What are the effects of temperature on injection molding prototyping molds, why does temperature affect the molding of injection molds, and how to properly select the temperature during mold forming? This article explains the effects of temperature on mold forming and several principles for temperature control. Let's first look at the impact of temperature on injection molding prototyping molds.
Mold temperature is a very important variable during the injection molding prototyping process. When the mold temperature is too low, it will reduce the fluidity of the melt, and the injection may be incomplete; the mold temperature affects the crystallinity of the plastic. For ABS, if the mold temperature is too low, the product surface will have a low gloss. Compared with fillers, plastics are more likely to migrate to the surface at high temperatures. Therefore, when the injection mold temperature is high, the plastic component is closer to the surface of the injection mold, and the filling is better, and the brightness and gloss are higher. However, the injection mold temperature cannot be too high, as it can cause sticking and obvious bright spots in certain areas of the plastic part. If the injection mold temperature is too low, it can also cause the plastic part to be too tight to the mold, and the part is easily damaged during demolding, especially the pattern on the surface of the part. For each type of plastic and plastic part, exceeding a certain surface temperature of the mold will result in one or more adverse effects.
On the other hand, when the mold temperature is too high, the melt will undergo thermal decomposition, and the shrinkage rate of the product will increase after it is made and exposed to the air. If the part size increases when the mold is used under low temperature conditions, it is generally caused by a low mold surface temperature. This is because the lower mold temperature speeds up the "frozen orientation" of the molecules, increases the thickness of the frozen layer of the melt in the mold cavity, and the low mold temperature hinders the growth of crystal formation, thereby reducing the forming shrinkage rate of the product. Conversely, if the mold temperature is high, the melt cools slowly, the relaxation time is long, the orientation is low, and it is beneficial to the formation of crystals, resulting in a larger actual shrinkage rate of the product.
As the hotter mold cavity does not freeze the plastic that enters the overflow edge area before high pressure is formed, the melt can overflow to the gap between the parting lines around the ejector pin. This indicates that good injection rate control is needed, and some modern flow control programmers can indeed achieve this. Therefore, a higher mold temperature means lower flow resistance, which means faster flow through the gate and mold cavity. Because the injection molding prototyping flow control valve used does not correct this change, faster filling can cause higher effective pressure in the gate and mold cavity, which may cause overflow flashes. This is because the hotter mold cavity does not freeze the plastic that enters the overflow edge area before high pressure is formed, and the melt can overflow around the ejector pin and overflow into the gap between the parting lines.
In order to ensure the production of high-quality plastic parts with high appearance quality requirements, stable dimensions, and minimal deformation in an effective time, the basic principles of mold temperature control should be clearly understood during design.
Different rubber materials require different mold temperatures. Different surface qualities and different structure molds require different mold temperatures, which requires targeted temperature control system design.
The mold temperature should be balanced without local overheating or overcooling. If the temperature of the front mold is higher than that of the rear mold, the temperature difference is generally about 2~3oC.
The front mold temperature with flame pattern requirements is higher than the front mold temperature required for general smooth surfaces. When hot water or hot oil needs to be passed through the current mold, the temperature difference is generally about 40oC.
When the actual injection molding prototyping mold temperature cannot reach the required mold temperature, the mold should be heated, so when designing the mold, the heat brought into the mold by the rubber material should be fully considered to meet the mold temperature requirements.
The heat brought into the mold by the rubber material, except for being consumed by thermal radiation and conduction, most of the heat needs to be taken out of the mold by circulating heat transfer medium.