Further development of injection molding equipment and continuous improvement of product quality requirements have put forward higher requirements for injection molding process. Correct selection of injection equipment, reasonable setting of molding process, and optimization of process conditions are keys to improving product quality.

 

When selecting specifications of injection molding machine, condition of production mold should be considered first, because same injection molding machine often needs to meet production of multiple pairs of molds of different sizes. Maximum clamping force and maximum injection volume, then select appropriate model according to specifications and models provided by injection molding machine manufacturer.
It is necessary to consider whether some special configurations are required. For example, special screws need to be selected when producing PA, PC and other materials, and corresponding devices must be equipped when forming molds with core-pulling or de-threading; again, it is necessary to determine whether it is necessary to choose some injection molding machines with special functions according to factors such as mold structure and product quality. For example, when molding thin-walled and long-flowing products (generally referred to as L/D300), a high injection speed injection molding machine should be used, and a precision fully closed-loop control injection molding machine should be used for precision electronic parts.

Theoretically, clamping force can be calculated as follows:
Fcm>=K×P average×A product×10
In formula: Fcm – clamping force, (KN) K – safety factor, generally take 1-1.2, P average – average cavity pressure (MPa), A product – maximum projected area of product on mold parting surface (c㎡)
In actual production, adjustment of clamping force should also consider influence of thermal expansion of mold in production, and generally a margin of 0.1-0.2mm should be reserved; setting principle of clamping force is that low clamping force is appropriate under premise of ensuring product quality.

Temperature of screw barrel is set according to performance of different plastic materials. Set temperature of barrel is generally 10℃-30℃ higher than melting point of plastic. It must be noted that materials provided by different manufacturers have different melting points and allowable residence times in barrel due to different synthesis methods or types of additives.
Mold temperature is generally cooled by circulating water, but when producing products with precise dimensions or high surface quality requirements, a mold temperature machine that can be accurately controlled should be used according to process requirements.

Injection time, holding 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 time for screw to complete injection stroke. Excessive injection time will not only produce negative effects such as mechanical wear and increased energy consumption, but also prolong molding cycle. Dwell time is set according to thickness of product, and thin-walled products can be formed without dwell time;

 

When setting holding time, as long as there is no obvious depression on the surface of product, it can also be determined by weighing method. Gradually prolong holding time until time when product quality does not change can be determined as the best holding time. Cooling time also needs to be determined according to product thickness, mold temperature, and material properties. Generally, cooling time required for amorphous polymers is longer than that for crystalline polymers.

Injection pressure setting should follow principle that it should be low and not high, as long as it can provide enough power to achieve required injection speed and enable melt to fill cavity smoothly. Too high pressure will easily cause internal stress in product; However, when molding products with high dimensional accuracy, in order to prevent excessive shrinkage of product, high pressure injection can be used to reduce shrinkage of product after demolding.
Injection speed will affect appearance quality of product, and its setting should be set according to geometry of mold, exhaust conditions, etc. Generally, under premise of ensuring a good appearance, injection speed should be increased as much as possible to reduce filling time.
In injection molding, when melt flows in mold, mold wall will form a solidified layer, thus reducing thickness of flowable channel. Generally, according to mold structure and injection speed, mold wall will have a solidified layer of about 0.2mm. Therefore, a faster injection speed is usually used in molding.

In molding, injection stroke must first be determined. In theory, injection stroke can be calculated as follows:
S1=4(CVp+Va)/ρDs2
In formula: injection stroke Vp – product volume ρ – resin density C – cavity number Va – gate volume Ds – screw diameter
In actual production, if total weight of "product + gate" is known, following formula can be used to calculate injection stroke S1=(M/Mmax)·Smax+(5~10)mm
In formula, S1: injection stroke, mm M – total weight of “product + gate”, g Mmax – maximum injection volume of injection molding machine, g/Smax – maximum injection stroke of injection molding machine.
Due to different geometric shapes of runner system and each part of mold, in order to meet product quality requirements, there are different requirements for flow state of filling melt (mainly referring to pressure and speed during flow) in different parts. In an injection process, when screw pushes melt to mold, it is required to achieve different pressures and speeds at different positions, which is called multi-stage injection molding.
Generally, it is more scientific to set at least three or more stages of injection when molding plastic parts, that is, first stage is main channel, second stage is shunt to gate, and product is filled with about 90% of cavity, third stage, remaining part is fourth stage. Calculation weight method can be used to determine switching position point of each stage;
In actual production, multi-stage injection process parameters should be scientifically analyzed and reasonably set according to product quality requirements, runner structure, and mold exhaust conditions. Usually, debugging and observation method can be used for setting. Set pressure/speed of switching position point to be 0 during injection, observe direction of melt and defect status of product, and adjust gradually until a reasonable position point is found. . However, in process of debugging and observation, we must pay attention to demoulding condition of under-injection product, so as to avoid mold sticking due to under-injection in some recessed parts of mold.

 

In injection molding, in addition to setting of several main parameters such as molding temperature, pressure, speed, time, multi-stage injection switching position, etc., there are many other process parameters, such as back pressure, screw speed, anti-casting of screw reverse cable and other action parameter settings, etc., and their settings cannot be ignored.

Taking production of nylon cable ties as an example, product quality requirements:
Product reaches specified tensile force standard; there are no silver wires, bubbles, sink marks and other undesirable phenomena on the surface; after molding, product has good tightness and no looseness. Material used is PA66, mold structure is hot runner type, and gate type is point gate.
First of all, according to product characteristics and mold structure to determine process parameter setting principle:
Due to long flow length of product, L/t (flow to wall thickness ratio) is 511, so high-speed injection molding should be selected;
Gate type is a point gate, and a higher pressure must be used to overcome resistance during flow;
In order to ensure that product can be filled smoothly, molten material must have good fluidity, and molding temperature should be appropriately high;
When high pressure and high speed are injected to the end, it is easy to produce flash, molding machine must have low inertia pressure and speed switching;
Due to small wall thickness of product, pressure holding is not required; formulate main molding process parameters
To formulate injection process parameters, it is necessary to understand information of equipment performance, mold structure, molding materials and product quality requirements, set various molding parameters scientifically and reasonably.
First of all, process parameters should be adjusted gradually according to molding conditions of product. Correct adjustment sequence is pressure→speed→temperature. Each time parameters are changed, input parameters have been confirmed by computer before changing next parameter, and it should be avoided to change more than two parameters at the same time;
Secondly, when product enters stable production, it is necessary to maintain stability of each parameter as much as possible, and a detailed record should be made. If change is too large, reason should be found in time. In addition, molding process must be fixed as much as possible each time mold is online, so as to facilitate quality control of finished product.