The Injection Molding
1. The 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 injection a method plastic into a mold cavity from a reservoir of melted material has been extremely difficult to solve for thermosetting plastic 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 powered 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 machine operation.
The advantage of injection molding are:
(a) a high molding speed adapter for mass production is possible;
(b) there is a wide choice of thermoplastic materials providing a variety of useful properties ;
(c) it is possible to mold threads,undercuts,side holes,and large thin section.
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,as shown in Figure 2-1.The screw acts as a combination injection and plasticizing unit.As the plastic is fed to the rotating screw,it passes through three zones as shown :feed,compression,and metering.After the feed zone,the screw-flight depth is gradually reduced,force the plastic to compress.The work is converted to heat 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 presents plastic under pressure from escaping back into the screw flight.
The clamping force that a machine is capable of exerting is part of the size designation 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°C~410°C.Thermosetting parts must be allowed to cool in the mold in order or remove them without distortion. Thus thermosetting cycles can be faster.Of course the mold must be heated rather than chilled,as with thermoplastics.
3. Basic Underfeed Mould
A simple mould of this type is shown in Figure3-1,and the description of the design and the opening sequence follows.The mould consists of three basic parts,namely: the moving half,the floating cavity plate and the feed plate respectively. The moving half consists of The moving mould plate assembly,support block,backing plate,ejector assembly and the pin ejection system.Thus the moving half in this design is identical with the moving half of basic moulds.
The floating cavity plate,which may be of the integer or insert-bolster design,is located on substantial guide pillars (not shown) fitted in the feed plate.These guide pillars must be of sufficient length to support the floating cavity plate over its full movement and still project to perform the function of alignment between the cavity and core when the mould is being closed.Guide bushes are fitted into the moving mould plate and the floating cavity plate respectively.
The maximum movement of the floating cavity plate is controlled by stop or similar device.The moving mould plate is suitably bored to provide a clearance for the stop bolt assembly.The stop bolts must be long enough to provide sufficient space between the feed plate and the floating cavity plate for easy removal of the feed system.The min. space provide for should be 65mm just sufficient for an operator to remove the feed system by hand if necessary.
The desire operating sequence is for the first daylight to occur between the floating cavity plate.This ensures the sprue is pulled from the sprue bush immediately the mould is opened.To achieve this sequence,springs may be incorporated between the feed plate and the floating cavity plate.The springs should be strong enough to give an initial impetus to the floating cavity plate to ensure it moves away with the moving half.It is normal practice to mount the springs on the guide pillars(Figure3-2) and accommodate them in suitable pocket in the cavity plate.
The major part of the feed system(runner and sprue) is accommodated in the feed plate to facilitate automatic operation,the runner should be of a trapezoidal form so that once it is pulled from the feed plate is can easily be extracted.Note that if a round runner is used,half the runner is formed in the floating cavity plate,where it would remain,and be prevented from falling or being wiped clear when the mould is opened. Now that we have considered the mould assembly in the some detail,we look at the cycle of operation for this type of mould.
The impressions are filled via the feed system(Figure3-1(a)) and after a suitable dwell period,the machine platens commence to open.A force is immediately exerted by the compression springs,which cause the floating cavity plate to move away with the moving half as previously discussed.The sprue is pulled from the sprue bush by the sprue puller.After the floating cavity plate has moved a predetermined distance,it is arrested by the stop bolts.The moving half continues to move back and the moldings,having shrunk on to the cores,are withdrawn from the cavities.The pin gate breaks at its junction with the runner(Figure3-1(b)) .
The sprue puller,being attached to the moving half,is pulled through the floating cavity plate and thereby release the feed system which is then free to fall between the floating cavity plate and the feed plate.The moving half continues to move back until the ejector system is operated and the moldings are ejected (Figure3-1(c)) .When the mould is closed,the respective plates are returned to their molding position and the cycle is repeated.
4. 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 remove from the flow-way itself in the process of extracted the molding.
A typical feed system for a four-impression,two plate-type mould is shown in Figure4-1.It is seen that the material passes through the sprue,main runner,branch runner and gate before entering the impression.As the temperature of molten plastic is lowered which going through the sprue and runner,the viscosity will rise;however,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 min. to reduce pressure and heat losses.It is for this reason that careful consideration must be given to the impression layout gate’s design.
4.1.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.
4.2.Runner A runner is a channel that guides molten plastic into the cavity of a mold.
4.3.Gate A gate is an entrance through which molten plastic enters the cavity.The gate has the following function: 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.
4.4.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 nozzle during the cooling and ejection phase.Perhaps of greater importance is the fact that it provides position means whereby 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.
5. 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; and(ⅲ) the method of ejection.
5.1. Ejector grid
The ejector grid(Figure5-1) is 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”.
The ejector plate assembly is that part of the mould to which the ejector e-lement is attached.The assembly is contained in a pocket,formed by the ejector grid,directly behind the mould plate.The assembly (Figure5-2)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 function 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.
5.2. Ejection 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 choose， 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 and(Ⅳ) air ejection.