The guiding elements in an injection mould are very important. They ensure that  both mould halves are already centred while closing against each other. Except in  special solutions, guide bolts are built into the fixed half and guide bushes are built  into the moving half. The tolerances between the cavity plates and the guide bolts  and bushes are so small that they are installed with a light press fit. The fixed half with the guiding bolts fits exactly, free of play, into the guide bushes  of the moving half. Only in this way is it guaranteed that both sides fit together on  top of each other precisely and repeatedly. If this were not the case, the mould  halves could move radially, which among other things can lead to different wall  thicknesses in the plastic parts. This is also called mould offset. Figure 2.5 shows what can happen when the guiding elements of an injection  mould are not exactly aligned.

Here are a few comparisons to get an idea of how important the accuracy of  the guiding is. The tolerances between the bolt and the plate have to be so  accurate that some light strikes are required when installing the bolt in the  plate. If the bolt is just 0.006 mm too thick, it will be very difficult to install. The tolerance between the guiding bolt and the guiding bush is even smaller.  The difference between free of play movement and getting jammed is a  maximum of 0.004 mm in diameter. If the centre distance between the guiding elements of the plates in the  upper part and the lower part differs by more than 0.02 mm it is difficult for  the mould to close. 

Anti-rotation Protection Today nearly all injection moulds are rectangular. For this reason normally four  guiding elements are installed, one in every corner. To prevent a false (rotated) assembly of the fixed half and the moving half, one of the guides is smaller or bigger  than the other three. In Figure 2.6 the fixed half of a mould is displayed: three guide bolts with diameter  (Ø) 18 mm and one guide bolt with Ø 20 mm. This should prevent a false (rotated)  assembly of the fixed half on the moving half.

The following is important for the length selection of the guide bolts: Before the  mould contours of the two halves approach, the guides must already fit into one  another. If the guides are too short, the mould contour could be damaged during  the closing action of the mould halves. In Figure 2.7 it is clearly visible that the guides are already sliding into one another before both sides can have contact.