Moulding thermoplastics

Injection/compression moulding - Thermoplastics and thermosets Blow moulding - Thermoplastics Rotational moulding - Thermoplastics... 

Tbe term structural foam was originally coined by Union Carbide to describe an injection moulded thermoplastic cellular material with a core of relatively low density and a high-density skin. The term has also been used to describe rigid foams that are load bearing. Today it is commonly taken to imply both of the above requirements, i.e. it should be load bearing and with a core of lower density than the skin. In this section the broader load-bearing definition will be used. Whilst structural foams are frequently made from polymers other than polystyrene, this polymer is strongly associated with such products and it is convenient to deal with the topic here. 

A method of injection moulding thermoplastics, which makes large-part moulding possible on injection equipment normally capable of moulding only small parts. The injection moulding machine fills the mould at relatively low pressures. See Injection Moulding. 

Recall that in the process of low-pressure moulding, thermoplastic polymers (polyolefins, polyamides, or their compositions) are loaded into a cylinder of the adiabatic extruder, plasticized and injected into a mould at low pressure. The formed article is cooled in the mould and removed due to shrinkage phenomena. 

The definition of the relationship between polymer structure and application, which has been at the centre of a significant proportion of grant applications or industrial programmes over my scientific lifetime, will need to move to a new level. We claim to understand what the relationship is in some of the more mature bits of the polymer industry - such as injection moulded thermoplastics - but we still seem to encounter problems in end use which run against received wisdom. 

Figure 7.10 shows the microhardnesses of several injection moulded starch samples processed at different temperatures with different initial water content. There is an increase in H with the injection moulding temperature in the range 80-110 °C, from 120 MPa to 140 MPa (Balta Calleja etal, 1999). These microhardness values are notably higher than those found for conventional injection moulded thermoplastic polymers like PE (50-60 MPa) (Rueda et al 1989 Balt Calleja et al., 1995). 

ASTM D4703-93 Practice for Compression Moulding Thermoplastic... 

ASTM D4703-93, Practice for compression moulding thermoplastic materials into test specimens, American Society for Testing Materials Publication, Philadelphia. 1993. 

In rotational moulding, thermoplastics are heated, shaped and cooled in the mould. Finely ground plastic powders are heated to melting and tumbled in a split, hollow mould. The inner surfeces of the mould are evenly coated with molten plastic due to centrifugal force and allowed to cool until hard. The final products are hollow with uniform thickness and relatively low moulding costs. Typical products include litter bins, traffic cones, fuel tanks and plastic balls. 

A typical helmet is expected to be between 5 and 10 mm thick, dependent on both material type and applicatioa The shell thickness is normally determined experimentally by the penetration test. The shell canbe made of either an injection-moulded thermoplastic or a pressure-moulded resin system, reinforced with ballistic resistance fabrics/fibres (Mills, 1996). Energy is absorbed when the helmet is impacted, with the shell bending and deformation of the underlying foam. The amount of the impact energy absorbed by the foam depends on ... 

P(6) The load transfer to injection moulded thermoplastic joint members shall be based on mechanical interlocking and not on adhesion, unless additional surface treatments specific to thermoplastic materials are applied. 

EN 763 Injection-moulded thermoplastics fittings - test method for visually assessing effects of heating. 

EN 803 Injection-moulded thermoplastics fittings for elastic sealing ring type joints for pressure piping - test method for resistance to a short-term internal pressure without end thrust. 

EN 12107 Injection moulded thermoplastics fittings, valves and ancillary equipment -determination of the long-term hydrostatic strength of thermoplastics materials for injection moulding of piping components. 

Long-fibre Injection moulding compounds improved surface treatment/coupling agents use of high-performance fibres in forms more suitable for injection moulded thermoplastics. 

These compounds screen out undesirable electrical frequencies by means of additives which form a shield. Typical applications are housings for equipment vulnerable to EMI. Because of the nature of the additive shield, they may require special processing, but better understanding of the technology is leading to development of formulations that are compatible with Injection moulded thermoplastics, such as ABS. 

The first commercial product made of injection-moulded thermoplastic starch was the drag-delivery capsule Capill, and further products are gradually appearing, e g. golf tees, cutlery, plates, and food containers [43]. In addition, extrusion has been applied to produce rigid foams, suitable for loose-fill packaging. 

All injection-moulded thermoplastics are prone to sink marks and voids in areas where sudden changes in section thickness occur, or over ribs and bosses. Semi-crystalline polymers such as PP are more prone to sinks and voids. Voids occur when the external skin of the moulding is rapidly cooled and becomes sufficiently rigid to support the contraction of the underlying melt. Sinking or surface depression occurs in localised thick sections where internal mass contains sufficient heat to keep the polymer in molten stage and erystallises slowly producing sink marks. 

PEG, being well above its glass transition temperature at room temperature, can act as a solvent for inks and paint (Hey et al., 1960) or a plasticizer for polymers, e.g. PVAC, CN or colophony (Powell, 1980). It may cause stress cracking of moulded thermoplastics such as PE. It does not appear to attack beeswax or gelatine. The presence and quantity of PEG in a solution or thin section can be assessed by colour staining (Hoffmann, 1983 Bilz et al., 1999), but distribution within an object is frequently assessed by extracting the PEG from a core and using instrumental analysis to quantify the concentration (Hoffmann, 1989). 

Hot Runners in Injection Moulds Thermoplastics are divided into two main groups ... 

Figure 6.10 Manifold of quadruple mould with eight injection points to mould thermoplastic rubber seals on inserted PP caps - theologically balanced mould [2]...

In electronics, polyimides are now extensively used in the form of self-standing films for flexible circuitry, deposited films for interlayer dielectrics, passivation and buffer coatings, moulding thermoplastic powders for PCBs, and adhesive pastes or tapes. The basic polyimide chemistry has been adapted to fulfill the specific requirements of these applications. A series of books provides complete information not only on the chemistry of polyimides but also on their utilization in electronics [4,23,24]. The following figures summarize the chemical formulae of the most important categories of polyimide precursors or precyclized polymers that are commonly used in electronics. 

Injection moulding technique for moulding thermoplastics similar to the pressure diecasting process. It consists basically of two operations, the softening of the thermoplastic by heat and its injection or transfer under pressure into a closed mould. 

Rotary vacuum drum and vacuum belt filters are extensively used on many process filtration operations, to provide filtration, clarification, dewatering or concentration. Moulded thermoplastic vacuum drum filters are particularly suited to providing continuous... 

---