Cost of an injection moulded part comprises material cost (Cmaterial), mould cost (Cmould) and processing cost (Cprocessing). The total cost of an injection moulded part (CIM) is given by
mould ,
IM material processing
p
C C C C
n (4.1) where np is the identical quantities of parts produced during the lifetime of the mould. The raw materials used in injection moulding are thermoplastic polymers. The material cost (Cmaterial) per part is given by
= 1+ ,
100
s material m m p
C c V l
(4.2) where cm is the material cost per unit mass, ρm is the density of the raw material, Vp is the volume of the injection moulded part and ls is the percentage loss of the material. As some material is always wasted, a suitable factor is multiplied by the actual mass. The procedure for determining the other two components of cost, i.e, mould cost and processing cost are described in Section 4.2.1 and Section 4.2.2, respectively.
4.2.1 Estimation of mould cost in injection moulding
The mould cost is the most dominating cost component in an injection moulding process.
Boothroyd et al. (2010) described the complete mould costing methodology that comprises the cost of the mould base and the cost associated with its manufacturing. The manufacturing cost of the mould is the cost required to convert it into a working mould.
The mould cost is given by
m o u ld m b m m ,
C C C (4.3) where Cmb is the cost of mould base and Cmm is the cost of mould manufacturing. A typical mould comprises a cavity plate, core plate, support plate, ejector plate and clamping plate as shown in Figure 4.1.
The material considered for the mould is steel of various categories such as stainless steel and hot-rolled steel. The molten material is injected into a stationary cavity plate. On the other hand, the movable core plate holds the core that facilitates the shaping of the inner side of the part. The core plate is attached to the support plate. Adjacent to the core plate
, there lie the support and the ejector plates. The ejector plate comprises ejection pins to push the part off
Figure 4.1 A schematic of a typical mould
the core during the opening of the mould. The rear clamping plate holds the mould with necessary force during the injection of the polymer material as well as ejection of the part.
To obtain an approximate cost of the mould base, Boothroyd et al. (2010) presented an empirical relation:
1000 0.45
0.4,
mb b p
C A h
(4.4) where Cmb is the cost of the mould base in $, Ab is the area of mould base cavity in cm and hp is the combined thickness of cavity and core plates in cm. Eq. (4.4) was developed in the year 1989 applicable to the United States of America (USA). The relation was developed based on a survey where several data were accumulated for different mould bases from industries in USA. For developing a similar formula from the perspective of an Indian manufacturer, it is necessary to obtain requisite set of data from Indian industry. However, for simplicity, Eq. (4.4) will be used for analysis in this thesis. It was observed that this equation provides reasonable values based on limited and approximate information. This equation is converted into Indian currency and its right-hand side is multiplied by the ratio of consumer price index (CPI) of India in 2019 to that in 1989. The obtained equation is again converted to dollar. Thus, Eq. (4.4) is modified, based on the data in references as (Inflation tool n.d., Thomas Cook n.d.)1940 0.87
0.4.
mb b p
C A h
(4.5)An appropriate mould base is selected based on the geometry of the part (area and depth) and the number of cavities contained in the mould. The minimum clearance between two adjacent cavities as well as the edges of the plate should be 7.5 cm. Thickness plate is taken as 15 cm larger than the cavity-depth (Boothroyd et al. 2010). The number of cavities to be fabricated in the mould base is inversely proportional to the size of the part. Increasing the number of cavities may increase the production output in one cycle, but it results in increased running cost due to the larger size of the injection moulding machine. The number of cavities for a large part (maximum dimension more than 100 mm) is usually taken as one (Chin and Wong 1996). In this work, only one cavity has been taken.
Cmm in Eq. (4.3) is the cost of manufacturing during the conversion of the mould base into a working mould. Mould manufacturing involves the cost associated with ejector pins, ejector systems and geometrical complexities of the part. The list of equations for estimating the mould manufacturing cost is provided in Appendix C.
4.2.2 Estimation of processing cost in injection moulding
The processing cost is based on the type of moulding machine, the setup time of the machine, moulding cycle time and the involvement of the operator during different stages of the manufacturing process. The processing cost (Cprocessing) per part is given by
,s e tu p c y c le
p r o c e s s in g o p e r a to r la b o u r o v e r h e a d
p c y
t t
C c c c
n n p
(4.6) where tsetup is the setup time of the machine, np is the required number of parts, tcycle is the cycle time, nc is the number of cavities in the mould and py is the production yield (fraction of non-defective parts to all parts produced). Further, coperator, clabour and coverhead are the operator, labour and overhead costs, respectively. The labour cost is calculated similar to the operator cost. Similar to the SLS process, overhead cost comprises machine depreciation, annual maintenance contract, factory rent and electricity charge. The expressions to estimate these cost components are given in Chapter 3.
Cycle time (tcycle) in Eq. (4.6) is given by the algebraic summation of the three components— injection time (tf), cooling time (tc) and mould resetting time (trs). Thus,
cycle f c rs
.
t t t t
(4.7) Injection time is the time taken to fill the molten plastic material inside the mould via the sprue, runners and gates. Cooling time is of the longest duration amongst all three components due to the low thermal conductivity of the polymers. The cooling process takes place by the conduction mode of heat transfer. Mould resetting time involves the openingof the mould, ejection of the moulded part and closing of the mould. A low value of trs seems to be economical, but this involves the fast movement of the mould that causes vibration in the machine and failure of the part. The equations for determining tf and tc are presented in Appendix D. Based on the Shing (1999), trs is taken as 10 s.