Quality Improvement of an Injection-Molded Product Using Design of Experiments: A Case Study

Figures & data

Figure 1. Bulldog with excessive flash.

Figure 2. Bulldog without flash.

Figure 3. Screw-type injection-molding machine.

Table 1. Factors with high and low levels

	Levels
Factors	Low (−1)	High (1)	Duplicates
Pack pressure (A)	150 psi	450 psi	4
Pack time (B)	1 sec	5 sec	4
Injection speed (C)	0.5 in./sec	2 in./sec	4
Screw RPM (D)	100 rpm	200 rpm	4

Figure 4. Flash measurement.

Figure 5. Defective bulldog.

Table 2. Design of experimental run and average flash

Run order	Pack pressure	Pack time	Injection speed	Screw RPM	Average flash (mm)
12	−1	−1	−1	−1	*
3	1	−1	−1	−1	5.625
9	−1	1	−1	−1	0.000
15	1	1	−1	−1	5.375
4	−1	−1	1	−1	6.000
1	1	−1	1	−1	5.500
16	−1	1	1	−1	6.150
7	1	1	1	−1	7.875
13	−1	−1	−1	1	0.425
2	1	−1	−1	1	4.625
5	−1	1	−1	1	0.500
14	1	1	−1	1	4.375
6	−1	−1	1	1	10.500
8	1	−1	1	1	9.000
10	−1	1	1	1	9.000
11	1	1	1	1	9.600

^*Indicates defective parts.

Figure 6. Normal probability plot of the effects.

Figure 7. Main effects plot for average flash.

Figure 8. Interaction plot.

Table 3. Estimated regression coefficients for average flash

Term	Coefficient	SE coefficient
Constant	5.321	0.2072
A	1.176	0.2072
C	2.632	0.2072
D	0.682	0.2072
A^*C	−1.135	0.2072
C^*D	0.890	0.2072
S=0.7904	R-Sq=96.4% and R-Sq(adj)=94.4%

Table 4. Analysis of variance (ANOVA) table

ANOVA for Average Flash
Source	DF	Sequential sum of squares	Adjusted sum of squares	Adjusted mean square	F ratio	P value
Regression	5	149.789	149.789	29.9578	47.96	0.000
Linear	3	116.582	115.610	38.5367	61.69	0.000
Interaction	2	33.207	33.207	16.6036	26.58	0.000
Residual error	9	5.622	5.622	0.6247
Lack-of-fit	2	1.42	1.42	0.7100	1.18	0.361
Pure error	7	4.202	4.202	0.6003
Total	14	155.411

Figure 9. Normal probability plot for residuals.

Table 5. Flashes produced for confirmation runs

Setting	Pack pressure	Pack time	Injection speed	Screw RPM	Flash (mm)
1	Low	High	Low	Low	0.3
2	Low	High	Low	High	0

Figure 10. Setting 1.

Figure 11. Setting 2.