Pressure provided by injection molding machine's pressure system (oil pump) or servo motor is mainly used in various action procedures such as injection, melting, mold opening/closing, ejection, injection unit, and core pulling. After inputting relevant parameters on injection molding machine's control panel, processor converts them into signals for each program's action, thereby controlling pressure required for each action procedure.
Principle for setting pressure is: corresponding force to overcome resistance of action, but parameter value needs to be adjusted accordingly to match speed of action.

 

Speed (flow rate of system hydraulic oil) required to complete each action procedure in conjunction with above pressure. Basic speed levels are: Slow 0.1-10, Medium 11-30, Medium 31-60, High 61-99.
1. Injection speed control involves setting different values for different product structures and materials. To avoid confusion, we won't differentiate between (engineering/general-purpose plastics, crystalline/amorphous plastics, high-temperature/low-temperature plastics, soft/hard plastics) here. Injection speed is a relatively difficult process element to control in injection molding, unlike other process elements which have standard data for reference (this will be explained in detail later).
Setting of injection speed values mainly follows these points:
Based on material's flowability; soft plastics such as PP, LDPE, TPE, TPR, TPU, PVC have good flowability and low cavity resistance during filling. Generally, a lower injection speed can be used to fill cavity. Commonly used medium-viscosity plastics such as ABS, HIPS, GPPS, POM, PMMA, PC+ABS, Q-type glue, K-type glue, and HDPE have slightly poor flowability. When product's gloss requirement is not high or product thickness is moderate (wall thickness or core thickness exceeding 1.5mm), a medium injection speed can be used. Conversely, injection speed should be appropriately increased according to product structure or appearance requirements.
Engineering plastics such as PC, PA+GF, PBT+GF, and LCP have poor flowability and generally require high-speed injection, especially materials with added GF (glass fiber). If injection speed is too slow, it will cause severe surface fiber floating (silver streaks).
2. Melt Speed Control: This parameter is one of the most easily overlooked processes in daily work, as most colleagues believe that this process has little impact on molding and that parameters can be adjusted arbitrarily to produce a product. However, in injection molding, melt parameters are just as important as injection speed. Melt speed directly affects melt mixing effect, molding cycle, and other important aspects.
3. Control of Mold Opening and Closing Speed: Different parameters are set for different mold structures. For example, for two-plate flat molds, adjusting to high-speed mold closing before starting low-pressure mold closing and adjusting to fast mold opening after product leaves mold cavity can effectively improve production efficiency. However, for molds with sliding parts, switching between fast and slow mold opening speeds needs to be determined based on height and structure of sliding parts. Special mold structures and core-pulling molds are explained in detail in later chapters due to their complexity.
4. Control of Ejector Pin Speed: This mainly depends on product's demolding condition. In principle, speed should be as fast as possible while ensuring that product does not exhibit whitening, excessive ejection height, or deformation. Otherwise, parameters need to be adjusted appropriately according to actual situation. Of course, under normal circumstances, initial adjustment of ejector speed should be at a medium-low speed (15%-35%), which can effectively extend service life of ejector pins and ejector cylinders.

Switching point between different speeds and pressures in various actions.
1. Control of injection position: During injection molding parameter debugging, injection position needs to be adjusted according to product's unit weight and structure. Adjusting position based on product's unit weight is commonly referred to as determining required amount of glue for product.
For example: A product weighs approximately 50g and is produced using a 90T injection molding machine. Theoretical injection volume of this machine is 120g, and melt stroke is 130mm. Approximately, weight of melt per mm is 120g ÷ 130mm = 0.92g. Therefore, injection distance for this product is 50 × 0.92 = 46mm. If melt termination position is set at 60mm, then product quality is basically OK when injection reaches 14mm. (Of course, above is based on experience and may have some inaccuracies, as it doesn't follow screw compression ratio calculation formula from textbooks—that's too complex, and I believe most colleagues wouldn't be able to calculate it.) Regarding how to control various defects in molded products using injection position.
2. Control of Melt Position: Generally speaking, this involves setting melt distance to match required injection volume for molded product. Most colleagues ignore three-stage switching positions of melt and only focus on endpoint position. Of course, for molded products of general difficulty, adjusting melt position doesn't necessarily require switching between fast/slow speeds or high/low back pressures to achieve desired product quality. However, when producing masterbatches or highly heat-sensitive plastics, appropriately switching melt speed and back pressure adjustment positions can better control product quality.
3. Mold Opening/Closing Position Control: Switching point is mainly set to match mold opening/closing speed requirements.
3.1 Generally, mold opening speed switching point is slow before molded part leaves mold cavity (approximately 5-15mm), then switches to fast speed to effectively shorten mold opening time. Finally, it switches to slow speed again (i.e., mold opening buffer position, generally 20-40mm from desired mold opening termination position is ideal). (Termination position depends on product structure and whether a robot is used). This effectively extends service life of injection molding machine's crankshaft and ensures stable mold opening action.
For some special mold structures, such as three-plate molds or core-pulling molds, mold opening speed needs to be determined according to actual situation. For example, in a three-plate mold, since product cavity is on the middle plate, first action during mold opening is on sprue plate. Sprue channel needs to be separated from product before male and female molds separate. Therefore, 1-2 switching points need to be added at mold opening position, in the order of medium speed—slow speed—high speed—slow speed. Larger tonnage machines can add more switching points as needed. Main principle is to ensure that quality of molded product is not affected during mold opening and that operation is smooth.
3.2 Setting of mold clamping position mainly depends on mold structure. For example, in a flat mold structure (i.e., parting surfaces of front and rear molds are both flat, without sliders/core-pulling, and without insert structures), mold clamping speed can be switched directly using four positions: "fast—medium speed—low pressure—high pressure". Principle for switching positions is that fast clamping stroke is preferably about 70% of mold opening stroke (fast termination position of a three-plate mold depends on mold's structural dimensions). Main function is to shorten mold clamping cycle. Medium speed setting then acts as a deceleration buffer for high-speed mold clamping (because it switches to low-pressure protection after medium speed).
Termination position of medium-speed mold clamping is crucial, as it determines starting position of low-pressure protection. Some experienced colleagues are unclear about low-pressure mold clamping, believing it can be set arbitrarily, which is incorrect. Improper low-pressure setting will completely disable protection function, which is fatal for molds in fully automated production.
4. Ejector pin position control: Theoretically, ejector pin extension length should be twice height of mold cavity (i.e., mold core). However, in actual operation, it is not necessary to strictly adhere to this method; primary consideration should be ease of product removal. When initially adjusting ejector pin position, length should be gradually increased, starting with 50% of ejector pin stroke, then adjusted based on product removal during production.

 

Essential conditions for plastic melting and mold heating
1. Control of barrel temperature: Generally, different types of plastics have their own relatively standard molding temperatures, such as: ABS = (high impact resistance 230-260, low impact resistance 190-230), SAN = 180-220, HIPS = 180-220, POM = 170-200, PC = 240-300. ABS/PC = 230-260, PMMA = 200-230, PVC = (high density 160-200, low density 140-180), PP = 180-230, PE = (high density 240-300, low density 180-230); TPE = (high density 170-200, low density 140-180), TPR = (high density 170-200, low density 140-180), TPU = (high density 160-200, low density 120-160), PA = 230-270, PA+fiber = 250-300, PBT = 200-240, PBT+fiber = 240-280. Additionally, molding temperature for materials with added flame retardants (i.e., fire-retardant materials) should be 20-30 degrees Celsius lower than that of ordinary materials. Specific operating temperature depends on production conditions, as molding temperature directly affects plastic's flowability, viscosity, mold temperature, color, shrinkage rate, and product deformation.
2. Mold Temperature Control: Mold temperature is primarily determined by different flowability characteristics of plastic. Simply put, it's a key process for overcoming poor flowability. For example, PC and PA+cellulose materials have poor flowability and high flow resistance during filling, requiring a faster injection speed.
Additionally, when producing transparent PC parts, a higher mold temperature is needed to improve surface defects such as air bubbles, rainbow marks, and internal air bubbles. When producing fiber-reinforced materials, a lower mold temperature will result in surface silver streaks (floating fibers).
Under normal circumstances, following data can be used to adjust the mold temperature: ABS = 30-50℃ (60-110℃ for products requiring high surface quality or controlled deformation); PC = 50-80℃ (85-140℃ for products requiring high surface quality or thin walls); HIPS = 30-50℃ (60-80℃ for transparent PS and products requiring high surface quality); PMMA = 60-80℃ (80-120℃ for thin-walled products and products requiring high surface quality); PP = 10-50℃, PE = 10-50℃ (mold temperature can be appropriately increased for high-density or thin-walled products) Rubber materials (TPE, TPR, TPU) = 10-50, PA, PBT = 30-60 (70-100 for materials with high surface quality requirements and those with added glass fiber).

Time required for each action
1. Control of filling time: Including injection time and holding time
1.1. Injection time: Generally, for products meeting quality requirements, the shorter injection time, the better. Injection time directly affects internal stress of product and production cycle. In principle, the thinner glue layer of product, the shorter injection time; conversely, for thick-walled products, injection time needs to be extended appropriately to control shrinkage.
Products using multiple injection stages and those with large speed transitions require longer injection times. Injection time setting must also be based on product volume (larger products require longer injection times). Properties of plastic used must also be considered. For example, for general-purpose ABS plastic with a product wall thickness of 2.0mm, moderate injection speed, and moderate barrel temperature, longitudinal flow rate is approximately 65mm/s (flow rate varies depending on mold structure or process).
1.2. Holding Pressure Time: In principle, holding pressure time mainly controls product surface shrinkage and structural dimensions. However, with complete mastery of holding pressure time control methods, it can also be used to adjust product's deformation (therefore, this adjustment process is a precision machine adjustment process, and its adjustment method will be described in detail in later chapters).
This section primarily explains how to use holding pressure to control product shrinkage. Choice of holding pressure depends on location of shrinkage. Not all shrinkage can be addressed with holding pressure. For example, if shrinkage is at the end of melt flow, using holding pressure will cause excessive stress near sprue, leading to ejection whitening, mold sticking, product warping.
2. Ejector Pin Delay: This controls dwell time of ejector pin during ejection, facilitating product removal by robotic arm.
3. Core Pulling Time: This controls action time of core pulling device on injection molding machine (mainly used when action stroke is controlled by time). If core pulling stroke is controlled by a sensor switch, a core pulling time setting is not required.