Injection Mold For Turnover Box

TheTurnover Box is a large deep cavity thin-walled shell component. The main raw materials are: PP and HDPE. Product features: large flat surface, densely distributed reinforcing ribs, handle / stacking snap catch, prone to warping, and the rib positions are extremely prone to trapping air. The basic mold forms are mostly single cavity and four-sided slider core-pulling type.

 

I. Main Mold Overall Structure Scheme

 

1. Parting Scheme

The parting surface is set at the maximum contour of the upper opening of the box body;

· Side wall handles and stacking limit clamps of the box body → Four-sided slider core pulling (the fixed mold slider is the most commonly used type)

· Inverted structure inside the box body (internal slot) → Inclined roof mechanism

 

2. Two routes for core-pulling drive

1) Inclined guide pillar slider: Low cost, commonly used for small and medium-sized boxes; prone to wear during high-speed production, with limited core-pulling force

2) Cylinder-driven four-sided slider: Preferred for large-sized turnover boxes, stable core-pulling, controllable stroke, suitable for automated production lines; the slider must have an independent cooling water system inside

 

3. Ejection system (the most crucial point, preventing white marks on the top and bottom depression)

Three mature solutions:

1. Push plate (top plate) ejection [Preferred]: Uniform force, large flat surface is less likely to be damaged; suitable for standard logistics turnover boxes

2. Push plate + pin combination ejection:增设 pins at the intersection of ribs, taking into account both the frame and the bottom reinforcing ribs

3. Secondary ejection structure： For ultra-high-depth boxes and products with a small demolding slope, pre-demolding first, then complete ejection to avoid damaging the box body

Design points： The pins avoid the appearance surface; symmetrically arranged; reserve air-pushing auxiliary ejection to reduce vacuum adhesion of the mold.

 

 

II. Pouring System (Comparison of Pouring Orifices)

 

scheme	applicable scene	relative merits
Bottom fan-shaped side gate (cold runner)	Standard shipping boxes, with limited budget	The melt is filled uniformly, reducing bottom shrinkage; manual cutting of the gate is required, resulting in waste of the handle part of the product.
Side-wall recessed gate	High appearance requirements and automated production lines	Automatic stop casting, no casting nozzle marks; not suitable for extremely large boxes; the process is long and prone to material shortage.
Hot runner multi-point injection (needle valve type)	Large Turnover boxes, high output, pursuit of short cycle	Balanced filling, weakened fusion marks, no handle; the cost of the mold has increased.

 

Key reminder: For large components, a mold flow analysis must be conducted to predict the location of weld marks and areas prone to gas trapping. Try to position the weld marks in non-load-bearing and non-visual areas.

 

 

III. Cooling System (Determining Warpage and Molding Cycle)

 

The biggest pain point of theTurnoverbox: uneven cooling → bottom arching and deformation of the four sides.

1. Moving core model: using a water-blocking plate / spray-flow waterway, achieving full cooling inside the deep cavity;

2. Fixed model cavity, four-sided sliders: each has an independent cooling circuit, temperature control is zone-based;

3. The waterway is 20-30mm away from the surface of the cavity, and the waterways are denser in the corners and rib areas;

4. The temperature difference between the inlet and outlet water should be ≤ 5℃; under ideal conditions, the molding cycle can be shortened by 20% - 30%.

 

 

IV. Exhaust system (solutions for the frequent defects of theTurnoverbox)

 

Common problem: Accumulation of gas at the end of the reinforcement ribs → Burnt edges, bubbles, poor bonding strength

1. Open vent slots around the parting surface, at the corners, and at the locations of the weld marks: depth 0.02 - 0.05mm, width 6 - 15mm;

2. Use inserts to seal the vent at the ends of all reinforcement ribs and the dead corners of the snap-in parts (0.015 - 0.025mm);

3. Reserve venting gaps at the sliding block mating surface and the inclined top guiding surface; for extremely gas-accumulated areas, breathable steel can be embedded.

 

 

V. Selection of Mold Materials (PP/HDPE without corrosion, with emphasis on wear resistance)

 

1. Economy type (within 100,000 mold cycles) Mold cavity / core: P20 / 1.2311 (HRC28-32); Mold base S50C

2. Mass production general model (mainly recommended) Mold cavity / core: 718H / 1.2738 (HRC33-38), with good polishing and etching performance, the service life is 40% higher than P20, and the HuangyanTurnover box mold is the most commonly used

3. Ultra-long-term mass production, with slight glass fiber filling Mold cavity undergoes nitriding treatment; the sliding block wear-resistant block uses copper alloy / hard alloy;

Food-gradeTurnover box, if rust prevention is required, can choose S136H, the cost will increase significantly, and ordinary logistics boxes generally do not use

 

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