a processing guide to silicone rubber extrusions.

by:Keyuan     2020-05-28
(
The first two issues of the series appear in the August and September 1998 issues of RW. )
When running continuously for a long time at a relatively high production speed, cooling is required.
Friction work and pressure accumulation of the screw-
At both the circuit breaker board and the extrusion head, it will help to heat-
In the equipment.
Since the mold is directly exposed to steam pressure, the CV vulcanization system actually needs to cool the head.
If the compound is allowed to reach a temperature of 120 [degrees]to 150 [degrees]
F. It is likely that the polymer is burnt or partially cross-linked.
It is recommended to keep the inventory temperature between 80 [degrees]and 100 [degrees]
If possible.
High Green strength inventory, long barrel extruder, high screw compression ratio, detailed screening ,(i. e. 100 or more)
When used, both the long track and the small hole die help to rub heat.
Although the above conditions are often necessary for quality and economy when appropriate selection, appropriate cooling allowances must be considered and given.
When considering the transformation of existing equipment or the purchase of new equipment, it is planned to cool in the barrel, screw and head.
It may not be necessary to use these three methods in all cases;
Flexibility is needed, however.
The two most commonly used methods for wire and cable manufacturers are CV or continuous steam vulcanization;
HAV or hot air vulcanization method mainly used by Profile extrusion manufacturers.
One preference over the other depends largely on the size of the wires and cables manufactured, the available space, the allowable expense, and the expected output of the lens.
The HAV unit requires a smaller footprint because the length of the HAV tunnel may be between 8 and 30 feet, while the length of the CV tube may be 250 feet.
The CV system is more expensive to install and requires more floor space.
The CV system will also generate more startup
Scrap has become an important consideration for short-haul transport involving larger-size cables.
On the other hand, the importance of the CV approach is no longer debated when it comes to the long running of repetitive items and when speed enters the economy.
Then, the economics of its operation quickly covered the initial overhead costs.
The CV system adopts the method of direct heat transfer, which is an efficient curing method.
Steam pressure is desirable from 150 to 250 psi.
The temperature of the HAV may be between 300 and 1,000. degrees]
F, depending on the length of the tunnel, the size of the conductor, the thickness of the insulation, the profile size, the concentration of the catalyst, the air circulation within the tunnel and the effectiveness of preheating the conductor or carrier.
As we all know, the forced air circulation within the HAV tunnel has doubled the production speed, which will definitely produce a more thorough treatment effect.
Air turbulence within the tunnel allows for better heat transfer and the removal of vulcanization
Products that directly solidify the atmosphere.
It is best to vent the HAV system whenever possible.
In order to ensure the best efficiency of the HAV device, silica must be used and cured
Regularly remove product deposits by brushing out the interior of the tunnel.
If accumulation is allowed, these deposits will affect the production rate and may result in insufficient curing due to the resulting heat transfer deficiency.
Brushing out of the tunnel is a fairly convenient and effective preventive maintenance control.
Depending on the type of conductor, the conductor preheating to be insulated can be done in many ways.
The decision whether to use preheating depends on the type of vulcanization and the requirements for the economy of production and the speed of operation.
The three most commonly used methods for preheating are: * resistance method; * gas flame;
* oven preheat.
The most effective and versatile of these three is probably resistance heating, as the current can be adjusted to compensate for changes in production speed.
Therefore, the best production control can be maintained conveniently.
The resistance heaters on the market today are equipped with built-in-
In the control of automatic stabilization temperature with little change in production speed.
Open gas flame method Although effective in some cases, there is a danger of open flame and it is easier to form oxidation and flame
Deposit of products on conductors.
The gas flame method is also prone to moisture, which cannot be tolerated in the HAV vulcanization system.
In some cases, however, neither the resistance nor the gas flame preheating method fills the bill.
Large cable diameter and fiber filler composition that are intertwined with the spiral wound conductor strands usually need to be preheated using the oven.
In this case, the conductor reel is placed in the oven for about 12 hours or more to ensure a thorough warm-up of the entire reel and to eliminate possible retention moisture.
When CV vulcanization is used, preheating of the conductor is not important;
However, this is a prerequisite when using HAV technology.
When exposed to the vulcanization temperature, absorb moisture along the insulated conductor and evaporate into steam.
This situation can cause blisters and, in serious cases, can lead to poor healing along the conductor.
Since the external pressure of the steam can prevent the insulation from foaming, the moisture captured is not important for steam vulcanization.
However, it is not advisable to retain moisture in any insulation material.
The vacuum pump provides two important advantages.
The vacuum should be applied inside the core rod before the conductor enters the head cavity.
Accessories can be adapted to most cores by tapping the core rod directly or attaching an extension at the inlet end without any effort.
When applied to the conductor, the vacuum?
Or extrusion, will eliminate blisters in the insulation caused by the interception of air.
Small, tightly wound conductors do not usually cause problems;
However, in most cases, when the vacuum is used in conjunction with the HAV system, large stranded cables and woven structures that require an external silicone rubber sheath will be handled more efficiently.
The trapped air phenomenon does not cause problems with CV methods.
The second case where a vacuum can be used is that when it is necessary to obtain extremely tight insulation around the conductor and fill the gap of the conductor more effectively than the pressure type extrusion mold, it is only allowed.
Early curing of silicone rubber oven, a post
Treatment is actually essential.
It is not enough just vulcanization.
However, this is no longer the case today.
Some modern silicone rubber compounds are designed to provide tough, high-strength extrusion without the need for a later periodcure.
In some cases, however, oven curing is required.
In the high temperature closed system, high temperature stability of performance, oil and pressure resistance, gas release, anti-response and various other specifications does require curing of the cured oven.
With the improvement of various other processing aspects of silicone rubber, the oven curing technology has also been significantly improved.
Post-curing in coil form, although still a common method of extrusion of large cables and profiles today, has been largely replaced by curing technology in reels or stainless steel baskets.
The most commonly used catalyst bis in the wire and cable industry (
2-chlorhexyl)
In the process of curing the oven, the peroxide is the system that is most likely to reverse the silicone rubber.
Other catalytic systems used in conjunction with silicone rubber, such as benzene peroxide (Di Cup 40C);
2,5 DME, 2,2-2 (t-butyl peroxy)hexane (Varox)
And adding treatment, it is not easy to support reversal.
However, these catalysts require a higher vulcanization temperature (300 [degrees]F)
By contrast, 240 [1 [degrees]F for bis (
2-chlorhexyl)
Peroxide and peroxide.
Of the above 5 catalysts, only bis (
2-chlorhexyl)
Peroxide and addition curing can be used with the HAV system. The (
2-chlorhexyl)
Peroxide is easier to support reduction than the other four catalysts.
Silicone rubber is a polymer material with a high average molecular weight, which is obtained by connecting the basic unit together through the polymerization process.
Under specific conditions, the polymerization reaction to form a polymer is reversible.
Long chains split into various low molecular weight parts.
Acid fragments and [H. sub. 2]O by-
If sufficient ventilation is not given, the product issued after curing will result in a reply.
Therefore, the oven curing in the form of a coil is the main method.
However, the new technology has now provided manufacturers with ready-made compounds or enhanced gums that can be treated with any catalyst system including bis (
2-chlorhexyl)
Peroxide that can be cured on six process reels
Randomly wound the inch segment of the product.
This anti-reply feature enables the manufacturer to eliminate double processing, as the reel or coil can be removed directly from the extruder and placed in the oven for post-curing.
This process also enables the manufacturer to obtain a longer continuous length of the finished product.
The application of fiber has its advantages, but sometimes two problems are encountered.
Low modulus and low durometer silicone rubber will have problems in the application of outer weaving.
Shares with a range of 45 to 65 durometer Shore a are the most troublesome.
The softness of the insulating material allows the tension of the woven fiber to extend down to the insulating material, resulting in a backup or aggregation effect.
The result is rough insulation and distorted weave pattern.
At more than 65 degrees, the silicone is woven like other insulating media.
The second problem that may arise is caused by the thermal expansion of silicone insulation during subsequent paint or resin applications.
The heat required to cure the resin coating will also cause thermal expansion of the rubber, and if there is not enough space between outer weaving and insulation, silicone will be squeezed out through the void.
While compounds with a low durometer range can be cumbersome if handled improperly, there are certain conditions that, when established, can enable manufacturers to effectively apply outer weaving.
Important note when weaving low durometer compounds: * keep the lowest effective Yarn tension.
* Use the weaving angle between 54 [degrees]and 65 [degrees].
Select the appropriate size of the fiber that allows the appropriate angle and allows 100% coverage.
* Use woven pins.
This technology requires the use of steel pins, which are inserted under the woven layer, allowing for about 1 inch of the gathering points of the woven layer to be highlighted.
Experience has shown that wires insulated with 1/32 silicone walls need about 0.
015 inch diameter pin.
Heavier insulation may require larger pins.
By allowing the weaving to be formed around a larger effective diameter, the tolerance is [
No continuation of the original text]
Ted Taylor is senior vice president and managing partner of professional silicone products.
He has 36 years of experience in silicone, including several positions at GE
Silicone Products Division.
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