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Deformation

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List of DURANEX® PBT grades This page's PDF (1.72MB)

4.2  Deformation

Uneven shrinkages of various sections of a molded part are the major cause of deformation. Inversely, if a part shrinks uniformly, then no deformation occurs.

Though all of the following three causes are major reasons for uneven shrinkage in a part, the anisotropy of the shrinkage between the flow and the transverse directions is the most important cause of deformations in reinforced grades of DURANEX® PBT.

1. Non-uniform section thickness

2. Non-uniform mold temperature and pressure of melt in a cavity

3. Anisotropy of shrinkage due to flow direction

 

(1) Shape Design vs. Deformation

Because the configuration greatly affects deformation, it is essential to design a part with shapes that minimize deformation so long as it conforms to its function. The typical deformation of some popular shapes are explained below.

 

(a) Disc –shaped Parts

Roundness and plane flatness of DURANEX 3300 is shown as follows:

 

Roundness :

One rule-of-thumb is that the roundness of a 60 mm diameter plate is 0.15 mm or less. When the diameter of a disc-shaped part is less than 60 mm, the roundness is nearly proportional to the diameter.

 

Flatness :

One criterion is that the flatness of a 60 mm diameter plate is 0.3 mm or less. The relationship to diameter is same as that of roundness (proportional).

 

(b) Box-type Parts

The relation between concave warpage and span length is shown in Fig. 4-2. There is a rather large scattering in the data, but the figure will give a rough estimation of the amount of concave warpage.

Figure 4-2

Measures to overcome concave warpage are providing the part with ribs to shorten the span length or designing molds with an inverse warpage. Proper gate design is also effective; i.e., a pin-point gate at the center functions better than a side gate as shown in Table 4-1. Core cooling is also useful; e.g. when the core is cooled, concave warpage is reduced to as little as 20 to 30% in case of DURANEX 2000 and 2002 and to about 60% in case of glass fiber reinforced grades.

Table 4-1  Concave Warpage vs. Gate Design  (Unit: mm)

Grade Side gate Pin-point gate at the center
2000 0.91 0.65
2002 0.90 0.69
3105 1.20 0.72
3200 1.15 0.94
3300 1.18 0.99
3400 1.09 0.92
6300B 0.80 0.65
7400W 1.05 0.80
 

Note 1: Measured on a box of 80(l)×40(w)×20(h)×2(t)mm

Note 2: Side gate: 5(w)×3(t)mm, Pin-point gate: 1mm dia.

Note 3: Mold temp.: 60°C

 

(c) L-shaped Parts

The deviation from right angle for various L-shaped sections is shown in Table 4-2. A triangular rib like that in shape #8 is found to be the most effective way to prevent deformation.

Table 4-2  Deviation from Right Angle (in degrees)

Mold
Temp.
(°C)
Grade Shapes
30 3300 2.5 2.5 2.5 2.5 3.0 2.5 2.5 0
80 3300 3.0 3.0 3.0 3.0 - 3.0 3.0 0
 

Molding conditions: Refer to term 2.1

Forced temperature differences between the core and the cavity mold halves at the corner are effective to reduce deviation from right angle for grades like DURANEX 2000, 2002 and 6300B, as seen in Table 4-3.

In case of glass fiber reinforced grade, deviation from right angle depends on the gate position as shown in Table 4-4, which indicates some instances where gates located at the corner produce less deviation than gates located at the tip, which is the conventional position.

Table 4-3  Effect of Corner Cooling on Deviation in L-shaped Parts  (Unit: degrees)

Grade Uniformly temperature controlled mold Water-cooled inner comer
2000 1.9 0.5
2002 2.7 0.7
3300 3.3 2.7
6300B 2.2 0.7
7400W 3.6 2.8

Table 4-4  Gate Location vs. Deviation in L-shaped Parts  (Unit: degrees)

Grade Gate at tip Gate at corner
2000 2.1 2.8
2002 2.4 3.0
3105 3.9 1.7
3200 4.1 1.7
3300 3.2 1.6
3400 3.1 1.4
6300B 1.9 2.1
7400W 3.2 1.8

 

(d) Ribbed Plates

One of the measures to improve stiffness of a part is to provide it with ribs, but some ribs tend to increase deformation of the part. Examples are listed in Table 4-5.

The deformations of shape #1 that has symmetrical ribs at both ends is similar to that of shape #6 that has no ribs. Plates having ribs on only one side have some warpage making the ribbed side convex. These results can be used to minimize warpage.

Table 4-5  Warpage (mm) of Ribbed Plates

Mold
temp.
(°C)
Grade Shapes
30 3300 0.04 0.07 0.44 0.40 0.50 < 0.04
80 3300 0.07 0.09 0.50 0.64 0.70 0.07
 

 

(2) Gate Design vs. Deformation

[Example 1] Impeller (3300)   (Unit: mm)

Number of gates One Two Three
Roundness 0.22 (0.05) 0.24 (0.07) 0.14 (0.03)
Flatness 0.72 (0.25) 0.94 (0.06) 0.58 (0.00)
 

Injection pressure: 98MPa

The figures in parentheses are R for n=3.

 

Gate: 1.5 mm dia. of pin gate

Diameter: approx. 65 mm

Thickness avg.: approx. 3 mm

[Example 2] Round parts (valve) No.2, housing (3300)   (Unit: mm)

Gate position Asymmetric 6 points Symmetric 4 points
Roundness 0.15 0.15
Plane flatness 0.44 0.27
 

Molding conditions:

Refer to No.6 in Table 4-2.

[Example 3] Round parts (valve) No.1, top and bottom lids (3300)  (Unit: mm)

(Unit : mm)

Number of gates One Two Three
Top lid Roundness of d1 0.07 0.18 0.05
Flatness of plane A 0.18 0.16 0.08
Deviation of flange from flat line (H) 0.22 0.26 0.14
Bottom lid Roundness of d2 0.14 0.27 0.10
Flatness of plane B 0.28 0.45 0.21
 

Injection pressure: 98MPa

The figures in parentheses are R for n=3.

 

 

 

Sketches of example 3

(i) Top lid

Gates were located at three vertexes of a regular triangle with their center at the center of the lid. The average thickness of the lid was approximately 2 mm.

Figure �\66-2

(ii) Bottom lid

Gates were positioned at three vertexes of an isosceles triangle with hteir center at the center of the lid. The average thickness was about 2 mm.

Figure �\66-3

 

(3) Molding Conditions vs. Deformation

Molding conditions which are closely related to deformation of a part are mold temperature, cooling time and those factors which affect pressure of melt in the cavity, such as gate size, injection speed and dwell time.

 

(a) Mold temperature

As examples, the results obtained from round parts made of DURANEX 3300 are listed in Table 4-6. There is an indication that the lower the mold temperature, the better the roundness and the flatness.

Table 4-6  
Effect of Mold Temperature on Roundness and Flatness of Valves (3300)

Valves Mold temp.
(°C)
Dia.
(mm)
Roundness
(mm)
Plane flatness
(mm)
Gate
No.2 45 - 48 59 0.14 0.26 Four pin-point gates
60 - 65 0.15 0.27
No.3 40 - 50 45.3 0.07 0.11 One pin-point gate
50 - 70 0.12 0.14
No.4 40 - 50 62 0.10 0.11 One pin-point gate
50 - 60 0.10 0.16
No.5 45 - 55 61 0.11 0.24 Four pin-point gates
60 - 68 0.13 0.26
No.6 40 63 0.16 0.21 Three pin-point gates
70 0.24 0.32

 

(b) Injection and dwell time

The relation between injection and dwell time and flatness of a disc 120 mm in diameter and 2 mm in thickness, molded through a center pin-point gate, is shown in Fig. 4-3.

This figure shows that if injection and dwell time is shorter than the gate seal time, deformation is greatly increased. This is a very important point in setting the molding cycle.

Figure 4-3

 

(c) Injection speed

The effect of injection speed on deformation is not so great. However, deformation tends to reduce as injection speed increases. Examples are shown in Table 4-7.

Table 4-7
Effect of Injection Speed on Roundness and Flatness of an Impeller (3300)

Injection speed
(mm/sec)
Injection pressure
(MPa)
Roundness
(mm)
Flatness
(mm)
3 98 0.20 0.65
8 98 0.14 0.58
17 68 0.14 0.65
80 68 0.11 0.63

 


   >>> [ Dimensional Tolerances ] [ Deformation ] [ Use of Regrind ]
Contents Introduction Molding
Conditions
Moldability Molded Product
Quality
Mold
Design
Safety Countermeasures