Plastic and nonmetallic article shaping or treating: processes – Pore forming in situ – By mechanically introducing gas into material
Reexamination Certificate
1999-05-25
2002-03-19
Kuhns, Allan R. (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Pore forming in situ
By mechanically introducing gas into material
C264S051000, C264S572000
Reexamination Certificate
active
06358446
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a method of forming an article via injection of plastics material into a mould.
BACKGROUND OF THE INVENTION
It is known to expand a mould containing foamable molten plastics material. For instance, in U.S. Pat. No. 3,793,415, to A. G. Smith, assigned to ICI and applied for on Nov. 16, 1971, formation of articles having a foamed core is described. A foamable composition is injected into a mould to fill it without foaming. A skin forms against the inside surface of the mould. Movable inserts in the mould are retracted. This allows areas of the mould to foam. Alternatively, these areas are filled without foaming and the volume of the mould is enlarged by partial opening of the mould to allow the core of the moulding to foam. A variety of possible blowing agents is set out. This method is hereinafter referred to as “the ICI Method”.
A more complex method is the TAF method, as described in Japanese patent No. 51-27266 to Ashani Dow Limited and applied for on Jul. 1, 1972. It addresses the then prevalent problem of exothermic blowing agents, namely that during injection of them with the plastics material into the mould, gas bubbles—in practice nitrogen since diazobonamide was the usual blowing agent—form close to the surface of the material and burst through the surface before it had frozen off against the mould. The TAF method addresses this by pressurising the mould with nitrogen so as to prevent foaming until the mould has been filled and a skin has formed against the inside surface of the mould. After injection the gas is vented, allowing foaming to start. The mould volume is expanded and the material foams to fill it. The advantage was blemish-free surfaces of the final product.
A similar process is described in British patent application No. 2,010,168 by USM Corporation, applied for on Dec. 12, 1978. Again a skin is formed against the inside surface of the mould.
In my International Application No. PCT/GB/96/01706, now published under No WO 87/03800, I described and claimed (as proposed to be amended):
A method of forming an article via injection of plastics material into a mould, the finish formed article having thin wall portion(s) and thick wall portion(s), the thick wall portion(s) being at least partially foamed, the method consisting in the steps of:
providing a mould tool defining in its closed state, between its cavity part and its core part, narrow gap portion(s) whose mould part gap is to be substantially reproduced in the thin wall portion(s) of the article and wide gap portion(s) whose mould part gap is less than the thickness of the thick wall portion(s) of the finish formed article;
closing the mould tool to define the narrow and wide gap portions;
injecting a plastics material mixture comprising a basic polymer and a foam producing additive into the mould tool;
allowing the plastics material mixture to at least substantially solidify in the narrow gap portions of the mould tool to produce the thin wall portions of the finish formed article;
withdrawing at least a portion of one part of the mould tool from the other part before the plastics material mixture has at least substantially solidified in the wide gap portion(s) of the mould tool to allow the mixture to expand by foaming and form at least some of the thick wall portion(s) of the finish formed article; and
ejecting the article from the mould tool.
This method is hereinafter referred to as “My Original Method”.
It is possible to work My Original Method in conjunction with the ICI Method. Indeed in my International Application No. PCT/GB96/01706 on My Original Method, there are described alternatives where movable mould parts are drawn back to allow local foaming from thick wall portions.
However these alternatives were not expected to produce finish formed articles with a foam structure different from that otherwise obtained in My Original Method, that is to say with a clearly perceptible skin and a foam structure, resembling that of conventional foam.
In developing My Original Method, with a view to making larger foamed sections, I designed the tooling described in respect of the first embodiment below and experienced the very surprising “rapid expansion foaming”, by withdrawing the movable part very early in the moulding cycle.
SUMMARY OF THE INVENTION
According to my present invention there is provided a method of forming an article via injection of plastics material into a mould, the method consisting in the steps of:
providing a mould tool defining in its closed state a mould cavity, the mould tool being adapted to be partially expandable from an injection volume for receiving a load of plastics material to a final, larger volume determining the finish formed shape of the article, the mould tool having at least one movable member, which is adapted to be withdrawn to increase the volume of the mould cavity, the difference in the injection and final volumes being represented by a void exposed in the mould cavity when the movable member is withdrawn,
closing the mould tool to delimit the injection volume,
injecting into the injection volume a plastics material mixture comprising a basic polymer and a foam producing additive,
withdrawing the movable member to allow the plastics material to take up the shape of the final volume and
opening the mould and ejecting the finish formed article;
wherein the withdrawal is effected with substantially minimal delay on filling of the injection space, whereby rapid expansion foaming occurs into the void on the withdrawal.
The actual delay after which rapid expansion foaming will not occur will depend upon the individual article being formed and details of the moulding tool. Too long a delay will result in a coherent skin being formed against the movable member, which inhibits the foaming from occurring throughout the final volume of the mould cavity. It is possible that a thin skin, which ruptures and loses coherence on foaming, may be allowed to form against the movable member.
In terms of conventional injection moulding practice, wherein the injection phase of the moulding machine cycle comprises an injection/filing stages followed by a packing/holding stage, the holding stage is minimised, if not dispensed with entirely. The delay between the filling stage and withdrawal of the movable member need only be sufficiently long to allow closure of a shut-off valve between the injection barrel and the moulding tool and/or sufficiently long to allow solidification of the plastics material in the gate of the moulding tool to prevent return flow of the material out of the tool. Therefore the movable member can be withdrawn immediately after the shut-off or gate solidification.
In making a small article, such as a cosmetics cream jar, I draw back the movable member within less than a second after filling of the mould, i.e. after the injection screw has ceased forwards movement. I have withdrawn the movable member within half a second of filling. Typically I can withdraw within one or two tenths of a second of filling; and I envisage that I could withdrawn without any delay, with a shut-off valve being closed at the same time as withdrawal. I would not expect to experience the rapid expansion foaming if the withdrawal was delayed for appreciably more than a second, at least without the movable member being insulated. For larger mouldings, it may be that longer delays can be tolerated due to the thermal mass of molten plastics material.
It is my present understanding that the foam forms spontaneously on withdrawal, with the movable member exposing molten plastics material to the void. To encourage this, I provide that the “original thickness” dimension, from the surface of the movable member through the material to the portion of the mould tool on the opposite side of it, sufficiently large for the body of the material to retain its heats and for little cooling effect to be experienced at the movable member. This does not necessarily mean that this original, expanding thickness of the material cannot be thin in comparison with
Coraltech Limited
Kuhns Allan R.
Ware, Freesola, Van Der Sluys & Adolphson LLP
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