Performance Aspect 3: Assembly Clearance Tolerances

In any assembly designed to have some form of relative movement between components, the precision and ease of this movement becomes an important performance issue. Here, PA 3 will refer to the combined relative accuracy between moving components in an assembly. If a product's design requires no relative component movement, then PA 3 is irrelevant and does not need to be measured. An example of such a product is any one that has a fixed soft-touch grip (e.g. toothbrush or power tool housings). On the other hand, for assemblies with features such as hinges or sliding tracks, PA 3 becomes a very important quantity. For this model, it is assumed that if relative movement is desired, then no bonding will occur between separate shots in the MMM variant. If bonding did occur, relative movement would not be possible. The definition and measurement of PA 3 will be detailed below.


Definition of Assembly Clearance Tolerances

Assembly accuracy could be measured in many relevant ways. Here, a straightforward approach involving clearances will be used. PA 3 is defined as the sum of the variations in clearances for all of the critical dimensions of an assembly. That is, each assembly dimension requiring a specified clearance will have an associated range of actual clearances when manufactured. These ranges, or "clearance bands" will be computed for each applicable dimension and summed to arrive at a value for PA 3.

In its simplest definition, a clearance is a gap that allows relative movement between two components in an assembly. For this model, only linear dimensions will be considered, so clearances (and the resulting PA 3) can be measured in units of length. Depending on the exact interface geometry, at least one or more dimensions would be considered as critical clearances for the assembly to function as intended. Figure 1 below illustrates identifying critical clearance dimensions between components.

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The first example (Figure 1) involves an assembly with a sliding mechanism. Although there is only one specified degree of freedom between parts A and B, there are actually three critical dimensions (Figure 1b) because of the geometry. Each one of these dimensions has an associated clearance value that must be within a specified allowable range for the assembly to function as desired (i.e. slide smoothly). The second example (Figure 1c) involves an assembly with a bearing joint. There are two degrees of freedom (rotation, and up-down translation) and two associated critical dimensions. Each assembly has its own specific critical dimensions, and these should readily be identified from the CAD files.

Now that the topic of critical dimensions has been discussed, clearance variances (PA 3) can be defined. Each critical dimensions, , has an associated clearance value, , that measures the gap size between components at the location under consideration. This clearance value must be within an acceptable specified clearance band for the assembly to function properly . Because the desired (or "nominal") value for each clearance is specified, the clearance variation can be defined as the deviation between the specified clearance and the actual clearance achieved by the manufacturing process. The mechanisms by which these clearance variations are formed are discussed below.


Causes for Clearance Variations

As with all manufacturing processes, injection molding produces parts with dimensional variations. These variations are caused by fluctuations in the process parameters, and more importantly, part shrinkage. It is important for the designer to understand shrinkage, and the tolerances realistically achievable for both process variants. Shrinkage is the tendency of plastic parts to become smaller during cooling. As the resin solidifies and become denser, the part's dimensions will start to decrease by a certain percentage. This occurs for all dimensions and along all directions, except where prevented by the mold. A simple example of part shrinkage is illustrated below in Figure 2:

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The example part in Figure 2 is a simple cylindrical pin with an internal blind hole produced via a protrusion in the mold's core. Right after injection of material A, the resin fills the cavity completely and takes on its shape. During the cooling phase, the resin tends to shrink away from the cavity and onto the core. Depending on how soon the part is ejected, the hole may or may not shrink as well. However, compared to the cavity-side shrinkage (which cannot really be prevented) the hole shrinkage should be small.

The effect of shrinkage is that the part dimensions will deviate from the mold's dimensions. A further complication is that the actual shrinkage percentage can vary from shot to shot. Part shrinkages can have unexpected results when attempting to assemble molded parts or perform subsequent shots (in MMM only). This is what leads to clearance variations. The actual values of these variations depend highly on the process variant and are discussed below.


Measurement of Clearance Variations

Clearance variations are specified by two values: 1) the desired or nominal clearance, and 2) the clearance variation achieved. Because the desired values are determined by the designers, measurement of the clearance variation only involves the prediction of the actual clearances produced by the process variant under consideration. This requires a thorough understanding of both process variants, and the associated shrinkage characteristics to consider. Specifically, each process variant has its own set of dimensional values which can or cannot be specified and or/controlled as a result of shrinkage.

An example of an assembly with one critical clearance dimension is illustrated below in Figure 3. It emphasizes the differences between SMM&A and MMM and derives the method for computing the associated clearance variations. The SMM&A variant involves molding both components separately and then attempting to insert the pin B into hole A. The MMM variant involves molding the hole A and then molding pin B directly into the cavity in material A. Pin B would then shrink away from the cavity and form a clearance between components.

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Although the above example involves a simple cylindrical assembly with only one critical dimension, the same reasoning could be used for any geometry, and the result would be the same. The key difference to note is:

1) The SMM&A variant involves two dimensions to specify along with associated actual dimensions which much be sufficiently controlled

2) The MMM variant involves only one specified dimension. Resultantly, only the corresponding actual dimension needs controlling, as the second dimension is entirely dependent on the first and will always shrink to form a clearance.

In essence, the SMM&A variant involves assembly of two unrelated components with independent dimensional uncertainties, whereas the MMM variant involves two directly-related components and only one real dimensional uncertainty. The end result of this difference is that for SMM&A, the clearance variation is controlled by two independent and uncertain variables where the clearance variation for MMM is only controlled by one variable. These dimensional variations are directly caused by the shrinkages of their respective material. This difference usually results in the MMM variant having easier-to-meet clearance requirements. If values for and are known, PA 3 can be calculated explicitly.

Unfortunately, it may not be possible to predict the exact dimensional variations because shrinkage can vary from shot to shot. However, it is customary to predict a certain percentage of shrinkage based on the material and nominal processing conditions. These numbers can then be used as representative shrinkage values and then PA 3 can be estimated for relative comparison purposes.

As long as the "hole" is molded before the "pin", this same reasoning can be used for most assembly geometries to determine PA 3. While each specific assembly must be analyzed on a case-by-case basis, the general analysis will be similar. The end result is usually that the MMM variant will provide a smaller clearance variation, and hence be more desirable in this respect.


Validity of Performance Aspect 3

The validity of PA was confirmed through preliminary shrinkage tests conducted at the University or Maryland's Manufacturing Automation Lab (MAL). Assembly clearance tests were conducted on MMO's molded in various shot sequences. In general, it was found that if a pin is molded inside of a hole using incompatible materials, the pin will shrink away from the hole and provide a clearance fit. These types of fits proved to exhibit equal or better clearances and freedom of movement when compared to assemblies composed of separately-molded components.