In plastic injection molding, injection mold is not only a molding equipment for plastic melt, but also acts as a heat exchanger. Mold temperature adjustment system directly affects quality and production efficiency of product. Due to different performance and molding process requirements of various plastics, requirements for mold temperature are also different. In particular, some injection molded parts are made of materials such as polycarbonate (PC), polymethyl methacrylate (PMMA) and glass fiber (glass fiber), and surface quality of product is strict.

Schematic diagram of mold temperature machine is shown in figure

 

Mold temperature machine structure diagram

Taking oil temperature machine as an example, heat flow outlet [1] is connected to mold, and after circulating in mold, it goes from mold to oil tank [13] through heat flow return port [2]; temperature sensor [14] measures temperature of thermal fluid and transmits data to local controller; controller [11] regulates temperature of thermal fluid, thereby indirectly regulating temperature of mold. Inflow heater connector [8] and oil float body [10] in figure are liquid level control system in mold temperature machine. When oil amount is sufficient, oil float body [10] floats up, liquid level control switch [9] is turned on, oil flows into heating and cooling system [7] through inflow heater connector [8] to work. If it is a water temperature machine, liquid level is controlled by measuring water pressure, water pressure line, etc.
If mold temperature machine is in normal production and mold temperature exceeds set value of controller, controller will open solenoid valve and connect water inlet pipe until temperature of hot fluid, that is, temperature of mold, returns to set value. If mold temperature is lower than set value, controller will turn on heater. Water temperature cooling of water temperature machine is direct water cooling. Oil temperature and oil temperature cooling is to heat oil temperature in cooling system 7 by cooling water intake. Mold warmers with open tank heating are suitable for water or oil warmers.

 

Type of mold temperature machine
According to use of heat transfer fluid (water or diversion oil), Oneng's mold temperature machines are divided into water temperature machines (usually outlet temperature is up to 120℃) and oil temperature machines (usually maximum outlet temperature is 180℃-300℃). Mold temperature machine can basically keep temperature of heat transfer fluid constant at 1℃, and temperature control accuracy of mold temperature machine should be determined according to quality requirements of injection molded parts.

Mold temperature is one of the most basic control parameters in injection molding process. After plastic is filled, it is cooled and hardened in mold cavity to obtain desired shape. Mold temperature affects flow behavior of plastic melt during filling and affects performance of plastic product. Mold temperature actually determines cooling rate of plastic melt. Mold temperature is usually controlled by liquid medium, which determines cooling rate of part.
Effect of mold temperature on appearance of product: Higher temperature can improve fluidity of resin, which usually makes surface of part smooth and shiny, improves appearance of part, and also improves strength and appearance of fusion line.
Influence of mold temperature on internal stress of product: formation of internal stress is basically caused by different thermal shrinkage rates during cooling. When product is formed, its cooling is gradually extended from surface to inside, surface first shrinks and hardens, then gradually goes to inside, during this process, internal stress is generated due to speed of shrinkage. When residual internal stress in plastic part is higher than elastic limit of resin, or under erosion of a certain chemical environment, cracks will occur on the surface of plastic part. Mold temperature is the most basic condition to control internal stress. A slight change in mold temperature will greatly change its residual internal stress. Generally speaking, each product and resin has its lowest mold temperature limit for acceptable internal stress. When forming thin walls or long flow distances, mold temperature should be higher than minimum during general forming.
Mold temperature to improve product warpage: Unreasonable design of cooling system of mold or improper control of mold temperature, insufficient cooling of plastic parts, and structure of plastic parts will cause plastic parts to warp and deform. With control of mold temperature, temperature difference between fixed mold and movable mold, mold core and mold wall, mold wall and insert should be determined according to structural characteristics of product. It is more inclined to bend in direction of traction on the side with higher temperature to offset difference in orientation shrinkage, prevent plastic parts from warping and deforming according to orientation law.
Mold temperature affects molding shrinkage rate of product: low mold temperature accelerates "freezing orientation" of molecules, which increases thickness of frozen layer of melt in mold cavity, and at the same time, low mold temperature hinders growth of crystallization, thereby reducing molding shrinkage rate of product. On the contrary, when mold temperature is high, melt cooling is slow, relaxation time is long, orientation level is low, and at the same time, it is beneficial to crystallization, and actual shrinkage rate of product is large.
Mold temperature affects thermal deformation temperature of product: for crystalline plastics, product is formed at a lower mold temperature, molecular orientation and crystallization are instantly frozen. Process of alignment and crystallization allows product to deform even well below heat distortion temperature of material.

A certain car headlight lens is made of PC. PC has advantages of good light transmittance, refractive index, etc. and good mechanical properties, is the first choice material for headlight spectroscopes. Surface quality of optical lens is high, there are no defects such as yellowing, welding lines, silver wires, bubbles, vibration lines, haze and white smoke. Plastic parts are also coated to increase surface weather resistance and hardness. It is relatively large and shape is in a special-shaped state, which brings certain difficulties to molding. PC material is an amorphous plastic, its melt viscosity is much larger than that of nylon and polystyrene, which has an impact on injection molding.
Because flow length decreases with increase of viscosity, its flow characteristics are close to that of Newtonian fluid. Melt viscosity is less affected by shear rate and is very sensitive to change of temperature. Therefore, in injection molding process, it is more effective to reduce viscosity by increasing temperature than increasing pressure. General use temperature is 290-310℃, and temperature is lower than 280℃, melt viscosity is large, fluidity is small, mold filling is insufficient and surface is dull; if it exceeds 310℃, overheating decomposition will make color of plastic part darker, there will be black spots and other defects, mechanical properties decline, even become brittle and scrapped. Without a mold temperature machine, product cannot be formed by its own material temperature, so mold temperature machine is used to control mold temperature. Usually, it is roughly proportional to difference between material temperature and mold temperature during cooling, and mold temperature should be as high as possible. PC products can be demolded smoothly at a mold temperature of 140℃, and mold temperature is generally maintained at 85 to 140℃. When mold temperature is too high, cooling of plastic part is slow, molding cycle is prolonged, and it is easy to stick to mold, which causes part to deform during demoulding process.

 

Car light mirror
Mold temperature control is very important to determine degree of filling, appearance, and residual stress of final part. Use a mold temperature machine to control mold temperature when molding PC. Higher mold temperatures tend to produce good flow, higher weld line strength, and less internal product stress, but molding cycle will be extended. If mold temperature is relatively low, it will cause high internal stress and damage optimal performance of part.
After practice, three mold temperature machines are used for control. According to material fluidity, workpiece surface and cycle cycle, better results can be obtained when mold temperature is about 140℃. Fixed mold core and movable mold core are controlled by a mold temperature machine at 80-110℃. Temperature is directly related to surface of plastic part. High mold temperature makes surface of plastic part shiny and stress is small. Fixed mold mold plate and movable mold plate are at (60 ± 5) ℃, and are specially controlled by one mold temperature machine. There is a protective barrier on the outside of movable and stationary mold cores to maintain temperature of stationary mold and movable mold core. Temperature of barrel is 285～310℃, and plastic parts can meet quality requirements.

In addition to good mechanical properties, PMMA has excellent weather resistance. It has been exposed to sun for many years in a tropical climate, its transparency and color change are small. After accelerated aging for 240 h, light transmittance is still 92%, and it only drops to 89% after 10 years of outdoor use. PMMA is an amorphous material, and a mold temperature machine is used to control mold temperature in injection molding.

Nylon 66 is widely used in electromechanical industry for its superior mechanical properties, heat resistance, wear resistance, self-lubrication, low temperature resistance, flame retardant and other properties. In order to improve mechanical properties of nylon 66, various modifiers are added, and glass fiber is one of them. Glass fiber is used for reinforcement and toughening of various high-performance engineering plastics due to its high cost performance. Nylon 66/GF30 is a product obtained by blending, extruding, cooling and pelletizing nylon 66 and 30% (mass fraction) glass fiber. Figure 3 is a kind of instrument cover. It is made of nylon 66/GF30, outer dimension is φ140.0 mm, height is 40.0 mm, and average thickness is 5.0 mm. It is mainly used for products with strength, toughness, flame retardant and other performance requirements, and it is also appearance part of product, which requires high surface smoothness. But in actual production, material is dried and injection temperature is 265-270 ℃, and all fibers of product are floating on the surface, which is relatively rough and loose.
After nylon 66 is added with glass fiber, length, dispersion, content, coupling agent, lubricant and additives of glass fiber will interact with matrix, factors such as process conditions will affect surface quality of plastic parts. Among them, if blending temperature is too low, combination of glass fiber and matrix will be poor, and coating effect will be poor; if temperature is too high, nylon 66 will be degraded, which will affect performance of product, which is reason for floating of glass fiber. After many tests, mold temperature machine is used to control fixed mold temperature of 80 to 100℃, sometimes 110℃, and moving mold is connected to cooling water to maintain temperature of 30 to 40℃. Pressure and speed were adjusted accordingly, cooling time was appropriately extended by 6-10 s, nylon 66 resin on the surface of plastic parts was more, corresponding fibers on the surface were less or went to inside of cover, and surface quality of plastic parts was good.
Glass fiber is irregular in nylon 66, its size and length are inconsistent, while fiber flow in melt does not change with change of temperature. Therefore, higher temperature can improve fluidity of resin, which usually makes production process more difficult. Surface of parts is smooth and shiny, especially to improve surface beauty of glass fiber reinforced resin parts. It also improves strength and appearance of weld line. Similar to other materials containing glass fiber, mold temperature can also be used to improve surface quality of plastic parts.

PP is a non-polar crystalline polymer, and mold temperature is the most important process parameter for injection molding crystalline polymers, which determines crystallization conditions. Near melting point, its volume changes greatly due to thermal shrinkage and crystallization. Progress of crystallization and increase of molding shrinkage are reasons for generation of bubbles, non-uniformity of shrinkage in all directions and internal stress caused by molecular orientation during mold filling are culprits of low strength and easy deformation of formed product. The thicker wall of product, the lower resin temperature, the higher mold temperature, the greater molding shrinkage. In order to reduce product shrinkage and deformation, lowering mold temperature is a way to release the most heat. In the past, cooling water was used for cooling. In summer (temperature is above 38℃), environment is very hot, and heat dissipation of mold is very poor. Only by extending cooling time to ensure production of plastic parts, benefit is very low.
A chiller is used to cool mold, and temperature of mold is controlled in the range of 50 to 60 ℃ to reduce influence of stress, so that water temperature of chiller is controlled at about 10 ℃, and cooling time is reduced from previous 100 s to 70 s. Cooling time is greatly reduced, product quality is guaranteed, and production cycle is shortened.

 

Air-cooled chiller
Chiller is an extension of mold temperature machine. Basic principle of chiller is to compress refrigerant (Freon) through compressor, reach condenser for heat exchange (heat absorption), then reach evaporator for heat exchange (heat release), and finally return to compressor. Use this circulating chiller to achieve effect of cooling and cooling. Chiller has a cold water tank. Cooled water is pressed by water pump to mold and then returned to water tank. After water temperature rises, compressor works to cool down and circulate in turn, and output water temperature is lower.

Modern production technology adopts mold temperature machine, and mold temperature is controlled by closed-circuit cold unloading medium. In injection molding process, mold temperature has a decisive effect on product quality and molding cycle of desired part. Function of mold temperature machine is to heat (cool) mold and maintain a certain working temperature to ensure stable quality of injection molded parts and optimize processing time.
Materials used for processing plastic parts are extremely wide, such as modified materials and alloy materials. Quality of parts processed by some materials needs to be improved urgently. Means is to solve material fluidity and crystallinity of product, solve problems of product shrinkage and deformation, improve quality of its products, and optimize process. This shows that mold temperature machine still has great potential to be tapped in injection molding process.