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Product Design / Deformation

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

List of DURACON® POM grades This page's PDF(0.97MB)

7. Product Design

7.3 Deformation

(a) Warpage of plates having ribs

Causes:

  • Shrinkage difference stemming from difference between base thickness and rib thickness
  • Difference in mold temperature between opposite sides of plate:
    A base with ribs on both sides (an H-shape in cross-section) is considered the best countermeasure, but Table 7-2 and Fig. 7-2 illustrates warpage when an H-shaped rib cannot be applied and a one-sided rib used instead.
  • Non-reinforced grades, bead-filler grades, powder-filler grades:
    Convex warpage occurs towards the rib side when rib wall thickness is less than base wall thickness. Concave warpage occurs towards the rib side when rib wall thickness is greater than base wall thickness. A plaque can be made through balancing the ratio of rib / base thickness in a range from 1 to 1.2.
  • Fiber filled grades:
    Convex warpage always occurs towards the rib side regardless of the ratio of rib / base.

 

Table 7-2 Warpage of plates with ribs

Cross-section 1 2 3 4 5 6
Mold temp. 30°C 0.04 0.06 0.29 -0.68 -0.55 0.04
80°C 0.05 0.08 0.61 -0.44 -0.04 0.04

  Warpage: Minus values represent concave deformation towards ribs.

 


Fig. 7-2 Warpage of plates with ribs

 

(b) Tilt deformation of L-shaped and U-shaped moldings

In general, L-shaped and U-shaped moldings suffer deformation towards the center.

Causes:

  • As mold temperature of core side in the corner section becomes higher than that of the cavity side, shrinkage of core side becomes larger.

 

Countermeasures:

  • Shape:
    Set triangular rib at the corner with a rib wall thickness from 1/3 to 1/2 of base thickness.
    Set through hole at the corner.
    Design T-shaped, not L-shaped.
    In U-shaped molding design, wall thickness / base wall thickness ratio is 1 : 2.
  • Mold:
    Cool the core.
    When using fiber-filled grades, set a gate at the corner section.

 

Table 7-3 Deformation of L-shaped moldings

Cross-section 1 2 3 4
M90-44 2.3 -0.7 3.4 2.4
GH-25 3.5 2.1 2.7 2.8

 

Cross-section 5 6 7
M90-44 1.6 0.2 0.0
GH-25 2.4 0.8 0.0

  The values are tilt angle. The unit is arc degree. Negative values represent outward tilting.

 

(c) Internal warping of box-type moldings

In general, box-type moldings suffer internal warping of outer wall. The causes are the same as those with L-shaped and U-shaped moldings: shrinkage in a corner side increases as mold temperature in the core side increases.

Countermeasures:

  • Shape:
    Set triangular ribs at appropriate pitch.
    Set a through hole at the corner.
    Set a circumference rib.
    When using non-reinforced grades, wall thickness / base wall thickness ratio is 1 : 2.
    Warp outer wall shape slightly outward.
  • Mold:
    Cool the core.
    When fiber-filled grades are used, a pinpoint gate at the center of base is more favorable than a side gate.

 

(d) Warpage of disk-shaped moldings

With disk-shaped moldings, surge deformation or umbrella-shaped deformation may occur.

Causes:
The difference in shrinkage between the radial and circumferential direction is generated by molecular orientation or the orientation of reinforced fibers:

  • Surge warpage takes place in case of shrinkage of radial direction > shrinkage of circumferential direction.
  • Umbrella-shaped deformation takes place in case of shrinkage of radial direction < shrinkage of circumferential direction.

 

Countermeasures:

  • Shape:
    With surge warpage, set H-shaped ribs along the circumference or reduce wall thickness in a doughnut shape.
    With umbrella-shaped warpage, set radial ribs in both sides.
  • Mold:
    It is recommended that cooling holes be radially laid near cavities.

The number and position of gates should be considered for improving roundness. In general, a better result is obtained when three or more pinpoint gates are employed rather than one-point side gate and one pinpoint gate. It is appropriate to arrange the three pinpoint gates at the points of an equilateral triangle. Roundness is improved when the gate position is set in the center or near the center of a molding and resin is flowed concentrically from the center to the circumference. As shown in Fig. 7-3, good accuracy can be obtained when the resin flows concentrically.


(a) Flow pattern by 3 gates at web section

(b) Flow pattern by 4 gates at bearing boss end
3 web gates
(1.0 mm dia.)
Boss end 4 gates
(1.0 mm dia.)

Fig. 7-3 Gate positions of gears and their flow patterns

 

(e) Warpage of cylindrical molding

Moldings of long cylindrical shapes tend to have the shape of a tsuzumi, a Japanese hand drum having outside diameters larger at both ends and smaller in the middle. This is because melted polymer solidifies sooner at the ends than in the middle, and such conditions can be improved by decreasing the wall thickness: for example, by adopting a double-wall cylindrical structure. For such a structure, when adopted, the thickness of ribs connecting the outer and inner walls should be made as thin as possible in order to reduce effects of sinking by the rib.

 

(f) Warpage of rectangular molding

When a gate is arranged on the longer side of a slender molding, the molding is apt to experience concave warpage towards the gate side. For this reason it is desirable to locate the gate on the shorter side of the molding. Examples are shown in Fig. 7-4.


Fig. 7-4 Gate design and deformation of rectangular molding

 

(g) Molding conditions and warpage

Molding conditions relating to warpage include injection and holding time, cooling time, injection speed and mold temperature. Careful attention should be paid to these factors:

(1) Injection and retention time

The sum of injection time and retention time is set longer than the gate seal time. If the sum of these times is set shorter than the gate seal time, warpage may increase, as shown in Fig. 4-8.

 

(2) Cooling time

As a rule, warpage decreases with an increase in cooling time.

 

(3) Injection speed

Depending on mold shape, warpage may vary with injection speed. In some cases, higher injection speed results in less warpage, but in other cases the opposite effect occurs. Therefore, it is necessary to determine optimum conditions for minimizing warpage by conducting tests at various speeds.

 

(4) Mold temperature

Warpage decreases with low mold temperature. However, when moldings are used at elevated temperature, warpage and dimensional change due to after-shrinkage may cause problems. The optimum mold temperature should be selected in consideration of these requirements.

 


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