plastic injection molding tolerance

Plastic Injection Molding Accuracy & Tolerance Standards

Plastic Injection Molding Accuracy & Tolerance Standards

Injection molding technology is one of the most common methods used in the plastics processing industry. It allows you to get ready-made elements of almost any shape.

For a material such as plastic, injection molding is a very efficient injection technology. It has been known since the first half of the last century. Since then, the technology has been constantly improved – in response to ever new types of plastics, the needs of subsequent industries seeking this method, and ever-increasing demands from the end users of the products.

Plastic injection molding - what is it?

Thermoplastic injection molding is a standard process that uses a mold. The volume of which is filled with a polymer melt. The injection molding machine allows injection pressure and speed, quality control and minor errors, so products are usually ready to ship immediately after production.

The plastic granulate is loaded into a cylinder where it is melted, compressed and injected into the mold channels. The hot material solidifies in the mold space and thus the cast element is created. Plastic injection technology means that ejectors eventually remove the part from the mold, which falls into the supply tank. As soon as the series is ready, the parts (or the main series) are immediately packed and shipped.

Generally, there are two indicators for precision injection molding:

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℃.

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Technical focus

Injection molding machine

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.


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.


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?

When injection molding plastic parts, which standard is better for tolerance?

Plastic Injection Molding Accuracy

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.
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Factors Affecting Tolerances of Plastic Parts  

Before discussing standards, it is necessary to discuss what factors affect the tolerances of plastic parts 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.

Plastic Properties Product Design Mold Design Injection Molding Process
(isotropic and anisotropic)
Product structure Mold accuracy Injection molding
machine performance
Dimensional stability Wall thickness Number of
mold cavities
Viscosity Draft angle Runner system Hold
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

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