Injection Moulding Process Optimization

What are the tips for injection moulding process optimization? How to optimize the injection molding process? 

The further development of injection molding equipment and the continuous improvement of product quality requirements have both put forward higher requirements for the injection moulding process optimization.

Correct selection of injection equipment and reasonable setting of the injection moulding process and injection moulding process optimization conditions are the keys to improve the quality of products. 

Correct selection of injection moulding machine in injection moulding process optimization

The performance of injection moulding machines directly affects the quality of injection molded products, and the price of injection moulding machines with different specifications and performance requirements can vary a lot. 

Injection moulding machine specifications selection 

When choosing injection moulding machine specifications, the first thing to consider is the condition of the production mold, because the same injection molding machine often has to meet the production of multiple molds of different sizes, the injection moulding machine specifications should be determined according to the weight of the parts, mold size, etc.,

That is, the maximum clamping force and maximum injection volume of the injection moulding machine, and then choose the appropriate model according to the specifications provided by the injection moulding machine manufacturer.

Most of the manufacturers provide customization service, which provides great convenience for the purchase of injection moulding machine; secondly, we should consider whether we need some special configurations, such as special screws for the production of PA, PC and other materials, and corresponding devices for the moulding of moulds with inlet and outlet cores or threads.

Again, according to the mould structure, product quality, and other factors to determine the need to use some special features of the injection moulding machine, such as moulding thin-walled long flow products (generally refers to L/D>300), the need to use high-speed injection moulding machine, precision electronic parts need to use precision full closed-loop control injection moulding machine, etc.

Clamping force setting in injection moulding process optimization

Theoretically, the clamping force can be calculated according to the following formula 

Fcm>=K × P average × A products × 10 

Where: Fcm – clamping force (KN) 

K-safety factor, generally taken as 1-1.2 

P average – average pressure of mold cavity (MPa) 

A product – the maximum projection area of the product on the mold parting surface (cm2) 

In actual production, the adjustment of clamping force should also take into account the influence of the thermal expansion of the mold in production, and should generally leave a margin of 0.1-0.2mm; the principle of setting the clamping force is to ensure product quality under the premise of low clamping force is appropriate. 

Injection moulding process parameters setting in injection moulding process optimization

Barrel temperature, mold temperature According to the properties of different plastic materials to set the screw barrel temperature, the barrel set temperature is generally higher than the plastic melting point 10 ℃ -30 ℃.

It is important to note that the melting point and the allowable residence time in the barrel may vary depending on the synthesis method or the type of additives added to the materials provided by different manufacturers.

Mold temperature is generally cooled using circulating water when setting, but when producing products with precise dimensions or high surface quality requirements, a mold temperature machine capable of accurate control should be used according to the process requirements.

Injection holding time and cooling time in injection moulding process optimization

The injection time, holding time and cooling time should be set according to the product thickness, mold temperature, material properties, etc. A long injection time will not only produce negative effects such as mechanical wear and tear and increased energy consumption but will also prolong the molding cycle.

The holding time is set according to the thickness of the product. Thin-walled products can be molded without holding time.

When setting the holding time, as long as there is no obvious depression on the surface of the product, it can also be determined by the weighing method, and the best holding time can be set by gradually extending the holding time until the product quality no longer changes.

The cooling time should also be determined according to the product thickness, mold temperature and material properties. Generally, amorphous polymers require longer cooling time than crystalline polymers. 

Injection pressure and speed in injection moulding process optimization

The injection pressure setting should follow the principle of low rather than high, as long as it can provide enough power to achieve the required injection speed and make the melt fill the cavity smoothly, too high pressure will easily cause internal stress in the product.

However, when molding products with high dimensional accuracy, in order to prevent excessive shrinkage, the high-pressure injection can be used to reduce the shrinkage of the product after demolding. 

The injection speed will affect the appearance quality of the product, and its setting should be set according to the geometric structure of the mold, exhaust condition, etc. Generally, the injection speed should be increased as much as possible to reduce the filling time under the premise of ensuring a good appearance.

In injection molding, when the melt flows in the mold, the mold wall will form a curing layer, thus reducing the thickness of the flowable channel. Generally, depending on the mold structure and injection speed, the mold wall will have a curing layer of about 0.2mm. Therefore a faster injection speed is usually used in molding. 

Injection stroke, multi-stage injection parameters 

In molding, the injection stroke must be determined first. Theoretically, the injection stroke can be calculated according to the following formula 

S1=4(CVp+Va)/ρDs2 

Where: S1 – injection stroke Vp – product volume 

ρ – resin density C – number of cavities 

Va-gate volume Ds-screw diameter 

In practice, if the total weight of the product + gate is known, the following formula can be used to calculate the injection stroke 

S1=(M/Max)-Smax+(5~10)mm 

Where: S1—injection stroke, mm 

M-Total weight of product+gates, g 

A mmax-Maximum injection volume of the injection machine, g 

Smax-Maximum injection stroke of injection machine, mm 

Due to the different geometries of the sprue system and mold parts, there are different requirements for the flow state of the melt in different parts of the mold (mainly referring to the pressure and speed when flowing) to meet the product quality requirements.

In an injection moulding process, when the screw pushes the melt into the injection mould, it is required to achieve different pressure and speed at different positions, which is called multi-stage injection moulding.

Generally, it is more scientific to set at least three or more than four injection stages in molding, i.e. the first stage at the main flow channel, the second stage from the manifold to the gate, the third stage when the product is filled with about 90% of the cavity, and the fourth stage for the remaining part, which can be used to determine the switching position of each stage by calculating the weight method.

In actual production, the multi-stage injection process parameters should be scientifically analyzed and reasonably set according to product quality requirements, runner structure, and mould exhaust condition.

Usually, debugging observation method can be used to set the pressure/speed of the switching position point required for injection is set to 0. Observe the directional position of the melt and the defective condition of the product, and gradually make adjustments until a reasonable position point is found.

However, in the process of debugging observation, we must pay attention to the demoulding condition of under-injected products to avoid sticking in some depressing parts of the mould due to under-injection. 

Other process parameters in injection moulding process optimization

In injection moulding process optimization, in addition to the setting of several main parameters such as molding temperature, pressure, speed, time and multi-stage injection switching position, there are many other injection moulding process optimization parameters such as backpressure, screw speed, screw inversion anti-flow delay, and other action parameters settings, which should not be neglected. 

Injection moulding process optimization parameter setting example 

The product quality requirements are: the product meets the specified tensile strength standard; the surface is free of silver, bubbles, shrinkage marks, and other kinds of undesirable phenomena; the product has good tightness and no loosening after molding.

The material used is PA66; the injection molding machine is JSW1000-EⅡ-SP, the mold structure is hot runner type, and the gate type is point gate. 

Firstly, according to the characteristics of the product and the structure of the mold, we will determine the principle of setting injection moulding process optimization parameters.

(1) Because of the long flow length of the product, L/t (flow to wall thickness ratio) is 511, so high-speed injection molding should be selected.

(2) The gate type is point gate, higher pressure should be used to overcome the resistance during flow.

(3) To ensure that the product can be filled smoothly, the melt must have good flowability and the molding temperature should be appropriately high.

(4) High pressure and high-speed injection to the end of the easy to produce flying edge, the molding machine must have low inertia pressure, speed switching.

(5) Because of the small wall thickness of the product, it is not necessary to use pressure-holding.

To formulate injection moulding process optimization parameters, we must understand the information of equipment performance, mold structure, molding materials and product quality requirements, and set each molding parameter scientifically and reasonably.

First of all, the injection moulding process optimization parameters should be adjusted step by step according to the product forming condition, and the correct order of adjustment is pressure → speed → temperature.

Each time the injection moulding process optimization parameters are changed, after the input parameters have been confirmed by the computer, the next parameter change should be made, and more than two parameters should be avoided at the same time.

Secondly, when the product enters into stable production, the injection moulding process optimization parameters should be kept stable as much as possible, and detailed records should be made, if the change range is too large, the reason should be found in time.

In addition, the molding process should be fixed as much as possible every time the mold is put on line to facilitate the quality control of the finished product.

After the read of the injection moulding process optimization article, you can get the tips of injection moulding process optimization.

You may also be interested in the below articles.

Summary Of 50 Injection Mold Structure Operation Dynamic Diagrams

What Is An Overmold?

Auto Parts Stamping Die Design Concept

18 FAQ Of Injection Molding Machine Mold Clamping

What Is Cold Forging?

Mechanical design and Taboos of mechanical design

Share: