How to control the hardness and tolerance of adhesive parts for humanoid robots

[Introduction]

The adhesive parts on humanoid robots have higher requirements for tactile sensation and consistency than ordinary industrial parts.

Fingertips should be just soft, soles of the feet should be just right, and pieces from the same batch should not be soft or hard. These requirements fall on the manufacturing end, which includes two indicators: hardness and tolerance.

1、 What does ± 3 Shore A mean The hardness of silicone components is usually represented by Shore A. The conventional production control capability is on the order of plus or minus 3 degrees - that is to say, for parts with a nominal temperature of 40 degrees, the actual batch produced is controlled between 37 and 43 degrees.

This number may not seem large, but people can feel it.

If the hardness of the rubber parts of the two feet and hands of the same robot differs by several degrees, the walking posture and grasping force feedback will be different. For parts that integrate tactile sensors, differences in hardness can directly affect sensor calibration.

So the hardness requirements for robot parts are often tighter than those for ordinary industrial parts. And this requires the manufacturing end to manage the variables that affect hardness one by one.

2、 Why does hardness float Degree of vulcanization. This is the most influential item. Insufficient vulcanization, the rubber material is soft, sticky, and the performance does not meet the standards; Excessive sulfurization leads to hardening and brittleness. Temperature, time, and product thickness jointly determine the actual degree of vulcanization - on parts with uneven wall thickness, the measured hardness of thin and thick areas may differ by a few degrees.

Post sulfurization. Secondary vulcanization will drive out residual low molecules and make cross-linking more complete, usually resulting in a slight increase in hardness. Whether to do it or not, how long to do it, and the hardness result are different.

Formula and dispersion. The dispersion uniformity of fillers (white carbon black) and the weighing accuracy of vulcanizing agents can cause differences between batches. The use of gas-phase silica gel, commonly known as fumed silica gel in the industry, has advantages in purity and mechanical properties, and is also a common choice when making parts with high tactile requirements.

Test conditions. This one is the easiest to overlook. The Shore hardness reading is affected by the thickness of the specimen, the reading time, and the contact state of the indenter. The same piece of adhesive, 1mm and 6mm samples, can be read immediately and 15 seconds after pressing, and the results can differ by several degrees.

So if there is no agreed testing condition for the requirement of "hardness of 40 degrees", it is normal for both parties to test differently, not for anyone to falsify it.

3、 Dimensional tolerance: The error comes from three places Mold Processing accuracy, wear, and consistency between each hole of the multi hole mold.

The rubber material shrinks. Silicone will shrink after vulcanization, and the shrinkage rate is usually in the range of a few percent, depending on the rubber grade, hardness, and vulcanization conditions. The shrinkage varies at different positions with different wall thicknesses, which is the fundamental reason why it is difficult to accurately measure the dimensions of complex structural components.

Process fluctuations. The fluctuations in temperature, pressure, and holding time within a batch will be reflected in the size.

The combination of these three factors determines the final tolerance level that can be achieved. For small components on robots, the common feasible range is between plus or minus zero and a few millimeters, depending on the complexity and size of the structure.

If the tolerance is marked on the drawing to a level beyond the process capability, there are only two results: either full inspection and selection will increase the cost, or both parties will repeatedly argue.

4、 The position of gas-phase silicone in this scene For robot parts that come into contact with humans and require high tactile feedback, the material used is usually silica gel reinforced with vapor phase white carbon black (VMQ). Compared to the precipitation method, its advantages are:

Higher purity, less impurities and black spots, cleaner appearance.

Better mechanical properties, better tear and tensile performance - especially required for thin-walled parts and tear prone areas.

It can achieve high transparency, and is almost a must-have when transparent or light colored items are needed.

The cost is higher than that of ordinary precipitation method rubber materials. So whether to choose depends on the requirements of the location: Fingertips, skin, and other areas that require high tactile and appearance requirements are worth using; Pure structural protective components can be evaluated for degradation.

5、 How to clearly define the requirements When agreeing on hardness, write three things: nominal value, allowable deviation, testing method (which standard, sample thickness, reading time).

When agreeing on dimensional tolerances, clearly state: what are the key dimensions, tolerance values, measurement environment, and measurement tools. Not all dimensions require tight tolerances - marking critical dimensions and relaxing non critical dimensions can significantly reduce costs.

Make limit samples when agreeing on appearance standards. Especially for parts with parting surface flash, welding marks, and slight color differences, it is difficult to define them in text description, and physical limit samples are the most convenient.

Consistency assurance provided by the manufacturing end If done according to specifications, it usually includes: first piece confirmation for each shift, process sampling (by frequency rather than full inspection), retention of samples for each batch, and batch records of key indicators such as hardness.

Sample retention is particularly important - in case of disputes or failure analysis in the later stage, it is impossible to trace without sample retention.

For products such as robots that are still being iterated, it is recommended to require suppliers to keep complete records of process parameters. Because these records are a reliable basis for comparing the differences before and after the redesign.

The last honest sentence The most problematic aspect of requirements such as hardness and tolerance is not the inability to meet them, but rather the fact that both parties did not clearly explain how to consider them qualified before starting work.

For procurement and engineering, writing a few extra lines on the drawings (testing methods, sample thickness, key dimension annotations) results in almost zero cost; For manufacturers, actively asking for clarification when encountering vague requirements is much more convenient than explaining after completion.

There is currently no specific standard for adhesive components in the field of humanoid robots, and agreements rely more on both parties writing them clearly in advance.