Key Design Points for Injection Molding Appearance of Electric Toothbrush Housings

As appearance parts for consumer electronics, electric toothbrush housings require excellent appearance quality, comfortable hand feel, precise assembly and stable mass production. The appearance design of injection molding must coordinate product modeling, material selection, mold structure and process control, while meeting daily grip, waterproof and wear resistance requirements. This article summarizes core design specifications in detail.

1. Overall Shape and Appearance Surface Design

Electric toothbrush housings are mainly divided into integrated structure, upper and lower split structure and multi-section combined structure. Smooth curved surfaces are preferred instead of sharp edges. Large flat surfaces are prone to sink marks, flow lines and weld lines. Design subtle camber or fine textures on flat areas to cover minor molding defects and improve anti-slip performance. All corners adopt arc transition with proper radius to avoid mold stress, material burning and collision damage.

Position parting lines and assembly lines on side surfaces, bottom or grooves to reduce visual impact. The curvature of grip areas changes gently to prevent uneven melt flow, color difference and uneven gloss caused by abrupt curvature. Keep overall lines coherent to maintain a unified appearance effect.

2. Wall Thickness and Rib Structure Design

Uniform wall thickness is the foundation of good appearance. The conventional wall thickness of electric toothbrush housings is 1.2mm to 2.0mm, and grip areas can be thickened to 1.8mm–2.2mm for higher structural strength. The wall thickness difference on a single part shall be controlled within 0.5mm. If thickness transition is unavoidable, use inclined surface for gradual transition, with transition length more than three times of thickness difference.

The thickness of internal reinforcing ribs, buckles and positioning columns is 0.5 to 0.7 times of the main wall thickness. Large fillets are set at joints between ribs and outer walls to prevent sink marks on appearance surfaces. Segmented ribs are equipped with exhaust grooves to improve filling effect. Keep internal screw posts and bosses away from main appearance surfaces, or use textures to cover corresponding areas.

3. Layout of Gates, Parting Lines and Exhaust Systems

Concealed gates are the first choice for appearance parts, including submarine gate, fan gate and horn gate. Gate marks are arranged on bottom, inner sides or assembly gaps. Avoid exposed direct gates and side gates on main visual areas. For large-area parts, adopt multi-point gates for balanced feeding to reduce weld lines. Optimize gate position for long shell products to shorten flow length and reduce flow lines.

Arrange parting lines along product contours and ensure high machining precision to prevent flash and misalignment. Set exhaust grooves at melt end positions, weld lines and dead corners. The depth of exhaust grooves is controlled according to raw material characteristics to avoid gas trapping, burning and material shortage. Design independent exhaust structure for deep grooves and blind holes to ensure uniform surface gloss.

4. Texture, Gloss and Matching of Secondary Processes

Main surface styles include high gloss, matte and fine texture. For high-gloss housings, adopt mirror polishing on mold cavities, and use materials with good fluidity. Medium temperature and medium speed filling are applied to avoid flow lines and foggy surfaces. Reduce tiny grooves on high-gloss surfaces to prevent dust accumulation. Matte and textured surfaces can cover minor molding defects. The texture direction is consistent with melt flow direction to prevent texture deformation and whitening.

Reserve structural space for spraying, electroplating, laser engraving and two-color injection molding in advance. Set steps at color dividing lines of two-color parts to prevent material overflow and color mixing. Add chamfers on edges of spraying areas to enhance coating adhesion. Reduce tiny holes and long grooves to avoid burrs and appearance damage during trimming.

5. Draft Angle and Demolding Design

Reasonable draft angle prevents surface scratching and whitening during demolding. The draft angle of high-gloss surfaces is no less than 1°. For textured surfaces, increase draft angle according to texture depth, generally 1.5° to 3°. Match draft angles for internal structures such as buckles and ribs. Polish deep-cavity molds and long grip parts to reduce demolding resistance.

Arrange ejector pins and ejector blocks on bottom or inner sides instead of main appearance areas. Use large-area ejection structure to avoid dents and marks on product surfaces caused by concentrated ejection force.

6. Material Selection and Color Matching Control

Common materials for electric toothbrush housings include ABS, ABS+PC, TPU and TPE. ABS features easy coloring and polishing, suitable for matte and textured products. ABS+PC has better heat resistance and toughness for products with charging structures. Set steps at joints of hard and soft materials to prevent flash.

For light color, transparent and high-gloss products, strictly control raw material purity and drying process to avoid silver marks and bubbles. Ensure uniform mixing of color masterbatch and raw materials, and clean barrels regularly to prevent color difference. For transparent housings, control wall thickness uniformity and exhaust performance to avoid foggy spots.

7. Structural Design for Assembly Matching

Arrange internal buckles and sealing grooves on inner assembly surfaces to avoid affecting appearance. Optimize tolerance of matching surfaces of multi-component housings to prevent uneven gaps and misalignment. Add reinforcement around functional holes such as button holes and indicator holes, and use chamfer transitions at hole edges to avoid sink marks and deformation.