jection molding process is a complex process involving many factors, such as material, mold, injection machine, molding process and so on. The defects of injection molding products are inevitable, so it is very important to find the inherent mechanism of defects and predict the location and types of defects, which can be used to guide mold design and formulate more reasonable process operation conditions. In this article, we discuss classification of injection molding defects and cycle time minimization of injection molding process & how to reduce cycle time in injection moulding.
1. Injection molding and its working process
Injection molding machine is mainly composed of clamping system, injection system, heating and cooling system, hydraulic system, lubrication system, electric control system, safety protection and monitoring system. The injection cycle is mainly composed of closing the mold, moving the injection seat forward, maintaining the injection pressure, pre plastic metering, cooling, opening the mold and ejecting the product.
2. Typical defects of injection molding
Generally speaking, there are three aspects to evaluate the performance of plastic products:
– Appearance quality, including integrity, color, luster, etc;
– The accuracy between size and relative position is the accuracy of size and position;
– The mechanical properties, chemical properties and electrical properties corresponding to the application, i.e. functionality.
3. Classification of injection molding defects
According to the above three evaluation criteria, the common defects of injection molded parts can be divided into two types:
– Appearance problems: Including under injection, flash, filling imbalance, shrinkage mark, shrinkage hole, weld mark, wave current mark, spray mark, gate halo, coke mark, bubble, craze, color difference, whitening, cracking, surface floating fiber, warpage, etc.
– Performance problems: embrittlement, residual stress, dimensional instability, overweight and underweight, i.e. poor weight repetition accuracy, etc.
On the premise of ensuring product quality, the shorter the cycle time is, the better the quality. The cycle time includes the following items: injection time, pressure holding time, melt time, cooling time, ejection time, low pressure time of mold locking, and even the time factor related to the opening and locking of the mold, and the speed of ejection.
1. Injection time
Generally speaking, the longer the injection time is, the more saturated the product will be. However, when we stress the quality, we also need to consider the production capacity. Moreover, too long injection time may lead to a series of problems such as too full product and too large sticking die ejection size. Therefore, we should consider comprehensively when setting the injection time and try to reduce the injection time when meeting the quality requirements.
2. Glue melting time
The length depends on the setting speed of the glue melting speed and the setting size of the back pressure, but there is one thing, the length of the glue melting time control must be shorter than the cooling time.
3. Cooling time
The length of cooling time directly affects the injection molding cycle. The longer the cooling time is, the longer the molding time will be and the lower the production capacity will be. Therefore, when we set the high-pressure cooling time, as long as we can ensure the smooth molding of the product and will not directly affect the deformation and other problems, the shorter the setting time is, the better.
4. Ejection time
The ejection time is generally suitable to cooperate with the manipulator. When the manipulator is used in full-automatic production, the ejection residence time is generally maintained at 1.5-2 seconds. In semi-automatic production, if the product can not be removed due to the falling or clamping of the thimble due to the return of the thimble, the residence time should be maintained at about 5 seconds.
5. Low voltage protection time
To protect our personal safety, mold safety plays a great role, with good mold low pressure position and low pressure adjustment, low pressure protection time should take 1-3 seconds, the shorter the protection time, the less harm may be caused.