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Product Design / Strength of Molded Products

Contents Introduction Safety Molding Machines Molding Conditions Recycling
Molding Characteristics Product Design Mold Design Countermeasures for Defects

Points  Accuracy  Deformation  Dimension Control  Strength of Molded Products

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7. Product Design

7.5 Strength of Molded Products

 

Table 7-4 Possible originating points of breakage

Point of breakage Principal cause of breakage
Sharp corner • Stress concentration (especially under impact load)
Weld area area • Reduction of elongation
Gate • Reduction of elongation and molded-in stress
Heavy-walled area with slender core • Blow-out molten material due to insufficient cooling
(reduction of elongation due to insufficient holding pressure)
Drastic change in thickness • Molded-in stress due to nonuniform cooling rate
Flash • Flash may become notches
Metal insert • Reduction of elongation due to gate or weld, Creep rupture
Thin flat section • Anisotropy in properties due to flow orientation
Others • Incomplete plasticization
• Cold slug
• Contamination of other resins
• Inadequate molding conditions
(e.g., material temperature, holding pressure)
• Problems with recycled resin
(e.g., incomplete plasticization, contamination with foreign substances, etc.)

 

Problems related to shape

(a) Sharp corners

Since strain at the time of molding and stress at loading is concentrated at sharp corners, sharp corners are apt to cause breakage. For this reason, corners should be provided with as large a curvature as possible within the allowable range (Figs. 7-6 and 7-7).


Fig. 7-6 Radius of curvature and stress concentration


Fig. 7-7 Dependence of dart drop impact strength on radius of curvature

 

(b) Weld line and gate

Table 7-5 shows tensile strength measured on test pieces having a weld line where stress is applied. This table shows that elongation at break decreases to approximately 1/2 to 1/3 that of normal test pieces, while tensile strength is similar to that of a sample with no weld line. Designing should be performed and gate position should be selected so that as little stress as possible is applied on the weld line and gate section. Since incomplete adhesion of the weld line also becomes a cause of breakage, attention should be paid to molding conditions.

Table 7-5 Effect of weld on tensile properties

  Tensile strength (MPa)
and retention rate
Tensile elongation (%)
and retention rate
M90-44 No weld 60 59
With weld 58
(98%)
27
(46%)
GH-25 No weld 129 2.2
With weld 55
(42%)
0.9
(41%)

  Test piece: ISO tensile test piece (2mm thickness)

 

(c) Flash and moldings containing metal insert

If flash occurs in an area where stress is applied, the flash may crack, and then these cracks may act as notches, which cause breakage. Wall thickness of resin in metal insert moldings should be determined in consideration of breakage from creep. When a metal insert has an edge and when flash occurs on the surface of the insert, creep life until breakage becomes so short that careful attention should be paid to the metal edge and flash.


Fig. 7-8 An example of creep breakage life of a metal insert molding

 


Points  Accuracy  Deformation  Dimension Control  Strength of Molded Products

Contents Introduction Safety Molding Machines Molding Conditions Recycling
Molding Characteristics Product Design Mold Design Countermeasures for Defects