The precision and accuracy of any molded plastic part hinge on the understanding and control of injection molding tolerances. These tolerances define the allowable range of variation in part dimensions resulting from the injection molding process. This guide will delve into the importance of injection molding tolerances, the factors that affect them, and how to optimize them for a specific project. We will also explore different measurement and inspection techniques used to verify these tolerances.

What are Injection Molding Tolerances?

Injection molding tolerances refer to the permissible limits within which the dimensions of a molded part can vary while still maintaining its functionality and fit. These tolerances are typically expressed as a plus or minus value from the nominal dimension. The standard injection molding tolerances range from +/-0.1mm for common parts to +/-0.025mm for parts demanding high precision.

These tolerances are crucial in plastic injection molding, as even slight deviations can cause assembly issues, functional problems, or aesthetic defects. For instance, if the holes in two parts meant to be bolted together are not accurately located or sized due to incorrect tolerances or mold tooling, it could lead to assembly errors and loss of function.

Why are Injection Molding Tolerances Important?

Tolerances play a vital role in the effective functioning of injection-molded parts. They ensure that all parts fit and function together as designed, even when there are slight variations in the molding process.

In any rapid prototyping process, a degree of variation is inevitable. Despite being a highly precise molding technique, injection molding is not immune to these variations. Hence, it is crucial to determine the acceptable range of deviation for the parts to function effectively after assembly.

When you control and optimize injection molding tolerances, you essentially prepare for the “worst-case scenario.” You determine the acceptable range of deviation that ensures optimal functioning of the product, thereby mitigating the risk of part failure or assembly issues.

What Are the Standards of Injection Molding Tolerances?

Consistent precision is fundamental when it comes to injection molding; striving for true injection molding tolerances facilitates superior design possibilities. Venturing into understanding possible plastic injection molding tolerances can significantly enhance the efficacy of the plastic mold design process. Tolerances can be limited by several factors, but there are always professional standards.

Plastic injection molding tolerance standards
											GB/T 14486-2008
														Measurement: mm
Tolerance Level		Dimension Range
		>0	>3	>6	>10	>14	>18	>24	>30	>40	>50	>65	>80	>100	>120
		~3	~6	~10	~14	~18	~24	~30	~40	~50	~65	~80	~100	~120	~140
Dimension with tolerance requirement
MT1	a	0.07	0.08	0.09	0.1	0.11	0.12	0.14	0.16	0.18	0.2	0.23	0.26	0.29	0.32
	b	0.14	0.16	0.18	0.2	0.21	0.22	0.24	0.26	0.28	0.3	0.33	0.36	0.39	0.42
MT2	a	0.1	0.12	0.14	0.16	0.18	0.2	0.22	0.24	0.26	0.3	0.34	0.38	0.42	0.46
	b	0.2	0.22	0.24	0.26	0.28	0.3	0.32	0.34	0.36	0.4	0.44	0.48	0.52	0.56
MT3	a	0.12	0.14	0.16	0.18	0.2	0.22	0.26	0.3	0.34	0.4	0.46	0.52	0.58	0.64
	b	0.32	0.34	0.36	0.38	0.4	0.42	0.46	0.5	0.54	0.6	0.66	0.72	0.78	0.84
MT4	a	0.16	0.18	0.2	0.24	0.28	0.32	0.36	0.42	0.48	0.56	0.64	0.72	0.82	0.92
	b	0.36	0.38	0.4	0.44	0.48	0.52	0.56	0.62	0.68	0.76	0.84	0.92	1.02	1.12
MT5	a	0.2	0.24	0.28	0.32	0.38	0.44	0.5	0.56	0.64	0.74	0.86	1	1.14	1.28
	b	0.4	0.44	0.48	0.52	0.58	0.64	0.7	0.76	0.84	0.94	1.06	1.2	1.34	1.48
MT6	a	0.26	0.32	0.38	0.46	0.52	0.6	0.7	0.8	0.94	1.1	1.28	1.48	1.72	2
	b	0.45	0.52	0.58	0.66	0.72	0.8	0.9	1	1.14	1.3	1.48	1.68	1.92	2.2
MT7	a	0.38	0.46	0.56	0.66	0.76	0.86	0.98	1.12	1.32	1.54	1.8	2.1	2.4	2.7
	b	0.58	0.66	0.76	0.86	0.96	1.06	1.18	1.32	1.52	1.74	2	2.3	2.6	2.9
Dimension without tolerance requirement
MT5	a	±0.10	±0.12	±0.14	±0.16	±0.19	±0.22	±0.25	±0.28	±0.32	±0.37	±0.43	±0.50	±0.57	±0.64
	b	±0.20	±0.22	±0.24	±0.26	±0.29	±0.32	±0.35	±0.38	±0.42	±0.47	±0.53	±0.60	±0.67	±0.74
MT6	a	±0.13	±0.16	±0.19	±0.23	±0.26	±0.30	±0.35	±0.40	±0.47	±0.55	±0.64	±0.74	±0.86	±1.00
	b	±0.23	±0.26	±0.29	±0.33	±0.36	±0.40	±0.45	±0.50	±0.57	±0.65	±0.74	±0.84	±0.96	±1.10
MT7	a	±0.19	±0.23	±0.28	±0.33	±0.38	±0.43	±0.49	±0.56	±0.66	±0.77	±0.90	±1.05	±1.20	±1.35
	b	±0.29	±0.33	±0.38	±0.43	±0.48	±0.53	±0.59	±0.66	±0.76	±0.87	±1.00	±1.15	±1.30	±1.45
Tolerance Level		Dimension Range
		>140	>160	>180	>200	>225	>250	>280	>315	>355	>400	>450	>500	>630	>800
		~160	~180	~200	~225	~250	~280	~315	~355	~400	~450	~500	~630	~800	~1000
Dimension with tolerance requirement
MT1	a	0.36	0.4	0.44	0.48	0.52	0.56	0.6	0.64	0.7	0.78	0.86	0.97	1.16	1.39
	b	0.46	0.5	0.54	0.58	0.62	0.66	0.7	0.74	0.8	0.88	0.96	1.07	1.26	1.49
MT2	a	0.5	0.54	0.6	0.66	0.72	0.76	0.84	0.92	1	1.1	1.2	1.4	1.7	2.1
	b	0.6	0.64	0.7	0.76	0.82	0.86	0.94	1.02	1.1	1.2	1.3	1.5	1.8	2.2
MT3	a	0.7	0.78	0.86	0.92	1	1.1	1.2	1.3	1.44	1.6	1.74	2	2.4	3
	b	0.9	0.98	1.06	1.12	1.2	1.3	1.4	1.5	1.64	1.8	1.94	2.2	2.6	3.2
MT4	a	1.02	1.12	1.24	1.36	1.48	1.62	1.8	2	2.2	2.4	2.6	3.1	3.8	4.6
	b	1.22	1.32	1.44	1.56	1.68	1.82	2	2.2	2.4	2.6	2.8	3.3	4	4.8
MT5	a	1.44	1.6	1.76	1.92	2.1	2.3	2.5	2.8	3.1	3.5	3.9	4.5	5.6	6.9
	b	1.64	1.8	1.96	2.12	2.3	2.5	2.7	3	3.3	3.7	4.1	4.7	5.8	7.1
MT6	a	2.2	2.4	2.6	2.9	3.2	3.5	3.9	4.3	4.8	5.3	5.9	6.9	8.5	10.6
	b	2.4	2.6	2.8	3.1	3.4	3.7	4.1	4.5	5	5.5	6.1	7.1	8.7	10.8
MT7	a	3	3.3	3.7	4.1	4.5	4.9	5.4	6	6.7	7.4	8.2	9.6	11.9	14.8
	b	3.2	3.5	3.9	4.3	4.7	5.1	5.6	6.2	6.9	7.6	8.4	9.8	12.1	15
Dimension without tolerance requirement
MT5	a	±0.72	±0.80	±0.88	±0.96	±1.05	±1.15	±1.25	±1.40	±1.55	±1.75	±1.95	±2.25	±2.80	±3.45
	b	±0.82	±0.90	±0.98	±1.06	±1.15	±1.25	±1.35	±1.50	±1.65	±1.85	±2.05	±2.35	±2.90	±3.55
MT6	a	±1.10	±1.20	±1.30	±1.45	±1.60	±1.75	±1.95	±2.15	±2.40	±2.65	±2.95	±3.45	±4.25	±5.30
	b	±1.20	±1.30	±1.40	±1.55	±1.70	±1.85	±2.05	±2.25	±2.50	±2.75	±3.05	±3.55	±4.35	±5.40
MT7	a	±1.50	±1.65	±1.85	±2.05	±2.25	±2.45	±2.70	±3.00	±3.35	±3.70	±4.10	±4.80	±5.95	±7.40
	b	±1.60	±1.75	±1.95	±2.15	±2.35	±2.55	±2.80	±3.10	±3.45	±3.80	±4.20	±4.90	±6.05	±7.50

Material	Tolerance
	Tolerance Requirement		No tolerance requirement
	Fine	Normal
ABS	MT2	MT3	MT5
CA	MT3	MT4	MT6
EP	MT2	MT3	MT5
PA	MT3	MT4	MT6
PA+30GF	MT2	MT3	MT5
PBT	MT3	MT4	MT6
PBT+30GF	MT2	MT3	MT5
PC	MT2	MT3	MT5
PDAP	MT2	MT3	MT5
PEEK	MT2	MT3	MT5
PE-HD	MT4	MT5	MT7
PE-LD	MT5	MT6	MT7
PESU	MT2	MT3	MT5
PET	MT3	MT4	MT6
PET+30GF	MT2	MT3	MT5
PF with inorganic fillers	MT2	MT3	MT5
PF with organic fillers	MT3	MT4	MT6
PMMA	MT2	MT3	MT5
POM	MT3	MT4	MT6
POM	MT4	MT5	MT7
PP	MT4	MT5	MT7
PP with inorganic fillers	MT2	MT3	MT5
PPE	MT2	MT3	MT5
PPS	MT2	MT3	MT5
PS	MT2	MT3	MT5
PSU	MT2	MT3	MT5
PUR-P	MT4	MT5	MT7
PVC-P	MT5	MT6	MT7
PVC-U	MT2	MT3	MT5
SAN	MT2	MT3	MT5
UF with inorganic fillers	MT2	MT3	MT6
UF with organic fillers	MT3	MT4	MT6
UP+30GF	MT2	MT3	MT5

What Factors Affect the Injection Molding Tolerances?

Several factors influence the injection molding tolerances. Understanding these parameters can help control and optimize tolerances to meet the specific requirements of a project.

Shrinkage

All plastic materials shrink to some degree during the cooling phase of the injection molding process (click to see how does injection molding process works). This shrinkage is a key factor affecting the final dimensions of a part, and hence, the tolerances. Different materials have different shrinkage rates, with crystalline materials typically exhibiting higher shrinkage than amorphous materials. This is due to the phase change from a densely packed crystalline structure to a less dense amorphous structure during melting. Plastic parts are widely used in various industries due to their versatility and cost-effectiveness.

Warpage

Warpage is the distortion of a part’s shape caused by non-uniform cooling and shrinkage. Parts with uniform wall thickness tend to shrink uniformly, minimizing the chances of warpage. However, parts with non-uniform walls cool and shrink at different rates, leading to warpage and consequently, affecting the tolerances. Injection pressure is a critical factor that affects the cooling and shrinkage of the part, and it is essential to maintain the optimal injection pressure to minimize warpage.

Thermal Expansion

The thermal expansion coefficient of the plastic material is another crucial factor. Plastics typically have a high thermal expansion coefficient, implying that their dimensions can change significantly with temperature variations. This characteristic can affect the ability of the part to hold tight tolerances, especially when used in environments with extreme temperature changes.

Part Design

The design of the part, including its geometry, size, and wall thickness, can significantly influence the tolerances. Larger parts and those with thick walls can have different shrink rates in different sections, making it challenging to control and maintain tight tolerances. Understanding the impact of part geometry on tolerances, specifically part size, is crucial for achieving high-quality parts.

Part Complexity

The complexity of a part can affect the material flow and tooling design. Complex parts can be more challenging to fill quickly during molding, maintain the right tooling temperature, and manage the cooling process, all of which are important for achieving tight tolerances and cnc tool design.

Tooling

Achievable tolerances are influenced by the design of the injection mold and the number of cavities. Failure to achieve a stable, reproducibly cooled mold design can result in increased shrinkage and repeatability tolerances. Injection molding service molds are crucial to the manufacturing process and play a significant role in achieving high-quality products.

How can to Optimize Injection Molding Tolerances to Meet Specific Project Requirements?

Optimizing injection molding tolerances requires a strategic approach encompassing product design, material selection, tooling design, and process control.

During the Design Phase

To minimize issues such as warping, shrinking, and misalignment during molding, it is critical to adhere to the principles of Design for Manufacturing (DfM). This approach involves designing the product in a way that facilitates easy and cost-effective manufacturability. A good DfM strategy can help limit the occurrence of problems affecting the tighter tolerances and fillers.

Overall Size

The total dimensions of the product or its components can considerably affect the tolerances. Larger products are more prone to distortions like warping or shrinking, affecting the tolerances. Hence, considering the size during the design phase is crucial.

Wall Thickness

Uniform wall thickness can help achieve a stable shrink rate, reducing defects such as warping, sinking, and cracking. It also aids in maintaining the desired tolerances. Thus, the design should avoid thick walls and sharp internal corners, use ribs for strengthening walls, and use radii on inside corners to alleviate warping.

Draft Angles

Incorporating draft angles in the design can facilitate the easy removal of a part from the injection mold, reducing the likelihood of shrinkage and parts getting stuck during ejection. A draft angle of 10 to 20 is generally suitable for most parts, although the exact requirement can vary based on the design and material.

Bosses

Bosses are used for fastening components during assembly. The design should ensure that the boss’s wall is not too thick, as it can lead to voids, sink marks, and increased cycle time. It’s also beneficial to core bosses, i.e., attach them to the nearest sidewall, to improve load distribution and material flow.

Select the Right Material

The choice of material is crucial in achieving the desired injection molding tolerances. The material should have a uniform shrinkage rate and be suitable for the part’s size and design. Different resins have different shrinkage rates, which should be factored into the material selection and mold design process.

Keep Mold Tools in Mind

The mold tool’s design is crucial in attaining the targeted tolerances. The tool should be oversized to account for material shrinkage and designed to ensure uniform cooling. Also, the location of the ejector pin and the gate should be carefully considered to avoid defects and ensure a smooth and even fill rate.

Execute Repeatable Process Controls

Process controls involve calibrating the variables that can affect part quality, such as temperature, pressure, and holding time. Embedding temperature and pressure sensors in the mold can provide real-time feedback and enable the execution of repeatable process controls.

What Are Some Measurement and Inspection Techniques Used to Verify Injection Molding Tolerances?

Various measurement and inspection techniques are used to verify injection molding tolerances. These include:

Visual Inspection: This is the simplest form of inspection, where the parts are visually examined for defects such as warping, sinking, and cracking.

Manual Measurement: This involves measuring the parts manually using tools such as calipers and micrometers. It is suitable for simple parts with few critical dimensions.

Automated Measurement: For complex parts with numerous critical dimensions, automated measurement systems like Coordinate Measuring Machines (CMMs) and Vision Systems are used. These systems can measure parts quickly and accurately, providing detailed reports on the part’s dimensions and tolerances.

First Article Inspection (FAI): This is a comprehensive inspection process performed on the first part produced in a production run. It involves measuring all the part’s dimensions and comparing them against the design specifications to ensure that the part meets the required tolerances.

Conclusion

Understanding and controlling injection molding tolerances is key to ensuring the quality, functionality, and fit of injection-molded parts. Factors such as shrinkage, warpage, thermal expansion, part design, part complexity, and tooling design can significantly impact these tolerances. By optimizing these factors through strategic design, material selection, and process control, it is possible to achieve the desired tolerances for any injection molding project.

Get Started With Seaskymedical

Injection molded parts often require precise dimensions to function correctly. Tight tolerances ensure that the parts fit together properly, perform their intended functions, and meet the required specifications.

Partnering with us at Seaskymedical may be the best decision you will ever make to manufacture high quality parts. With more than 10 years of experience, SeaskyMedical has accumulated expertise in precision mold manufacturing and precision injection molding. We specialize in producing medical-grade products that meet stringent quality and regulatory requirements, and we have extensive experience and knowledge in this regard.