Mold Customization

The mass of the mold is then estimated as:

= ⋅ ⋅ + ⋅ ⋅ ⋅

If a DME#3 (AISI P20) steel is used for the mold base construction, then the cost of the mold is $5.25 per kg. The cost of the mold base is then estimated as:

= + ⋅ =

mold_base

US$830 538 kg 5.25 $ $3,700 C kg

3.3.3 Mold Customization

The mold base customization includes many design, machining, and assembly steps. Some of the specific steps in the mold customization include:

Cutting pockets and bolt holes in the mold plates to receive the core and cavity inserts.

This cost is proportional to the number of mold cavities and the mold dimensions.

Milling a cold runner system into the mold plates, or purchasing a hot runner system and modifying the mold accordingly. This cost is related to the type of feed system, the number of gates, and the mold dimensions.

Drilling, tapping, and plugging the cooling lines in the mold. This cost is related to the number and layout of the cooling lines, which is related to the number of cavities and their geometry.

Drilling and reaming holes in the core inserts and support plates to accept ejector pins, and providing appropriate counter bores in the ejector retainer plate. This cost is related to the number of ejector pins, which is related to the number of cavities and their geometry.

Milling holes in the ejector plate and the ejector retainer plate to provide support pillars, if needed. This cost is related to the number of cavities and their geometry.

Designing and machining other necessary mold components such as stripper plates, slides, core pulls, etc. These costs are related to the specific part geometry and application requirements.

56 3 Mold Cost Estimation

A detailed cost analysis of all the customizations is too lengthy to present given the necessary discussion of the assumptions and equations. However, a review of the above customizations indicates that the costs are generally related to the size of the mold base, the cost of the inserts, and the specific technologies required. Accordingly, a reasonably simple model is:

= ⋅

∑

∑

customization cavities cavity_customizingi mold_base mold_customizingi

i i

C C f C f (3.18)

where the coefficients fcavity_customizingcorrespond to the factors governing the costs of customizing the cavity inserts, and the coefficients, fmold_customizing, correspond to the factors governing the costs of modifying the mold base. The summation over i represents the added customization for each of the mold subsystems, with i∈ [feed system, cooling system, ejector system, structural system, and miscellaneous]. It should be noted that these customization factors have been developed so as to include the procurement cost of the required components and system assemblies, such as hot runners, fittings, core pulls, etc.

Feed systems are discussed in detail in Chapter 6. The cost factors associated with modify-ing the cavity inserts and mold base for accommodatmodify-ing different types of feed systems are provided in Table 3.7. A simple molding application with one to four cavities might use a two plate cold runner system with fcavity_customizingequal to 0.05 and fmold_customizingequal to 0.1. For a molding application with high production volume and sixteen or more cavities, a thermally gated hot runner might be used with fcavity_customizingequal to 0.5 and fmold_customizing

equal to 1.0.

Cooling systems are discussed in Chapter 9. The cost factors for various cooling system designs are provided in Table 3.8. Many molds use straight lines with o-ring and fittings, adding 5%

to the cost of the cavity inserts and 20% to the cost of the mold base. As the cooling system becomes more complex, the implementation cost increases.

Ejector systems are discussed in Chapter 11. The cost factors for various ejection system designs are provided in Table 3.9. Most molds can be assumed to use a mix of round ejector pins, blades, and sleeves though ejection requirements will vary significantly depending on the part geometry and application requirements.

Table 3.7: Feed system cost coefficients

Feed system design Cavity cost coefficient,

feed_system

Two plate cold runner system 0.05 0.1

Three plate cold runner system 0.1 1.0

Hot runner system with thermal gate 0.4 2.0

Hot runner system with valve gates 0.5 4.0

Hot runner stack mold with thermal gates 0.5 8.0

Hot runner stack mold with valve gates 0.9 12.0

57

The structural design of molds is detailed in Chapter 12. The cost factors for various structural system designs are provided in Table 3.10. Most molds with high production volumes can be assumed to use support pillars and parting plane interlocks. In this cost estimation method, the sealing of the cavity by the core insert and cavity insert is considered as part of the structural system. The cost of the mold will increase with the complexity of the parting surface, the design of which will be discussed in Section 4.1.

There are many other customizations that can be performed on the mold. Some of these factors are provided in Table 3.11, and are applied as necessary. For most molds, none of these customizations are required.

Table 3.8: Cooling system cost coefficients

Cooling system design Cavity cost coefficient,

cooling_system Straight lines with o-rings and fittings 0.05 0.2 Straight lines with bubblers or baffles,

o-rings, and fittings

0.10 0.2

Circuitous cooling lines with o-rings, plugs, and fittings

0.15 0.4

Circuitous cooling lines with bubblers or baffles, o-rings, plugs, and fittings

0.20 0.4

Complex cooling line layout with thermally conductive inserts or contoured cooling inserts

0.25 0.8

Table 3.9: Ejector system cost coefficients

Ejector system design Cavity cost coefficient,

ejector_system

Mix of round ejector pins, blades, and sleeves

0.2 0.2

Stripper plate 0.2 0.4

External slide or lifter 0.2 0.4

Internal slide or lifter 0.4 0.4

Actuated core pull 0.4 0.5

Reverse ejection system 0.5 1.0

3.3 Mold Cost Estimation

58 3 Mold Cost Estimation

Example: Estimate the cost of customizing the mold base and inserts for the laptop bezel.

The mold will use a hot runner with thermal gates, so the appropriate customization factors are:

The mold will use a cooling system with circuitous cooling lines, o-rings, and plugs so the appropriate customization factors are:

Table 3.10: Structural system cost coefficients

Structural system design Cavity cost coefficient,

structural_system

Complex, contoured parting surface 0.4 0.2

Support pillars 0.0 0.1

Support pillars and interlocks 0.1 0.2

Split cavity mold 0.5 1.0

Table 3.11: Other customization cost coefficients

Required mold customization Cavity cost coefficient,

miscellaneous

59

The mold will use an ejector system with a mix of round ejector pins, blades, and sleeves, so the appropriate customization factors are:

=

The mold will use a structural system with support pillars and interlocks. Also, the mold will require a stepped parting plane to form the details along the side of the molding as shown in Figure 3.5. As such, the appropriate customization factors are:

= + =

The mold will use a melt thermocouple at the end of flow and a melt pressure transducer near the gate for process control purposes, so additional customization factors are:

= + =

The cost of all customizations may then be calculated as:

= ⋅ + + + +

+ ⋅ + + + + =

customizations $27,900 (0.4 0.15 0.2 0.3 0.1)

$3700 (2 0.4 0.2 0.2 0.2) $43,200 C

To summarize the above analysis, the total cost of the mold is estimated as:

= + +

= + + ≈

total_mold cavities mold_base customization

$27,900 $3,700 $43,200 $74,800

C C C C

The estimate seems reasonable for a mold produced in the United States. On the other hand, this result may over estimate the cost of the mold if made in Asia, especially if not including a hot runner system. Accordingly, the analysis could be repeated for a cold runner mold with different labor cost coefficients from Appendix D.

3.3 Mold Cost Estimation

60 3 Mold Cost Estimation