Accuracy and Mold Direction ofPTFE Products
1. Introduction

Table1

G eneral tolerance for polytetrafluoroethylene（machine cut）
Unit: mm

In general, the accuracy of PTFE product is not easy toGrade 1

Grade 2higher than that of metals, and its one of volume transition

1up to16±0.1±0.3

More than 16

up to

40±0.2±0.6

temperature is around room temperature causes volume

More than 40

up to

63±0.3±0.8

More than 63

up to

100±0.4±1.0

More than 100

up to

160

±0.5±1.2

More than 160

up to

250±0.6±1.4

because not only PTFE is flexible and elastic material,

More than 250

up to400±0.7±1.7

but also residual stress remains after molding sometimes

More than 400up to630±1.0±2.0

More than 630

up to 1000±1.5±2.5

control because its coefficient of linear expansion ischanges approximately 1 to 2%. Moreover, thin-walled
PTFE products are known to be difficult to machining

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Tolerance

Categories of nominal
dimension

deform due to frictional heat generated during themachining process or due to aging after machining. Suchbe taken into account:deformation could influence dimensional accuracy.

1. PTFE has low thermal conductivity.

Regarding processing accuracy, PTFE products are

2. PTFE has a high coefficient of linear expansion.

sometimes required the same permissible dimensional

3. PTFE’s volume changes markedly（by approximately

tolerance as for a metal material. In such cases, the1 to 2%）at around 23°C.

characteristics of PTFE described above could cause

4. PTFE is elastic.

troubles between users and manufacturers. With this

5. PTFE sometimes has residual stress.

background, this report explains the processing
accuracy of PTFE.

From the above, PTFE’s dimensional minimum
tolerance is approximately ±0.05 mm or half the value

2. General Permissible
Dimensional Tolerance

stipulated in JIS K 6884（grade 1）, although PTFE’s
machining accuracy depends on the size and shape.
However, because of PTFE’s elasticity, an accurate

Table 1 shows“General Tolerance for Polytetrafluoroethylene

value could be varied if the end of a measuring device（Machine Cut）”stipulated in JIS K 6884-1971.

is pushed strongly against a PTFE specimen. ForThis standard stipulates dimensions ranging from 1

example, a difference of at least 0.1 mm in measured

mm to 1000 mm for when the material of a PTFE

values sometimes occurs depending on how amolded product is machined through compression or

micrometer is pushed against a PTFE specimen. Users

extrusion molding. The term“general”used in thisand manufacturers should consider this point.

standard means that the standard can be applied whena blueprint shows no figures or symbols.
When measuring the processing accuracy of PTFE,
the following essential characteristics of PTFE should29

Technical Papers

3. Effects of Annealing Treatment
Usually, free sintering (baking) process is applied toTable2

Categories of roughness

New JIS symbol
Symbol Arithmetical mean roughness

Former JIS symbol
Applicability standard
for machining
Symbol Maximum roughness

12.5 to 25 a

50 to 100 S

3.2 to 6.3 a

12.5 to 25 S

Acceptable

PTFE after compression molding. During sintering,

Acceptable

PTFE’s internal stress could be decreased compare

0.4 to 1.6 a

1.6 to 6.3 S

with molded products with coining process (a process（Conditionally acceptable,in the case of sheet）

0.012 to 0.2 a

0.05 to 0.8 S

Unacceptablein which a material is sintered in a mold and then（Unit:μm）

cooled under pressure) whose outer layer is quenched.
However, annealing* treatment is applied to theThe comparison of the two symbol types（new andmaterial in case high dimensional accuracy is requiredformer JIS symbols）shown in Table 2 is not strictlyor the product shape is complex.

precise; it is shown for convenience. Even today, many

Eliminating the internal stress generated during

companies continue to use the former JIS symbols.

molding process is an effective way to improve

The former symbols were introduced approximately

dimensional accuracy and to prevent from its change

60 years ago, and so are well known. It will take timeover time.

for the new symbols to become known among
peripheral manufacturers. In addition, in the case offunctional parts, existing techniques tend to be

are slowly cooled at a given temperature to remove

followed. Therefore, it is important to understand theinternal stress generated by heat or mechanicalrelationship between the new and former symbols.

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*Annealing: A procedure in which molded products

stress.
Roughness profile

4. Surface Roughness
AsstipulatedinGeneral ToleranceforPolytetrafluoroethylene（Machine Cut）
, material characteristics should be taken

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into account when setting a surface-roughness value.
Since resin is affected by heat during machining on thesurface and has elasticity, the surface-roughness value

Figure1

Type of surface roughness: calculated
average roughness（Ra）

could not equal to the machined metal surface.
Generally, a difference in the finish of surface roughness
Peak lineis caused by the machining conditions including
rotational and feeding speed and cutting tools（blades）.
Table 2: Standard values of surface roughness of PTFE
Figure 1: Calculated average roughness（Ra）

Valley line

Figure 2: Maximum peak（Rmax）

Note:In calculating Ry, only the average roughness profile area inreference length should be used, because too high or low peaks
might be caused by abnormal reason such as scratch.

Figure 3: A machining method for round bars andFigure2

Type of surface roughness: maximum peak（Rmax）

sleevesFigure 4: A machining method for sheet (plate material)

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Accuracy and Mold Direction of PTFE Products

molding is called the molding direction（MD）,while thedirection perpendicular to MD is called the crosssectional direction（CD）.
Some PTFE materials are filled PTFE whose material
is mixed with filler. Since fibrous filler orients duringmolding, and has markedly different physical properties
from those of PTFE, filled PTFE should be designedand used with caution.
Figure3

The following are characteristics that are changed in

Machining of outside diameter

filled PTFE.
Compression-creep characteristics:	The MD value becomes greater than the CDvalue.
Tensile strength and elongation:	MD tensile strength becomes greater thanCD tensile strength. CD elongation becomes
greater than MD elongation.

Figure4

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Coefficient of linear expansion:

Machining of a surface	The MD value becomes greater than the CDvalue.

5. Direction of Molded Products

When material shapes are sleeves and rods, theMolded PTFE products have various directions

molding direction can be visually confirmed, and so thedepending on their molding methods, and the direction

processing can be started based on the direction.

influences the physical properties. One of the common

However, when the material shapes are plates andmolding methods for PTFE is the compression molding

complex processing is required, it is necessary tomethod, as shown in Figure 5.

visualize the direction including marking the direction
during the initial processing stage.

Material

6. Conclusion

PowderPTFE is chemically inactive, has excellent chemical
resistance, can be used at a wide range of temperature,and has the lowest friction coefficient among solidMoldmaterials, and so has been used for various purposes.

PressRaw material input

Figure5Compression molding

Baking

The compression molding method

Also, PTFE is much easier to machine than metals. Itis also relatively softer than other resins and has goodaffinity when combined with other materials for use.

In the compression molding method, the material isRecently, many PTFE products have also been used inplaced in a mold and force is applied from both above

the precision-instrument industry, and so stricter

and below the mold to press the material together.

dimensional accuracy is required. We will developThis generates directionality. The direction towardmachining techniques in line with the accelerating

the compressed side in the material at the time ofprogress of machine tools and industrial tools. To do31

Technical Papers

this, we will ask users and manufacturers for theirrequests for dimensional accuracy and tolerances
according to their intended uses.

7. References
1. Valqua Handbook, Technology Ver., September 2010
2. F luororesin DuPont™ Teflon Handbook, Du PontMitsui Fluorochemicals Company, Ltd.
3. JIS K 6884-1971

Shigeko Kawai

Nobuyuki OtaCorporate Research andDevelopment Group
Development DivisionHigh Performance
Plastics Development
Division
Technical
Service Section No.3

Corporate Research andDevelopment Group
Development DivisionHigh Performance
Plastics Development
Division
Technical
Service Section No.3

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No.32

Correction and apology
VALQUA would like to announce and apologize for a partial
correction to“Explanation on PTFE Linear Expansion
Coefficient,”3. Example calculation for dimensional correction
in VALQUA Technology New Summer No. 31.
Incorrect: Dimensional change
Incorrect:Dimensional change＝1000×（100－0）
×20×10−5＝20
Correct:Dimensional change＝1000×（0－25）
×20×10−5＝−5
Therefore, the length at 0°C is 995 mm (shrunk).

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