There are two main systems used in the retreading process: Pre-cure Tread System (Cold process) and Mold Cure System (Hot Process). Both systems are well known and each of them requires tire retreading machines at different investment levels. Since Pelmar has already been engaged in establishing retreading plants through its vast collection of tire retreading equipment, we would be glad to assist and advise regarding any of retreading projects through its well exprienced tire retreading experts.
Cold Process
Today, most retread plants, primarily those retreading truck tires, use the Precure Tread System. This system is a process using a fully tread pattern and curing this “precured” tread pattern onto a tire casing using a thin bonding layer of uncured rubber. The “precured” tread rubber has the tread design cured into the finished shape before the rubber is shipped to the retreader. The thin layer of uncured rubber is sandwiched between the buffed tire casing and the buffed backside surface of the precured tread. The tire is then vulcanized with heat, time, and pressure in a curing chamber which bonds the tread to the casing. That thin layer of uncured rubber is called bonding gum or cushion gum.
Hot Process
Another retread process still widely used is the Mold Cure System. Once the primary method of retreading, it is now used mainly by those retreaders doing passenger retreading. It is also used by large volume truck retreaders who have tires being retreaded of similar size and manufacturer and who require a minimum tread pattern selection. This system uses aluminum molds with a tread design cast or machined into it. The curing of the tires is similar to the technology of new tire manufacturing. Uncured rubber is applied to these tires and the design is molded onto the tire during the vulcanization process. The uncured rubber can be applied by a strip winding extruder machine, a slab extruder machine providing a specified width and thickness or die size rubber can be purchased in a specified shape. With over than 40 years in tire business, Pelmar can supply machinery and consulting services to those we wish to set-up retreading facilities with customized solutions as per the given requirements.
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Retreading should be carried out by few sequence steps. Each step in the process should be followed by the next one. The production flow is arranged in the following steps:
Worn tires are brought into this designated area and first examined. Tires are visually inspected and all readily apparent non-retreadable tires are rejected in this area. Visually accepted tires for retreading are arranged in rows, stored, and stages in this area for continued processing.
Tires are mounted on inspection spreaders allowing for a thorough examination of the internal and external casing. Tires are inspected and injuries are identified on the tire casings and are accepted or rejected from the process accordingly.
The worn tire is placed on a buffing machine for removal of the original tire tread design from the tire. This process provides a textured surface to the casing for maximum adhesion of the new retread material to be applied. The buffing operation is carried out by a buffer - a wide used tire retreading machine. It shapes the tire casing to the proper contour and profile. Buffing insures correct fitment of the tire to the retread system being utilized.
Fig 1.Truck tire buffing machine manufactured by Bandag
Tires can be positioned on a buffer or can be positioned at a separate work station with rotating roller arms. Suspect damaged areas are isloated to determine the extent of the damage to the tire casing. After determining the extent of injury, the void is filled-in with cushion gum or rope rubber and aligned with the buffed surface using a patch. This process provides for a smooth buffed surface without voids after the buffing operation.
Fig 2. Cushion Applicator manufactured by VMI-AZ
Pre-Cure Building
The buffed and cemented tire is placed on the precure building machine. The building process is accomplished by applying precure cushion gum to the prepared casing. The cushion gum is the bonding agent between the buffed casing and the pre-cure tread. After the pre-cure cushion is applied, the pre-cured tread is cut to the proper length and stitched to the cushion gum and buffed casing.
Enveloping (Pre-Cure) - Each buffed, cemented and built tire, prior to loading in the curing chamber, must have an envelope applied (stretched) over the tire. Envelopes provide a flexible mechanical pressure to the pre-cure tread so that there is a good flow of cushion gum that bonds the pre-cure tread to the buffed casing during the cure cycle. Different size and type envelopes are required depending on types of pre-cure systems utilized.
Fig 3. Cold Process - Truck tire building machine manufactured by Bandag
Mold Cure Building
The buffed and cemented tire can be placed on a strip winding building machine. Uncured strips of rubber are continuously wound around the cemented and buffed tire to fit a specific built profile.
Fig 4. PCR Orbitread Extruder machine manufactured by SCV
6. Curing
Cold Process Curing
The tires are loaded into precure curing chamber that have generally a 60” inside diameter, and can hold anywhere from 12-15-23-25 tires at one time. Chamber head is adjustable from 210°F to 300°F. Cure times vary from 2-1/2 to 4 hours. Chamber heating source can be steam or electric. Air Pressure inside the chamber depends on curing chamber, usually an autoclave which is one of common machines of tire retreading equipment.
Fig 5. Cold Process - 22 Tires Bandag Autoclave
Mold Curing Process
The buffed and built tires are loaded into metal matrix curing molds available in many different versions:
Common curing temperature is usually 300°F. Heating of metal molds can be either by steam or electric. Curing times will vary depend on tire sizes
The finished cured tire is further examined for cosmetic flaws and defects. Finished tire is inspected to insure that the tire meets customer and government regulations. Finished tire is treated for appearance by painting and adding markings if required.
Pelmar can offer its customers a wide variety of tire retreading equipment for hot and cold processes. It includes buffers, tire builders, tire spreaders, retreading presses, autoclaves, cushion gum applicators etc.
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