Accessory products of driving recorder are shown in Figure 1. Maximum dimensions of product are 97.80 mm * 37.80 mm * 74.00 mm; average thickness of plastic part is 1.50 mm, material of plastic part is ABS, shrinkage rate is 1.005, and weight of plastic part is 40.60 grams. Technical requirements for plastic parts are that there should be no defects such as peaking, underfilling, flow lines, pores, warping deformation, silver streaks, cold material, and jetting lines, and meet ROSH environmental protection requirements.

 

Figure 1 Product map of driving recorder accessories

 

Figure 2 3D mold structure diagram
It can be seen from Figure 1 that plastic part is a tall flat shell with a lock on one side. Parting surface here needs to be made with a step difference to facilitate insertion of front and rear mold cores. This is difficulty of this set of mold design. Another difficulty in mold design is ejection of plastic part. Plastic part is a closed shell with a large ejection force, which requires sufficient ejection force to make plastic part demould smoothly.

 

Figure 3 Die parting surface diagram
When designing mold, it is first necessary to analyze plastic part according to principle of three elements. Three elements that determine mold structure are design of parting surface, design of runner and gate, and ejection method of plastic part. For any set of molds, after three elements are determined, mold structure is basically determined. Parting surface of this set of mold plastic parts is a plane, which is the simplest kind of parting surface. The only difficulty is design of step difference at parting surface. Step difference here is not shown in customer's product drawing. It needs to be analyzed and added by mold design itself, and confirmed by customer.

 

Figure 4 Explosion diagram of mold
Cavity design of mold is 4 cavities, gating system is a point gate, and top surface of plastic part is glued in one point, as shown in Figure 1. Due to high cylinder of plastic part, it is easy to trap air. Cutting insert from the bottom is ideal mold design for this deep cavity barrel. See bottom plate of front mold cavity in Figure 2, and fix it directly on through frame of plate A. This insert has two advantages, the first is easy to exhaust, and the second is convenient for cavity processing. When deep cylinder cavity is processed, bottom is limited by tool, it is difficult to CNC process it in place at one time, and it is easy to deposit carbon when sparking, so it is necessary to cut inserts to solve processing problem.
Mold base is DCH4050, straight body mold base, three-plate mold, point gate. Plastic has a great tightness on rear mold. Therefore, ejector mechanism is designed with ejector pin ejection and push block ejection. Position of push block is shown in Figure 4. Three push blocks are E1A, E2A and E2B. Bottom of small push block is ejected with a push rod. At this time, push rod should be reliably connected to prevent it from rotating. E1A is a large push block, which is ejected with two push rods.

 

Figure 5 Die structure diagram

 

Figure 6 Injection molding conditions
Mold is basically a heat exchanger. It continuously takes away heat of melting through cold water channel, and a properly designed mold can improve efficiency of heat exchange.
However, ice water cooling can shorten "cooling time" when allowed. If mold is cooled by ice water, dew point can be lowered by drying fan and sealed mold clamping device to prevent condensation. The second is cooling of mold, that is, heat exchange system, which is the most important factor affecting injection molding cycle. Customer survey and various complaints show that molds of many companies are not doing well enough in the important part of cooling system, which reflects weakness of design force. Because a well-designed cooling system can not only ensure that product is cooled evenly and quickly in mold, but also greatly shorten injection molding cycle, and it is easier to ensure that product with stable dimensions and qualified quality is obtained. Cooling time is the most critical control point in the entire injection molding cycle.
In this increasingly competitive business society, improving production efficiency is a matter of great concern. If diameter of cold runner is larger than thickness of finished product, cooling time has to wait for runner to cool to a certain extent before opening mold, but finished product has already cooled, resulting in waste. Runner of hot runner mold does not need to be cooled, and finished product determines cooling time. Amount of plastic in cold runner is a percentage of finished plastic, and some are even heavier than weight of finished product, so injection and feeding time will be longer. Use of hot runner molds saves injection and feeding time required for runner plastics. There is a nozzle in cold runner, and mold opening stroke needs to be increased. During multi-cavity injection molding, cold runner does not guarantee whether finished product will fall or not. It needs to be taken out by a mechanical arm (automatic operation) or manually (semi-automatic operation), and cycle will be slowed down.
Cooling system and cooling cycle of mold will gradually improve as number of mold trials increases. Haitian 200-ton injection machine was used for the first trial mold of this set of molds, and injection molding conditions are shown in Figure 6.