POSSIBLE CAUSE

RECOMMENDED ACTION

The extrudate is slipping in the caterpillar belts

The extrudate must be firmly clamped between the upper and lower caterpillar belts.  When setting up the machine, check that it is not possible to pull the extrudate out from between the belts. If it is, increase the nip pressure.  In addition, check that the extrudate is not wet or slippery.  Finally, make sure that the caterpillar belts are in good condition.  Worn belts should be replaced. 

The extrudate is being stretched

The extrudate must be under a constant tension as it enters the caterpillar belts. This is particularly critical for materials which easily stretch, e.g. foam rubber, silicone and very small diameter flexible plastics.  Ideally, the tension on the material should be as low as possible.

Caterpillar belt speed is varying

The infeed belt must be set at a constant linespeed to achieve best results.  Do not adjust the caterpillar speed unnecessarily.  Leave it set to a constant speed.  Similarly, do not adjust the nip pressure during a run.  Any changes in infeeder speed or nip pressure will immediately influence cut length accuracy.

Cutter guide bush set-up is incorrect

The guide bush bore should be a reasonable tight fit to the product diameter.  If too tight, it may cause a product hold-up as the extrudate is pushed through by the infeeder.  If too loose, it may allow the product to move from side-to-side.  For very flexible extrudates, the distance between the end of the caterpillar belt lean-in is kept to a minimum.  It is also important that the knife blade brushes the bush face during cutting.

Material not exiting the guide bush

Any material hold-up in the exit bush may cause compression of the on-coming extrudate end.  This will effect the accuracy of the oncoming length.  Making sure cut lengths are free to exit the bush.  Do this by either reducing the length of the exit bush, putting an internal cone into the bush or using air to blow the cut pieces out of the bush.

POSSIBLE CAUSE

RECOMMENDED ACTION

Blade gap is too large

It is critical that the knife blade actually brushes each cutter guide bush during the cut.  The bushes should be as tight as possible to the blade, whilst still able to pass through the gap when the blade head is turned by hand.

Bush edge is not sharp

The bush faces, which are in contact with the blade, should be straight and clean.  The 90° angle between the bush face and the product bore should be as sharp – under no circumstances should there be a bevel or radius on this edge.  It is recommended that you check for wear on this edge from time-to-time.

Blade not sharp

Check the cutting edge.  Check the double bevel is even.  Replace the blade if appropriate.

Blade gap too large

Check that the blade is touching both bush faces as it passed through the gap between them.  Reset the bushes if necessary.  If the gap is too wide, the material can be pushed down into the gap by the blade, causing a jam.

Blade sticking

For many product, particularly rubbers and flexible PVC, lubricating the blade greatly assists the cutting action and eases the passage of the blade through the material.  Fill the integral blade lubrication system with a lubricant, e.g. water with a dash of dish-washing up liquid.

POSSIBLE CAUSE

RECOMMENDED ACTION

Inlet bush is too short

For very flexible extrudates, make sure that the distance between the end of the caterpillar belt nip and the bush lead-in is kept to a minimum.  It may be necessary to turn the end of the inlet bush into a cone, so that it can fit in between the caterpillar rollers.  In extreme cases, use a small tube to fit right into the nip point and guide the material into the inlet bush.

Inlet bush is too long

When cutting short lengths or very flexible materials, make sure that the exit bush is not over long.  If too long, material will have to be pushed an excessive distance through the bush, causing drag and product hold-up.

Bush bore is not smooth

Make sure that the internal bush bore is smooth and free from machining rings and other potential drag points.  If possible, polish the bore or use a very low friction material (e.g. Teflon) as an insert in the bush.  Alternatively, use low pressure air, blown down the bush to create an air cushion around the extrudate to minimise drag.

Infeed guide in is not straight

Check that the extrudate is straight as it enters the caterpillar.  If the material has been coiled before it is cut, it may have a tendency to try to twist as it passes through the inlet guide rollers.  If this is a problem, add additional guide-in rollers to holder the infeeder.  In very bad cases, use a tube guide to direct the extrudate right to the caterpillar inlet belt nip.

Blade is incorrect shape

Because the extrudate is trying to move forward continuously during the cutting action a wide blade may cause excessive product hold-up resulting in a jam.  Check that you are using the narrowest blade possible.  If necessary, grind away the back of the blade to reduce hold-up.

The material to be cut should be fed into the cutter from a similar operating height.  It is not recommended that the material be pulled off the floor.

For very flexible extrudates, which are easily stretched, it is strongly recommended that a relaxing loop of material be allowed to form before the infeeder.  This will ensure that the extrudate is not stretched as it enters the infeeder.

It is also important to ensure that any pre-coiled material is allowed to un-twist before it goes into the cutter.  An un-wind table or stand should be used to remove any twist.

Speed stability

Infeeder speed variation should be avoided as far as possible.  Do not adjust the speed unnecessarily during a production run.  Allow time for the caterpillar to accelerate from rest to production speed before checking cut length accuracy.

The more stable the infeeder speed the better the accuracy on cut length.

Caterpillar belts

Check that your caterpillar belts are in good condition.  Very worn belts, or belts where the surface joint has come apart should not be used. 

Even dirty belts can effect cut length accuracy.  Ensure that the belt is clean and free from grease or any other matter.

Keep your belts in good condition.

Belt nip pressure

Check that the nip pressure based with the infeeder/take-off is adequate to avoid extrudate slippage and snaking within the belts.  This is especially important if the infeeder is pulling material from a drum or coil, particularly if the extrudate has a curved ‘memory’, which encourages it to attempt to turn within the belts.

However, do not use excessive nip pressure as this may damage the extrudate and place the caterpillar infeeder under unnecessary load.

Avoid changing the nip pressure during a production run.  This may affect the cut length being produced, causing apparent cut length inaccuracy.  If possible set the belt nip pressure to the same setting for each product size from run-to-run.

Set the belt nip pressure sufficiently to avoid extrudate slippage or snaking.

Product support

Cutting plastic, rubber or any other material with a flying knife type cutter such as the Servo-Torq requires a device to support the material while the cut is in progress.

The cutter guides or bushes are cylindrical metal devices which have been bored or otherwise machined to match the outer diameter or cross-sectional profile of the material to be cut.

They serve the following functions:

a.  Guide the product to the cut point.

b.  Provide support for the material as it is cut.

c.  Guide and support the knife.

Although frequently overlooked, cutter guide bushes are extremely important in obtaining a clean cut on extruded tubes,hoses, pipes and profile.

Boring the guide bushes

Have the cutter guide bushes machined or bored to suit the product outer diameter or cross-sectional profile.  Clearance must be provided to permit the material to freely slide through the opening.  However, the clearance must not be great enough to permit excessive movement of the product material.  This may cause irregular or angular cuts.

Excessive clearance will allow the material to move laterally and may cause irregular or angular cuts.

The opening in the pair of guide bushes should be continuous.  Any misalignment will cause feeding problems.

Do not enlarge the entrance of the down-stream guide bush unless the product is being held-up on the edge at each cut.  The more square the entrance, the better the cut quality.

Positioning the guide bushes

The upstream (inlet) side of the cutter guide bush should be as close as practicable to the discharge point of the caterpillar infeeder which precedes the cutter.

For the optimum results the inlet end of the inlet cutter bush should be coned so that it can fit in between the upper and lower belts of the infeeder.  This ensures that it reaches right into the exit of the caterpillar nip giving the product no opportunity to snag or jam.

Generally, the more flexible the extrudate the nearer the cutter guide bush has to be to the caterpillar belt.

The most significant factor to successful cutting is a sharp knife!

Different products will require different knife blade thicknesses.

As a general rule –

The thinnest knife blade possible should be selected when cutting flexible extrudates.  The more rigid the product, the greater the thickness of blade required.

Despite the first statement in this section, it is sometimes found that when cutting rigid materials a better cut quality is achieved with a blade which has had its edge taken off.

The following thickness blades are available from the GILLARD factory:

                       0.25 mm         (0.010”)

                       0.38 mm         (0.015”)

                       0.46 mm         (0.018”)

                       0.60 mm         (0.024”)

                       0.80 mm         (0.031”)

                       1.60 mm         (0.062”)

Please e-mail us at sales@gillard.co.uk for information on ordering blades.

Possibly the most difficult selection concerning knife blades is whether the cutting action should be chopping or slicing.

Chopping directly through with a straight edged blade causes the least amount of engagement time and, therefore, the least interruption to the extrudate as it is continuously pushed forward by the caterpillar infeeder belt.

Slicing through the product with a curved blade tends to give a better-cut quality but can considerably increase extrudate interruption time.

Using a static cutter, such as the Servo-Torq®, with a continuously moving product demands a compromise between slicing angle and engagement time.

The type of blade you should use will obviously depend upon your application.  However, in our experience, approximately 90% of products can be cut adequately with a straight edged ‘chopping' blade.

The remaining 10% are best served with a curved ‘slicing’ blade.  Typical products would be thin-wall tubes or profiles, or profiles with an intricate shape where a chopping action might cause material distortion or collapse.

We can offer you a selection of straight or curved blades, or produce a custom blade design especially for your product.

Additional advice is always available from GILLARD.  Experienced staff would be pleased to share their knowledge to help you achieve the best possible results from your Servo-Torq®.