Mould internal features

The decisive factor for all quality features that are concerned with dimension and weight is the internal pressime of the mould. Constant maintenance of this pressure curve in every cyele guarantees imiformity of the quality of injection moulded parts. If the mould internal pressure emwe is maintained at a eonstant, all of the negative factors mentioned above are compensated. 

Loss of torque against time has been well documented with plastic caps on plastic bottles. This does not necessarily mean that the closure becomes ineffective, but there are occasions when a low torque only needs the vibration effects of transportation to become an unsatisfactory (loose/leaking) closure. Certain tamper-evident/resistant features such as sealed diaphragms and ratchet-type closure systems are likely to overcome such problems. Having the right combination of plastics may also eliminate or reduce this loss of torque. However, there is the converse where the use of the incorrect combination of plastics, or plastics with undesirable constituents such as lubricants, mould release agents, internal release agents, may actually exacerbate the loss of closure torque. This loss may be associated with thread forms, area of thread contacts, cold flow or creep of plastics involved, lubricants, etc. 

The smooth flow of the melt in a mould is interrupted by sharp comers and abrapt changes in wall thickness. Both types of feature have other undesirable effects, and should be avoided where possible. Sharp internal comers act as stress concentrators, and are one of the most frequent sites for crack initiation in moulded parts. Stress concentration factors can be calculated using standard formulae given in books on stress anafysis. They should preferably be limited to a value of 1.S, and when a sharp comer cannot be avoided, the radius should be not less than 0.5 mm. 

Every plastic manifests a limited shear resistance, determined as the permissible shear rate exceeding this value causes mechanical destruction and tearing of molecules of the plastic as a result of excessive internal friction, which has a bearing on the mechanical, electrical or thermal properties of the moulded part. The plastics with the greatest shear resistance are those of low viscosity, e.g., readily fluid versions of PP and PE, PA, etc. Low shear resistance is a feature of plastics like PP (of a high viscosity), PC, PSU and PPS. PVC, CA, CAB, EVA, POM and fluoroplastics have a particularly low shear resistance. 

In view of the advantages of HR systems with internal heating (described in Chapter 4.2.2), they are also used in stack moulds. Of particular importance here is the potential to reduce the mould height. Figure 9.14 shows a fragment of a stack mould with a long sprue bushing, internally-heated manifold and two-way torpedo. A characteristic feature of this system is that the manifold is the mould plate. 

After a component has solidified and cooled down, it needs to be removed from the mould cavity. Ideally, this is done by gravity and the part falls to the floor as shown in Figure 4.25. However, some components with design features such as undercuts, adhesion or internal stresses may have to be removed fi om the mould manually or by robots. 

Special features Do not use silicone based mould release agents with PSU. Post cure of the component is recommended to reduce the internal stresses and improve the mechanical properties up to 5 min. in oil or glycerine, up to 5 hours in air at approx. 165 °C. 

---