Key Gate Design Points for Spline Mold

Spline plastic parts are mostly long arc special-shaped curved structures with variable cross-sections, commonly used in home appliance decorative strips, sealing strips, handrail trim strips, mechanical arc brackets and other products. They feature smooth appearance, uneven wall thickness and easy bending and warpage, and are extremely sensitive to glue feeding position, filling flow direction, gate size and form. Gate design is the key to the success of spline molds, which directly determines smooth filling, shrinkage and bubble defects, weld line position, deformation and bending, and gate residual marks. The following sorts out the core design points in detail.

1. Gate Form Selection Matching Spline Structure(1) Side GateIt is the most commonly used form, suitable for ordinary straight splines with non-direct appearance parts for glue feeding. It has simple structure, convenient processing and easy trimming, suitable for multi-cavity mold layout. The disadvantage is obvious gate residual marks, which cannot be arranged on the main curved surface of the appearance. It is suitable for ordinary decorative splines with uniform wall thickness and moderate length.

(2) Submarine Gate

Suitable for splines with no allowable gate marks on the appearance surface, realizing automatic gate breaking without manual trimming. It can be hidden at bone positions, end faces and inner undercuts. The design focus is to keep the feeding angle smooth, and avoid directly impacting the thin-walled curved surface to prevent jet flow and surface scuffing. Elongated splines prefer submarine gates to ensure appearance integrity.

(3) Point Gate

Suitable for high-gloss, transparent splines with high appearance requirements, feeding from the center from top to bottom with uniform shunting and low internal stress, which can effectively reduce bending deformation. The disadvantage is that the mold needs a three-plate structure with high cost, suitable for long-diameter ratio splines with large arc curvature and easy warpage.

(4) Fan GateFor wide and thin flat splines and large-area arc trim strips, the gate gradually widens from narrow to wide for smooth transition feeding. It avoids excessive flow velocity causing jet flow and ripple, fills large-area curved surfaces steadily, and reduces internal stress and warpage deformation.

2. Selection Principles of Gate Feeding Position(1) Feeding at the Maximum Wall ThicknessSpline parts are generally thick in the middle and thin at the edges. Feeding must be arranged at the thick wall position to allow the melt to flow naturally from thick to thin. It prevents premature condensation of thin walls leading to insufficient filling and shrinkage depression on the surface. Direct feeding at the thinnest cross-section is strictly prohibited, which is prone to material shortage, whitening and flow lines.

(2) Arrange Gate Along Spline Arc TrendArrange the gate along the length and arc tangent direction of the spline to allow the melt to fill along the curved streamline. Direct lateral impact on the curved wall will easily cause turbulent flow and air flow marks, increase internal stress of plastic parts, and lead to large bending deformation after molding.

(3) Avoid Main Appearance Surface and Assembly Matching PositionGate residual marks and trimming burrs shall not appear on the outer visible curved surface, assembly fitting surface and buckle mounting position. Priority is given to arranging at end faces, hidden side positions, inner bottom bone positions and rounded non-main viewing areas.

(4) Place Weld Lines in Non-key AreasFor multi-gate or long-distance shunting, predict the weld line position in advance, and deliberately place the weld line at the end, side, non-stress and non-appearance area, avoiding the middle main viewing surface and stress-bearing section of the spline.

3. Gate Size and Flow Control(1) Gate Thickness Matches Plastic Part Wall ThicknessThe thickness of ordinary side gate is 0.6～0.8 times the wall thickness of spline parts. Excessive thickness leads to slow cooling, large water gap residues and high internal stress; too thin gate freezes easily with large pressure loss, causing difficult filling and insufficient material supply.

(2) Gate Width Adjusts with Spline LengthThe gate of elongated splines should not be too narrow. Appropriately increase the width to reduce injection pressure and shorten filling time. For slender small cross-section splines, narrow the gate to prevent flash and local stress concentration.

(3) Minimize Gate Length

The runner section of the gate should not be too long, reducing pressure loss and condensed layer thickness, ensuring rapid mold filling and easy gate shearing, while lowering residual stress and reducing subsequent bending deformation of splines.

4. Flow Direction, Exhaust and Deformation Prevention Design

(1) Maintain One-way Smooth Flow and Avoid Turbulent ImpactThe spline curved surface is complex. The gate layout should ensure a single main flow direction, and try to avoid melt impact caused by multi-point feeding. Otherwise, obvious weld lines and internal bubbles will be generated, and torsional deformation will be aggravated.

(2) Arrange Exhaust at the Far End of Gate FillingExhaust grooves must be set at the end of melt filling, arc ends and dead corners, matching the gate feeding direction to discharge internal air in time, preventing burning, material shortage and air marks.

(3) Adjust Warpage by Gate PositionArc splines are most prone to inward or outward bending. The bending amount can be corrected by offsetting the gate position and adjusting the feeding speed to balance shrinkage difference, which is a common method to adjust deformation without modifying product structure.

5. Demolding and Mass Production Adaptation Points(1) Avoid Ejection Pins Near Gate PositionDo not arrange ejection parts near the gate to prevent cracking and whitening at the gate caused by pulling during ejection, ensuring normal separation of plastic parts and water gaps.

(2) Convenient for Automatic Falling or Simple Manual TrimmingPriority is given to submarine gate and three-plate point gate for mass production to realize automatic gate breaking. Ordinary side gates are designed at positions easy to shear and polish, reducing post-processing working hours and ensuring consistent batch appearance.

ConclusionThe core of spline mold gate design is to select the correct gate form, find the thick feeding position, follow the arc flow direction, control reasonable size, and avoid appearance and matching positions. Meanwhile, take into account flow direction exhaust and deformation correction. As long as the gate form, position and flow are well matched with the structural characteristics of long arc splines, it can effectively solve common defects such as flow lines, weld lines, shrinkage shortage and bending warpage, and ensure the appearance, size and mass production stability of spline plastic parts.