The further development of injection molding equipment and the continuous improvement of product quality requirements all place higher demands on the injection molding process. The correct choice of injection equipment and the rational setting of the molding process and the optimization of process conditions are the key to improving product quality.

Correct choice of injection molding machine

The performance of the injection molding machine directly affects the quality of the injection molding products. The injection molding machines with different specifications and performance requirements will also have much different prices.

Injection molding machine specifications

When selecting the specifications of the injection molding machine, we must first consider the status of the production mold, because the same injection molding machine is often required to meet the production of multiple pairs of different molds, the size of the injection molding machine should be determined according to the weight of the parts, mold size, etc., ie the injection molding machine Maximum clamping force and maximum injection volume, then select the appropriate model according to the specifications provided by the injection molding machine manufacturer. Most manufacturers provide customized services, which provide great convenience for the purchase of injection molding machines; secondly, whether it needs some special configuration, such as the production of PA, PC and other materials need to use a special screw, forming with suction Need to equip the core or de-threading mold with corresponding devices; again, according to the mold structure, product quality and other factors to determine whether you need to use some special injection molding machines, such as the formation of thin-walled long flow products (generally referred to L /D??300), need to use high injection speed injection molding machine, precision electronic parts need to use precision full closed-loop control injection molding machine.

Clamping force setting

In theory, the clamping force can be calculated as follows:

Fcm>=K*P average*A product*10

Where: sFcm - clamping force, (KN)K - safety factor, generally take 1-1.2 P average - cavity average pressure (MPa) A product - the maximum projected area of ​​the product on the mold parting surface (cm2)

In the actual production, the adjustment of the clamping force should also take into account the impact of thermal expansion of the mold in the production, generally there should be a 0.1-0.2mm margin; the setting principle of the clamping force is to ensure the quality of the product It is advisable to use a low clamping force.

Injection molding process parameter setting

Barrel temperature, mold temperature

According to the performance of different plastic materials to set the screw barrel temperature, the barrel set temperature is generally higher than the plastic melting point of 10 °C -30 °C. It must be noted that the materials provided by different manufacturers may differ in their melting point and allowable residence time in the cartridge, depending on the method of synthesis or type of additives added. Table 1 on the next page compares the melting points of Solutia's PA66 (designated as 21SPC) and that of Rhodia's PA66 (designated as 25AE1) with the allowable residence time at each temperature.

The mold temperature is generally set to use circulating water for cooling, but in the production of products with high precision or high surface quality, a mold temperature controller capable of accurate control should be used according to the process requirements.

Table 1 Comparison of melting point and allowable residence time of different brands of PA66

Injection pressure holding time, cooling time

Injection time, dwell time and cooling time must be set according to product thickness, mold temperature, material properties, etc. Injection time is generally set to be slightly longer than the time required for the screw to complete the injection stroke. Excessive injection time will not only produce adverse effects such as mechanical wear, increased energy consumption, but also prolong the molding cycle. The setting of holding pressure time is set according to the product thickness. The thin-walled product can not be used for holding time during molding. When the holding pressure is set, as long as there is no obvious depression on the surface of the product, it can be determined by weighing method. Prolonging the holding time until the time when the product quality no longer changes can be determined as the optimal holding time. The cooling time also needs to be determined based on product thickness, mold temperature, and material properties. Generally, amorphous polymers require longer cooling times than crystalline polymers.

Injection pressure, speed

The injection pressure should be set according to the principle of appropriate low and high, as long as it can provide enough power to achieve the required injection speed, so that the melt can successfully fill the cavity, excessive pressure can easily cause internal stress in the product; but When molding high-precision products, in order to prevent excessive product shrinkage, high-pressure injections can be used to reduce the shrinkage of the product after demoulding.

The injection speed will affect the appearance quality of the product. The settings should be set according to the mold geometry, exhaust conditions, etc. Generally, under the premise of ensuring a good appearance, the injection speed should be increased as much as possible to reduce the filling time. In injection molding, when the melt flows in the mold, a solidified layer is formed on the mold wall, thereby reducing the thickness of the flowable passage. Generally, according to different mold structures and injection speeds, the mold wall has a solidified layer of about 0.2 mm. Therefore, faster injection speeds are usually used in molding.

Injection stroke, multi-stage injection parameters

In molding, the injection stroke must first be determined. In theory, the injection stroke can be calculated as follows:

S1=4(CVp+Va)/ρDs2

Medium sS1 – Injection stroke Vp – Product volume ρ – Resin density C – Number of cavities Va – Gate volume Ds – Screw diameter

In actual production, if the total weight of “product + gate” is known, the following formula can be used to calculate the injection stroke?s

S1=(M/Mmax)·Smax+(5~10)mm

In the formula?s S1---injection stroke, mm M-“product + gate” total weight, g Mmax – maximum injection volume of the injection molding machine, g Smax – maximum injection stroke of the injection molding machine, mm

Due to the different geometry of the runner system and various parts of the mold, in order to meet the product quality requirements, there are different requirements for the flow state of the mold-filling melt (mainly the pressure and velocity during the flow) in different parts. In an injection process, when the screw advances the melt toward the mold, it is required to realize different pressures and speeds at different positions, which is called multi-stage injection molding. It is more scientific to set at least three or more injections of plastic parts during molding. That is, the main section is the first section, and the runner-to-gate section is the second section. The product is filled with about 90% of the cavity. The third section, the remaining part is the fourth paragraph, can be used to determine the switching point of each section by the calculation of weight; actual production, should be based on product quality requirements, flow channel structure, mold exhaust conditions, etc. on the multi-stage injection process parameters Scientific analysis, reasonable setting. Usually can be set by the debugging observation method, when the injection needs to find the switching point pressure / speed is set to 0, observe the trend of the melt and the position of product defects, and gradually adjust until find a reasonable location . However, in the process of debugging and observation, attention must be paid to the mold release condition of the under-injected product, so as to avoid mold sticking due to lack of injection in certain depressed portions of the mold.

Other process parameters

In injection molding, in addition to the setting of several major parameters such as molding temperature, pressure, speed, time, and multi-stage injection switching position, there are many other process parameters, such as back pressure, screw speed, and anti-flow screw reversing cable. The delay and other settings of the operating parameters cannot be ignored.

Process parameter setting example

Take the production of nylon cable ties as an example, the product quality requirements?s products meet the specified pull standards; the surface is free of silver, air bubbles, sink marks, and other unfavorable phenomena; after the molding, the product is tight and has no loosening phenomenon. The material used is PA66; the injection molding machine is JSW1000-EII-SP, the mold structure is #p# page header titled #e# hot runner type, and the gate type is point gate.

First of all, according to product features and mold structure to determine the process parameters set principles? s (1) due to a longer product flow, L / t (flow and wall thickness ratio) is 511, should choose high-speed injection molding; (2) gate The type is the point gate, the higher pressure must be used to overcome the flow resistance; (3) In order to ensure the smooth filling of the product, the molten material must have good fluidity, and the molding temperature should be appropriately high; (4) high-pressure high-speed When the injection to the end is easy to produce flash, the molding machine must have low inertial pressure, speed switching; (5) due to the small wall thickness of the product, do not use pressure; formulate the main molding process parameters shown in Table 2.

Table 2 belt molding product process parameters

In order to formulate injection process parameters, it is necessary to understand information on equipment performance, mold structure, molding materials, and product quality requirements, and scientifically and reasonably set various molding parameters. The first step is to adjust the process parameters according to the product molding conditions. The correct adjustment sequence is pressure → speed → temperature. Each time the parameters are changed, the input parameters have been confirmed by the computer before the next parameter change, and it should be avoided to change more than two parameters at the same time; secondly, when the product enters into stable production, the parameters should be kept as stable as possible, and detailed Record, if the change is too large, you should find out why. In addition, the molding process should be fixed as much as possible each time the mold goes on line, which facilitates the quality control of the finished product.