Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
  • B22D17/00—Pressure die casting or injection die casting, i.e. casting in which the metal is forced into a mould under high pressure
  • B22D17/20—Accessories: Details
  • B22D17/22—Dies; Die plates; Die supports; Cooling equipment for dies; Accessories for loosening and ejecting castings from dies
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
  • B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
  • B29C45/00—Injection moulding, i.e. forcing the required volume of moulding material through a nozzle into a closed mould; Apparatus therefor
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
  • B29K—INDEXING SCHEME ASSOCIATED WITH SUBCLASSES B29B, B29C OR B29D, RELATING TO MOULDING MATERIALS OR TO MATERIALS FOR MOULDS, REINFORCEMENTS, FILLERS OR PREFORMED PARTS, e.g. INSERTS
  • B29K2905/00—Use of metals, their alloys or their compounds, as mould material
  • B29K2905/08—Transition metals

Definitions

• This invention relates to methods of forming from a fluid substance an article within a cavity and more particularly, but not exclusively, is concerned with methods of injection moulding of polymeric substances and methods of pressure die-casting metals.
• the cavity For pressure forming, it is usual to form the cavity from a die composed of two separable parts in order that the article formed within the cavity should be removable following the forming operation without destroying the die defining the cavity.
• the die- parts meet and contact one another along a parting line which traces a path around the wall of the cavity.
• close contact of the die-parts one with another along the parting line is essential if the fluid substance is not to leak from the cavity.
• the need for closeness of contact between the die- parts increases as higher pressure differentials are employed. Accordingly, it has hitherto been customary to manufacture die-parts for high pressure forming methods by careful machining of ferrous, or other high melting point alloy, stock within very fine tolerances.
• the resulting die-parts can be used without detriment in long production runs of pressure formed articles.
• the machining operations needed to manufacture the die-parts are expensive and, consequently, articles produced in short production runs by such dies are -also expensive.
• Metals which are used commercially on a large scale are not pure substances but rather are solutions of amounts of various solute elements within a solvent substance. When these metals freeze, they throw out primary solid of a composition which is somewhat different from that of the liquid, and generally the primary solid comprises dendritic particles.
• the metals freeze over a freezing range of temper ⁇ atures between their liquidus and their solidus. During freezing, as the temperature descends through the freezing range, the dentritic particles of primary solid interact with one another at the same time as the metal is changing its dimensions as a result of the drop in temperature, and this tends to produce defects such as porosity and segregation in the resulting casting. It is generally believed that these phenomena, unless rigorously controlled, are responsible for weaknesses in the casting and give rise to uncertainty in the dimensions thereof, so rendering castings unsuitable for immediate use as articles whose strength and precise dimensions, surface detail and texture are of crucial importance.
• a "slurry” means a homogeneous or substantially homogeneous metallic mixture whose continuous phase is liquid and whose dispersed phase is solid, the temperature of the slurry being one at which the liquid and the solid can co-exist in equilibrium.
• the slurry can be, for example, a eutectic alloy, in which case the dispersed phase comprises two or more solid phases. More typically, however, the dispersed phase comprises a single primary solid phase which appears over at least the upper part of the freezing range of the alloy. Usually, the freezing range is from 60° to 100°C and the primary solid is dendritic.
• the substance which forms the slurry is acted upon when it is in its freezing range (normally by stirring) in order to maintain the dispersed phase as a multitude of discrete particles rather than as a massive solid deposit around the sides of the vessel in which the slurry is contained.
• OMPI /., WIPO - , - H - to herein are about 0? by weight solid so that, at the moment of their casting, 5055 of their latent heat has already been evolved and 50? of their freezing contraction has already taken place.
• the pattern should be of a metal whose coefficient of thermal expansion in the relevant temperature range is similar to that of the alloy being cast.
• the pattern is heated prior to casting the slurry against it. It is considered that preheating the pattern helps to reduce the extent of dimensional change which the pattern undergoes during casting while it is in contact with the slurry, and so assists to.provide a close fit between the pattern and the casting. Also, such preheating should reduce the rate of cooling of the slurry when it first contacts the pattern and so should assist in main- taining the slurry fluid long enough for it to fill the cavity.
• the first die-part should be heated prior to casting the second die-part against it.
• the second die-part is cast against the first die-part.
• the metal from which the die-parts are formed can be an alloy of aluminium.
• the pattern against which the die-parts are cast can, for example, be of aluminium or of brass .
• the present method is satisfactory for use in short experimental production runs.
• the applicants had previously supposed that die-parts, if made in this way, would deteriorate as soon as they became subject to the conditions of temperature and pressure prevailing in a pressure forming method to such an extent that it would not be worthwhile to produce such die-parts.
• the die-parts have shown themselves capable of retaining sufficient integrity to remain serviceable after a large number of articles have successively been pressure formed within them.
• the die-parts formed in the present method may not have such a long life as the conventional die-parts, they are generally much cheaper to produce. They are expected to be particularly useful in design experiments, pre-production runs and short produc- tion runs where small numbers of pressure formed articles are " required. Even on long production runs they may be useful as they can be cheaply replaced at regular intervals.
• Aluminium die halves formed in the present method been found suitable for use as cavities in the pressure die-casting of "Mazak” (Registered Trade
• An advantage of the method of the present invention is that cooling channels, and even a complex network of such channels, can be set into the die-part during the casting process. Networks of cooling channels are feasible which it might not be possible readily to provide by the conventional machining processes. The provision of precisely located cooling channels is of increasing importance in the injection moulding industry .
• Figure 1 is a view, in perspective, of an aluminium pattern for a door handle to be formed in nylon by an injection moulding method according to the present invention
• Figure 2 is a vertical section of a pair of moulding boxes prepared for use in one stage of a method according to the present invention
• Figure 3 and H are vertical sections similar to that of Figure 2, of further stages in the method
• Figure 5 is a vertical section of a pair of die halves which have been cast in the moulding boxes, showing schematically how they can be used in an injection moulding process to produce a nylon handle correspon- ding to the aluminium pattern.
• Figure 1 there is shown an aluminium pattern 11 for a design of door handle.
• Marked on the pattern 11 is a parting line 12 which corresponds to the edges of the projection of the pattern 11 onto a plane Z being the plane in which lies the base 13 of the pattern.
• the parting line 12 traces out a continuous loop around the circumference of the pattern 11. While there are no re-entrant surfaces on the pattern 12, the method is applicable to patterns having such re-entrant surfaces, use being made of cores in known manner, whenever it is necessary to do so.
• a moulding box 1* was filled with moulding sand 15 and the pattern 11 was placed on the top surface 16 of this sand.
• the pattern 11 was fixed against movement relative to the sand 15 by screws projecting downwardly into the sand 15 from holes tapped in the pattern 11. These screws are not shown in the drawing in order that the drawing should suffer no loss of clarity.
• Sand was banked up around the pattern 11 until its surface corresponded closely with the parting line 12.
• Four conical depressions (not shown) were made in the sand on the parting line. (These generate conical projections on one die half which themselves generate conical depressions in the other die half on subsequent casting of it against the first die half, thereby to locate the die halves accurately against one another).
• a proprietary mould coating was.applied to the exposed area of the pattern 11 above the sand 15 and to the top surface 16 of the sand in the area of sand surrounding the pattern to provide these areas with a coating of a rough, pimpled texture.
• the pattern and coated area of sand were heated.
• An upper mould box 17 was filled with a quantity 18 of moulding sand and a cavity 19 was formed within the sand 18.
• the cavity 19 was sufficiently large to provide a clearance all around the pattern 11 with the upper box 17 in its working position above the box 1*1.
• Two runners 20 were cut in the sand to link
• the slurry was cast against the exposed surface of the pattern 11 in the cavity 19.
• the air adjacent the pattern 11 was displaced by the incoming alloy, and its escape from the cavity 19 was assisted by the rough texture of the mould coating.
• the escape of the air prevented the formation on the casting of defects caused by entrapped air.
• the alloy cast within the cavity 19 was allowed to cool, and it and the pattern 11 were subsequently removed from the moulding boxes.
• the shape of the casting was found to conform accurately to the shape of that part of the pattern 11 which was contacted by the slurry during the casting operation and the pattern fitted snugly within the corresponding recess in the casting.
• the surface smoothness of the casting depends upon the texture of the mould coating.
• Figure 3 shows the casting produced as described above employed as a heat sink 22 and set within a quantity 23 of moulding sand within the lower moulding box 14.
• the pattern 11 was placed within the recess of corresponding shape in the heat sink 22 and, if desired, secured therein by the use of screws projecting through the heat sink 22. Again, the screws are not shown for reasons of clarity.
• a cylinder 2*1 of mild steel having a ridged cylindrical surface was placed on the top surface 25 of the heat sink.
• a sprue for the subsequent injection moulding process was later machined within this steel cylinder 2 .”
• the exposed surfaces of the pattern 11 and the top surface 25 of the heat sink 22 were given a coating of the proprietary mould coating of control ⁇ led roughness,as above. As above, the coated areas were heated, along with the cylinder 2h .
• a number of cooling tubes 28 of a copper alloy were arranged within the upper moulding box 17 in position such that, when the box 17 was filled with moulding sand and the desired cavity 27 was formed in the sand, the cooling tubes 28 spanned the cavity 27.
• runners 20 were cut in the sand 26 to link the cavity 27 with the upper surface of the
• the upper box 17 was placed in its working position above the lower box 1*1 and aluminium slurry was cast into the cavity 27, as above, to form the first die half.
• the cast metal was allowed to cool.
• the first die half was found to conform closely in shape to the area of the pattern 11 above the heat sink 22 and to the top surface 25 of the heat sink. The pattern was a snug fit over the corresponding area of the first die half.
• the first die half 29 carrying the pattern 11 was located within a quantity 30 of moulding sand within the lower moulding box 1*J.
• a second die half provided with cooling tubes 32 was cast against the pattern 11 and the top surface 33 of the first die half 29. The casting of the second die half against the first die half helps to provide a closeness of fit of the two die-parts against one another that is of a high order.
• the first die half 29 and the second die half 31 prepared as above were used as an injection moulding die (otherwise called "tooling") in an injection moulding process as shown in Figure 5-
• tooling also called "tooling"
• Preparation of the die halves for use as tooling in such a process involved machining in the cylinder 2k a channel 33 for use as a sprue, and forming in one of the die faces 33 and 3 1 *! a runner 35 linking the die cavity 36 with the sprue 33.
• the back faces of the die halves were machined parallel to facilitate their mounting in an injection moulding machine.
• the dies were then mounted in the injection moulding machine, the cooling channels provided by the tubes 28 and 32 connected to the cooling system thereof and the sprue 33 connected to a supply of synthetic polymeric material under pressure.
• the die halves were then brought into contact and the polymeric material injected into the die cavity through the sprue 33 and runner 35 while maintaining a flow of cooling fluid through the cooling channels of the die halves.
• the die halves were subsequently separated to free the moulding from within the cavity.
• the die halves were examined for damage and the moulding inspected. No defects were found so another moulding was pressure formed within the cavity. 1000 mouldings were formed successively in a single experimental run. The last few mouldings to be formed were satisfactory and the ultimate life of the dies used for injection moulding has yet to be ascertained.
• Example 1 The procedure of Example 1 above was followed in order to produce a pair of die-halves similar to those of Example 1.
• the die halves were mounted in a machine for pressure die-casting of zinc alloys.
• the die halves were used to produce successively 200 castings of Mazak alloy. Although the last few mouldings to be produced showed some evidence of deterioration of the die halves, all the mouldings were nevertheless satisfactory.
• the mould coating used in the above Examples comprised a suspension in water of finely divided zirconium oxide, aluminium oxide and colloidal graphite with a small amount of sodium silicate as a binding medium.
• Other mould coatings can be used, provided that, at the working temperature, they are stable and compatible with the pattern and the slurry.
• Cooling tubes within the die-parts can be provided as a single length or as several separate lengths of cooling tube.
• the arrangement of cooling tubes can be chosen with a view to establishing a specific pattern of cooling within the cavity, for example, to ensure that the article being pressure-formed solidifies directionally.

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• Engineering & Computer Science (AREA)
• Mechanical Engineering (AREA)
• Moulds For Moulding Plastics Or The Like (AREA)
• Molds, Cores, And Manufacturing Methods Thereof (AREA)

Applications Claiming Priority (2)

Publications (1)

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• 1979
  • 1979-06-27 GB GB7922368A patent/GB2024067B/en not_active Expired
• 1980
  • 1980-01-07 WO PCT/GB1980/000004 patent/WO1981001972A1/en unknown
  • 1980-01-07 EP EP80900097A patent/EP0042834A1/de not_active Withdrawn
  • 1980-01-07 JP JP80500143A patent/JPS56501841A/ja active Pending
• 1981
  • 1981-09-03 DK DK390681A patent/DK390681A/da unknown

Non-Patent Citations (1)

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