Pressure
The operating pressure provided by the injection molding machine's pressure system (oil pump) or servo motor is mainly used in various procedures such as injection molding, melting, mold opening/closing, ejection, injection unit, and core pulling. After relevant parameters are input into the injection molding machine's control panel, the processor converts them into signals for each procedure, thereby controlling the pressure required for each action.
The principle for setting pressure is: the corresponding force to overcome the resistance of the action, but the parameter values need to be adjusted accordingly to match the speed of the action.
2. Speed
The operating speed (flow rate of the system hydraulic oil) required to complete each action procedure in conjunction with the pressure mentioned above. Basic speed levels are distinguished as follows: 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).
The setting of injection speed values mainly follows these points:
Based on the material's flowability; soft plastics such as PP, LDPE, TPE, TPR, TPU, and PVC have good flowability and low cavity resistance during filling. Generally, a lower injection speed can be used to fill the 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 the product's gloss requirement is not high or the product thickness is moderate (wall thickness or core thickness exceeding 1.5mm), a medium injection speed can be used. Conversely, the injection speed should be appropriately increased according to the 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 the 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 the 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 the 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 the product leaves the mold cavity can effectively improve production efficiency. However, for molds with sliding parts, the switching between fast and slow mold opening speeds needs to be determined based on the height and structure of the 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 the product's demolding condition. In principle, the speed should be as fast as possible while ensuring that the product does not exhibit whitening, excessive ejection height, or deformation. Otherwise, parameters need to be adjusted appropriately according to the actual situation. Of course, under normal circumstances, the initial adjustment of the ejector speed should be at a medium-low speed (15%-35%), which can effectively extend the service life of the ejector pins and ejector cylinders.
3. Position
The switching point between different speeds and pressures in various actions.
1. Control of the injection position;
During injection molding parameter debugging, the injection position needs to be adjusted according to the product's unit weight and structure. Adjusting the position based on the product's unit weight is commonly referred to as determining the required amount of glue for the product.
For example: A product weighs approximately 50g and is produced using a 90T injection molding machine. The theoretical injection volume of this machine is 120g, and the melt stroke is 130mm. Approximately, the weight of melt per mm is 120g ÷ 130mm = 0.92g. Therefore, the injection distance for this product is 50 × 0.92 = 46mm. If the melt termination position is set at 60mm, then the product quality is basically OK when the injection reaches 14mm.
(Of course, the above is based on experience and may have some inaccuracies, as it doesn't follow the 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 the melt distance to match the required injection volume for the molded product. Most colleagues ignore the three-stage switching positions of the melt and only focus on the endpoint position. Of course, for molded products of general difficulty, adjusting the melt position doesn't necessarily require switching between fast/slow speeds or high/low back pressures to achieve the desired product quality. However, when producing masterbatches or highly heat-sensitive plastics, appropriately switching the melt speed and back pressure adjustment positions can better control product quality.
3. Mold Opening/Closing Position Control;
The switching point is mainly set to match the mold opening/closing speed requirements.
3.1 Generally, the mold opening speed switching point is slow before the molded part leaves the mold cavity (approximately 5-15mm), then switches to fast speed to effectively shorten the mold opening time. Finally, it switches to slow speed again (i.e., the mold opening buffer position, generally 20-40mm from the desired mold opening termination position is ideal). (The termination position depends on the product structure and whether a robot is used). This effectively extends the service life of the 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, the mold opening speed needs to be determined according to the actual situation. For example, in a three-plate mold, since the product cavity is on the middle plate, the first action during mold opening is on the sprue plate. The sprue channel needs to be separated from the product before the male and female molds separate. Therefore, 1-2 switching points need to be added at the 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. The main principle is to ensure that the quality of the molded product is not affected during mold opening and that the operation is smooth.
3.2 The setting of the mold clamping position mainly depends on the mold structure. For example, in a flat mold structure (i.e., the parting surfaces of the front and rear molds are both flat, without sliders/core-pulling, and without insert structures), the mold clamping speed can be switched directly using four positions: "fast-medium speed-low pressure-high pressure". The principle for switching positions is that the fast clamping stroke is preferably about 70% of the mold opening stroke (the fast termination position of a three-plate mold depends on the mold's structural dimensions). The main function is to shorten the mold clamping cycle. The medium speed setting then acts as a deceleration buffer for high-speed mold clamping (because it switches to low-pressure protection after medium speed).
The termination position of the medium-speed mold clamping is crucial, as it determines the starting position of the 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 the protection function, which is fatal for molds in fully automated production.
4. Ejector pin position control;
Theoretically, the ejector pin extension length should be twice the height of the mold cavity (i.e., mold core). However, in actual operation, it is not necessary to strictly adhere to this method; the primary consideration should be ease of product removal. When initially adjusting the ejector pin position, the length should be gradually increased, starting with 50% of the ejector pin stroke, and then adjusted based on product removal during production.
4. Temperature
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, the 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. The specific operating temperature depends on the production conditions, as molding temperature directly affects the plastic's flowability, viscosity, mold temperature, color, shrinkage rate, and product deformation.
2. Mold Temperature Control;
Mold temperature is primarily determined by the different flowability characteristics of the 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, the following data can be used to adjust the mold temperature:
ABS = 30-50°C (60-110°C for products requiring high surface quality or controlled deformation)
PC = 50-80°C (85-140°C for products requiring high surface quality or thin walls)
HIPS = 30-50°C (60-80°C for transparent PS and products requiring high surface quality)
PMMA = 60-80°C (80-120°C for thin-walled products and products requiring high surface quality)
PP = 10-50°C, PE = 10-50°C (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)
5. Time
The 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 the injection time, the better. Injection time directly affects the internal stress of the product and the production cycle. In principle, the thinner the glue layer of the product, the shorter the injection time; conversely, for thick-walled products, the 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. The injection time setting must also be based on the product volume (larger products require longer injection times). The properties of the 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, the longitudinal flow rate is approximately 65mm/s (the flow rate varies depending on the 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 the 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. The choice of holding pressure depends on the location of the shrinkage. Not all shrinkage can be addressed with holding pressure. For example, if the shrinkage is at the end of the melt flow, using holding pressure will cause excessive stress near the sprue, leading to ejection whitening, mold sticking, or product warping.
2. Ejector Pin Delay
This controls the dwell time of the ejector pin during ejection, facilitating product removal by the robotic arm.
3. Core Pulling Time
This controls the action time of the core pulling device on the injection molding machine (mainly used when the action stroke is controlled by time). If the core pulling stroke is controlled by a sensor switch, a core pulling time setting is not required.
