Injection Molding Tolerances: An In-Depth Look

Injection Molding Tolerances: An In-Depth Look

Injection molding is one of the most common production technologies and is used in almost every industry from automotive to aerospace. Maintaining tight dimensional consistency is critical across high production volumes, especially if the injection-molded parts are used in larger assemblies. This article will discuss the importance of injection molding tolerances and their relationship to tolerance stack-ups, how part tolerances are specified, and what factors influence them.

Tolerances and Tolerance Stackups

Injection molding can be used to produce parts for both low volume and high volume applications. Often, these parts are used in assemblies and must interface precisely. To ensure that each part has consistent dimensions and will support assembly, designers need to define a part’s acceptable dimensional variations or tolerances. Specifying part dimensions without tolerances isn’t practical since there will always be some deviation, whether becaus of raw material differences, processing parameters, tooling wear, or other factors. 

A tolerance stack-up refers to how all of the manufactured parts and the limit of their individual tolerances must fit together. Consider the example of three parts bolted together through a hole. Even even if all three parts are manufactured with holes at the maximum or minimum tolerance value, the three holes must align so that a fastener can pass through. 

Download Fictiv’s Tolerance Analysis Calculator

Types of Injection Molding Tolerances

Injection molding tolerances are represented in a number of ways and are usually defined for specific feature types. Some of the more common feature types are listed below:

1. Dimensional: Overall sizing of the part. Increased part size results in a larger shrinkage during cooling and dimensional tolerances must account for the change — this is why injection molding tolerances are listed for specific size ranges.

2. Straightness or Flatness: Covers the general warping of large, flat areas. Mold design features like gate location and uniform cooling can reduce warpage.

3. Hole Diameter: Larger holes need a larger tolerance range due to increased shrinkage. 

4. Blind Hole Depth: Blind holes require cantilever-type features in the mold. If the plastic is injected at high pressure, the pin inserts can deflect, and the deeper the hole, the higher the likelihood of deflection.

5. Concentricity/Ovality: A large cylindrical part with a thin wall can shrink unevenly so that the part loses its circularity. 

Commercial vs. Fine Tolerances

Injection molding tolerances can be broken down into two main categories, namely commercial and fine tolerances. 

• Commercial tolerances tend to require lower-cost molds and produce lower-cost parts.
• Fine tolerances, also called precision tolerances, provide a tighter tolerance band which ultimately makes the mold and subsequent parts more expensive. 

Typical Mold Tolerances

Tables 1, 2, and 3 below contain tolerances (both commercial and fine) for some common injection molding materials. These guidelines were defined by the Society of the Plastics Industry (SPI), a U.S.-based trade association that’s now known as the Plastics Industry Association (PIA, or PLASTICS).

Note: These values are a general guideline, and your mold manufacturer must be consulted to confirm these values.

Dimensional Tolerances (mm)
Type:	Commercial				Fine
Feature Size:	1–20 mm	21–100 mm	101–160 mm	For every 20 mm over 160	1–20 mm	21–100 mm
ABS	0.100	0.150	0.325	0.080	0.050	0.100
ABS/PC
HDPE	0.125	0.170	0.375	0.100	0.075	0.110
LDPE
PA	0.075	0.160	0.310	0.080	0.030	0.130
PA GF 30%	0.060	0.120	0.240	0.080	0.030	0.100
PC
PMMA	0.075	0.120	0.250	0.080	0.050	0.070
POM	0.075	0.160	0.310	0.080	0.030	0.130
PP	0.125	0.170	0.375	0.100	0.075	0.110
SAN	0.100	0.150	0.325	0.080	0.050	0.100

Table 1: Dimensional Tolerances

Straightness/Flatness Tolerances (mm)
Type:	Commercial Tolerance		Fine Tolerance
Feature Size:	0–100 mm	101–160 mm	0–100 mm	101–160 mm
ABS	0.380	0.800	0.250	0.500
ABS/PC
PA	0.300	0.500	0.150	0.250
PA GF 30%	0.150	0.200	0.080	0.100
PC
POM	0.300	0.500	0.150	0.250
PP	0.850	1.500	0.500	0.850
SAN	0.380	0.800	0.250	0.500

Table 2: Straightness/Flatness Tolerances

Hole Diameter Tolerance (mm)
Type:	Commercial Tolerance				Fine Tolerance
Feature Size:	0.3	3.1–6	6.1–14	14–40	0.3	3.1–6	6.1–14	14–40
ABS	0.050	0.050	0.080	0.100	0.030	0.030	0.050	0.050
ABS/PC
HDPE	0.050	0.080	0.100	0.150	0.030	0.050	0.050	0.080
LDPE
PA	0.050	0.080	0.080	0.130	0.030	0.040	0.050	0.080
PA GF 30%	0.050	0.050	0.080	0.080	0.030	0.040	0.050	0.050
PC	0.050	0.050	0.080	0.080	0.030	0.040	0.050	0.050
PMMA	0.080	0.080	0.100	0.130	0.030	0.050	0.050	0.080
POM	0.050	0.080	0.080	0.130	0.030	0.040	0.050	0.080
PP	0.050	0.080	0.100	0.150	0.030	0.050	0.050	0.080
SAN	0.050	0.050	0.080	0.100	0.030	0.030	0.050	0.050

Table 3: Hole Diameter Tolerance

How Is Part Shrinkage Calculated?

Injection molding shrinkage compensation is normally done in the mold design phase — which means the part is designed to nominal dimensions and the injection mold is scaled up by the expected shrinkage. Shrinkage is normally determined by testing a material’s linear shrinkage rate in accordance with a specific standard like ASTM D955. Basically, a sample is injection molded and allowed to cool down over a period of time, then the shrinkage is calculated as per the equation below:

Shrinkage Rate =100 % (Lc – Lp) / Lp

Lc: Cavity Length

Lp: Part Length After Cooling

This equation only calculates the linear shrinkage and does so for materials with asymmetric properties like long, fiber-filled resins. The calculated value represents a decreased shrinkage in the melt flow direction, which can result in an overestimation of shrinkage in a part’s transverse direction. 

Mold Flow Analysis

One of the best ways to determine part shrinkage for complex components is with a mold flow analysis. This software simulation can show how a resin will fill the mold during the injection molding process. Mold flow analysis is also used to help designers locate difficult spots to fill within the mold.

The image below shows shrinkage variation of an injection molded part, as simulated within Autodesk Moldflow. 

Typical Shrinkage Values

Table 4 below lists the shrinkage range (percentage) of the typical injection molding resins offered by Fictiv. These values represent a general range; the specific material datasheet must be consulted for more accurate values. 

Material	Shrinkage Range
ABS	0.7–1.6
PC/ABS	0.5–0.7
Acetal/POM (Delrin®)	1.8–2.5
ASA	0.4–0.7
HDPE	1.5–4
HIPS	0.2–0.8
LDPE	2–4
Nylon 6/6	0.7–3
Nylon 6/6 Glass Filled (30%)	0.5-0.5
PBT	0.5–2.2
PBT Glass Filled (30%)	0.2–1
PEEK	1.2–1.5
PEEK Glass Filled (30%)	0.4–0.8
PEI (Ultem®)	0.7–0.8
PET	0.2–3
PMMA (Acrylic)	0.2–0.8
PC	0.7-1
PC Glass Filled (20–40%)	0.1–0.5
Polyethylene Glass Filled (30%)	0.2–0.6
Polypropylene Homopolymer	1–3
Polypropylene Copolymer	2–3
PPA	1.5–2.2
PPO	0.5–0.7
PPS	0.6–1.4
PPSU	0.7-0.7
Rigid PVC	0.1–0.6
SAN (AS)	0.3–0.7
TPE	0.5–2.5
TPU	0.4–1.4

Table 4: Shrinkage Rates

Another important consideration is that semi-crystalline resins will typically have higher injection molding shrinkage rates when compared to amorphous resins as per the mold shrinkage chart in Figure 1 below:

Figure 1: Semi-Crystalline vs. Amorphous ShrinkageImage Credit: The Tool Hub

How To Maintain Injection Molding Tolerances 

To ensure injection molded parts are manufactured with your desired tolerance, you should account for the following factors:

1. DFM (Design For Manufacturing)

Following standard DFM principles in injection molding is the best way to ensure that parts behave predictably and stay within defined tolerance parameters. Following these principles ensures that warping, deformation, and excessive shrinkage are avoided. Another important consideration is parting-line-mismatch tolerance; dimensions measured across a parting line cannot be held to the same precision as a feature not bisected by the mold parting line. 

2. Material Selection

Different injection molding materials will shrink at different rates, As such, it’s critical to use the correct shrinkage factor when designing the mold. It’s also possible for materials from different batches and material suppliers to have slightly different shrinkage rates. Long, fiber-reinforced polymers will shrink less in the direction of the melt flow when compared to the transverse direction. This is known as asymmetrical shrinkage. Asymmetrical resins can create significant challenges for maintaining tolerances in complex parts and should be avoided when possible. 

3. Process Control

Injection pressure, holding time, material density, and mold temperature all have a significant effect on a part’s overall shrinkage. As such, keeping these parameters consistent ensures that the injection molded parts have repeatable and predictable shrinkage.

Plastic injection molding tolerances is a complex topic that entails specialized knowledge. When designing an injection molded part, it’s best to work with an injection molding expert early on in the process so that the part is designed within typical injection molding parameters.

Sourcing Simplified – Start Your Next  Project With Fictiv

For all your injection molding needs, Fictiv has you covered. Our injection molding experts are well versed in managing shrinkage and maintaining tight dimensional consistency for both high and low volume productio
n runs. We produce custom mechanical parts in a variety of materials and we simplify part sourcing with intelligent, streamlined, automated workflows. 

Fictiv is your operating system for custom manufacturing that makes part procurement faster, easier, and more efficient. If you need assistance dialing in your tolerances or selecting the proper material for your next project, create an account, upload your design and talk to one of our experts. We’re here to help, so find out what our instant quote process, design for manufacturability feedback, and intelligent platform can do for you.

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