine between facing primary and secondary mold portions, to produce the handle; (b) upon a hardening of the handle and separation of the primary and secondary mold portions from each other, causing a tertiary mold portion of the machine and the primary mold portion to face ach other with the primary mold portion still containing the handle, the tertiary mold portion including a multiplicity of elongate cells and an adjoining clearance facing the primary mold portion whereby said cells and said clearance form a second cavity bounded by the handle; ; (c) injecting the modified rubber into the second cavity to produce the set of bristles integral with the common backing layer fused to the handle; (d) separating the tertiary mold portion from the primary mold portion to expose the molded bristles, with retention of the molded brush on the primary mold portion; and (e) ejecting the molded brush from the primary mold portion.

10. A method of forming a brush using an injection molding machine substantially as hereinbefore described with reference to the accompanying drawings.