Four techniques – one result
There are various ways of applying rubber to a metal core in order to manufacture a roller. The most common techniques are:
- Wrapping a prepared calendared sheet of rubber around the core, that is held between two points in a lathe
- Extruding an endless rubber strip and applying it to a slowly rotating core that is fixed between two points in a spin station
- Sending the core through a cross head extruder, where the rubber is flowing round the core, thus covering it
- Putting the core between the rolls of a Three-roll-builder and covering it with an endless calendared sheet of rubber – a roll of which is placed in the Three-roll building machine.
There are advantages and disadvantages to each of these methods and certain circumstances where one may be preferable to another.
The ‘traditional’ wrapping starts with cutting the rubber sheet according to the needs of the roller (width and diameter) and putting the prepared core into the lathe. The core is turned slowly as the rubber sheet wraps around it.
Depending on the diameter required and the geometry of the calendared sheets this wrapping has to be repeated several times. After reaching the final diameter (plus overbuild) a bandage can be wrapped around the roller in the same machine for curing purposes.
This method is cheap and valid for rollers of a medium size both in length and diameter. It is flexible and, given a roller of 1500 x 500 mm, takes roughly 15 minutes covering time including all set up and handling work.
In the case of series production this method does not provide any possibilities of cost reduction. It also requires calendared rubber sheets, which are more expensive than rubber batches.
Care must be taken as the connecting line between two rubber sheets and the lack of pressure that sticks the rubber to the core and itself, often causes air bubbles. Due to the relatively low investment required, this wrapping technique is used in nearly every start up company.
Compared to traditional wrapping, the strip extrusion method provides several advantages. Firstly there is no need to use the more expensive calendared rubber; batches that can be fed to the extruder are sufficient.
Secondly there is no limitation in the roller size. This technique can be used for any roller width and diameter and it is the only productive method to cover big paper mill rollers.
Thirdly there is no set up time for different rollers as long as identical compounds are used; changing width and diameter do not affect the process at all.
What’s more the compound remaining in the machine or cut away at the ends of the roller after wrapping can be used again without additional treatment.
It’s also possible to reduce the wrapping time per roller by 10 to 20%, with staff preparing the next core while the process continues. And finally, the reject rate can be cut down to about 50%.
There are, however, some disadvantages. Due to the fact that the compound applied to the roller tends to flow to the ends, overbuilds of up to 16 mm (on the diameter) is required.
Significant investment – up to ten times higher than for traditional wrapping – and highly skilled personnel are also required. On average it takes a worker six to twelve months’ training to become a professional in extrusion.
Despite the improvement in rejects, there are still two percent or more of rejects caused by air bubbles. Undoubtedly there is a gain in flexibility and cost reduction, but strip extrusion is not the final solution in rubber roller covering.
For series production – beginning with lots of 15 – 20 identical rollers and quantities in excess of one hundred rollers with identical compound – cross head extrusion is a fantastic alternative.
It also uses cheap rubber batches and can reduce the covering time per roller (1000 mm wide) to about 2 minutes. Even if at least two, normally three, workers support the process, the personnel time per roller can be cut down to 5 – 6 minutes, one third of the time required for the techniques mentioned above.
Although the advantages are obvious, crosshead extrusion is not a popular choice for roller manufacturers. The main reason is the wide range of different rollers manufactured by the average roller covering company in combination with the small amount of identical rollers. There are also diameter limitations with the crosshead extruder.
Additionally these machines cost more than £200.000 GBP and small or start-up companies can rarely afford such an investment. The skills required are also far beyond the average knowledge of roller covering. Even experienced companies meet new challenges with this technique from time to time.
Other unsolved problems with cross head extrusion are, among others:
- Concentricity of the (non fixed) core flowing in the rubber
- Precise control of the overbuild
- Reliable bonding of the rubber to the core when the manufacturing speed is increased
- Trumpet effects at the roller ends
- The need for an additional machine for bandaging
- Cleaning the extruder
- Feeding the extruder automatically
Three roll builders allow a simplification of the covering process in many respects. The main difference to the techniques above is the positioning of the core, which is placed between two rolls opposite a roll of calendared rubber.
The three roll builder has many advantages:
- There is no fixation between the points
- No tools are required
- The bearing seats are not damaged
- No protection needed for the ends of the cores
- No time consumption for set up
When the core is ready, the calendared rubber is torn to it and fixed to cover the width of the roller. Then the third roller comes down and applies pressure to the rubber sheet. Driving the rolls turns the core, thus wrapping rubber from the roll onto the core.
At the same time the polythene film between the rubber sheets is collected on an additional roll and thus does not trouble the process as in traditional covering.
It’s possible to pre-define the diameter and the machine stops automatically when this is achieved. To finish the operator cuts the (endless) rubber sheet and trims the roller ends if the width of the rubber sheet surpasses that of the roller.
The covered roller is taken out of the machine, put into a simple device for bandaging and after no more than 2 minutes the roller is ready for curing.
Although effective the three roll builder has its limitations. It does not work with extremes of widths or diameter. While the width should be between 300 mm and 1700 mm, the diameter can vary from 20 mm to 400 mm.
For loading the three-roll builder a crane can be used, though the full speed of this method can only be achieved with hand loading. The full speed also depends on the availability of calendared rubber in the width required.
World wide there are compounding companies which provide calendared rubber up to 1800 mm wide, but unfortunately most of them are limited to lesser width, probably 1300 to 1500 mm. For rollers with strongly differing widths the rubber roll has to be replaced with another one whose width fits better. This means that the roller manufacturer must stock the same compound in different widths according to the range of rollers to be produced – or do the calendaring in-house!
Despite these limitations the advantages of the three roll building technique are appealing:
- The time per average roller can be reduced easily to less than 5 minutes – often to less than 3 minutes including the bandaging process
- The applied pressure almost eliminates the risk of air bubbles induced by the covering process
- As a result of the applied pressure it’s possible to reduce the overbuild to 6 mm on the diameter
- No more additional layer on the ends
- No more concavity in the middle – sometimes you can even refrain from bandaging at all
- The set up time for changing compound rolls or the roll for the polythene film is negligible – less than 2 minutes for two experienced workers
- The predefinition of the diameter is achieved in seconds – especially with a digital control system
- The flexibility of the three roll building method is unique and so are the cost savings. This technique is used by many roller-covering companies who depend mainly on the quality of their products and who cannot risk rejects.
- There is a tradition of three roll builders; most of them are, unfortunately, very sophisticated and expensive, providing features like bandaging in the machine, that are not necessarily needed. Only very few of these machines are available at a price fitting to simplicity and flexibility.
Exclusivity or technique mix
All covering techniques are related to a specific product range. While the roller dimensions restrict the use of crosshead extrusion and three roll building, it is the amount of identical roller to be manufactured that defines the value of cross head extrusion.
Traditional wrapping is limited according to the maximum size of rubber sheets to be handled and so this technique cannot be applied successfully to very big rollers. Strip extrusion is the only covering method that does not face any substantial restrictions. Both small and narrow, big and wide rollers can be rubber covered this way and so the manufacturer is well equipped with a strip extruder.
Considering the economic superiority of the strip extruder to the traditional wrapping, the wide product range that can be covered, and the reduced complexity and price regarding rubber supply as well, the strip extrusion technique is the best one available for the average rubber re-covering company – if the latter can afford the investment.
This statement is correct as long as the roller company only has one covering device. As soon as there is a second one needed, the question of specialisation arises. Should there be a second, more or less identical, strip extruder or do the quantity and geometry of the rollers allow division of the work between two different machines?
This strategy can only be profitable if the second covering unit is less expensive (per roller) than strip extrusion – for a certain product range. This condition is given for series roller production and the crosshead extrusion and it is valid for small and medium sized rollers and the three roll building.
The few cases where cross head extrusion can be regarded as a serious alternative; do not need to be discussed deeply. It is sufficient to know that, even a small, cross head extruder easily produces 200 rollers (< 1000 mm wide) per shift. Keep in mind that only OEM suppliers and printing roller manufacturers usually deal with quantities to use such a device profitably.
The three roll builder alternative
Normally the technical and productive alternative to strip extrusion is three roll building. Knowing this, many companies should decide to invest in three roll builders when they grow their covering capacity. But for some strange reason, many don’t.
Besides strip extruders you will find traditional wrapping – which is expensive, or a second strip extruder, which is an offence to the rule of specialisation.
It’s common sense to keep an old lathe in function as reserve capacity if there is new investment in a more productive strip extruder. However it’s unwise to invest in an ineffective lathe machine after investing in a strip extruder. If this is the case, it proves that this branch of industry is not yet cost oriented to an extent that other industries have lived with for decades.
The only alternative to this conclusion would be a lack of knowledge regarding the real costs and possibilities of three-roll building. Rubber roller manufacturers simply do not know what interesting alternative is available in the market. Thus they do not decide consciously against the three-roll builders; they are just unaware of the possibilities and advantages.
To close this information gap we will show a cost comparison between strip extrusion and three-roll building.
Cost comparison: Strip extrusion vs. three-roll building
As mentioned above the ideal roller for three-roll building is less than 1500mm wide. For our comparison let’s take an average graphic or industrial roller:
- 1000 mm wide,
- Outer core diameter: 60 mm,
- Rubber layer required: 10 mm.
For material and production we take the following assumptions:
Issue Strip extruder Three roll builder
- Volume of investment 80.000 GBP 18.000 GBP (including bandaging unit)
- Maintenance costs 1.000 GBP/a < 100 GBP
- Rubber overbuild (diameter) 10 – 16 mm 6 – 8 mm
- Production time per roller 6 min. 2 min.
- Set up time (material change) 20 min. 5 min.
- Reject risk 2% <0,1 %
- Rubber price/kilo 3.5 GBP 4.0 GBP
- Specific weight 1,08 1,08
- Personnel cost/minute 0.15 GBP 0.15 GBP
- Machine + admin cost/minute 0.21 GBP 0.13 GBP
- Amount of rollers
- in same compound 30 30
Under these assumptions the covering of one average roller causes remarkably differing costs if affected alternatively by strip extrusion and three roll building:
|Strip extruder||Three roll builder|
|Material costs (rubber)||19.30 GBP||15.40 GBP|
|Process time costs||2.40 GBP||0.60 GBP|
|Reject risk costs||0.95 GBP||0.04 GBP|
|Total||22.65 GBP||16.04 GBP|
The material cost considers the differing overbuilt and the differing price per kilo. To even the material costs would demand a price per kilo of 1.16 GBP, a mere theoretical deliberation.
Process time costs consider both the differing time per roller and the differing minute rates of the machines plus the set up time and an average lot size of 30 rollers after which the compound has to be changed.
One reject roller causes costs of material and the whole manufacturing work, because the failings normally get detected only at the last production step – the grinding or grooving.
We calculated the overall production time to be 50 minutes and the overall minute rate to be 0.50 GBP. The likelihood for a reject with the strip extruder is 20 times higher than with a three-roll builder and thus the costs differ remarkably.
The result of our detailed deliberations is that three roll building, when it is a realistic alternative, requires costs that undercut those of strip extruding by 30%. In other words: Strip extrusion is nearly 50% more expensive than three roll building (with a simple and cheap machine).
The cost difference should not lead to a wrong conclusion. Three roll building is not a technique generally preferable to strip extrusion. In most cases it is a superior complement to an extremely flexible basic covering method. And so it should always be regarded as a first step towards specialisation.
We define a certain range of rollers, not too large in width or diameter, to this we provide an enhanced simple technique with the chance of supple productivity increase. At the same time we leave the strip extruder with all big and complicated rollers, with rollers for which we have no fitting calendared compound, with narrow rollers, etc.
The advantages of strip extrusion as basic covering technology are unbeatable at the moment. But similarly unbeatable is three roll building as complementary covering technique. It is far cheaper and thus can earn the money to provide the flexibility of strip extrusion to unforeseeable customer needs.
Three roll building can strengthen and grow the capabilities of a company in total and can increase the usefulness of strip extrusion. When you do not need to cover all rollers with this method, you work far more efficiently with the remaining complicated work – while the three roll builder next door makes profit.
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