Abstract
An envelope for tyre retreading comprises a rubber pad located on the crown of a green envelope, the pad and the envelope being integrally attached to each other by vulcanising and curing in a mold, so as to render the said pad an integral part of the envelope; a one way valve \ passing through the envelope body and the said pad, with an adaptor attached to the said valve, whereby the said pad serves as a reinforcement at the adaptor base and also prevents pinching of the envelope body at the said base.
Specification
This invention relates to an envelope for tyre retreading.
Envelopes are used for bleeding out air from tread grooves and air trapped between envelope and built tyre. Envelopes are used in retreading primarily to hold the tread and cushion assembly till it gets bonded. Envelopes also prevent the steam/hot air penetrating into the cushion gum during curing.
In a tyre retreading process, the worn out tread portion of a used tyre is buffed to a uniform crown profile for attaching a new vulcanized pre-cured tread. A pre-cured tread having bufted surface at bottom is attached to the buffed crown portion of the casing with a lamina of uncured cushion gum. The built tyre is then covered with an envelope which will hold the built tyre assembly tightly, in intimate contact with the entire surface of the tread and casing. The tyre is then placed in a curing chamber for a specified cure cycle time at an elevated temperature under differential pressure to cure the gum and positively bond the casing and tread together. The envelope and other accessories are removed after the cure cycle and the retreaded tyre is ready for use.
An important step in the retreading process is the removal of air in
between the envelope and the built tyre. The air present in tread grooves will interfere with heat transfer to un-vulcanized cushion under the tread and has to be evacuated through an outlet connected to a one way valve screwed to an adaptor fixed on the crown of the envelope. The envelope helps proper heat transfer to cushion and effects uniform differential pressure acting on entire tread surface and grooves, giving a uniform bonding of tread without distortion.
Thus to bleed out air and ensure complete evacuation of air there is a need for efficient air bleeding through the adaptor fixed to the crown of the envelope. While doing so, because of the vacuum created, there is a strain induced around the adaptor attached to the envelope on the crown. Upon continuous usage, this strain around the metal base increases, leading to tearing of the rubber around the adaptor base.
This invention, in order to overcome the aforementioned drawback, proposes the provision of a rubber pad on the crown of the envelope as an integral part of the envelope.
Conventionally, the adaptor is inserted into the envelope at the crown, after envelope molding, through a hole punched on the envelope. The adaptor is then tightened in place using a plate washer and nut. However, the adaptor causes pinching of the envelope body at the base of the adaptor when the plate washer comes in direct contact with heat during repeated use. This leads to premature failure of the
envelope and reduces its life considerably resulting even in failure of the processed tyre due to envelope leak at the adaptor base.
To overcome the aforesaid problem, this invention proposes the fixing of a rubber pad at the crown, on a green inflated envelope and curing the pad and envelope assembled unit in a separate envelope mold. Thus, the rubber pad gets attached to the crown of the envelope during curing process and reinforces the area where the adaptor is to be fixed. The rubber pad becomes an integral part of the envelope and serves as a reinforcement at the adaptor base. Such a reinforcement at the adaptor base completely eliminates pinching of the envelope body at the said base.
The envelope for tyre retreading, according to this invention, comprises a rubber pad located on the crown of a green envelope, the pad and the envelope being integrally attached to each other by vulcanising and curing in a mold, so as to render the said pad an integral part of the envelope; a one way valve passing through the envelope body and the said pad, with an adaptor attached to the said valve, whereby the said pad serves as a reinforcement at the adaptor base and also prevents pinching of the envelope body at the said base.
This invention will now be described with reference to the accompanying drawings which illustrate by way of example, and not
by way of limitation, one of possible embodiments of the envelope proposed herein,
Fig. 1 illustrating the green envelope in inflated condition with rubber pad located on the crown
Fig.2 illustrating the said envelope with rubber pad after molding and the adaptor ( screwed to the one way valve) fixed in place on the crown of the envelope through a hole punched through the envelope body and the rubber pad.
A rubber pad R is located on the crown of a green envelope E, the pad and the envelope E (in green inflated condition) being integrally attached to each other by vulcanising and curing in a mold, so as to render the said pad an integral part of the envelope.
A one way outlet valve which allows air from between the envelope and the built tyre to pass out through the envelope body through a hole H (shown in broken outline) punched in the envelope body and the rubber pad. The adaptor A is screwed to the said valve V. The said pad serves as a reinforcement at the adaptor base B, and thus prevents pinching of the envelope body at the said base.
This invention therefore not only eliminates failure due to pinching of the envelope at the adaptor bas, but also gives an effective air bleeding function with enhanced life for the envelope and avoids process failures during retreading.
We Claim:
1, An envelope for tyre retreading comprises a rubber pad located on
the crown of a green envelope, the pad and the envelope being
integrally attached to each other by vulcanising and caring in a mold,
so as to render the said pad an integral part of the envelope; a one way-
valve passing through the envelope body and the said pad, with an
adaptor attached to the said valve, whereby the said pad serves as a
reinforcement at the adaptor base and also prevents pinching of the
envelope body at the said base.
2, An envelope for tyre retreading wherein the said pad is made up of
heat resistant rubber.
3.An envelope for tyre retreading substantially as herein described
