Runner Design and Structural Specification of Injection Mold for Wire Grommet
Wire grommet is a flexible thin-walled plastic fitting used for cable protection, mostly made of TPU or TPE soft materials. The runner system directly affects melt filling, product surface quality and molding stability. Improper runner design will bring problems such as incomplete filling, weld lines, excessive internal stress and product sticking to mold. Formulating standardized runner and mold structure specifications can reduce trial mold adjustment difficulty and improve long-term mass production stability of wire grommet molds.
Material Characteristics and Runner Design Pre-evaluation
Soft TPU and TPE materials have high viscosity and strong sensitivity to shear heat. Excessive shear in the runner will lead to material decomposition, precipitation and product surface defects. At the early stage of mold design, material hardness, shrinkage rate and processing temperature range need to be confirmed. For thin wire grommet with double-sided flange structure, the wall thickness is thin, so the melt flow resistance is relatively large. The number of cavities should be determined according to injection machine capacity, and the balance of runner layout is the priority factor to ensure consistent filling of each cavity.
Main Runner and Branch Runner Structure Specification
The main runner taper shall be controlled within the standard range to facilitate smooth demolding of sprue. The inner surface of main runner needs fine polishing to reduce melt adhesion. Branch runners adopt circular or trapezoidal cross-section. Circular runner has the best fluidity and less material waste, which is preferred for multi-cavity wire grommet molds. The runner size cannot be too small to avoid excessive pressure loss, and cannot be too large to extend cooling cycle and increase material waste. Runner layout adopts balanced layout, making the melt flow distance from main runner to each cavity basically consistent, so that all cavities complete filling at close time.
Gate Type and Position Selection Rules
Wire grommet commonly use edge gate or submarine gate. Gate size needs to match soft material characteristics. Too small gate will cause high shear and material burning; too large gate makes gate residue difficult to remove and easily pull the product during demolding. The gate position shall avoid the sealing lip of grommet, prevent weld lines on the main sealing area. For circular double-sided grommet, the gate is usually arranged on the outer flange. The gate thickness shall be set reasonably to balance filling speed and gate break effect.
Cooling Structure Matching Specification for Runner and Cavity
Soft plastic is sensitive to mold temperature. Uneven temperature distribution will lead to inconsistent shrinkage and product deformation. Cooling water channels are arranged around runner plate and mold cavity. The distance between water channel and runner surface shall be kept uniform, avoiding local overheating. For multi-cavity molds, the cooling pipeline of each cavity is arranged symmetrically. The mold temperature control keeps stable within the production range, preventing excessive temperature fluctuation from affecting melt viscosity and filling balance. The runner plate also needs independent cooling control to reduce heat transfer from runner to mold insert.
Exhaust System Design Requirements
During the filling of TPU and TPE melt, gas is easy to accumulate at the end of filling. Poor exhaust will cause burning spots, weld line strength reduction and sticking mold. Exhaust grooves are set at the product parting line and the terminal position of melt flow. The depth of exhaust groove must be controlled strictly according to material characteristics, preventing melt overflow and flash. For the multi-cavity wire grommet mold, each cavity is equipped with independent exhaust structure. Regular cleaning of exhaust groove shall be reserved in later maintenance specification to avoid accumulation of decomposed precipitates.
Demolding and Insert Structure Specification
Wire grommet belongs to soft thin-walled product, which is easy to deform and tear during ejection. The mold adopts ring ejector sleeve structure instead of scattered small ejector pins, realizing uniform force during ejection. The ejection speed is controlled smoothly to prevent local stress concentration. The mold insert adopts split insert structure, which is convenient for later disassembly, cleaning and maintenance. The matching clearance between insert and mold base considers thermal expansion of steel, avoiding jamming after temperature rise. The parting surface sealing area is designed with appropriate allowance to balance flash risk and hot state matching.
Mold Trial and Mass Production Verification Specification
After mold processing is completed, cold inspection of runner size, gate and cavity matching shall be carried out first. In the trial mold stage, adjust injection speed and pressure step by step, check the filling balance of each cavity, gate quality and product appearance. Record molding parameters and optimize runner size if cavity imbalance appears. After qualified trial production, formulate regular maintenance specification, including runner cleaning, surface polishing and exhaust groove inspection, to keep long-term stable production of wire grommet mold.
