- 上一篇：模具热处理英文文献和中文翻译
- 下一篇：机械结构英文文献和中文翻译
Polymer left in the barrel or hot runner system may often be pressurized. Care should be taken whenever resin flow is interrupted due to blockage or mechanical problems.
If the molding machine will be shut down for an extended period of time (30 minutes or longer), lower the heats, purge the machine or cycle it until the lower temperature is reached before shutting it down, leaving the screw full of resin and in the retracted position.
Table 4. Suggested start-up conditions (based on general purpose melt index/flow rate products)
Electricity
Molding machines utilize high electrical voltage and have warning signs pointing out electrical hazards; do not ignore these signs. Keep water away from these areas. A periodic inspection of all electrical devices and connections for wear, looseness, etc. is very important.
Machinery motion
There is considerable mechanical motion during the injection molding process. Neckties and loose fitting clothing should not be worn around molding machines since these can be caught by the equipment movement and lead to physical injury. Do not reach around, under, through, or over guards while the equipment is operating.
Be sure all people working near the injection molding machine know where the Emergency Shut Off button is located. Never disengage any of the safety mechanisms or interlocks on the injection molding machine.
Some machines store energy (hydraulic, pneumatic, electrical, or gravitational) which can be present even when the machine is turned off. Consult the manufacturer’s operating guide for methods to properly de-energize the equipment. As with any piece of potentially hazardous equipment, a suitable lock-out/tag-out procedure should be implemented and enforced.
The injection molding process and its effect on part performance
The molding cycle
As detailed in the section on the Injection Molding Process, there are several steps in the production of injection molded parts. In most cases, the injection molding cycle begins with the mold open, ejector pins/slides retracted, and the screw/ ram ready with the next shot of material. The cycle then proceeds as follows:
1. MOLD CLOSE: Mold closes and clamp develops full closing pressure
2. INJECTION: Material is injected into the mold cavity
3. PACKING: Material is packed into the mold to fill out the part
4. PLASTICATION: Screw begins to rotate (or ram retracts) to develop the next shot of material
5. COOLING: Coinciding with the start of plastication, the cooling cycle begins (Note: Since coolant continuously circulates through the mold, cooling technically starts as soon as the melt contacts the cavity during injection)
6. MOLD OPEN: Mold opens, slides retract, ejector pins activate, part(s) are ejected.
The length of each of these steps will depend on the complexity of the mold, the size of the machine, and the geometry and end-use requirements of the part. A typical cycle for a four cavity, 16 oz. stadium cup can be found in (Figure 34) while one for a single cavity, 12 lb. bumper fascia can be found in(Figure 35).
Regardless of part size, weight, or mold complexity, nearly half of every injection cycle is spent cooling the part(s) to a temperature sufficient to allow ejection without postmold distortion. Factors that affect the cooling rate of the part(s) will be covered later.
In almost all cases, part quality is the result of steps 2-5. The highest quality parts begin with a homogeneous plastic melt in terms of temperature and composition.
Therefore, the quality of the next part to be produced is the result of the development of the shot during the current cycle. Shot size should be sufficient to produce a cushion of material at the end of the step 3 of 0.1-0.5 in. (2-13 mm). This cushion will keep the screw from ‘bottoming out’ and help maintain plastic pressure within the cavity.
Achieving a homogeneous melt is controlled by many factors including: screw design, screw speed, screw & barrel wear, back pressure, shot to barrel capacity ratio, soak time, and heater band settings.