Tube Burr Issue, How to Set Clamping Force for Injection Molds
Tube parts are typical rotary plastic workpieces. Flash along parting surface and flange edge is a common defect during injection molding. Many operators simply attribute flash to insufficient clamping force and blindly increase locking pressure. Excessive clamping force will cause template elastic deformation, accelerate wear of guide pins and mold plates, and shorten mold service life. For cylindrical tube sleeves, the melt will generate radial expansion pressure on the core during filling, so the clamping force must be calculated and adjusted combined with product projected area, melt pressure and structural features, matched with mold polishing and injection process optimization, to restrain flash while protecting mold structure.
1. Basic Theoretical Calculation of Clamping Force
The core formula of clamping force: Clamping Force ≥ Projected Area of Cavity × Melt Pressure Inside Cavity. Calculate the projected area of the bushing part. The bushing is a rotary part, and the projection adopts the circular area corresponding to the maximum outer diameter of the product. For multi-cavity molds, combine the projected area of all cavities, and include the projected area of runners and gates instead of only calculating the plastic part itself. Determine injection melt pressure. For ordinary PP and PE soft materials, the cavity pressure is generally selected from 30MPa to 50MPa. For ABS and POM materials, it ranges from 50MPa to 70MPa. For glass fiber reinforced materials and high-viscosity engineering plastics, the cavity pressure increases to 70MPa~100MPa. Reserve safety factor. The conventional production safety factor is 1.1~1.3. For thin-wall bushings and high-speed filling conditions, the safety factor is adjusted to 1.3~1.5 to avoid the mold opening of parting surface caused by instantaneous pressure peak during filling.
2. Correct Clamping Force Setting Combined with Bushing Structure
Under the action of injection pressure, the core of cylindrical bushing will bear radial expansion force. The locking force calculated by the basic formula sometimes is not enough to restrain the opening of parting surface. For long cylindrical and large-diameter thin-wall bushings, the lateral pressure during melt filling will transfer to the parting surface, so the pre-tightening clamping force needs to be appropriately increased. For bushings with steps and flanges, the flange accounts for a large proportion of projected area and is also the high-incidence area of burrs. All flange areas should be included in the projected area during calculation. For multi-cavity bushing molds with asymmetric cavity layout, the force distribution is uneven. It is impossible to simply average the clamping force, so it is necessary to appropriately increase the clamping margin to prevent burrs generated by the opening of unilateral parting surface.
3. Clamping Force Debugging Steps in Mold Trial Stage
Before formal production, the clamping force needs to be adjusted gradually from low to high. It is strictly forbidden to directly set to the maximum clamping force of the machine. Initial setting: Set the clamping force according to the lower limit of theoretical calculation, carry out short shot mold test and observe the filling state to confirm the position of melt front. Gradually increase the clamping force with small adjustment each time. After each adjustment, inject a complete product to check whether there are burrs on the parting surface and flange edge. When the burr just disappears, record this clamping force value, and increase it slightly by 5%~10% as the benchmark parameter for production. Observe the mold opening and closing state. If the clamping force is too large, the template will deform, which may lead to small inner diameter of the product and difficult demolding. Meanwhile, the thermal expansion after mold heating will further increase the template stress.
4. Distinguish Insufficient Clamping Force from Other Burr Inducements
Many bushing burrs are not caused by insufficient clamping force. Blindly increasing clamping force cannot solve the problem. If the matching gap of parting surface is too large, the fitting gap of inserts exceeds standard, or the template stiffness is insufficient to produce elastic deformation, burrs will still occur even if the clamping force reaches maximum. Too good material fluidity, excessive injection speed and high holding pressure will also cause flash. During debugging, reduce injection and holding parameters first, and then adjust the clamping force. Excessively high mold temperature will reduce melt viscosity and make flash easier to occur. It is necessary to control mold temperature synchronously instead of only relying on clamping force to suppress flash.
5. Dynamic Maintenance of Clamping Force in Mass Production
During long-term continuous production, the mold temperature keeps rising, the template expands by heat, the matching state of parting surface changes, and the clamping effect fluctuates. Check the burr on product edges respectively at the initial production stage, stable mass production stage and restart stage after shutdown. The wear of clamping cylinder and crank arm of equipment will cause attenuation of actual clamping force, and there is deviation between the value displayed on the equipment panel and real clamping force. Regularly verify the clamping force of injection molding machine. When the material batch is replaced or the product wall thickness is changed, recalculate and debug the clamping parameters. Reasonable setting of clamping force is an important means to solve bushing flash. It needs to cooperate with mold polishing and injection molding process. Matched clamping force can restrain parting surface flash, reduce long-term heavy load on the mold, reduce template deformation, guide column wear and other faults, and stabilize the molding quality of bushing mass production.
