Analysis of Dimensional Control for Injection Molded Parts

With the rapid development of the mold industry in recent years, along with the continuous expansion and advancement of the application scope of new technologies and processes, there has been a qualitative change of Analysis of Dimensional Control from traditional experiential accumulation to software development applications for injection molded parts.

The widespread application of CAD, CAM, and CAE has opened up space for the geometric dimension control technology of our molds and molded products. Due to the significant differences in market demand for molds and molded products, as well as their diverse types and considerable variations in aspects such as shape, size, material, and structure, we have encountered many problems and difficulties in the production process of molds and products. Among them, how to effectively control the geometric dimensions of molds and products is prominently placed before us.

Different types of molds and products require different control technologies and methods. Today, I will share some insights on the dimensional control of injection molded products, which naturally involves discussing injection molds. Generally, I start from the following aspects:

I. Injection Molded Parts Control in Mold Design:

Firstly, it is essential to fully understand the technical requirements of users in various aspects such as mold structure, material, hardness, and precision, including whether the shrinkage rate of the molding plastic is correct and whether the 3D dimensions of the product modeling are complete, and conduct reasonable processing and analysis.

Fully consider the areas affecting the appearance of injection molded parts such as shrinkage holes, flow marks, draft angles, weld lines, and cracks.

While not compromising the functionality and pattern formation of injection molded products, simplify the mold processing methods as much as possible.

Carefully choose the parting surface to ensure proper mold processing, forming appearance, and removal of burrs from the molded part.

Evaluate whether the ejection method is appropriate, considering the use of ejector pins, stripper plates, ejector sleeves, or other methods, and whether the positions of ejector pins and stripper plates are appropriate.

Assess the suitability of the side core pulling mechanism, ensuring flexible and reliable operation without any jamming.

Determine the most suitable method for temperature control, including the choice of temperature control oil, temperature control water, cooling fluid circulation structure, the size of cooling fluid holes, quantity, and positioning, for better adaptation to plastic products.

Evaluate the gate form, size of runners and sprue, and whether the gate positions and sizes are appropriate.

Assess the influence of various module and core thermal deformation effects and the suitability of standard parts selection.
Evaluate whether the injection volume, injection pressure, and clamping force of the injection molding machine are adequate, and whether parameters like nozzle radius and sprue bushing diameter are appropriately matched.

Comprehensive analysis and preparation from these aspects should be carried out from the initial stage of product development.

II. Injection Molded Parts Control in Manufacturing Process:

Although comprehensive considerations and arrangements are made during the design phase, various problems and difficulties may still arise during actual production. We should strive to adhere to the original design intent as much as possible during production and identify more effective and economically rational process means during actual machining.

Select economically suitable machine tools and devise 2D and 3D machining plans.

Consider using appropriate jigs and fixtures for auxiliary preparation work during production, rational use of cutting tools to prevent part deformation, fluctuation in part shrinkage rate, and demolding deformation, thereby improving mold manufacturing precision, reducing errors, and preventing changes in mold precision.

Regarding the causes of dimensional errors in molded parts and their distribution proportions, mentioned by the British Plastics Federation (BPF): A: Mold manufacturing errors account for about 1/3, B: Errors generated by mold wear account for 1/6, C: Errors caused by uneven shrinkage rates of molded parts account for about 1/3, D: Errors caused by inconsistent predetermined shrinkage rates and actual shrinkage rates account for about 1/6. The total error = A + B + C + D, thus indicating that mold manufacturing tolerance should be below 1/3 of the part size tolerance to ensure the geometric dimensions of the molded part.

III. Injection Molded Parts Control in Production:

Geometric dimensional fluctuations of molded parts after molding are a common and frequent problem.

1， Temperature control of materials and molds:

Different grades of plastics have different temperature requirements. The use of plastics with poor flowability and the use of mixed materials can lead to different situations. Mold temperature control is relatively more complex. Different geometric shapes, sizes, and proportions of wall thickness of molded parts have certain requirements for cooling systems. Mold temperature largely controls the cooling time. Therefore, it is advisable to keep the mold at an allowable low temperature to shorten the injection cycle, improve production efficiency, and maintain stable dimensions.

2， Adjustment and control of pressure and exhaust:

The appropriate injection pressure and matching clamping force should be determined during mold debugging. Air in the gaps formed in the mold cavity and core and gases generated by plastics must be exhausted from the mold. Inadequate exhaust may result in insufficient filling, weld lines, or burns. When there are thick-walled sections around thin-walled parts of molded parts, insufficient filling will occur if the mold temperature is too low, and burns will occur if the mold temperature is too high.

Usually, there are simultaneous appearances of weld lines in burnt areas. Exhaust slots are often overlooked and are generally too small. Therefore, under normal circumstances, as long as there are no burrs, the depth of the exhaust shoulder should be as deep as possible, and larger ventilation slots should be opened at the rear of the shoulder to allow gases to be quickly expelled from the mold after passing through the shoulder. If necessary, exhaust slots can be opened on the ejector pins. The principle is the same: to avoid burrs and ensure rapid exhaust to achieve good results.

3， Supplementary and reshaping control of injection molded part dimensions:

Some injection molded parts may undergo various deformations and warping due to changes in temperature and pressure after demolding. In such cases, auxiliary fixtures can be used for adjustment. Prompt remedial measures should be taken after demolding, and good correction and adjustment effects can be achieved after natural cooling and shaping. If strict management is ensured throughout the entire injection molding process, the dimensional control of injection molded parts can be ideally achieved.