Moulding defects

Moulding defects up to 5% are allowed in the moulding operation. The common moulding defects and their causes and remedies are given in the following table 14.2. 

Backrinding Rupture at parting line. Cure at low temperature and high pressure. 

Blisters Air trapped in blanks. Modify the processing to a low temperature level and prick out the blisters while in unvulcanized state. 

Flow cracks. Incorrect shape and size of blank, incomplete mould flow high viscosity stock and excessive mould lubricant. Modify the shape, reduce temperature, modify curing system, peptize and pre warm the stock. 

Porosity-generally inside the centre Under cure and reduced pressure. Increase time, temperature and pressure. 

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Moulded plastics will also have crack initiation sites created by moulding defects such as weld lines, gates, etc and by filler particles such as pigments, stabilisers, etc. And, of course, stress concentrations caused by sharp geometrical discontinuities will be a major source of fatigue cracks. Fig. 2.72 shows a typical fatigue fracture in which the crack has propagated from a surface flaw. 

In addition to this, the use of a direct gate means that a number of other points also have to be observed, without which perfect moulding production is not possible either. In particular, attention has to be paid to using a mixing head that works absolutely free of prefeed and overrun and conveys the mix into the mould cavity completely free of turbulence. And yet it is above all these problems of prefeed and overrun which have still not been very satisfactorily solved today, continually resulting in moulding defects under production conditions. 

Mould quality Checking, solving and preventing moulding defects... 

Replacing or re-working worn tooling before it causes moulding defects. 

Fig. 9.2. Some causes of typical moulding defects in injection - moulded TPU.

Many processes are available for the production of plastic products and the choice depends upon the relationship between material properties, processing method and end-product properties, in addition to the choice of forming method, which all have technical and economic aspects. Three important features of plastic melts during processing are shear viscosity, melt flow index (MFI) and melt elasticity. MFI is the quantity of polymer extruded under specific load and temperature conditions in a given time. The elastic properties of the melt are a major factor in determining the residual strain and moulding defects [8]. 

A tool was commissioned and the first prototypes in acrylic proved encouraging (Fig. 10.18), although many moulding defects were present, such... 

Hourly checks are therefore carried out on one complete lift (eight mouldings) for weight - 1% tolerance allowed flexibility - each moulding is bent back through 160 so that points A and P touch. A brittle moulding (defective) will break. 

Stress optical analysis for stress concentrations around moulding defects Stress/creep relaxation Adhesion studies... 

Disruptions to the Operation of HR Moulds and Typical Moulding Defects... 

It is a lot easier to identify disruptions to the operation of an HR monld system which occur directly, than it is to establish the real causes of moulding defects associated with them. Many of the canses of disrnptions lie outside the HR system, and some occur independently of the nse of an HR system. The most important disrnptions are discnssed in this chapter, together with possible canses and remedies. 

Another design aspect of tooling is the need to provide vents for compressed air and gases to escape during moulding. Trapped air and gases can cause a variety of moulding defects which are more fully described in Chapter 9, such as ... 

Injection moulding defects are classified into seventeen sections ... 

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