Key Precision Molding Points of Injection Molds for Lipstick Cases and Mascara Tubes
1. Confirmation of Product Tolerance and Appearance Benchmark
Lipstick and mascara components are multi-part sets. The thread fit clearance, coaxiality, roundness and wall thickness uniformity of tube body, middle bundle, inner core, outer cap and base directly determine the hand feel of screwing. The appearance surface generally requires Grade A mirror, without weld lines, sink marks, silver streaks, ejector pin marks and flow marks. Some customers require gate-free appearance. Before mold design, it is necessary to distinguish appearance surface, fit surface and hidden surface, and lock the standard of thread profile, pitch, starting angle and stop tooth position. Common materials include PMMA, SAN, PC, ABS and transparent AS. Transparent materials are highly sensitive to impurities, bubbles and weld lines, while black solid parts are prone to flow marks and uneven gloss. Material characteristics shall be incorporated into the molding scheme in advance.
2. Selection of Mold Steel and Surface Treatment
High mirror corrosion-resistant mold steel is preferred for the cavity. S136 and NAK80 are the most commonly used grades for cosmetic packaging. S136 is preferred for transparent parts, which can reach HRC48–52 after quenching, with stable polishing performance, resistance to corrosion by plastic additives, and less prone to rust and pitting. Thread inserts, sliders and core pins are also made of steel with good polishing performance. The thread profile is a precision forming part, and insufficient hardness will cause rapid wear, deformed thread profile and stuck screwing. Polishing shall be carried out in accordance with the mirror grade standard. Transparent parts generally reach A2 mirror or above, and orange peel and scratches shall be avoided during polishing.
3. Gating System and Gate Design
Gate position is the top priority of appearance control. Gates for cosmetic housings shall be arranged on non-visible surfaces such as bottom, inner side and hidden stop tooth area. Common gate types include submarine gate, tab gate and banana gate. Tab gate is suitable for large-area transparent housings, with stable filling and low shear heat to reduce weld lines and internal stress. Banana submarine gate can automatically break the gate and reduce manual cutting procedures, which is suitable for mass production. Hot runners are widely used in multi-cavity lipstick molds to balance filling, improve consistency of each cavity, reduce color difference and size difference, and save raw materials. Hot runner nozzles adopt pointed or side gating structure to avoid nozzle marks on the appearance surface.
4. Control of Venting and Weld Lines
Many lipstick and mascara housings are annular tubular structures, which are easy to form weld lines on the back after melt flow around. The vent depth shall be set strictly according to materials. For transparent AS and PMMA, the vent depth is generally controlled at 0.01–0.02 mm. Too deep vents will cause flash, while too shallow vents will lead to trapped gas and burning. The gate position can be adjusted to transfer the weld line to non-appearance and non-stressed areas. The vent of each cavity of multi-cavity molds shall be balanced independently, and the vent grooves are easy to accumulate carbon and need regular cleaning and maintenance.
5. Uniform Design of Cooling System
Although the wall thickness of lipstick and mascara housings is thin, the annular structure is easy to accumulate heat locally. Uneven cooling will lead to roundness deviation, warpage and inconsistent gloss. The cooling water channel shall be close to the cavity surface, and the annular cavity adopts surrounding water channels. The moving and fixed molds are controlled by independent temperature controllers. Thread inserts have poor heat dissipation, so built-in water wells or straight water channels are preferred to prevent thread deformation and sticking caused by long-term high temperature.
6. Precision Design of Thread Demolding Structure
Rotating thread demolding, rack and pinion thread demolding and motor synchronous thread demolding are commonly used for threaded caps. Synchronization error of multi-cavity molds will lead to thread pulling and tooth collapse. Thread profiles are processed by CNC turning as a whole to reduce splicing seams that may produce fine flash. A stop tooth structure is set to control the end position of rotation, and the demolding speed shall not be too fast, because high-speed demolding of transparent hard plastic parts is easy to cause scratches and whitening.
7. Ejector System and Appearance Protection
Ejector marks are common defects of cosmetic appearance parts. Sleeve ejection or annular ejector plate is adopted to disperse ejector force and avoid depression and whitening caused by single-point ejector pins. The ejector mechanism shall run stably without jitter, which will scratch the inner wall of products. Ejection shall be carried out after the product is fully cooled and shaped to prevent deformation under high temperature.
8. Trial Mold and Mass Production Control Points
During trial molding, the appearance consistency, thread screwing hand feel, size and roundness of each cavity shall be confirmed first. Transparent parts shall be checked for bubbles, yellowing and internal stress. The process window shall be stable during mass production. The cavity shall be rust-proofed, and the thread core and polished surface shall be inspected regularly for wear and scratches. Precision molding of lipstick and mascara molds is the coordinated control of appearance, matching thread and dimensional accuracy. Reasonable gate layout, steel polishing, synchronous thread demolding structure and uniform cooling can meet the high appearance and assembly requirements of cosmetic packaging.
