Film spool end

Single impression tool, centrally filled from a sprue gate (i.e. no runner needed, the cavity filling from the sprue directly). The central boss part is of greater thickness than the rest of the moulding it is cooled with a large volume of ambient temperature water to balance its cooling rate with that of the other parts. 

These samples are produced at PPITB Training Centre on an older machine than that used for the bracket. It is a Windsor 130 tonne machine with manually operated, relay-activated control. There are only two injection speeds available. The more sophisticated machine requires a lot more understanding of the process to realize its potential, but its automated control system then delivers highly reproducible and accurate results. The older machine is at the mercy of disturbance factors which it is unable to accommodate, especially variability in operator performance. 

Sample 1. Again, the tool contains cores, which give small holes at the centre of the product. There are three holes, asymmetrically placed, at the centre of the product and this leads to uneven flow, side to side. 

Sample 2. In addition, it is necessary to get just the correct amount of packing to ensure that the thin sections outside the five large holes are not weak spots. Inspection of the illustration shows that there are meeting points at these, as the melt flows round, where perfect welds must be formed. 

a series of short shots is used to show the progress of the melt. Plate 15. 

---

The examples (a support bracket and a film-spool end) were made at the Plastics Processing Industries Training Board (PPITB) centre at Cardiff, now located at Telford. They also feature in the Open University course on Polymer Engineering (PT 614), which is a module of their new Continuing Education course on Manufacturing. 

We have now observed the importance of control of moulding conditions in the manufacture of two injection-moulded products. Next let us examine these process variables, and the way in which a modern machine gives control over them. In the account of the moulding of the support bracket and the film-spool end it became clear that control of injections, speed and pressure, were vital to the production of satisfactory mouldings. One other feature, not specifically discussed previously, but clearly also of vital importance, is the supply of consistent polymer melt delivered by the plasticizing unit at the specified rate. 

---