Product diagram of motor mounting box is shown in Figure 1. Maximum size of product is 209.48 mm * 168.40 mm * 138. 15mm, average thickness of plastic part is 2.40mm, material of plastic part is fireproof ABS, shrinkage rate is 1.005, and weight of plastic part is 235.70 grams. Technical requirements for plastic parts are that there must be no defects such as peaks, underfilling of injection molding, flow lines, pores, warpage deformation, silver streaks, cold materials, jet lines, etc.

 

 

It can be seen from Figure 1 that structure of plastic part is relatively complex, with three undercuts on outside and one undercut inside. Difficulty of mold design is design of four sliders. Other requirements for mold design are a mold life of 800,000 times, an injection cycle of 40 seconds, and an injection molding machine tonnage of 450 tons. Mold design requirements are shown in Figure 3 mold design specifications.
Size of plastic part is large, and there are four slider cores. Mold design cavity is layout 1 cavity. Mold base is a non-standard mold base 4550. Plastic material is ABS fireproof material, which is corrosive to mold core, so front and rear mold cores are made of 2316 steel and heat treated. Four corners of front and rear mold cores are positioned by tiger mouths to improve accuracy of mold clamping. Size of front and rear mold core is 250*290. Due to its large size and high height, front and rear mold cores and mold base have high precision frame matching requirements, frame is difficult to match. Therefore, squeezing block on the edge of mold core is used for positioning, and squeezing block is designed on two sides away from base angle of mold core.
Due to large size of mold, clamping force of front and rear molds is also very large, but cavity of front mold core is deeper, contact area of front and rear mold cores is small, only edge of plastic part cavity is in contact. In long-term injection molding production, parting surface will be crushed, which will block exhaust groove of parting surface and cause poor injection molding. In order to solve this problem, balance weights are designed on the four corners of mold base, and  balance weights are installed on movable mold B board, see mold design diagram 2.
Gating system of mold uses a single hot nozzle, and glue is injected at the top of plastic part. Injection method of gating system affects injection cycle. Hot runner and hot nozzle injection can significantly speed up filling process of molten plastic and effectively shorten injection cycle.

 

Outer undercut adopts a movable mold slider to pull core and is driven by an inclined guide post. Due to large thickness of A plate, inclined guide post is fixed on A plate by a pressing plate. Inner undercut uses a T-slot to drive inner sliding block to release undercut. When mold is opened, push plate first bounces up by 50, and under action of T groove, inner sliding block moves diagonally to release undercut. In second fractal, parting surface is opened, and movable mold slide block completes outer undercut ejection.
Because there is no mutual moving mold plate between fixed mold mold plates, it is difficult to design fixed mold slider core pulling. Therefore, a core-pulling mechanism in which cylinder is vertically designed in movable mold is adopted. Before mold is opened, oil cylinder drives shovel, then shovel drives slider to complete fixed mold core pulling. It is necessary to pay attention to design, shovel must be designed in guide sleeve to make core-pulling movement smooth.
Cooling of plastic parts adopts surrounding and direct water cooling. Corresponding cooling circuits are designed for movable and fixed mold and large sliding block of movable mold.
Ejector of plastic part is ejected by a cylinder and a thimble.

 

Figure 1 Product Picture of Motor Mounting Box

 

 

 

 

 

 

Figure 2 Mould diagram of motor mounting box

 

 

 
 
Figure 3 Mould design requirements for motor mounting box
 
Figure 3 Mould design specification book for motor mounting box

In a sense, injection mold is also a displacement device, that is, plastic melt enters mold cavity and simultaneously replaces air in mold cavity. In fact, air in mold is not limited to cavity, especially three-plate injection mold. Air in runner cannot be ignored. In addition, plastic melt will generate a small amount of decomposition gas. These gases must be discharged in time.

 

If mold's exhaust performance is poor, it is easy to produce bubbles, silver streaks, clouds, filling dissatisfaction, surface burn marks, intermittent injection and other defects. Therefore, mold must be equipped with a reasonable exhaust structure to avoid product defects caused by poor exhaust.

For molds for forming large and medium-sized plastic parts, amount of gas that needs to be removed is large, and exhaust groove is usually opened on the side of concave mold on parting surface. Location of exhaust slot is better at the end of melt flow, size of exhaust slot is based on principle that gas can be smoothly discharged without overflow. Width of exhaust groove is generally about 3-5mm, depth of front end of exhaust groove is less than 0.05mm, and length is generally 0.7-1.0mm. Depth dimension of common exhaust groove can be verified in <Plastic Mold Technical Manual>.

 

For small molds, parting surface gap can be used to vent, but parting surface must be at the end of melt flow.

For combined dies or cavities, split gap can be used to vent.

Use matching clearance between push rod and mold plate or core to exhaust, or intentionally increase clearance between push rod and mold plate.

Powder unsintered alloy is a material sintered with a spherical particle alloy, which has poor strength, but has a loose texture and allows gas to pass through. Placing a piece of such alloy in part that needs to be vented can meet requirements of venting, but diameter of bottom vent hole should not be too large to prevent it from being squeezed and deformed by pressure of cavity.

On the outside of plastic melt confluence, a cavity is provided to allow gas to be discharged into it, and a good exhaust effect can also be obtained.

Set exhaust rod in part where gas is enclosed. This method has good exhaust effect but will leave traces of rod on plastic part, so exhaust rod should be located in a hidden place of plastic part.

1. When exhaust is extremely difficult, inlay structure is used. If dead corners of mold are not easy to open exhaust groove, first of all, mold should be appropriately changed to inlay processing without affecting appearance and accuracy of product. This will not only help processing exhaust but also improve original processing difficulty and facilitate maintenance
2. In closed shape of plastic product, a thimble must be added to exhaust gas, prevent burns and melting marks.
3. For product molds with complex geometric shapes, it is best to determine opening of exhaust slot after several mold trials.
4. The thinner product, the farther away from gate, opening of exhaust groove is particularly important.
5. For small parts or precision parts, we should also pay attention to opening of exhaust groove, because it can avoid surface burns and insufficient injection volume of product, and can also eliminate various defects of product.

 

Properly opening exhaust groove can greatly reduce injection pressure, injection time, holding time and clamping pressure, so that molding of plastic parts is changed from difficult to easy, thereby improving production efficiency, reducing production costs, and reducing machine energy consumption.