DFM | hot runners
Nozzle with tip
Above left: Figure 1: Visual appearance of an open nozzle
Above right: Figure 2: Visual appearance of a needle valve gate. The needle point is marked with a yellow arrow. The red arrow marks the nozzle tip witness line
that the feed point can be quite visible (Figure 1) and if the cooling performance of the tool is not optimised the gates can tend to string, especially on faster cycle times.
A needle valve system will give a more controlled
gate point because the forward movement of the needle after injection and packing of the cavity allows the gate point to be closed positively. A needle valve system will also allow a slightly faster cycle time because it is possible to open the mould before or during the decompression phase of the moulding cycle. Figure 2 shows a typical needle valve gate, with the yellow arrow indicating where the needle has penetrated a little way into the component after the valve gate has closed (this recess can be adjusted in depth if and when needed). The red arrow marks the witness line of the nozzle tip, which guides the needle. It is important to note that the steel around the gate point acts as both the guide and shut-off point for the needle. Depending on the hardness of the mould steel and the abrasiveness of the materials being processed – a glass reinforced PA being much worse in this respect than a PC/ABS blend, for instance – this area will wear. If the shut-off is machined directly into the cavity steel repairs can be time consuming and costly; it may be possible to make a welded repair but in the worst case a new cavity will be required. A better option is to place an easily-replaceable sub-insert in the
Figure 3: A disc placed in the centre of the cold runner
allows a needle valve system to be guided using a
standard nozzle tip
Valve gate nozzle
nozzle tip area of the cavity. If the nozzle tip is placed onto a cold runner it is
important to create a small disc in the system to avoid the nozzle tip making a steel shut-off with the core side, as the nozzle will grow in length due to thermal expansion (Figure 3). While determining how many gate points will be needed and where they will be placed on the compo- nent, it is worth checking with the toolmaker or hot runner supplier on the minimum possible pitch between drops. This is especially important if you plan to use a valve gate system as the actuators that drive the needles are larger in diameter the nozzles. The product designer should also take a look at any
design features close to the anticipated direct gate point or points. Remember that the nozzle bores will be quite large compared to the typical gate point. A basic principle in tool design is that holes, for instance, will be inserted with core pins. If these pins need to be established on the cavity side they may interfere with the nozzle bore. This becomes an even bigger challenge when you have to cope with undercuts to be demoulded from the cavity side of the tool. Hot runner tools are not rocket science and there are
plenty of suppliers and systems available to suit even the most complicated applications. But fi xing gate points in the early stage of the DFM phase is very important and it is useful for a component designer to have some insight into this tool technology before running into a problem as the design matures. Hot runner suppliers and manufacturers have plenty of expertise; it is good DFM practice to make use of it.
About the author: André Eichhorn is general manager of Germany-based AST Technology. This is the latest instalment in a series of articles in which he discusses how part and moulding problems can be overcome at the start of any project by the application of Design for Manufacturing techniques. You can read the most recent articles in this series here, here and here.
www.ast-tech.de
52 INJECTION WORLD | July/August 2013
www.injectionworld.com
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