Plastic Injection Molding Accuracy & Tolerance Standards

injection molding Accuracy

The precision of injection molding usually refers to the precision requirements of dimensional accuracy, appearance quality and physical properties of injection molding products.

In the process of injection molding, there are many factors affecting the molding accuracy, including raw materials, injection molding equipment, mold design and manufacturing, process parameters and so on. 

Plastic Injection Molding Accuracy

The following are some of the main factors affecting the accuracy of injection molding:

Raw Material

The raw material of injection molding is thermoplastic, whose composition and quality have important influence on the molding accuracy. The raw material should have a certain fluidity and stability to ensure that the temperature and pressure during the molding process are controlled within the appropriate range.

Injection Molding Machine

The stability, precision and flexibility of injection molding machine have great influence on the molding accuracy. A suitable injection molding machine should be able to control temperature, pressure, speed and other parameters, and have sufficient closed-loop control and data acquisition capabilities.

Mold Design and Manufacturing

The precision and quality of the mold directly affect the size and appearance quality of the molding product. The mold should have sufficient stiffness and precision to ensure the accuracy and surface finish of the molded product.

Technological Parameter

The process parameters of injection molding include injection temperature, pressure, speed, cooling time, etc. The setting of these parameters has great influence on the molding accuracy and physical properties. These parameters should be set reasonably according to the characteristics of specific products and raw materials.

Plastic Injection Molding Tolerance Standards

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The tolerance standard of injection molding is determined according to the design requirement, function requirement and application requirement of the molding product. In the development of tolerance standards for injection molding, the following aspects generally need to be considered:

Design requirements of molding products:

The design requirements of molding products include size, shape, surface finish, material, color and so on. According to the design requirements to determine the tolerance standard can ensure that the molding products meet the design requirements

Functional requirements of molding products:

Functional requirements of molding products include bearing capacity, sealing performance, wear resistance, corrosion resistance, electrical conductivity and so on. The tolerance standard can be determined according to the functional requirements to ensure that the product has sufficient functional performance.

Application requirements of molding products:

Application requirements of molding products include environmental requirements, safety requirements, regulatory requirements and other aspects. According to the application requirements, the tolerance standards can ensure that the molding products meet the application requirements and safety standards.

Two Indicators For Precision Injection Molding:

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One is equipment accuracy, and the other is mold error. The former is difficult to compare due to the difference in size and product thickness, which represents the comprehensive level of injection molding machines.

Generally, the weight repetition error of ordinary injection molding machines is about 1%. , a better machine can reach 0.8%, less than 0.5% is a precision machine, and less than 0.3% is an ultra-precision machine. According to relevant data reports, the international highest level is less than 0.15%. 

The other is mold accuracy. In addition to the experience of engineers and technicians, the accuracy of molds also needs to be completed by high-precision mold processing equipment.

Therefore, first-class mold processing equipment is a necessary condition for precision injection molding.

Influencing Factors

① Consider the shrinkage rate of the plastic.

② Product shape and size Shape and size of plastic parts.

③ Precision of mold plastic parts.

④ Mold machining accuracy.

⑤ The injection molding machine has high precision requirements for the products.

Specific Requirements

First of all, the injection pressure should be greater than 25MPa; the injection speed should be greater than or equal to 300mm/s; the dimensional accuracy of the product should be 0.001mm; the standard deviation coefficient (change rate) of the product quality should be less than 0.1%.

The injection position accuracy (pressure holding end point) is less than 0.03mm; the force balance of the tie rod is less than 1%; the pre-molding position accuracy is less than 0.03mm. The maximum mold force is less than 0.005 mm; the temperature control accuracy of the barrel and screw is less than ±0.5℃.

Technical Focus

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Clamping force: In the process of precision injection molding, high injection pressure is generally required, otherwise it is difficult to mold. [2] 

Measurement accuracy: To maintain high stability of plastic products, the hot melt plastic injected into the cavity must be equal each time.

Injection speed: Injection speed is an important indicator of precision injection molding machines.

Injection pressure: Precision injection requires a large injection pressure to improve the compactness of plastic parts, reduce shrinkage and deformation, and ensure the accuracy of parts.

Holding pressure: holding pressure has a great influence on precision plastic parts.

Mold temperature: The temperature change of the cavity surface should be controlled within ±1°C. 

Process Features

In order for the injection molding process to proceed smoothly and the resulting product to be of good quality, it is necessary to ensure the appropriate parameters during the implementation of each injection molding cycle.

An important element of the plastic injection molding process is duration. That is, the time that the plastic remains in the mold under high pressure while solidifying. Usually it is from a few seconds to about twenty. Most often, this time depends on the type of material, as well as the size and weight of the product.

Too short a pressure may cause the surface of the product to collapse or cause other damage. Leaving the material for too long is not beneficial. High pressure that is maintained for a very long time can cause the material to be over-sealed and make it difficult to remove the product from the mold cavity.

Plastic injection molding involves the use of special molds. They usually consist of two parts: one is attached to the table of the casting machine, and the other is movable, from which the finished product is removed.

If the forming object is large, then the form will have one nest. However, when small parts are produced, it happens that the mold has several or even about ten forming cavities. This increases the efficiency of the entire process. Depending on the number of seats, the molds can have a different number and different lengths of channels that distribute the plastic mass.

Types Of Molds

This system ensures that the molten material is delivered directly to the forming cavity. According to the design features of gating systems, the following molds are distinguished:

– Cold channel: Differs in low cost, simplicity of operation, high reliability. However, it has a significant drawback associated with the design features. During the processing of raw materials, the sprue remains cold, therefore, after cooling, the product falls out of the mold along with it.

– Hot runner: When casting using this design, the gate remains in a molten state, so it does not fall out in a solidified form along with the finished molded part. Thus, raw materials are saved, and production becomes waste-free. However, the costs need to be weighed: hot runners are much more expensive, often fail and require a lot of energy for cooling.

Types Of Molds By The Number Of Plates

Cold runner mold systems, in turn, differ in the number of working plates:

 Double plates: A simple device with only one connector.

– Three-slab: In this case, the device has 2 connectors, which makes it possible to determine the location of the sprue. The liquid polymer is evenly distributed over the plane of the mold, eliminating the possibility of manufacturing a defective product. Used to create parts whose production requires high precision?

Which standard is better for tolerance?

When designing engineering drawings of plastic parts, many engineers will face the confusion of how to set the relevant dimensional tolerances.For example, for a length dimension with a value of 100mm, is the tolerance set to +/-0.1mm, +/-0.2mm , +/-0.3mm, +/-0.4mm, or +/-0.5mm?

Tolerances are set too finely, which injection molding suppliers say are difficult and costly.

The tolerance is set too loose, and there is concern that the appearance, function and reliability of the product will be affected after assembly.

In short, too precise and too loose tolerance settings will have problems, and will be considered unprofessional, do not understand injection molding, and will be despised.

Factors Affecting Injection Molding Tolerances

Before discussing standards, it is necessary to discuss what factors affect the injection molding tolerances from the perspective of the injection molding process. There are many factors that affect the tolerance of plastic parts, including four aspects: plastic characteristics, product design, mold design and injection molding process.

Document
Plastic Properties Product Design Mold Design Injection Molding Process
Shrinkage
(isotropic and anisotropic)
Product structure Mold accuracy Injection molding
machine performance
Dimensional stability Wall thickness Number of
mold cavities
Injection
pressure/speed
Viscosity Draft angle Runner system Hold
pressure/speed
Whether to add
Reinforcing fiber
Symmetry Ejection system Melting temperature
and mold temperature
Surface treatment Cooling system Clamping force
Size Die Design/Layout Repeatability

Of all the factors above, two are the most critical:

Shrinkage rate of plastic materials ;plastic materials with small shrinkage rate are easy to obtain higher dimensional accuracy; plastic materials with large shrinkage rate have relatively low dimensional accuracy;

Whether the dimension is affected by the mold movement in the mold; if the dimensions are all in the same mold cavity and the dimension is not affected by the mold movement, then higher dimensional accuracy can be obtained.

If the dimensions are on both cavities (or On the slider with the inclined pin), there will be an additional assembly error of the two cavities (or with the inclined pin slider), and at the same time, it will be affected by the movement between each other, and the dimensional accuracy is relatively

Author

Gavin Leo is a technical writer at Aria with 8 years of experience in Engineering, He proficient in machining characteristics and surface finish process of various materials. and participated in the development of more than 100complex injection molding and CNC machining projects. He is passionate about sharing his knowledge and experience.

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