Limit

To maximize control in setting tolerances there is usually a minimum and a maximum limit on thickness, based on the process to be used. Available from the literature and material suppliers is extensive information on tolerances based on plastic related to fabricating process. Examples are provided in Tables 6.3 to 6.5. Each specific plastic has its own range that depends on its chemical structure and melt-processing characteristics. Outside these ranges, melts are usually uncontrollable. Any dimensions and tolerances are theoretically possible, but they could result in requiring special processing equipment, which usually becomes expensive. There are of course products that require and use special equipment.

One factor in tolerances is shrinkage. Generally, shrinkage is the difference between the dimensions of a fabricated part at room temperature and the cooled part, checked usually twelve to twenty-four hours after fabrication. Having an elapsed time is necessary for many plastics, particularly the commodity TPs, to allow parts to complete their inherent shrinkage behavior. The extent of this postshrinkage can be near zero for certain plastics or may vary considerably. Shrinkage can also be dependent on such climatic conditions as temperature and humidity, under which the part will exist in service, as well as its conditions of storage.

Plastic suppliers can provide the initial information on shrinkage that has to be added to the design shape and will influence its processing. The shrinkage and postshrinkage will depend on the types of plastics and fillers and/or reinforcements. The amount of filler and reinforcement can significantly reduce shrinkage and tolerances.

Another influence on dimensions and tolerances involves the coefficient of linear thermal expansion or contraction. This CLTE value usually has to be determined at the part's operating temperature. So it is important to include in the design specifications the operating temperature conditions, to specify a plastic that will do the job. Plastics can provide all the extremes in CLTEs, including graphite-filled compounds that could work in reverse. Upon heating, they contract rather than expand, and vice-versa.

To assist the designer a Society of the Plastics Industry (SPI) bulletin is available that specifies the limits for certain dimensions. Each material supplier converts this data to suit their specific plastics.

Tsblr : :;, Wall thickness tolerance guide for thermoplastic moldings

Dimensions, in.

ABS

Acetal

Nylon

Polycarbonate

Commercial

Fine

Commercial

Fine

Commercial

Fine

Commercial

Fine

To 1.000

0.005

0.003

0.006

0.004

0.004

0.002

0.004

0.0025

1.000-2.000

0.006

0.004

0.008

0.005

0.006

0.003

0.005

0.003

2.000-3.000

0.008

0.005

0.009

0.006

0.007

0.005

0.006

0.004

3.000-4.000

0.009

0.006

0.011

0.007

0.009

0.006

0.007

0.005

4.000-5.000

0.011

0.007

0.013

0.008

0.010

0.007

- 0.008

0.005

5.000-6.000

0.012

0.008

0.014

0.009

0.012

0.008

0.009

0.006

6.000-12.000, for each

0.003

0.002

0.004

0.002

0.003

0.002

0.003

0.015

additional inch add

0.004

0.002

0.004

0.002

0.004

0.003

0.003

0.002

0.003

0.002

0.004

0.002

0.005

0.003

0.003

0.002

0.000-0.125

0.002

0.001

0.002

0.001

0.002

0.001

0.002

0.001

0.125-0.250

0.002

0.001

0.003

0.002

0.003

0.002

0.002

0.015

0.250-0.500

0.003

0.002

0.004

0.002

0.003

0.002

0.003

0.002

0.500 and over

0.004

0.002

0.006

0.003

0.005

0.003

0.003

0.002

0.000-0.250

0.003

0.002

0.004

0.002

0.004

0.002

0.002

0.002

0.250-0.500

0.004

0.002

0.005

0.003

0.004

0.003

0.003

0.002

0.500-1.000

0.005

0.003

0.006

0.004

0.005

0.004

0.004

0.003

0.000-3.000

0.015

0.010

0.011

0.006

0.010

0.004

0.005

0.003

3.000-6.000

0.030

0.020

0.020

0.010

0.015

0.007

0.007

0.004

Total Indicator Reading

0.009

0.005

0.010

0.006

0.010

0.006

0.005

Polyethylene, high-density

Polyethylene, low-density

Polystyrene

Vinyl, flexible

Vinyl, rigid

Dimensions, in.

Commercial Fine Commercial Fine Commercial Fine Commercial Fine Commercial Fine

Dimensions, in.

Commercial Fine Commercial Fine Commercial Fine Commercial Fine Commercial Fine

To 1.000

0.008

0.006

0.007

0.004

0.004

0.0025

0.011

0.007

0.008

0.0045

1.000-2.000

0.010

0.008

0.010

0.006

0.005

0.003

0.012

0.008

0.009

0.005

2.000-3.000

0.013

0.011

0.012

0.008

0.007

0.004

0.014

0.009

0.010

0.006

3.000-4.000

0.015

0.013

0.015

0.010

0.008

0.005

0.015

0.011

0.012

0.007

4.000-5.000

0.018

0.016

0.017

0.011

0.010

0.006

0.017

0.012

0.013

0.008

5.000-6.000

0.020

0.018

0.020

0.013

0.011

0.007

0.018

0.013

0.014

0.009

6.000-12.000, for each

0.006

0.003

0.005

0.004

0.004

0.002

0.005

0.003

0.005

0.003

additional inch add

0.006

0.004

0.005

0.004

0.0055

0.003

0.007

0.003

0.007

0.003

0.006

0.004

0.005

0.004

0.007

0.0035

0.007

0.003

0.007

0.003

0.000-0.125

0.003

0.002

0.003

0.002

0.002

0.001

0.004

0.003

0.004

0.003

0.125-0.250

0.005

0.003

0.004

0.003

0.002

0.001

0.005

0.004

0.004

0.003

0.250-0.500

0.006

0.004

0.005

0.004

0.002

0.0015

0.006

0.005

0.005

0.004

0.500 and over

0.008

0.005

0.006

0.005

0.0035

0.002

0.008

0.006

0.006

0.005

0.000-0.250

0.005

0.003

0.003

0.003

0.0035

0.002

0.004

0.003

0.004

0.003

0.250-0.500

0.007

0.004

0.004

0.004

0.004

0.002

0.005

0.004

0.005

0.004

0.500-1.000

0.009

0.006

0.006

0.005

0.005

0.003

0.006

0.005

0.006

0.005

0.000-3.000

0.023

0.015

0.020

0.015

0.007

0.004

0.010

0.007

0.015

0.010

3.000-6.000

0.037

0.022

0.030

0.020

0.013

0.005

0.020

0.015

0.020

0.015

Total Indicator Reading

0.027

0.010

0.010

0.008

0.010

0.008

0.015

0.010

0.010

0.005

Table 6.4 Wall thickness tolerance guide for thermoset plastic moldings

Minimum

Average

Maximum

Thickness in. (mm)

thickness in. (mm)

thickness in. (mm)

Alkyd-glass filled

0.040 (1.0)

0.125 (3.2)

0.500 (13)

Alkyd-mineral filled

0.040(1.0)

0.187 (4.7)

0.375 (9.5)

Diallyl phthalate

0.040 (1.0)

0.187 (4.7)

0.375 (9.5)

Epoxy-glass filled

0.030 (0.76)

0.125 (3.2)

1.000(25.4)

Melamine-cellulose filled

0.035 (0.89)

0.100 (2.5)

0.187 (4.7)

Urea-cellulose filled

0.035 (0.89)

0.100 (2.5)

0.187 (4.7)

Phenolic-general purpose

0.050(1.3)

0.125(3.2)

1.000 (25.4)

Phenolic-glass filled

0.030 (0.76)

0.093 (2.4)

0.750(19)

Phenolic-fabric filled

0.062 (1.6)

0.187 (4.7)

0.375 (9.5)

Silicone glass

0.050(1.3)

0.125 (3.2)

0.250 (6.4)

Polyester premix

0.040(1.0)

0.070(1.8)

1.000(25.4)

Table 6.5 Wall thickness tolerance guide for thermoplastic extruded profiles

PVC

LDPE

HIPS

PC, ABS

PP

Rigid

Flex.

Wall thickness (%, =)

10

8

8

8

8

10

Angles (Deg., =)

5

2

4

4

2

5

Profile dimensions (in., ±)

To 0.125

0.012

0.007

00.10

0.010

0.007

0.010

0.125 to 0.500

0.025

0.012

0.020

0.015

0.010

0.015

0.500 to 1

0.030

0.017

0.025

0.020

0.015

0.020

1 to 1.5

0.035

0.025

0.027

0.027

0.020

0.030

1.5 to 2

0.040

0.030

0.035

0.035

0.025

0.035

2 to 3

0.045

0.035

0.037

0.037

0.030

0.040

3 to 4

0.065

0.050

0.050

0.050

0.045

0.065

4 to 5

0.093

0.065

0.065

0.065

0.060

0.093

5 to 7

0.125

0.093

0.093

0.093

0.075

0.125

7 to 10

0.150

0.125

0.125

0.125

0.093

0.150

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