Multi-layer mouldings

It is becoming common practice to have the cross-section of a plastic moulding made up of several different materials. This may be done to provide a permeation barrier whilst retaining attractive economics by having a less expensive material making up the bulk of the cross-section. To perform stress an ysis in such cases, it is often convenient to convert the cross-section into an equivalent section consisting of only one material. This new section will behave in exactly the same way as the multi-layer material when the loads are applied. A very common example of this type of situation is where a solid skin and a foamed core are moulded to provide a very efficient stiffness/weight ratio. This type of situation may be analysed as follows  

Example 2.8 A polypropylene sandwich moulding is 12 mm thick and consists of a foamed core sandwiched between solid skin layers 2 nun thick. A beam 12 mm wide is cut from the moulding and is subjected to a point load, H, at mid-span when it is simply supported over a length of 200 nun. Estimate tiie depth of a solid beam of the same width which would have the same stifihiess when loaded in the same way. Calculate also the weight saving by using the foam moulding. The density of the solid polypropylene is 909 kg i and the density of the foamed core is 6(X) kg/m.  

Solution The first step in analysing the foamed sandwich type stracture is to calculate the second moment of area of the cross-section. This is done by converting the cross section to an equivalent section of solid plastic. This is shown in Fig. 2.18. 

From the equivalent section the second moment of area can then be calculated as 

The solid polypropylene beam which would have the same stiffness when loaded in the same way would need to have the same 2nd moment of area. So if its depth is d then 

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Any recyclate derived from multi-layer mouldings will consist of a combination of the materials used to make up the individual layers. This does not usually present a problem as the layers are all of suitable properties for blow moulding and compatibilised with each other. 

A multi-layer expansion-moulded article is obtained by moulding a multi-layer parison comprising a foamed resin... 

The multi-layer article is made using moulds between which a cavity clearance is freely set. A skin material lined with foam is placed between the upper and lower moulds and molten PP containing a chemical blowing agent is supplied through a resin melt conduit in the lower mould when the cavity clearance is between (C plus 15) mm and (C plus 50) mm, where C is the thickness of the skin material lined with the foam. The upper mould is lowered at a specific rate and the molten resin is pressed at a specific pressure to fill the cavity ends with the molten resin to complete the moulding of the resin body. The body is pressed for a certain time to form a skin layer, the upper mould is lifted up to decrease the compression pressure of the skin material lined with the foam to a pressure lower than the blowing pressure of the PP resin to form and solidify the foamed body, the upper mould is lowered to apply pressure to the moulded article and finally the article is cooled in the mould. 

This issue has been addressed by the second type of aseptic filling system in which a polyethylene bottle (which can be multi-layered to add a gas or light barrier) is moulded under sterile conditions. While the bottle is still sterile, and the polyethylene is soft, the trim material above the neck finish (moil) is crimped closed to seal the bottle. The sealed, sterile, empty bottle can then be packed and transported to a filling line, which may be in another location from... 

Polypropylene (PP) bottles have the advantage that they are inherently hot-fillable (at 85°C) and retortable to 120°C. The bottles are normally extrusion blow-moulded, and can also be made with multi-layering to include barriers against oxygen permeability. This technology is commonly used for sauce bottles and juice products. Since most of the bottles are of an oval shape, product contraction resulting in volume reduction is countered by allowing die bottle to become more oval under the effects of vacuum. 

Nodax C4/C6 is said to be a natural fit for injection moulding and extrusion of sheet or film. The polymer has mechanical properties similar to a polyolefin and surface properties much like PET, including high receptivity to printing inks and dyes. Adhesion to LDPE and PP is good enough to avoid tie layers in multi-layer structures. Nodax s oxygen barrier property approaches that of EVOH. 

Among the latest techniques is the Japanese Cuius blow moulding process which produces a preclosed tube from an extrusion process. It is argued that the absence of a base pinch off eliminates the weakest part of a conventionally moulded bottle. The process is suitable for single layer and multi-layer bottles with a wide variety of shapes. 

CIRTM This process allows two or more resins to be simultaneously injected into the mould. Advantages include the ability to create multi-layer and multi-resin structures. CRTM Also known as injection compression moulding, the process involves a gap between the mould surface and the fibre perform (Figure 3.10). Resin is injected into the gap and the mould closed to compress the resin into the fibre preform. This reduces the injection pressure and fill time required in RTM. 

A preaccelerated, thixotropic, LSE orthophthalic resin offering rapid wet-out and a low exotherm. For hand-lay and spray/projection lamination. Fully cured laminates show good water resistance and mechanical properties. Ideal for multi-layer laminates and for boat hulls, vehicle bodies and industrial mouldings. Approved by Lloyd s Registry. 

Multi-layer blow mouldings, give the process added versatility by allowing the use of layers of different materials that can be tailored for purpose and economy. Multi-layers also provide a means of incorporating recyclates and masking the inherent colour of the recyclate itself. For example, consider a two-layer bottle structure that used blue virgin material as the outer visible layer and black recyclate as the inner layer. If you actually looked into the bottle you would see the black recyclate material, but all of the outer surface would be blue. 

Multi-layer extrusion blow moulding Complex Inflation Closed Single layers Very low... 

Injection blow moulding is commonly used to produee eontainers sueh as PET earbonated drinks bottles. In this method, preforms are injection moulded and then inflated in a separate operation. Core bars are used to transfer mouldings from the moulding station to the inflation station. Multi-layer for extrusion blow moulding has been practised for some time with nine or even more distinct layer configurations possible. Multi-layer with injection blow moulding is a newer development. 

K.K. Lim and A. lanakiev. Modelling of Rotational Moulding Process Multi-Layer Slip Flow Model, Phase-Change and Warpage, Polym. Eng. ScL, 2006, 46, 7 pp. 960-969. 

The Battenfeld solution to three channel moulding is shown in Figure 10.7. This utilises machine configurations also used in multi-shot techniques as will be described in later sections. The vertical unit can be used to feed the third material into a eombined nozzle system based on their two layer technology described in Section 10.8.2.3. 

The Mono-Sandwich technique for co-injection moulding was described in Section 10.8.2.3 to which the reader should refer for machinery details. This technique can also be used for over-moulding by using the core back technique, again described earlier. In this technique termed the monosandwich 5 process, an additional valve is required in the runner system that ean release different valves as necessary . Once the melt is layered, the first component is injeeted. The valve is switched within the mould to expand the cavity and then the rest of the shot is injected to ereate a multi-shot component. 

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