Mould surface

The surface finish of the specimen may also affect impact behaviour. Machined surfaces usually have tool marks which act as stress concentrations whereas moulded surfaces have a characteristic skin which can offer some protection against crack initiation. If the moulded surface is scratched, then this protection no longer exists. In addition, mouldings occasionally have an embossed surface for decorative effect and tests have shown that this can cause a considerable reduction in impact strength compared to a plain surface. 

Finishing and polishing the mould surfaces is also extremely important because the melt will tend to reproduce every detail on the surface of the mould. Finally the mould will have to be hardened to make it stand up to the treatment it receives in service. As a result of all the time and effort which goes into mould manufacture, it is sometimes found that a very complex mould costs more than the moulding machine on which it is used. Several features of the mould are worthy of special mention. 

A manufacturing method predominately used by the thermoplastic industry for the manufacture of hollow components, also used for thermosetting rubber. The polymer is forced against the inside of the mould surface by gas pressure applied either by a hollow needle or by the decomposition of a blowing agent. 

Any substance used to prevent sticking of rubber to a mould surface, thus facilitating removal of the product without risk of tearing or splitting. The lubricant may be painted or sprayed on the mould surface or applied to the surface of the... 

A wax-like polymer with resistance to a wide range of temperatures, high resistance to chemical attack and very low frictional resistance. It is widely used in O-rings, seals and gaskets it may also be sintered on to a mould surface to assist the release of the product. Polythene... 

Many release agents build up on the metal mould surface and can form hard deposits combined with compound degradation products. This leads to the need for frequent mould cleaning. 

Silicone resins, which after being sprayed onto the mould surface form a resin under the influence of the heat of the metal mould, are effective and relatively long lasting release agents. A single application of the resin will remain viable for a number of products lifts before requiring reapplication. 

In addition to in-mould transfer pots, loose pots are also used which sit on top of a mould. These save on mould costs and weight but some excess compound can escape between pot and mould surface. 

Pre-impregnated rovings, tapes, tows or fabrics, cut to the right shape and size, are laid-up by hand or machine onto a mould surface. 

A formed laminate is made by relative movement of first and second contoured mould surfaces toward each other to compress a flexible sheet and vertically spaced film with a foamable liquid mixture therebetween that has substantially completed expansion but not yet substantially cured so as to form the flexible sheet and provide a collapsed foam layer bonded to the sheet. The flexible sheet is preferably porous cloth, vinyl or leather. A film which is preferably a nonadherent plastic such as PE is preferably positioned in a vertically spaced relationship with the flexible sheet either above or below the sheet with the foamable liquid mixture received therebetween for the expansion prior to the compression that provides the formed laminate. The formed laminate... 

Tyres are normally cured in a modification of the compression mould where a bladder or an inflated airbag forces and holds the green rubber stock of the tyre against the mould surface during vulcanization. This force reproduces the design of the tyre tread and the heat from the steam is introduced into the bladder to effect the vulcanization. Small size rubber expansion joints used in piping systems are moulded by compression moulding... 

When it is the product rather than the material that is being investigated, it is obviously desirable to make tests, wherever possible, on the actual finished product rather than on specially prepared test pieces, which may have been produced under rather different conditions. Apart from the difficulty of having sufficient bulk in the product to obtain standard test pieces, extra operations may be involved which are time consuming and are likely to lead to lower test results because of destruction of the moulded surface. However, these difficulties can often be overcome satisfactorily by the use of miniaturised test pieces and by careful use of cutting and buffing apparatus. 

In rubber testing, the surface finish of metals is of importance, for example on mould surfaces and compression set plates. There are a number of standards in the ISO Geometric Product Specification series but the most relevant is ISO 428729 which covers terms, definitions and surface texture parameters relating to the profile method of measuring surface finish. There are apparently over 1000 different parameters to characterize surface finish30 but only a few are generally encountered. The most commonly found is Ra (previously called CLA) which is the mean deviation of the surface profile above and below the center line, followed by Rz, a measure of the peak to valley height. For example, the arithmetic mean deviation (Ra) of the compression plates for compression set tests must be better than 0.2 m. 

ASTM D575105 does not primarily recommend the generation of a stress/strain curve but details two methods - force at given deflection and deflection at given force. The test piece is a cylinder 28.6 0.1 mm diameter and 12.5 0.5 mm thick from which all moulded surface layers have been removed. The removal of moulded surfaces is presumably to eliminate skin effects, although such effects, if significant, would be present in a moulded product and one would expect to test in the same condition. 

The appearance of the surface of the component will be reflected in the appearance after painting—if, say, it is a grained pattern rather than smooth, a grainy effect will be obtained. To some extent this difficulty can be overcome with a thick coat of primer, but obviously it will be helpful from the point of view of quality if the mould surfaces and the natural finish of the material chosen are in keeping (rather than conflict) with the final finish required. 

Karahadian, C., Lindsay, R.C. 1987. Integrated roles of lactate, ammonia, and calcium in texture development of mould surface-ripened cheese. J. Dairy Sci. 70, 909-918. 

Furthermore, the friction forces acting in the flow field can induce phase segregation at the mould surface [189]. As pointed out by Cakmak and Cronin [191], in PP/EP blends with a high content of EP particles even shear amplification phenomena may occur due to the presence of the small rubber particles. The shear amplification results from considerable shear fields occurring in small gaps between rubber particles which in turn are subjected to the macroscopic shear field extended over the whole width of the sample. 

Thermoforming a material for a unit change in temperature. Values for plastics range from 0.01 to 0.2 mil/in. Any process of forming thermoplastic sheet which consists of heating the sheet and pulling it down onto a mould surface. 

Therefore the reaction rate for an in situ surface modification during a moulding process has to be very fast, as can be concluded from the model assumption in Fig. 18. The chemical coupling of substances has to be finished after a very short time because at the moment of contact of the hot melt front with the tempered or rather "cold" mould surface the temperature drops rapidly and as a result an exponential decrease of the reaction rate should be observed (Arrhenius equation). 

After evaporation of the solvent from the tempered mould surface the mould was closed. 

The hot plastic melt was injected into the mould followed by a fast reaction with the modifier on the mould surface by a polymer surface modification. 

INJECTION MOULDING SURFACE ROUGHNESS BLOW MOULDING EXTRUSION... 

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