At present, plastics industry has achieved rapid development in China, plastic molding technology is developing in direction of "precision, thinness, lightness and smallness". Cost of plastic usually accounts for a larger proportion of cost of product, and thinning of product can help reduce ratio. Due to miniaturization and portability of consumer electronic devices such as mobile phones, MP3 players, digital cameras, and palmtop computers, many plastic products used in above-mentioned devices to support light, sound and electricity are also required to be thinner, smaller and lighter.
These products have different shapes, small size, short product life, and fast replacement. Therefore, quality requirements for thin-walled products are getting higher and higher. However, thin-walled plastic products will bring many problems to molding, such as insufficient filling, warping deformation, etc. At the same time, in order to make plastic melt fill cavity in the shortest time during molding of thin-walled products, higher injection pressure and speed are required, requirements for molds and injection molding machines are also higher. Therefore, injection mold industry urgently needs a new device that can solve defect of thin-walled products.

At present, there is no unified definition for thin-wall molding. Generally speaking, thickness of molded product is less than 1mm or flow length to thickness ratio (that is, ratio of flow length from melt entering mold to the farthest point of filled cavity to corresponding average wall thickness) is more than 100 can be called thin-wall molding.
Thin-wall molding technology generally includes molding injection molding machine technology, molding die design technology, molded product design technology, and molding material selection.
Thin-walled injection molding injection molding machines have high requirements for melt filling time, pressure and speed control, and melt injection volume. Therefore, mechanical equipment manufacturers and research institutions need to cooperate to develop special injection equipment. When forming thin-walled products, special molds are generally required. Compared with injection molds for conventional products, mold structure, gating system, cooling system, exhaust system, and demolding mechanism of thin-walled products have changed. Therefore, special Injection moulds for thin-walled products are also more complicated; while products and their materials mainly consider rigidity, impact resistance and thermal stability.
In order to form high-quality thin-walled products, problems such as insufficient filling, warping and welding lines should be avoided as much as possible. If above problems are considered, it will lead to higher production costs and complex structures. A thin-walled partial molding device is now proposed, that is, through a simple structure, core at partial thin wall of molded product is moved according to melt filling condition, and volume of cavity is adjusted after melt injection is completed, thereby solving problem of thin-walled products. Defects of injection molding reduce requirements of production process on molds and injection molding machines.

Local injection mold molding device has a clever structure, which is mainly composed of a hydraulic cylinder mechanism 1, a core positioning mechanism 2, a limit mechanism 3, a core 4 and a mold base 5. Structure is shown in Figure 1.

 

(A) Overall structure

 

(B) Cut-away top view structure

Figure 1 Injection molding device for partial thin-walled products

 

(B) Partial enlargement of limit mechanism
1. Hydraulic cylinder mechanism 2. Core positioning mechanism 3. Limit mechanism 4. Core 5. Mould base 6. Hydraulic cylinder body 7. Piston 8. Retaining ring 9. Guide rail 10. Slider 11. Spacer 12. Type Core limit block 13. Baffle 14. Connecting block 15. Seal ring 16. Positioning block 17. Mounting frame 18. J-type copper sheet 19. First limit contact 20. Second limit contact
Working principle of forming device: hydraulic cylinder body 6 of hydraulic cylinder mechanism 1 is fixed on mold base 5 by screws, hydraulic cylinder body 6 is provided with a retaining ring 8 and a sealing ring 9 to ensure that oil in hydraulic cylinder body 6 does not leak; 7 is installed in hydraulic cylinder body 6, one end of connecting block 11 is fixed on piston 7 by threads, and the other end is connected to slider 12 of core positioning mechanism 2 through an I-shaped structure; guide rail 13 of core positioning mechanism 2 is fixed on mold base 5 by screws, spacer block 15 and core limit block 16 are fixed on slider 12 by screws. Slider 12 can move along guide rail 13 under action of piston 7; J-shaped copper sheet 18 in limit mechanism 3 is fixed on slider 12 by screws, and mounting frame 17 is fixed on mold base 5 by screws. Position of first limit contact 19 and second limit contact 20 are adjustable. They can move along sliding groove of mounting frame 17 and are fixed on mounting frame 17 by nuts; core 4 is installed at one end of slider 12, is set in space between spacer block 15 and core limit block 16, The three cooperate with each other through surface contact. Lateral displacement is limited by mold base 5, and longitudinal movement can be moved back and forth under action of sliding block 12.

 

(A) Position of molding device before melt injection

 

(B) Position of molding device after melt injection

Figure 2 Position of molding device

4. Core 15. Spacer 16. Core limit block
Before melt injection, relative positions of core 4, spacer block 11 and core limit block 16 are shown in Figure 2(a). At this time, J-shaped copper sheet 18 is in contact with first limit contact 19, inclined surface of core 4 matches inclined surface of spacer block 11, height inside cavity is relatively large, 1.5 mm.
After melt is injected, hydraulic cylinder mechanism 1 drives sliding block 10 to move forward until J-shaped copper sheet 18 contacts second limit contact 20, hydraulic cylinder mechanism 1 stops working, and sliding block 10 stops moving; spacer 11 and core limit block 12 are fixed on slider 10 by screws and move accordingly. When slider 10 stops moving, relative positions of core 4, spacer block 11 and core limit block 12 are shown in Figure 2(b). At this time, horizontal plane of core 4 is matched with horizontal plane of cushion block 11, height inside cavity is relatively small, 0.5 mm, which realizes effect of forming a local thin-walled product.
After one injection molding cycle is completed, hydraulic cylinder mechanism 1 drives slider back until J-shaped copper sheet 18 contacts first limit contact 19, hydraulic cylinder mechanism 1 stops working, slider 10 stops retreating, inclined surface of core 4 matches inclined surface of cushion block 11, and inner height of cavity is restored to state before melt injection.
First limit contact 19 and second limit contact 20 can move along chute of mounting frame 17, and are fixed on mounting frame 17 by nuts. Positions of the two determine position of core 4 before and after melt injection, thereby determining inner height of cavity corresponding to thin wall of product. Adjusting installation positions of first limit contact 19 and second limit contact 20 can achieve effect of adjusting local wall thickness of molded product.
Local thin-wall molding device of injection mold can move core 4 after melt completely fills cavity to reduce internal height of cavity, and form local thin-walled products, which solves problem of insufficient melt filling at thin wall during molding process.