GB2231670A

GB2231670A - Method and apparatus for testing

Info

Publication number: GB2231670A
Application number: GB8908679A
Authority: GB
Inventors: Kenneth Charles Mepham; Vivian Walter Slinn
Original assignee: ROBERT HORNE GROUP PLC
Current assignee: ROBERT HORNE GROUP PLC
Priority date: 1989-04-18
Filing date: 1989-04-18
Publication date: 1990-11-21
Also published as: GB8908679D0

Abstract

A method of testing paper comprises preparing a plurality of test samples of paper, selecting some of the test samples and folding each of the selected ones under predetermined fold conditions, performing a tensile strength test on each sample, and comparing the results of said tests on folded paper with those on unfolded paper. Some of said test samples may be heated prior to the folding step. A press for stamping folds in paper comprises a fluid operable piston and cylinder arrangement (8, 14) and a plate (11) operatively connected thereto to descend towards a fixed plate for stamping paper placed between said plates. Pressure regulation means (7) set a pressure to be applied to the piston and cylinder arrangement (8, 14) and thereby to the movable plate. Timing means (1) reverse the flow of fluid to the cylinder to lift the movable plate after a predetermined period of time. <IMAGE>

Description

METHOD AND APPARATUS FOR TESTING The present invention relates to a method and apparatus for testing paper. More particularly, but not exclusively, it relates to a method and apparatus for measuring reproducibly the strength of folds in paper, most particularly in heat-set web offset papers.

Papers for use in heat-set web offset printing are becoming increasingly used, and consequently it is becoming increasingly important to be able to determine the durability of the material at a fold in the paper. During the heat-setting of the ink, the paper web may reach 1500C and, before it has had any chance to recondition, it reaches the folding section seconds later. In some papers, such conditions result in a weak fold, usually where the fold lies parallel to the grain direction. In normal strength tests of paper, the paper is tested at standard conditions of temperature and moisture content and such tests are not appropriate for papers which undergo the above conditions.

It is an object of the present invention to provide a method and apparatus for testing paper which is able to measure reproducibly the strength of folds made when in both conditioned and/or heated states, and which can give a result which can be related to the behavior of the material after treatment.

According to a first aspect of the present invention there is provided a method of testing paper comprising the steps of preparing a plurality of test samples of paper, selecting some of the test samples and folding each of the selected ones under predetermined fold conditions, performing a tensile strength test on each sample, and comparing the results of said tests on folded paper with those on unfolded paper.

The method preferably includes also a step of selecting others of said test samples and heating them prior to the folding step. In this case the tensile strength of the heated and folded samples may be compared with those of the unheated unfolded and/or folded samples.

The step of heating may included placing each sample to be heated on a platen at a temperature in the region of 1500C for a time in the region of 5 seconds, and folding the heated samples approximately 7 seconds afterwards.

The step of folding is preferably carried out in a press adjusted to deliver a pressure in the region of 40 psi to the fold for a period in the region of 2 seconds.

It is preferred that the samples are allowed to condition for a period in the region of 24 hours after heating and/or folding before they are subject to the test for tensile strength.

According to a second aspect of the present invention there is provided a press for stamping folds in paper comprising a fluid operable piston and cylinder arrangement, a plate operatively connected thereto to descend towards a fixed plate for stamping paper placed between said plates, pressure regulation means for setting a pressure to be applied to the piston and cylinder arrangement and thereby to the movable plate, and timing means for reversing the flow of fluid to the cylinder to lift the movable plate after a predetermined period of time.

An embodiment of the present invention will now be more particularly described by way of example and with reference to the accompanying drawings, in which: FIGURE 1 is a schematic perspective view of a folding press embodying the invention; and FIGURE 2 is a schematic diagram of the means to operate the press.

Although other uses can be foreseen, the preferred use of the present method and apparatus is in determining tensile strength of heated and folded paper, such as for use in heat-set web offset printing. In order to obtain meaningful results, it is preferred to test the tensile strength of: unfolded samples; folded samples; and heated and folded samples. To this end, a sheet of paper to be tested is cut into strips, usually in the cross grain direction. The preferred size of strips to be cut is 300 x 25 mm.

One set of strips is left unfolded.

A second set of strips is folded in half using a press in a manner more particularly described below.

Each of a third set of strips is placed on an electrically heated, smooth hot plate, thermostatically controlled to maintain a temperature of 1500 + 50C, and equipped with a weighted felt pad to place even pressure on the strip against the hotplate. The preferred heating time is 5 seconds. Following the heat treatment, each strip is removed, looped and folded in the stamping press after a period of 7 seconds.

Referring now to the Figures, the stamping press comprises a fixed plate 10 and a movable plate 11. This pair of plates 10 and 11 operate within an enclosure 12 of transparent or translucent material having an opening 13.

The movable plate 11 is connected to a piston 14 movable within a cylinder 8 under the influence of compressed air.

The compressed air is supplied via a solenoid valve 7 provided with flow recirculators 9, and to which air is supplied via a pressure regulator 6. The air pressure supplied to the valve is indicated by means of a pressure dial 3.

Flow of compressed air is initiated by closure of start button 4 which signals, through timer 1, to the solenoid valve for compressed air to flow through passage 15 to cause the piston and movable plate 11 to descend.

After a predetermined time, set by the timer 1, the flow of compressed air is reversed and passes instead through passage 16 to cause the movable plate 11 to rise. The timer 1 is preferably fed with alternating electric current by means of an on/off switch 5 and illuminated indicator 2.

In order to use the folding press, each strip to be tested is looped and placed into the opening 13. Start button 4 is operated and the test strip is folded under a predetermined pressure, indicated by dial 3 and for a predetermined time set by timer 1. After this time, the strip may be withdrawn. The preferred stamping pressure is 40 psi, and the preferred stamping time is 2 seconds. This applies to both the unheated and the heated sample strips.

Once the strips have been treated, either by folding alone or by heating and folding, they are left under controlled conditions for 24 hours or such other period as may be appropriate. The sets of strips are then assessed using a tensile tester. This is set with a jaw separation of 100 mm. The speed of the jaw separation is adjusted so that breaking of the sample occurs 20 seconds + 5 seconds after initiation of the process. The speed will depend on the condition of the sample, but average speeds for separation using a 100 gsm mechanical content sheet are: 20 mm/min for an unfolded sample; 3 mm/min for a cold folded sample; and 1 mm/min for a hot folded sample.

As stated above, the samples are tested in sets of, possibly, six strips for each set and a mean result in Newtons is calculated for each set. The tensile strength results for the cold and hot folded samples may then be expressed as a retained percentage of the strength of the unfolded strips.

Claims

CLAIMS:

1. A method of testing paper comprising the steps of preparing a plurality of test samples of paper, selecting some of the test samples and folding each of the selected ones under predetermined fold conditions, performing a tensile strength test on each sample, and comparing the results of said tests on folded paper with those on unfolded paper.

2. A method as claimed in claim 1, further comprising a step of selecting others of said test samples and heating them prior to the folding step.

3. A method as claimed in claim 2, wherein the tensile strength of the heated and folded samples is compared with those of the unheated unfolded and/or folded samples.

4. A method as claimed in either claim 2 or claim 3, wherein the step of heating includes placing each sample to be heated on a platen at a temperature in the region of 150 C for a time in the region of 5 seconds, and folding the heated samples approximately 7 seconds afterwards.

5. A method as claimed in any one of the preceding claims, wherein the step of folding is carried out in a press adjusted to deliver a pressure in the region of 40 psi to the fold for a period in the region of 2 seconds.

6. A method as claimed in any one of the preceding claims, wherein the samples are allowed to condition for a period in the region of 24 hours after heating and/or folding before they are subject to the test for tensile strength.

7. A method of testing paper substantially as described herein with reference to the accompanying drawings.

8. A press for stamping folds in paper comprising a fluid operable piston and cylinder arrangement, a plate operatively connected thereto to descend towards a fixed plate for stamping paper placed between said plates, pressure regulation means for setting a pressure to be applied to the piston and cylinder arrangement and thereby to the movable plate,, and timing means for reversing the flow of fluid to the cylinder to lift the movable plate after a predetermined period of time.

9. A press for stamping folds in paper substantially as described herein with reference to the accompanying drawings.