Mould wear in the age of high-speed injection moulding In the Good Old Days, say 20 years ago, a 28 gram preform for a CSD package may have run at a cycle time of 24 seconds, wrapping up just over one million cycles per year, even if the tool was running all year round. At that time mould wear was not much of an issue.

Years ago, running at a cycle time of 24 seconds, it is more likely that the package had become obsolete before the mould had worn out. Today, a super-lightweight preform for a 500 ml water package may run as fast as 6.5 seconds, accumulating well over 4 million cycles per year. Tools may run for about 8 million cycles on horizontal machines and up to 15 million cycles without qualification as to what maintenance is required and what criteria are used to determine how long the tool can be used.In an incomrehensive survey we are given numbers between 5 and 12 million cycles but in the end it all depends hhow mould wear is measured. We can say for sure that some moulders are forced to buy new tools or major refurbishment every two years and this bi-annual capital expense has certainly caught their attention.s two-part article we will examine all of the issues surrounding this increasingly hot topic.

What is mould wear?

There are a number of components in preform moulds that lose all or part of their functionality over time. Valve-gate cylinders and stems, heaters, and nozzle tip insulators can be a nuisance to repair during a tight production schedule but nevertheless can be put right in a reasonable amount of time, and by processors themselves. The type of mould wear we will discuss in this article however concerns the parting line in the neck area. This parting line is formed by the two neck insert halves which form the outside of the threads. The neck inserts slide open during ejection, releasing the thread-form that would otherwise create an undercut. They are held together by two matching tapers. One (the male part) is an integral part of the neck inserts, the other, female part is integrated in the cavities. This so-called taper lock transmits the clamping force onto the mould plates, counteracting the opposing injection force that tries to open the mould during the injection and hold phase. There is considerable pressure on each taper lock and the tapers wear off over a certain number of cycles, depending on a variety of factors to be discussed later. When they do wear they fail to close tightly around the thread-form allowing plastic to flow into the gaps thus created. This is known as neck flash. As time goes on the increasing gap allows neck flash to grow until the tool has to be renewed or refurbished. There are considerable differences between users with respect to the time at which the size of the neck flash necessitates tool renewal. The PCO neck finish drawing limits the amount of flash to 0.13 mm (0.005”) per side and the larger brand owners may well expect this limit from their suppliers. On the other hand an in-house operation may tolerate much more flash, and regional differences certainly play a role. Practical limits are what the capping machines can tolerate and how much consumers who drink directly from bottle may find the flash uncomfortable.

 

Why do taper locks wear?

 

In a perfect world we should expect negligible wear on these parts. Made from hardened and precision-ground tool steel the compressive force exerted during the moulding cycle is well below what the material can bear. In the real world things are not quite as perfect. For one thing, there are manufacturing tolerances in the area of +/- 0.005 mm (0.0013”) on the tapers as well as all the positioning bores in the plates. Add to those tool positioning tolerances as well as machine plate offsets in two directions and the result is that not all tapers in a multi-cavity tool match perfectly. A further aggravation is that the clamping force may not be evenly distributed across the entire tool because of small differences in the castings or an offset in the centre location. As a result of these imperfections some tapers will be more engaged than others, leading to uneven wear over the millions of cycles that the tool is operating. Another factor may be dirt that has accumulated on the tapers, forcing the metal to shift slightly and once again pressurising certain parts more than others. On horizontal machines gravity is another detriment to proper alignment. Only very rigid tools set on perfectly straight guide bearings can hold the tool in place during the closing of the clamp as gravity exerts a force 90º to the closing action causing a sagging effect on less-than-perfect moulds. Vertical machines have an advantage in this respect, as they experience no side load during clamp movement. In summary, there are a number of imperfections that locally exert excessive stress on some parts of the tapers causing more wear than would be expected if everything was perfect. There is another stress factor that is more arbitrary, but should nevertheless be mentioned. The main culprit is failure of the mould protection system. To understand mould protection we need to look at how machines control clamp movement. On some machines one or two cylinders that are just strong enough to move the platen facilitate movement of the clamp from uully open to almost closed. On toggle-type clamps a measuring device senses the position of the clamping cylinder and engages mould protection at a certain point.  The function of mould protection is to provide the necessary speed and force to close the mould to the point where the mould faces are in direct contact with each other and prevent the high force clamp pressure to be applied prematurely. Once the mould protection is engaged the machine now senses any resistance exceeding a preset limit aborting the closing sequence if thta is the case. The mould protection device ensures that no obstacle, such as a partial preform stuck in the cavity, is present when the clamping cylinder engages. This prevents damage to the tool but operators can make mistakes when setting the system up and significant tool damage may ensue. It is also paramount that sensors check the presence of preforms after ejection to avoid preforms being stuck in cavities or on cores that would cause trouble during the next cycle.