Lesson 15 Runnerless Moulds Nozzle types,The purpose of the nozzle is to provide a flow path for the plastic melted from the machines cylinder to the mould. In the simplest design the nozzle butts on to the sprue bush of the mould.,There are two standard designs in common use and they differ only with respect to the form of seating that is made with the sprue bush.,One design incorporates a hemispherical end and the other design is flat-ended.,The small length of reverse taper in the bore at the front end of the nozzle is such that the sprue is broken just inside the nozzle.,This helps to keep the nozzle face clean and assists in maintaining a leak-free sealing face. The nozzle types are as follows:,extended nozzle (Figure 15-1), barb nozzle, antechamber design, internally heated sprue bush (Figure 15-2), internally heated extension nozzle, multi-nozzle manifold.,Figure 15-1 Extended nozzle,Figure 15-2 Internally heated sprue-bush Hot-runner unit mould,This is the name given to a mould which contains a heated runner manifold block within its structure.,The block, suitably insulated from the rest of the mould, is maintained at a closely controlled elevated temperature to keep the runner permanently as a melt.,The polymer material can thereby be directed to the mould extremities without loss of the heat and without the pressure loss associated with temperature variations.,The hot-runner unit (see Figure 15-3) is mounted adjacent to the cavity plate and accommodated in a suitably design grid.,The polymer material enters via a centrally positioned sprue bush, passes through the flow-way and leaves the unit via a secondary nozzle in line with the impression.,When the mould is opened the molding is pulled from the cavity, and the sprue is broken at the small diameter end.,The remainder of the feed system remains heated within the unit, ready for the next shot.,Figure 15-3 Hot runner unit mould,An insulated runner mould (Figure 15-4) is one in which the melt flows through a large-diameter runner machined in the butting surfaces of the cavity plate and the feed plate of a three-part mould.,The two plates are attached together by quick-release swing latches during the normal molding operation.,Figure 15-4 Insulated runner mould,Polymer melt flows from a standard or extended nozzle into a large-diameter runner, and finally into the impression via the reverse tapered sprue and gate.,The mould is operated as a simple two-part mould in that the runner system is not removed during the normal cycle of operations.,As the mould opens, the molding is extracted from the cavity and the gate is broken at its junction with the reverse tapered sprue.,This technique is only practicable because thermoplastics have such good insulating properties.,The outer layer of the melt solidifies against the cold runner wall which forms an insulating shell, while the central core of the material remains in the melt state.,Provided that the molding cycle permits the melt to pass intermittently through the runner system without extended hold-ups, the mould can operate continuously in this fashion.,Naturally, solidification of the runner system will occur if the production cycle is interrupted for any reason.,When this happens the latches are released, the respective plates are separated, the solidified runner is removed.,The insulated runner mould is similar in construction to the underfeed mould.,Basically, the only difference between the two is that the former design incorporates a larger-diameter runner, a larger-diameter reverse tapered sprue, and a swing latch system.,The diameter adopted for the runner is in the range 13mm25mm, and a correspondingly large diameter is chosen for the reverse tapered sprue.,From the above comments it will be apparent that an underfeed mould can be readily modified to an insulated runner design, if required.,The development of the insulated runner mould is credited to Philips Chemical Company (USA) who developed the system for polyethylene.,The conditions required for molding this material are not critical, but the coolant flow-way holes should be positioned close to the runner so that reasonably accurate temperature control can be achieved in this region.,The mould needs to be fairly warm when starting up, but to achieve an economic cycle the temperature is then progressively reduced until the required conditions are reached.,Note that in this basic design of insulated runner mould no heat is introduced to the material in the mould. The only heat supplied is by the injection cylinder heating elements.,It follows that, even allowing for the excellent insulation provided by the polymer shell within the runner, the temperature of the melt at the outlet point is considerably lower than the temperature of the melt which leaves the nozzle.,There is always the possibility, therefore, that the melt will freeze at the gate and thereby interrupt production.,To achieve bett