Accuracy and Mold Direction of PTFE Products

1. Introduction

In general, the accuracy of PTFE product is not easy to control because its coefficient of linear expansion is higher than that of metals, and its one of volume transition temperature is around room temperature causes volume changes approximately 1 to 2%. Moreover, thin-walled PTFE products are known to be difficult to machining because not only PTFE is flexible and elastic material, but also residual stress remains after molding sometimes deform due to frictional heat generated during the machining process or due to aging after machining. Such deformation could influence dimensional accuracy.

Regarding processing accuracy, PTFE products are sometimes required the same permissible dimensional tolerance as for a metal material. In such cases, the characteristics of PTFE described above could cause troubles between users and manufacturers. With this background, this report explains the processing accuracy of PTFE.

2. General Permissible Dimensional Tolerance

1.This standard stipulates dimensions ranging from 1mm    to 1000 mm for when the material of a PTFE molded product is machined through compression or extrusion molding. The term“general”used in this standard means that the standard can be applied when a blueprint shows no figures or symbols.

When measuring the processing accuracy of PTFE, the following essential characteristics of PTFE should

be taken into account:

1.   PTFE has low thermal conductivity.

2.   PTFE has a high coefficient of linear expansion.

3.   PTFE’s volume changes markedly（by approximately 1 to 2%）at around 23°C.

4.   PTFE is elastic.

5.   PTFE sometimes has residual stress.

From the above, PTFE’s dimensional minimum tolerance is approximately ±0.05 mm or half the value 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 value could be varied if the end of a measuring device is pushed strongly against a PTFE specimen. For example, a difference of at least 0.1 mm in measured values sometimes occurs depending on how a micrometer is pushed against a PTFE specimen. Users and manufacturers should consider this point.

3. Effects of Annealing Treatment

Usually, free sintering (baking) process is applied to PTFE after compression molding. During sintering, PTFE’s internal stress could be decreased compare with molded products with coining process (a process in which a material is sintered in a mold and then cooled under pressure) whose outer layer is quenched.   

However, annealing* treatment is applied to the material in case high dimensional accuracy is required or the product shape is complex.

Eliminating the internal stress generated during molding process is an effective way to improve dimensional accuracy and to prevent from its change over time.

*Annealing: A procedure in which molded products are slowly cooled at a given temperature to remove internal stress generated by heat or mechanical stress.

4. Surface Roughness

As stipulated in General Tolerance for Polytetrafluoroethylene （Machine Cut）,material characteristics should be taken into account when setting a surface-roughness value.

Since resin is affected by heat during machining on the surface and has elasticity, the surface-roughness value could not equal to the machined metal surface.

Generally, a difference in the finish of surface roughness is caused by the machining conditions including rotational and feeding speed and cutting tools（blades）.

The former symbols were introduced approximately 60 years ago, and so are well known. It will take time for the new symbols to become known among peripheral manufacturers. In addition, in the case of functional parts, existing techniques tend to be followed. Therefore, it is important to understand the relationship between the new and former symbols.