(2) Internal undercut only。

(3) Internal thread only。

(4) External undercut and internal undercut。

(5) External undercut and internal thread。

5。1。1。4。 Material class。  Plastic material used for  mould- ing the part is classified according to its material constant n [20]。 There are four positions for the fourth   digit。

(1)  Smooth,  having  a  uniform  diameter  along  the entire Position 1 2 3 4

height。 Material  constant n 0。6 0。7 0。8 0。9

(2) Cone, diameter stepped to one   end。

(3) Having various diameters along the entire   height。

Class 4

(1) Curved  rotating axis。

(2) More than one parallel rotating axes。

(3) More than one non-parallel rotating   axes。

Class 5

(1) Rectangular  with  no deviation  in corner。

(2) Rectangular with one deviation right angle or trian- gular。

(3) Rectangular  with  circular deviation。

(4) Flat part, regularly arched or dished。

(5) Irregular contoured flat part。

Class 6–7

(1) Straight axis, uniform cross-section, no deviation in corner。

(2) Straight axis, uniform cross-section, with one devia- tion。

(3) Straight axis, uniform, other than rectangular cross- section。

(4) Straight axis, varying cross-section stepped to one  end。

(5) Straight axis, varying cross-section along the entire height。

(6) Curved axis。

5。1。1。3。 Undercut feature。 Mould designers are frequently encountered with demoulding problems resulting from undercuts existing on the plastic parts。 An undercut can be defined as any interference occurring between the mould and the moulded part when the part is knocked out from the mould in the withdrawal direction。 The design of an appropriate mould for this type of products is inevitably more complicated than for those without any undercuts。 In general undercuts can be classified into three  types, external  undercut,  internal  undercut  and  internal  thread。

5。1。2。 The second section: the relationship between the part and the mould

This section consists of only one digit—the fifth digit。 It describes the relation between the directions of  the part axis and the mould parting line and is classified as   below。

(1) Part axis is perpendicular to the mould parting    line。

(2) Part axis is parallel to the mould parting    line。