Injection molding

Injection molding

Injection moulding will often be an interesting choice of production method at larger series, or when extensive or complicated machining can be eliminated. In injection moulding, plastic granulate is screwed through a cylinder. The material is melted with the help of heating bands around the cylinder, and by the shear forces in the cylinder. The melt is assembled in front of the retreating screw and is then injected into a closed mould. After a certain time, the (sometimes cooled) mould is opened and the part ejected. Cycle times can vary from seconds to minutes.

Some noteworthy aspects will be mentioned. Injection moulds are in general rather expensive to make. Mould lifespan is a function of several parmeters, for example mould material, wear and maintenance. Not seldom there will be some evaluation steps in the production of a mould, with trial runs and optimisations, and the process can be rather lengthy. Some of the difficulties lie in optimising machining tolerances, consideration of shrinkage, outlay of cooling, and special geometries. Software is often used to simulate the mould filling. Injection moulding can be problematic if the part has heavy walls or has great differences in wall thicknesses.

An alternative route is sometimes to compression mould parts, which are end machined to tolerance.

One should remember that material grades for injection moulding are not always usable for extrusion (semi-finished goods) and vice versa. The number of grades available to injection moulding is in general much greater than for extrusion, and thus also the array of tailored properties. A number of factors influence the processability, for instance melt viscosity, melt strength, crystallinity, thermo-oxidative stability etc. Thus, when specifying material grade for an application, some foresight as to production method is recommendable.

It should also be noted that two seemingly identical parts, made from the same material grade, one through extrusion/machining and one through injection moulding, may have very differing properties.

There are many reasons for this, of primarily morphological character. Principally, the parts can not be considered to be homogeneous and isotropic, since core and surface have different properties. This is caused during the processing, by thermal gradients and surface (tribological) effects. Thus there will be gradients through the thickness relating to crystallinity and relaxation (the degree of frozen in tensions). These are in turn affected by heat generation at machining, environmetal effects (heat, water, chemicals, radiation etc) which can negatively affect a number of properties. To eliminate these effects by process optimisation is either technically or economically undoable. Some tricks to remedy these effects are to anneal extrudates, temperate details at machining and control mould temperatures closely.