In plastic injection molding process, there would be problems like warping, excessive shrinking, and part misalignment, these problem will affect injection molding parts.
Injection molding tolerances means the tolerance maximum intended dimensions range of injection molded parts. Injection molds are machined by CNC commonly, the technology tolerance is usually ±0.03mm unless more restrictive tolerance are designed.
It is the key to predict the injection molding tolerance at the early stage of design to prevent from expensive cost at stage of production, to reduce the deformation and parts misalignment. If it is not correctly determined and optimized, the assembly wil be a fault.
The regular injection molding tolerance parts expense will be less than high precision injection molding parts. The excessively high precision tolerance requirement not only raise the difficultly of injection molding process, but also raise the cost significantly. Therefore, to balance the precision with the cost control is very important, to determine reasonable injection molding tolerances for each parts and optimize them during design, production, and maintaining is the key to achieve cost-effective production with ensured quality.
Tolerance Stackup
Tolerance stackup means multiple injectioned parts has there individual tolerance, when they needs to be assembled together, we need to consider the cumulative effect of dimensional deviations. In the range of tolerance stackup, the different parts will be combined sucessfully, otherwise, they will have problem to fit with each other.
For instance, we have many holes in one part, and need another part to fit it, even one of them has the minimum tolerance, the another part has the maximum tolerance, they should combined with each other correctly as well.
Types of Injection Molding Tolerances
Dimensional Tolerance: It refers the range of dimensions of the parts may vary. The shrinkage will be larger due to the increase of part dimension during the cooling process.
Straightness or Flatness Tolerance: It is used to control the general shrinkage of large flat surface in the range of acceptable deformation. It is important for parts needed to be assembled, even slightly flatness tolerance can lead to misalignment.
Hole Diameter Tolerance: As the hole diameter increases, the tolerance becomes larger. Specificly important for structures with mounting screws, bolts, or locating pins. Proper hole diameter tolerance design can assure proper assembly, and avoid loosening or assembly difficulties caused by shrinkage.
Blind Hole Depth Tolerance: When plastic is injected at high pressure, the pin inserts can deflect. The deeper of the hole is, the tolerance defect risk will be higher.
Concentricity or Ovality Tolerance: The most critical tolerances for cylindrical components are concentricity and ovality, wherein circularity must be maintained. Concentricity refers to the degree of alignment between the centre axis of the cylindrical part and the reference axis, while ovality denotes the roundness of the component.
Commercial Tolerances and Precision Tolerances
Commercial tolerances are less precise, suitable for parts that require no strict precision. These parts can be injected by a low-cost injection mold to reduce overall cost.
Fine tolerances are also called precision tolerances, Its acceptable deviation range is narrower, ensuring greater dimensional consistency, but this means it requires strict precise injection mold and the overall cost is higher.
In part engineering drawings, tolerance grades are typically indicated alongside dimensions in the form of letters or numerals corresponding to the dimension, to specify the precision level or permissible error range for each feature.
British Plastics Federation (BPF) Breakdown of Causes and Proportional Contributions to Moulded Part Dimensional Errors:
A: Mould manufacturing errors account for approximately 1/3
B: Errors arising from mould wear account for 1/6
C: Errors arising from uneven shrinkage rates in moulded parts: approximately 1/3
D: Errors arising from discrepancies between intended and actual shrinkage rates: approximately 1/6
Total error = A + B + C + D. It is therefore evident that mould manufacturing tolerances should be less than one-third of the dimensional tolerances for moulded parts; otherwise, the mould cannot reliably guarantee the geometric dimensions of the moulded components.
DIN 16901 (and others) defines general tolerances for different materials and various positions of manufactured components. Moreover, ISO 20457:2018 – Tolerances for plastic moulded parts – Dimensional tolerances for moulded parts. This International Standard provides guidance on general dimensional tolerances for plastic moulded parts. It outlines general tolerances based on plastic type and part dimensions.
Shrinkage
When plastics release heat during the curing process, the molecules tend to contract back to a relaxed state, which is referred to as shrinkage.
The shrinkage rate defines the percentage decrease in a typical linear dimension as the molten polymer cools to ambient temperature. It is vital to fully understand it and compensate the shrinkage behaviour to achieve the accuracy of injection molding parts.
According to a standard such as ASTM D955. Shrinkage Rate = 100 % (Lc – Lp) / Lp ( Lc: Cavity Length, Lp: Part Length After Cooling)
The calculation results indicate a reduction in shrinkage along the flow direction of the melt, suggesting that shrinkage in the transverse direction of the moulded part may be overestimated during computation. As this equation is a linear function of length, deformation caused by shrinkage is relatively minor for smaller features.
Different materials have different shrink rates, if a material has a large shrink rate, there will be difficulty in controlling the injection molding tolerance.
Crystalline materials like PA, PP are usually harder to control tolerance, compared with amorphous materials like PE, PS.
Because crystalline materials undergo a phase transition during moulding from a crystalline solid state (high-density molecular arrangement) to an amorphous molten fluid (low-density molecular structure). This phase transition induces volume changes, thereby causing greater dimensional shrinkage.
Shrinkage and Dimensional Compensation
At the design phase, we need to compensate in advance for shrinkage because the parts shrink during the injection molding cooling process. High-precision cutting is achieved through CNC machining, with typical machining tolerances ranging from ±0.03 to 0.1 mm.
Upon injection into the mould, the molten plastic undergoes shrinkage during cooling,
with the shrinkage rate determined by the properties of the resin or plastic material employed.
In the design, this means designing the part to its nominal dimensions and enlarging the mold dimensions according to intended shrinkage dimensions, so that the injection molded parts will have accurate dimensions.
Mold Flow Analysis
For complex parts, the best practice is useing mold flow analysis, CAE softwares simulations show how plastic material fills injection moulds. Mold folw analysis can also help designers to identify mold areas that are difficult to fill.
Thermal Expansion
For most plastic, they have high thermal expansion rate, it is a challenge to keep strict injection molding tolerance. It’s very important especially when the part need to be used in huge temperature changing environments.
How to Reduce the Effect of Thermal Changes
Engineers need to know what kind of plastic material should be used for their injection molding parts, as using environment and outter climate change will affect material performance. For example, which is a project we have done for a Netherlands company. PP has better heat resistance than ABS, PP can endure 120℃-150℃, but ABS parts will have large deformation in environment that exceed 100℃.
Design For Manufacturability (DFM) — How To Control Injection Molding Tolerances
Warpage
It is the key to keep uniform wall thickness for wholes part, to prevent the uneven shrinkage rate. If the shrinkage rate is uneven, it will cause part deformation and affect the strict injection molding tolerance control.
In the process of mold cooling, all plastic parts will shrink. The parts with uniform thickness will shrink evenly and prevent defects like warpage and sink marks. Parts with uneven thickness will shrink in different rate, and the deformation will be higher.
Wall Thickness and Warpage
During solidification, the thicker sections shrink more than thinner sections because they have longer time for cooling down. Thick sections tend to sink, this could lead to exterior surface divots. Molten core shrinks inward and pulls the exterior surface with towards it. Warpage is common for injection molding parts with uneven thickness.
How to Reduce Warpage
Avoid to use thick walls because they will reduce the colling rate. In most cases, structure designs like support ribs or gussets are more effective than increase thickness for strength.The best way is to keep the uniform thickness for whole part. If you have to use uneven wall thickness, the chage in thickness should not exceed 15% of the nominal wall thickness and always use smooth or tapered transition.
Bosses
Bosses are a important part, it is the structure used to connect or fasten parts.
Avoid sharp internal corners, long unsupported spans or improper boss designs. In addition, bosses should be cored out and, whenever possible, attached to the nearest side wall. By doing this will make more uniform stress distribution, enhance the rigidity of the part and improving the flow properties of the material.
Draft angles
Draft angle is the slight angle that applied to the surface at the same direction of the mold pull direction. It is essential for ensuring the part will eject smoothly and prevent from shrinkage and scratch of the surface.
1.5-2 degrees of draft is commonly the minimum safe value in the design.
Gate Location
Gate is the location where the injection molding materials enter the mold.
The gate must be located properly, otherwise, there will be some problems:
- Uneven fill rate, it will cause warping and shrinkage of the part.
- Appearance defects, the injection molded part poor surface or visible flow lines.
The gate should be put at the deepest location of the part or the deepest cross-section, it will reduce sink marks and void formation, ensuring smooth flow of the resin. Meanwhile, the gate should be located at the thick-walled areas so that we can achieve adequate material packing and proper compaction during the holding stage.
More Tips for Mold Design in Achieving Tolerances
- A well-designed mold should also have several features:
- Low-shear/turbulence gating, good venting system, enough cooling channels and proper cavity surface finishes. These design elements are helpful to achieve minimum warping, shrinkage, voids/taps and other defects, and ensure the injection molding tolerances are under control, injection molding parts will have a consistent dimension range.
- Sharp corners and thick sections can cause stress concentration, so radius corners and gradual wall thickness transitions are necessary.
- Gate and venting system should ensure the optimum melt folw, the shortest flow path, adequate packing and proper welding line formation.
- Draft angles should be provided on all vertical faces in the direction of draw to allow easy part ejection.
- Undercut features should be minimized or combined whenever possible, and sliders should be used when necessary to facilitate smooth ejection.
- The mold should be designed with fewer cavities, shorter and simpler runners, and a higher surface finish than standard molds to improve part release.
Material Selection
Thermoplastic materials have a wide range of shrinkage rates and thermal expansion coefficients, which directly affect the accuracy with which the solid polymer replicates the cavity shape after molding.
Different injection molding materials have different shrinkage rates; even materials from different batches or suppliers may vary in shrinkage behavior.
For long fiber–reinforced polymers, shrinkage is smaller in the melt flow direction, which often results in anisotropic (non-uniform) shrinkage in molded parts.
Since molds are typically designed with slightly oversized cavities to compensate for material shrinkage, it is crucial to determine the type of plastic material to be used early in the design phase. This helps reduce cosmetic and structural defects such as warpage, sink marks, cracking, and twisting, while improving the durability and dimensional stability of injection molded parts.
When controlling dimensional accuracy, it is important to carefully evaluate the key resin properties, such as the melt flow index (MFI) — the flow rate of molten polymer under specified test conditions — viscosity, and cooling behavior at both the micro and macro levels.
Injection molding is compatible with a wide variety of plastic resins, whose mechanical properties can be enhanced or modified through the use of additives, fillers, and stabilizers to improve performance and reduce molding shrinkage.
Although the shrinkage rate of a resin is generally predictable, small variations in formulation or batch composition can still affect shrinkage behavior and consequently the final part tolerance and assembly precision. Additionally, the larger the part, the more noticeable the shrinkage variation becomes.
Therefore, it is essential to consider differences in shrinkage rates across materials to ensure that individual parts fit together as designed.
A thorough understanding of the physical and processing properties of specific plastic materials is the key to maintaining consistent dimensional accuracy and long-term stability in plastic injection-molded parts.
Process Control
Injection pressure, holding time, material density, and mold temperature, all of them is very important for the injection molded part overall shrinkage. By adequate pilot injection molding, monitoring and adjusting these parameters can achieve the best injection molding results.
By controlling these injection molding parameters steadily, we can ensure there will be repeatable and predictable shrinkage of parts.
Tool Colling/Mold Cooling
It is the process that the heated plastic polymer cools evenly before the part ejection from the mold.
Different part shapes, sizes and wall thicknesses have different requirements for mold cooling system, the mold temperature controls the cooling time largely.
Cooling process must maintain uniform, otherwise, there will be defects like shrinkage, sink marks, jetting, warping, etc.
When the mold temperature is high, the molded parts have a high temperature as well, the actual shrinkage rate will be increased.
We need to keep the mold temperature as low as possible in an acceptable condition, within the recommended range so that the plastic resin maintains in best physical properties.
This will help to reduce the injection cycle, improving product efficiency. When the mold temperature is kept low, the shrinkage rate will be low and the part dimensions will be accurate, avoiding deformation, poor gloss and cooling marks of molded parts.
To achieve uniform mold cooling, the mold designers should put the cooling channels at proper locations, this will ensure adequate heat dissipation and temperature control.
By inspecting injection pressure, plastic resin viscosity, and mold fill time, engineers can monitor and optimize injection parameters, ensure proper pressure, heating and cooling during the process. Mold fill analysis is very important to predict mold temperature, heating, cooling, shrinkage and wrapping.
Once these parameters are under proper control, mold can produce parts with tight tolerances and minimal deviation.
Pressure and Venting Control
The proper setting of injection pressure and the matching of clamping force should be determined during the mold trial stage.
Holding pressure has a significant influence on the dimensional accuracy of precision parts.
Specifically, holding pressure helps compensate for shrinkage and reduce part deformation.
A stable screw end position is the key factor that determines the effectiveness of the holding phase.
Air trapped in the cavities and cores, as well as gases generated by molten plastic, must be vented through exhaust grooves. If venting is inadequate, it can lead to incomplete filling, weld lines, or even burn marks on the part surface.
Post-Molding Dimensional Correction and Adjustment
Some injection molded parts will have deformations and wrapping when the temperature and pressure change after ejected from the mold.
Use auxiliary fixtures timely when the part is ejected, we can get good correction when the part is cooling down.
Regular Maintenance and Quality Inspection
By inspecting and maintaining regularly, we can find and fix mold wear. Mold wear is the major problem that will cause tolerance issues.
It is important as well to implement quality control actions, such as dimensional inspection and statistical process control (SPC). This will be helpful to find and correct tolerances in the early stage.
Sourcing Simplified – Start Your Next Project With YonLion
Ready to optimize the injection molding process, our expert YonLion can guide you with complex injection molding tolerances management, offer parts that fully comply with your requirements.
Project Evidence: Tolerance Is a Functional Decision
In a four-mold interlocking toy project, machine limits, soft-material shrinkage and mating fit were reviewed together. Successive samples were measured and assembled before handover.
Build a Feature-Level Tolerance Plan
Send the 3D model, resin, appearance zones, critical dimensions and expected volume. We will identify the inputs needed for DFM, tooling and trial planning.