I. Mandrel & Inner Rubber: The stripping step determines inner bore smoothness
Hoses are manufactured over a mandrel. A steel or rigid nylon mandrel is inserted into the inner rubber tube; the outer diameter of the mandrel defines the inner diameter of the finished hose. Before fitting the inner tube onto the mandrel, release agent must be applied in the correct dosage. Insufficient release agent makes demolding difficult after vulcanization, and forced pulling will scratch the inner bore. Excess release agent leaves residues inside the hose, complicating subsequent cleaning and impairing adhesion between the rubber liner and steel wire reinforcement.
Mandrel stripping force is an indicator seldom listed in product datasheets. Even with the same rubber compound, varying vulcanization levels can lead to large differences in stripping force. Any scratch on the inner surface will act as a fixed crack initiation point during impulse testing.
II. Braid & Spiral Reinforcement: Uneven tension means certain wires fail first
The steel wire layer is formed by spindles rotating circumferentially. The coordination of spindle count, rotational speed and haul-off speed defines the braid angle and coverage rate. Minor fluctuations in haul-off speed alter the braid angle, resulting in inconsistent pressure-bearing capacity along a single hose.
Tension control is far more challenging than angle control. Tension must be uniform across dozens of spindles. If any single wire is looser or tighter than others, the steel wires will elongate asynchronously under pressure. The wires reaching their limit first will fracture, creating stress concentration and triggering failure of the entire reinforcement layer. Many failures observed in impulse tests do not originate from the middle of the hose, but from sections with locally loose steel wires.
For spiral constructions, there is an additional design choice: whether to add intermediate rubber between wire layers. Intermediate rubber prevents direct metal-to-metal contact and reduces fretting wear. Without intermediate rubber, the hose achieves greater flexibility at the cost of direct friction between steel wires. This is a trade-off between design performance and manufacturing cost.
III. Vulcanization: One-shot forming, no rework possible
Vulcanization converts raw rubber into cured elastomer. Once the three key parameters — temperature, pressure and dwell time — are set, the hose is fully formed after exiting the autoclave and cannot be reworked.
Under-vulcanization results in low rubber strength and poor bonding between inner rubber and steel wires, which may cause delamination once put into service. Over-vulcanization makes the rubber brittle and prone to cracking at low temperatures. Temperature distribution inside the autoclave is inherently non-uniform, especially for long hoses: the heating profile differs between areas near the autoclave door and the tank bottom. Reliable vulcanization relies on loading layout and temperature logging, not just the rubber formula.
IV. Testing: Hydrostatic, Burst and Impulse Tests serve different purposes
Three tiers of tests are carried out before delivery. Every single hose undergoes hydrostatic testing at 1.5 to 2 times the working pressure, with hold time to check for leakage and dimensional change. Burst testing is performed on sampled batches at 4 times the working pressure, to examine the location and pattern of rupture. Impulse testing takes the longest: hoses are cycled to failure under standard conditions, to evaluate service life rather than ultimate strength.
Among the three test types, hydrostatic testing identifies sealing defects, burst testing reveals structural flaws, and only impulse testing answers the critical question: how long will this hose last? Therefore, the sampling frequency of impulse testing strongly reflects a manufacturer’s commitment to quality.
