Injection molding is a complex process, and many factors affect molding, such as injection pressure, speed, position, temperature, time, and so on. In this article, we will analyze from these five perspectives, how we can better complete the plastic injection molding process.

Pressure

The action pressure provided by the injection molding machine’s pressure system (oil pump) or the servo motor is mainly used in the injection device, melt device, mold opening, and locking device, ejector device, injection table device, core extraction device, and other action procedures.

The processor converts the relevant parameters into signals for each program after inputting them into the control panel of the injection molding machine, thus controlling the pressure required for the execution of each program.

The pressure setting principle is the corresponding force to overcome the resistance of the action, but to match the speed of the action, the parameter value should be adjusted accordingly.

Speed

The speed of each action program (the flow rate of the hydraulic oil in the system) is set by the above-mentioned pressure. The basic levels of speed are as follows: slow flow 0.1-10, slow speed 11-30, medium speed 31-60, high speed 61-99.

Control of injection speed

Apply to different product structures and materials to set the size of the value, here for the time being do not distinguish (engineering / general plastic, crystalline / non-crystalline plastic, high temperature / low-temperature plastic, or soft/hard plastic) is easy to confuse.

A more understandable explanation is that injection speed is a difficult process element to control in injection molding, unlike other process elements that have standard data for reference.

The numerical setting of injection speed mainly follows the following points.

Depending on the fluidity of the material; such as PP, LDPE, TPE, TPR, TPU, PVC, and other soft plastics have better fluidity, and their cavity resistance is small when filling, so generally, a lower injection speed can be used to fill the cavity.

Such as ABS, HIPS, GPPS, POM, PMMA, PC + ABS, Q glue, K glue, HDPE, and other common medium viscosity plastics whose fluidity is slightly worse.

In the product appearance, gloss requirements are not high or the product flesh thickness is moderate (product wall thickness or bone thickness up to 1.5MM or more) in the case of injection speed can be used to fill the medium speed, and vice versa, according to the product structure or appearance requirements to properly Increase the filling speed.

Such as PC, PA + GF, PBT + GF, LCP, and other engineering plastics fluidity is poor, in the filling of the general need for high-speed injection, especially to increase the GF (glass fiber) materials, if the injection speed is too slow to cause the product surface floating fiber (surface silver pattern) serious.

Melting speed control

This parameter in the daily work is one of the most easily ignored processes because most people think that the process has little impact on the molding, the parameters can be adjusted casually to make injection molded parts.

But in the injection molding melt parameters are as important as the injection speed, melt speed can directly affect the melt mixing effect, molding cycle and other important links.

Control of mold opening and locking speed

Different parameters are mainly set for different mold structures. For example, adjusting the high-speed clamping before starting the clamping at low pressure and adjusting to fast mold opening after the product leaves the mold cavity can effectively improve the production efficiency.

However, when adjusting the clamping speed for molds with line position, the switching of fast and slow clamping speeds should be set according to the height and structure of the line position. The special mold structure and core mold are complicated, so we will explain them in the next section.

Control of ejector speed

In principle, it is necessary to ensure that the product does not appear top white and top high, or deformation of the premise as fast as possible, otherwise it is necessary to adjust the parameters as appropriate to the actual situation.

Of course, in general, the first time adjusting the top true speed when the low speed (15%-35%) is better, can effectively extend the service life of the ejector and ejector cylinder.

Position

The switching point between fast and slow speed, high and low pressure of each action

Injection position control

In the injection molding parameters debugging, the injection position needs to be adjusted according to the product weight and structure, in consideration of the product weight to adjust the position that is often said that the product needs how many plastic raw materials.

For example, the single weight of a product about 50G, using 90T injection molding machine production, theoretical injection volume of 120G, melt stroke 130MM, about the weight of the melt per MM for theoretical injection volume 120G ÷ melt stroke 130MM = 0.92G, that is, the product injection distance of 50 × 0.92 = 46MM position.

If the melt termination position is set at 60MM, then the injection glue to 14MM when the product quality has been OK.

(Of course, the above for the experience, more or less deviation, because not by the book screw compression ratio formula, that is too complicated, I believe most people do not know how to calculate), as for how to use the injection position to control a variety of molding products bad phenomenon, the later chapters, will explain in detail.

Melt glue position control

Most of our colleagues ignore the three switching positions of the melt glue and only focus on the end position of the melt glue.

Of course, it is not necessary to switch the melt speed and backpressure when adjusting the melt position for difficult molded products to achieve the required product quality, but it is possible to control the product quality better by properly switching the melting speed and backpressure when producing masterbatches and heat-sensitive plastics.

Position control of mold opening and clamping

The switching point is mainly set according to the speed of the mold opening.

Generally speaking, the switching point of the mold opening speed is slow before the molded product leaves the mold cavity (about 5-15MM), then turn fast, which can effectively shorten the time needed to open the mold, and finally turn slow (i.e., the opening buffer position, generally 20-40MM from the desired opening termination position is good to start switching.

The termination position depends on the product structure and whether to use robots, which can effectively extend the life of the injection molding machine strand and the stability of the mold opening action. (The life span and stability of mold opening action can be effectively extended.)

Some of the special mold structure factors such as three-plate mold or core mold, the mold opening speed needs to be determined according to the actual situation, such three-plate mold because the product cavity in the middle palate, the first action in the water mouth plate when opening the mold, the water mouth runner and product separation after the separation of the male and female mold.

So in the opening position of the mold need to increase 1-2 switching points, for medium speed – slow speed -High speed – slow speed, the larger tonnage of the injection molding equipment can be increased according to the need for more switching points.

In short, the quality of the injection molded products in the process of opening the mold is not affected and the action process is smooth.

The setting of the clamping position mainly depends on the structure of the mold, such as a flat mold structure (i.e., the front and rear mold parting surface are flat, no slider/core, no insertion structure) in the clamping speed switch can be directly used to 4 positions “fast-medium speed – low pressure -High pressure”, the position of the

The switching principle is: that clamping fast stroke is about 70% of the opening stroke is good, (three plates mold fast termination position depends on the size of the mold structure), the main role of shortening the clamping cycle.

After turning medium speed, the role of high-speed clamping mold deceleration buffer, (because after the medium speed will switch to low-pressure protection function)

The termination position of the clamping medium speed is very important, which determines the starting position of the clamping low-pressure protection, some of our colleagues in the old capital force are very vague about the clamping low-pressure, think as if randomly set can lock the mold it is not.

If the clamping low pressure is not set properly, the protection function will be completely lost, which is fatal to the fully automatic production of the mold.

Control of ejector position

Theoretically, the ejector length of the ejector pin is 2 times the height of the model cavity (i.e. core), but in practice it is not necessary to set the position exactly in this way, specifically to facilitate the removal of the product.

But in the initial adjustment of the ejector position need to gradually increase the length, first ejected 50% of the mold ejector stroke, and then depending on the production process of the product out of the situation.

Temperature

The necessary injection molding conditions for molten plastic and heating of the mold temperatures

Control the temperature of the material tube

In general for different properties of plastics have their relative standard molding barrel temperature, such as: ABS = (distinguish high impact material 230-260, low impact material 190-230), SAN = 180-220, HIPS = 180-220, POM = 170-200.

PC = 240-300, ABS/PC = 230-260, PMMA = 200-230, PVC= (distinguish high density 160-200, low density 140-180), PP=180-230, PE= (distinguish high density 240-300, low density 180-230).

TPE= (distinguish high density 170-200, low density 140-180), TPR= (distinguish high density 170-200, low density 140-180).

TPU= (distinguish high density 160-200, low density 120-160) PA=230-270, PA+fiber=250-300, PBT=200-240, PBT+fiber = 240-280, where the other added flame retardant (i.e. fireproof material) molding temperature should be 20-30 degrees lower than the ordinary material.

The specific temperature depends on the production situation because the molding temperature directly affects the fluidity of plastic, viscosity, mold temperature, color, shrinkage, product deformation, etc.

Control of nozzle temperature

The nozzle temperature should be lower than the barrel temperature. If it is too high– melting plastic will drool in the nozzle and the plastic will decompose if the temperature is too low. Materials may also block the nozzle– causing bad components.

Control of mold temperature

Mold temperature is mainly for different plastic fluidity depends, simple to understand is to overcome the key process of poor fluidity, such as PC and PA + fiber material fluidity is poor, its flow resistance in the filling process is larger, it needs a faster injection speed to fill.

In addition, such as the production of PC transparent material plastic parts, to improve the product surface airlines, rainbow marks and internal bubbles and other bad problems need a higher mold temperature, the production of fiber material, if the mold temperature is low, will appear on the surface of the silver wire pattern (floating fiber).

In general, the mold temperature can be adjusted by referring to the following data.

ABS = 30-50 (surface quality requirements or the need to control the deformation of the product can be increased to 60-110 degrees) PC = 50-80 (surface quality requirements or thin-walled products can be increased to 85-140 degrees) HIPS = 30-50 (transparent PS and products with high surface quality requirements can be increased to 60-80 degrees)

PMMA = 60-80 (thin-walled products and products with high surface quality requirements can be increased to 80-120 degrees) PP = 10-50, PE = 10-50 (high-density or thin-walled products can be appropriately increased mold temperature) Rubber (TPE, TPR, TPU) = 10-50, PA, PBT = 30-60 (high surface quality requirements and the addition of glass fiber materials can be increased to 70-100).

Time

The time for each action to be performed

Control of filling time

Include injection time and holding time

1.1. Injection time

In principle, the thinner the glue position of the product, the shorter the injection time; conversely, thick-walled products need to extend the injection time as appropriate to control the shrinkage problem.

In addition, the use of multi-stage and fast and slow speed switching range of products needs a longer injection time, the injection time needs to be set according to the volume of the product to set (the larger the product the longer the required injection time) in this also need to consider the nature of the production of plastic, such as general-purpose plastics ABS in the wall of the product.

For example, the flow rate of general plastic ABS is about 65mm/sec when the product wall thickness is 2.0MM, the injection speed is moderate and the temperature of the material tube is moderate (the flow rate varies for different mold structures or processes).

1.2. Holding time

In principle, the holding time mainly controls the product surface shrinkage and product structure size, but after fully mastering the holding time control method can also use the holding plastic pressure to adjust the deformation of the product (so the adjustment process is a precision tuning process, which will be introduced in detail in the later chapters to adjust the way).

Here we mainly explain the use of pressure-holding to control the shrinkage of the product, the general choice of pressure-holding to control the shrinkage of the product depends on the location of the product shrinkage, not all shrinkage can be solved with pressure-holding.

For example shrinkage position at the end of the melt filling, the use of pressure-holding to control shrinkage will cause excessive stress in the position near the mouth of the water, resulting in top white, sticky mold, or causing warpage deformation of the injection-molded products.

Ejector delay time

The main control of the ejector pin ejection dwells time, is to facilitate the robot to pick up the product.

Core extraction time

Control the action time of the core extraction device of the injection molding machine (mainly used in the time control action stroke), if the core extraction stroke is controlled by an induction switch, the core extraction time can not be set.