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Process Selection for a Spark Plug

Introduction

The anatomy of a spark plug is shown schematically in Figure 1. It is an assembly of components, one of which is the insulator. This is to be made of a ceramic, alumina, with the shape shown in the figure: an axisymmetric-hollow-stepped shape of low complexity. It weighs about 0.05 kg, has an average section thickness of 2.6 mm and a minimum section of 1.2 mm. Precision is important, since the insulator is part of an assembly; the design specifies a precision of ~ 0.2 mm and a surface finish of better than 10 mm (RMS roughness) and, of course, cost should be as low as possible. The design specification is provided in Table 1.

Figure 1 Sparkplug Insulator

Design Requirements

Material Class	Ceramics
Process Class	Primary, Discrete
Shape Class	Prismatic-axisymettric-hollow-stepped
Mass	0.05 kg
Minimum Section	1.2 mm
Precision	0.2 mm
Surface Finish	10mm
Batch Size	100,000

Table 1 Spark Plug Insulator: design requirements

The Selection

The starting point of the selection exercise is the idea that all processes are potential candidates until shown otherwise. A short-list of candidates is extracted in two steps. The first eliminates processes which cannot meet the design specification. In the second, economic batch size is used to select the processes which would be economic choices. We set up four selection stages as follows... The first stage (Figure 2) establishes that the process is a primary one (one which creates a shape, rather than one which finishes or joins) and that it can cope with the section-thickness of the insulator (1 to 4 mm).

Figure 2 A chart of section thickness range against process class

The second stage (Figure 3) deals with shape and precision: processes capable of making "prismatic-axisymmetric-hollow-stepped" shapes are plotted, and the selection box isolates the ones which can achieve tolerances better than ~ 0.2 mm.

Figure 3 A chart of tolerance against shape class

The third stage (Figure 4) combines the requirements of material and component mass. Here we select the sub-set of ceramic-shaping processes which can produce components with a mass range of 0.04 kg to 0.06 kg, bracketing that of the insulator.

Figure 4 A chart of Ceramic Shaping processes against component mass

Results

The processes which satisfy the design specification for the insulator (i.e. pass all of the selection stages so far) are shown in Table 2.

Processes Passing All Stages

Die pressing and sintering

Powder Injection Moulding (PIM)

Chemical Vapour Deposition (CVD)

Table 2 Possible processes for manufacturing the spark plug insulator

The final stage (Figure 5) shows the "economic batch size" plotted for all discrete (rather than continuous) processes. The labelled processes are the ones which passed all the previous selection stages. It can be seen that Chemical Vapour Deposition (CVD) - is technically feasible but impractical for such a large batch size. CVD is not suitable for mass production because it is slow.

Figure 5 A chart of economic batch size against process class

PostScript

The insulator is commercially made using die pressing followed by sintering. According to the Process Selector, PIM is a competitive alternative. Spark plugs have a very competitive market and, therefore, the cost of manufacturing should be kept low by choosing the cheapest route. More detailed cost analysis would be required before a final decision is made. Of course, if you only wish to make a few spark plug insulators (or similar components), instead of 100000, then Chemical Vapour Deposition may well be the best manufacturing route.