GB2305387A - Unitary support block in injection moulds - Google Patents

Abstract

An injection mould ejector grid structure comprising a unitary support block 70 comprising a back plate portion, at least two riser portions and a support plate portion, fabricated from a single block of material, and defining an ejector plate cavity therebetween. The manufacture of the ejector grid structure is substantially simplified over existing ejector grid assemblies by reduction of the number of parts and machining processes required, and provides a more rigid structure while maintaining ease of assembly of ejector plate and pin assemblies housed within the ejector grid structure.

Description

IMPROVEMENTS IN AND RELATING TO INJECTION MOULDS The present invention relates to injection moulds, and in particular to an ejector grid and supporting structure of a moving mould plate of a die set for such injection moulds.

The design of die sets for injection moulding has evolved over a considerable number of years. In a typical design, such as that shown in figure 1, a die set 5 typically comprises a front plate 10 having a suitable aperture and fixings to enable attachment of an injection system 11.

Attached to the front plate 10, and supported thereby, is a fixed mould plate 12. A corresponding moving mould plate 14 is brought into position adjacent the fixed mould plate 12 to form the mould cavity 15 prior to carrying out the injection process. The moving mould plate 14 is supported on a support plate 16 whose function, like that of front plate 10, is to resist bending of the mould plate 14 or 12 under the very high pressures encountered during the injection stage of the injection moulding process.

The moving mould plate 14 and the support plate 16 include a number of apertures 60,61 through which pass ejector pins 20,21 which forcibly eject a moulding 22 (shaded) out of the mould cavity 15 when the injected moulding material has set and the mould plates 12, 14 have been separated. The ejector pins are held by a retaining plate 24 and driven via an ejector plate 26 by way of an actuator rod 29 which is axially moveable within a space 35 defined by the support plate 16, support blocks or risers 27,28 and back plate 30. Front plate 10, fixed mould plate 12, moving mould plate 14, support plate 16, support blocks 27,28 and back plate 30 are all maintained in suitable axial alignment by way of suitable fixing arrangements as generally indicated along the lines of fixing centres 32,33, as are well known in the design of injection moulds.

The present invention is particularly concerned with the design of the ejector grid, ie. the support for the moving mould plate 14 and the ejector plates 24,26 which comprises the support plate 16, support blocks or risers 27,28, back plate 30. These components are depicted in more detail in the perspective view of figure 2, which includes a cut-out portion in the leading edge to illustrate internal detail.

With reference to figure 2, there is shown an in-line ejector grid comprising the back plate 30, support blocks or risers 27,28, and support plate 16. The functions of these components is, broadly speaking, as follows. The back plate 30 provides a suitable platform for holding the remaining components onto a moving part of the injection mould unit, and includes an aperture 34 for passage of the actuator rod 29 (figure 1). The support blocks 27,28 are required to create a cavity 35 into which the ejector plates 24,26 and ejector pins 20,21 may be individually installed and coupled to the actuator rod. The support plate 16 is used to bridge the support blocks 27,28 to provide a rigid backing surface for the mould plate 14.The support blocks 27,28 must be sufficiently closely spaced to provide adequate support for the support plate 16 to resist bending under the closing and injection pressures of the mould, but while still providing an adequately sized cavity 35 to contain the ejector plates 24,26, actuator rod 29 and ejector pins 20,21.

In some prior art arrangements, the actuator rod 29 is not physically connected to the ejector plates 24,26, but merely presses against them in one direction of travel only. In this case, a spring bias mechanism is used to provide a return force on the ejector pins instead of the actuator rod providing a pulling action.

Because of the complexity of assembling the ejector plate 26, retaining plate 24, actuator rod 29 and ejector pins 20,21 (see figure 1), the whole ejector grid assembly has typically been constructed of a large number of components, eg. twelve components as shown in figures 2 and 3.

These include the support plate 16, back plate 30, support blocks 27, 28 and various fixing mechanisms which will now be described. As shown in the cut-out section on the leading corner of the ejector grid of figure 2, fixing screws 40 pass through the back plate 30, support block 27 and support plate 16 to affix the mould plate 14 (not shown in figure 2) to the ejector grid, and the components of the ejector grid together.

Although only one example of the fixing screw 40 is shown in figure 2, it will be understood that this is replicated in at least the four corners of the assembly with holes 4144, as suggested by figure 3.

In addition, provision must be made for the entire ejector grid assembly to axially slide with respect to the fixed part of the injection mould, ie. the front plate 10, and the fixed mould plate 12. To achieve this, holes 51-54 must be drilled in the support plate 16, support blocks 27,28 and back plate 30. These holes are then very precisely axially registered with one another by bushings 50 (only one shown). This is necessary in order to provide the very high degree of accuracy of axial movement required to provide adequate accurate mating of the two mould plates 12,14.

The support plate 16 must also include a number of holes or slots 61-64 to allow passage of ejector pins (eg. 20,21 in figure 1) through the moving mould plate 14.

An exploded view of the at least twelve parts needed to assemble the ejector grid of figure 2 is shown in figure 3.

It can readily be seen that the construction of an ejector grid assembly is therefore extremely intensive in machining processes. Each of the parts: back plate 30, two support blocks 27 and 28, support plate 16 and four bushings 50 must be cast or extruded, cut, milled or turned, deburred, drilled, heat treated, and reamed or ground, prior to complete assembly of the constituent parts. These manufacturing steps are illustrated schematically in figure 5(a). Those skilled in the art of injection moulds will understand that many of the machining steps involved above must be carried out to very high tolerances in order to achieve the very high accuracy of alignment required of injection mould plates. This places a very high overhead on inspection stages in the manufacturing processes.

It is an object of the present invention to considerably simplify the manufacturing of such an ejector grid assembly.

According to one aspect, the present invention provides an injection mould ejector grid structure comprising a unitary support block comprising a back plate portion, at least two riser portions and a support plate portion, fabricated from a single block of material, and defining an ejector plate cavity therebetween.

According to a further aspect, the present invention provides a method of manufacturing an ejector grid structure for an injection mould unit comprising the steps of: using a single block of suitable material to form a back plate portion, at least two riser portions and a support plate portion, defining an ejector plate cavity therebetween, by casting, cutting and milling said single block.

Embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings in which: Figure 1 shows a schematic side view of a prior art injection mould assembly; Figure 2 shows a perspective view, with cut away section showing inner detail, of a prior art ejector grid assembly; Figure 3 shows an exploded perspective view of the components of the ejector grid assembly of figure 2; Figure 4 shows a preferred embodiment of an ejector grid according to the present invention; Figures 5(a) and 5(b) show process flow diagrams illustrating the respective number of manufacturing steps to be taken in the manufacture of an ejector grid of the prior art compared with an ejector grid according to the present invention; and Figures 6(a) and 6(b) show bar charts indicating manufacturing time for an ejector grid of the prior art compared with an ejector grid according to the present invention.

According to the present invention, despite a long tradition in ejector grid design, it has been recognized that ejector grid assembly can be fabricated as a unitary structure 70 whilst still overcoming the difficulties of assembly of the ejector plates 24,26, ejector pins 20,21 and actuator rod 29 within a cavity 35 in the unitary structure 70.

The back plate 30, support blocks 27,28 and support plate 16 are, as shown in figure 4, manufactured as a unitary structure 70. In doing this, the number of manufacturing steps required has been cut from twenty-eight steps (see figure 5(a)) to only five steps (see figure 5(b)).

Although the complexity or length of some of those five steps are greater than their counterparts in the prior art manufacturing process, as shown in figures 6(a) and 6(b), the overall manufacturing process time for the unitary structure is less than 50% of the prior art manufacturing process.

In order to be able to satisfactorily assemble the ejector plates 24, 26, the ejector pins 20, 21 and connect the actuator rod 29, it has been found desirable to provide an oversized access aperture 75 in the back plate portion of the unitary structure 70. Although this aperture 75 necessarily reduces the rigidity of the structure compared with one requiring only a small actuator rod aperture, it is found in practice that the use of unitary structure of comparable size to the prior art ejector grid assembly provides such an large increase in rigidity and strength over one assembled from a number of parts, that this increased size of aperture 75 is more than compensated for by the increased rigidity of the structure 70.

The ejector plates 24,26, ejector pins 20,21 and actuator rod 29 assembly are preferably pre-assembled outside the cavity 35 and passed en-bloc through aperture 75. Alternatively, the ejector plates may be inserted through end aperture 78 and assembly then takes place within the cavity 35 using the access hole 75 and/or end aperture 78.

The ejector grid structure 70 is drilled with guide holes 71-74 which carry out the same function as the holes 51-54 drilled in each of the support plate 16, support blocks 27,28 and back plate 30 and bushings 51 in figure 2.

In prior art ejector grid design (figure 2), the pattern of holes or slots 61-64 is determined by the particular mould configuration or design, and therefore, upon changing mould plate 14, a support plate 16 with a different configuration of holes is typically required. This is achieved either by drilling new holes 61-64 in the support plate 16 or, alternatively changing the support plate. In the former case, the operation is essentially the same when dealing with a unitary structure of the present invention, since new holes 81-84 can readily be drilled into the support plate portion of the structure 70. In the latter case, this requires a change of the complete unitary structure 70 rather than just a support plate 16, but because of the substantially faster manufacturing process, this is not found to be a significant disadvantage.

This particular problem of replacing the entire ejector grid may readily be overcome by providing a standardized matrix or array of ejector pin holes 81-84 to suit a multiplicity of ejector pin arrays (ie. a considerably greater number than those illustrated, required for a single mould design). Thus, a matrix of holes 81-84 would be provided over a large part of the area of the support plate portion of which only a few holes would be selected for actual passage of ejector pins for any given mould plate 14. The designer of a mould would be free to use any position in this standardized matrix of ejector pin holes when designing the mould.

Claims (10)

1. An injection mould ejector grid structure comprising a unitary support block comprising a back plate portion, at least two riser portions and a support plate portion, fabricated from a single block of material, and defining an ejector plate cavity therebetween.

2. An ejector grid structure according to claim 1 further including an ejector plate within said cavity for coupling an actuator rod to a plurality of ejector pins, and an access aperture provided in the back plate portion adapted to allow passage of the ejector plate into the cavity.

3. An ejector grid structure according to claim 1 or claim 2 including guide means for coupling the ejector grid to the moving part of an injection mould unit.

4. An ejector grid structure according to claim 3 in which the guide means comprise only holes drilled through the entire structure between support plate portion and back plate portion.

5. An ejector grid structure according to claim 1 further including an ejector plate within said cavity for coupling an actuator rod to a plurality of ejector pins, and an end aperture provided between said riser portions adapted to allow passage of partially assembled ejector plate and ejector pin assembly into the cavity.

6. An ejector grid structure according to claim 1 further including a matrix of regularly spaced holes through the support plate portion and covering a substantial proportion of the area of said support plate portion, adapted to allow selection of ejector pin positioning at a large number of sites within said area.

7. A method of manufacturing an ejector grid structure for an injection mould unit comprising the steps of: using a single block of suitable material to form a back plate portion, at least two riser portions and a support plate portion, defining an ejector plate cavity therebetween, by casting, cutting and milling said single block.

8. A method of manufacturing an ejector grid structure according to claim 7 further including the steps of cutting an access aperture in the back plate portion, and inserting an ejector plate therethrough.

9. A method of manufacturing an ejector grid structure according to claim 7 further including the steps of passing a pre-assembled ejector plate and ejector pin assembly through an end aperture between said risers, and completing coupling of said assembly to an actuator rod through an aperture in the back plate portion.

10. An ejector grid structure substantially as described herein with reference to the accompanying figure 4.