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Mold Construction

1. The injection molding and machine

1.1The injection molding

Injection molding is principally used for the production of the thermoplastic parts, although some progress has been made in developing a method for injection molding some thermosetting materials. The problem of injecting a melted plastic into a mold cavity from a reservoir of melted material has been extremely difficult to solve for thermosetting plastics which cure and harden under such conditions within a few minutes. The principle of injection molding is quite similar to that of die-casting. The process consists of feeding a plastic compound in powdered or granular form from a hopper through metering and melting stages and then injecting it into a mold. After a brief cooling period, the mold is opened and the solidified part ejected. Injection-molding machines can be arranged for manual operation, automatic single-cycle operation, and full automatic operation. The advantage of injection molding are: (i) a high molding speed adapted for mass production is possible; (ii) there is a wide choice of materials providing a variety of useful properties; (iii) it is possible to mold threads, undercuts, side holes, and large thin sections.

1.2 The injection-molding machine

Several methods are used to force or inject the melted plastic into the mold. The most commonly used system in the larger machines is the in-line reciprocating screw . The screw acts as a combination injection and plasticizing unit. As the plastic is fed to the rotating screw, it passes through three zones: feed, compression, and metering. After the feed zone, the screw-flight depth is gradually reduced, forcing the plastic to compress. The work is converted to heat by shearing the plastic, making it a semifluid mass. In the metering zone, additional heat is applied by conduction from the barrel surface. As the chamber in front of the screw becomes filled, it forces the screw back, tripping a limit switch that activates a hydraulic cylinder that forces the screw forward and injects the fluid plastic into the closed mold. An antiflowback valve prevents plastic under pressure from escaping back into the screw flights.

The clamping force that a machine is capable of exerting is part of

the size designation .The injection-molding machine ,the

reciprocating-screw injection system and is measured in tons. A

rule-of-thumb can be used to determine the tonnage required for a particular job. It is based on two tons of clamp force per square inch of projected area. If the flow pattern is difficult and the parts are thin, this may have to go to three or four tons.

Many reciprocating-screw machines are capable of handing

thermosetting plastic materials. Previously these materials were handled by compression or transfer molding. Thermosetting materials cure or polymerize in the mold and are ejected hot in the range of 375~410℃. Thermoplastic parts must be allowed to cool in the mold in order to remove them without distortion. Thus thermosetting cycles can be faster. Of course the mold must be heated rather than chilled, as with thermoplastics.

Machines are available for molding sandwich parts. One cylinder and plunger injects measured amount of skin material into the die, and then a second cylinder squirts the filler inside the mass. Finally, a final spurt from the first cylinder clears the core material from the sprue. The aim is to produce composites with optimum properties. Either case or core may be foamed.

2.Feed system

It is necessary to provide a flow-way in the injection mould to connect the nozzle (of the injection machine) to each impression. This flow-way is termed the feed system. Normally the feed system comprises a sprue, runner and gate. These terms apply equally to the flow-way itself, and to the molded material which is removed from the flow-way itself in the process of extracting the molding.

A typical feed system for a four-impression, two plate-type mould. It is seen that the material passes through the sprue, main runner, branch runners and gate before entering the impression. As the temperature of

molten plastic is lowered while going through the sprue and runner, the viscosity will rise; therefore, the viscosity is lowered by shear heat generated when going through the gate to fill the cavity. It is desirable to keep the distance that the material has to travel down to a minimum to reduce pressure and heat losses. It is for this reason that careful consideration must be given to the impression layout and gate’s design.

Sprue

A sprue is a channel through which to transfer molten plastic injected from the nozzle of the injector into the mold. It is a part of sprue bush, which is a separate part from the mold.

Runner

A runner is a channel that guides molten plastic into the cavity of a mold.

Gate

A gate is an entrance through which molten plastic enters the cavity. The gate has the following functions: restricts the flow and the direction of molten plastic; simplifies cutting of a runner and moldings to simplify finishing of parts; quickly cools and solidifies to avoid backflow after molten plastic has filled up in the cavity.

Cold slug well

The purpose of the cold slug well, shown opposite the sprue, is theoretically to receive the material that has chilled at the front of the

nozzle during the cooling and ejection phase. Perhaps of greater importance is the fact that it provides positive means whereby the sprue can be pulled from the sprue bush for ejection purposes.

The sprue, the runner, and the gate will be discarded after a part is complete. However, the runner and the gate are important items that affect the quality or the cost of parts.

3 .RUNNER

The runner is a channel machined into the mould plate to connect the sprue with the entrance to the impression. The wall of the runner channel must be smooth to prevent any restriction to flow. Also, as the runner has to be removed with the molding, there must be no machine marks left which would tend to retain the runner in the mould plate.

There are some other considerations for the designer to bear in mind: (i) the shape of the cross section of the runner, (ii) the size of the runner ；(iii) the runner layout.

Runner cross-section shape

The cross-sectional shape of the runner used in a mould is usually one of four forms: fully round, trapezoidal, modified trapezoidal and hexagonal.

The criterion of efficient runner design is that the runner should provide a maximum cross-section area from the standpoint of pressure

transfer and a minimum contact on the periphery from the standpoint of heat transfer . The ratio of cross-sectional area to periphery will, therefore, give a direct indication of the efficiency of the runner design; the higher the value the greater the efficiency.

Runner layout

The layout of the runner system will depend upon the following factors: (i) the runner of impression, (ii) the shape of the components, (iii) the type of mould, (iv) the type of gate.

When designing a runner system there are two main considerations. The runner length should always be kept to a minimum to reduce pressure losses, and the runner system should be balanced.

Runner balancing means that the distance the plastic material from the sprue to the gate should be the same for each molding. This system ensures that all the impressions will fill uniformly and without interruption providing the gate lands and the gate areas are identical.

Runner layout

It is not always practicable, however, to have a balanced runner system and this particularly applies to moulds which incorporate a large number of differently shaped impressions. In these cases（as shows examples ）balanced filling of the impression can be achieved by varying the gate dimensions that is by balanced gating.

4 .GATES

The gate is a channel or orifice connecting the runner with the impression. It has a small cross-sectional area when compared with the rest of the feed system. This small cross-sectional area is necessary so that: i) The gate freezes soon after the impression is filled so that the injection plunger can be withdrawn without the probability of void being created in the molding by suck-back. ii) It allows for simple degating and in some mould this degating can be automatic.ⅲ) After degating only a small witness mark remains.ⅳ) Better control of the filling of multi-impressions can be achieved.ⅴ) Packing the impression with material in excess of that required to compensate for shrinkage is minimized.

The size of the gate can be considered in terms of the gate

cross-sectional area and the gate length, the latter being known as gate land. The optimum size for a gate will depend on a number of factors including (i) the flow characteristics of the material to be molded (ii) the wall section of the molding, (iii) the volume of material to be injected into the

impression, (iv) the temperature of the melt (v) the temperature of the mould.

No theoretical size exists for the ideal gate. The gate size chosen in practice for a particular component is normally based on past experience. However, the reader may not have this experience upon which to base a decision and, therefore, a guide to the dimensions for each gate type is given. To the general case of a molding with a wall section between 0.75mm and 4 mm.

Figure 4-1 Types of gate

Type of gate

To obtain the optimum filling conditions the type of gate must be

carefully chosen. On most occasions, however, the choice will be obvious as only one type of gate will meet the particular requirements for the molding on hand. The types of gate (see Figure 4-1) commonly used are: sprue

gate(f), edge gate(n), overlap gate(a), fan gate(b), diaphragm gate(g), ring gate(h), film gate(c), pin gate(d), submarine gate(m) and tab gate(e).

5. Parting surface

The parting surfaces of a mould are those portion of both mould plates, adjacent to the impressions, which butt together to form a seal and prevent the loss of plastic material from the impression. The parting surface is classified flat and non-flat.

FLAT PARTING SURFACE

The mature of the parting surface depends entirely on the shape of the component. The position of the parting surface will therefore be at the top of the molding, the parting surface itself being perfectly flat. For appearance this is the ideal arrangement as the parting line is not noticeable unless flash develops.

A further consideration is that the parting surface must be chosen so that the molding can be removed from the mould. The suitable choice for the parting line is on the centre of the double-bevel which allows for half of the required form to be die-sunk into each of the two mould halves.

The parting line must occur along the line round the position of

maximum dimension when viewed in the draw direction. Then is this line lies on a plane the parting surface will be flat.

NON-FLAT PARTING SURFACE

Many moldings are required which have a parting line which lies on a non-planar or curved surface. In these cases the mould’s parting surface must either be stepped, profiled or angled.

6. MOULD CAVITIES AND CORES

The cavity and core give the molding its external and internal shapes respectively, the impression imparting the whole of the form to the molding. We then proceeded to indicate alternative ways by which the cavity and core could be incorporated into the mould and found that these alternatives fell under two main headings, namely the integer method and the insert method. Another method by which the cavity can be incorporated is by means of split inserts or splits.

6.1 Integer cavity and core plates

When the cavity or core is machined from a large plate or block of steel, or is cast in one piece, it is termed an integer cavity plate or integer core plate. This design is preferred for single-impression moulds because of the strength, smaller size and lower cost characteristics. For

multi-impression moulds there are other factors such as alignment which must be taken into consideration. Of the many manufacturing processes available for preparing moulds only two are normally used in this case. These

are a direct machining operation on a rough steel forging or blank using the conventional machine tools, or the ‘precision’investment casting technique in which a master pattern is made of the cavity and core. The pattern is then used to prepare a casting of the cavity or core by a special process. A 4.25% nickel-chrome-molybdenum steel (BS 970-835 M30) is normally specified for integer mould plates which are to be made by the direct machining method. The precision investment casting method usually utilizes

a high-chrome steel.

6.2 Inserts for cavity and core

For moulds containing intricate impressions, and for multi-impression moulds, it is not satisfactory to attempt to machine the cavity and core plates from single blocks of steel as with integer moulds. The machining sequences and operation would be altogether too complicated and costly. The insert-bolster assembly method is therefore used instead. The method consists in machining the impression out of small blocks of steel. These small blocks of steel are known, after machining, as inserts, and the one which forms the male part is termed the core insert and, conversely, the one which forms the female part the cavity insert. These are then inserted and securely fitted into holes in a substantial block or plate of steel called a bolster.

In the majority of cases the insert-bolster method of construction is used, the ease of manufacture, mould alignment, and resulting lower mould

costs being the overriding factors affecting the choice.

7. Splits

The mould designer is frequently confronted with a component design that incorporates a recess or projection which prevents the simple removal of the molding from the mould. The mould design for this type of component is inevitably more complex than for the in line of draw component, as it necessitates the removal of that part of the impression which forms the undercut prior to ejection.

The splits can be incorporated in the mould design in several ways. The designer considers more complex systems where the splits are retained on the mould plate and actuated automatically. There are two basic designs: sliding and angled-lift splits. In both designs there are moving parts and it is necessary to arrange for (i) guiding the splits in the desired direction, (ii) actuating the splits, (iii) securely locking the splits in position prior to the material being injection into the mould.

Sliding splits

In this design, the splits are mounted in guides on a flat mould plate and they are actuated in one plane by mechanical or hydraulic means. The various methods which can be used to actuate the splits in relation to the mould plate consist of finger cam, dog-leg cam, cam track, compression spring and hydraulic. The basic operation with cam actuation is as follows.

As the mould is opened, the cams attached to the fixed mould half cause the splits to slide across the moving mould plate.

Angled-lift splits

In this design the splits are mounted in a chase-bolster which forms part of the moving half of the mould. The splits are caused to move out with an angular motion, the outward component of which relieves the undercut portion of the molding. The splits are normally actuated by the ejector system. This shows the moving half of an angled-lift split mould for producing a spool. It will be noted from this illustration that the guiding of the angled-lift is not as critical as for guiding the sliding splits. The alignment of the splits, when closed, is accomplished by their being seated in the chase-bolster. The main requirement of the guiding system is that the split must be restrained to move smoothly in the required plane.

The various alternative systems with respect to particular actuating methods are: (i) angled guide dowel actuating system, (ii) cam track actuating system, (iii) spring actuation.

8. Ejection

A molding is formed in mould by injecting a plastic melt, under pressure, into an impression via a feed system. It must therefore be removed manually. Furthermore, all thermoplastic materials contract as they solidify, which means that the molding will shrink on to the core which

forms it. This shrinkage makes the molding difficult to remove.

Facilities are provided on the injection machine for automatic actuation of an ejector system, and this is situated behind the moving platen. Because of this, the mould’s ejector system will be most effectively operated if placed in the moving half of the mould, i.e. the half attached to the moving platen. We have stated previously that we need to eject the molding from the core and it therefore follows that the core, too, will most satisfactorily be located in the moving half.

The ejector system in a mould will be discussed under three headings, namely: (i) the ejector grid; (ii) the ejector plate assembly; (iii) the method of ejection.

Ejector grid

The ejector gridis that part of the mould which supports the mould plate and provides a space into which the ejector plate assembly can be

fitted and operated. The grid normally consists of a back plate on to which is mounted a number of conveniently shaped‘support blocks ’.

Ejector plate assembly

The ejector plate assembly is that part of the mould to which the ejector element is attached. The assembly is contained in a pocket, formed by the ejector grid, directly behind the mould plate .The assembly consists of an ejector plate, a retaining plate and an ejector rod. One end of this latter member is threaded and it is screwed into the ejector plate. In this particular

design the ejector rod functions not only as an actuating member but also as a method of guiding the assembly. Note that the parallel portion of the ejector rod passes through an ejector rod bush fitted in the back plate of the mould.

Ejector techniques

When a molding cools, it contracts by an amount depending on the material being processed. For a molding which has no internal form, for example a solid rectangular block, the molding will shrink away from the cavity walls, thereby permitting a simple ejection technique to be adopted. However, when the molding has internal form, the molding, as it cools, will shrink onto the core and some positive type of ejection is necessary.

The designer has several ejection techniques from which to chose ，but, in general, the choice will be restricted depending upon the shape of the molding. The basic ejection techniques are as follows: (i) pin ejection; (ii) sleeve ejection; (iii) stripper plate ejection ；(iv) air ejection.