Ensuring correct heat setting for lasted footwear
How to make sure that the heat setting process is operating to the greatest possible efficiency.
by Peter Allen
The uppers of most footwear are constructed from flat components of leather or synthetic materials, and the final shape of the footwear is produced by forcing the completed upper over a last. This is known as the lasting process, which not only gives the overall shape to the style but also is important in providing the internal footwear space required to help ensure that the footwear fits correctly. The final shape of the upper will depend on a number of factors, including the shape of the last, the material, the construction of the upper, the correct conditioning of the upper and the correct application of the lasting process. However, another important factor is how much of the lasted shape is retained in the upper after the last is removed.
During the lasting process, there will be some permanent distortion (set) of the upper, although it will still remain partially elastic. As a result, when the last is removed, the remaining elastic component will have the effect of causing the upper to lose shape over time as it attempts to return to its original, flat, state. Before SATRA developed the process of heat setting in the 1960s, the amount of shape loss was limited by leaving the last in the footwear for a considerable time – typically 14 days. Heat setting has provided a significant reduction in production time and the number of lasts required in the production process, as footwear can be removed in a fraction of the time compared to production without heat setting.
Heat setting applies heat and moisture for controlled periods of time. This has the effect of relaxing (de-stressing) the fibres of the upper, relieving much of the residual stress from the lasting process. Hence, the percentage of set retained in the upper increases.
There is a wide range of different types of heat setting machines installed in shoe manufacturing facilities worldwide. To ensure that heat setting is effective, it is important to verify that the heat setter maintains the correct temperature and humidity over the specified time for the upper material being used. It is not always sufficient to rely on machine settings. This is because the actual temperature and moisture level delivered to the lasted upper in the heat setter can be different to that indicated on the heat setter controls.
SATRA recommends that an effective maintenance schedule is developed and implemented for all heat setters on a production line. Many problems are due to faulty machinery or inaccurate temperature indicators. If the machine is not working correctly, there is little chance of heat setting being effective. High velocity air (HVA) heat setters circulate large amounts of air which can carry dust into the machine. This can build up on the heating elements, leading to inefficiency. Water tubes can become blocked, and paying careful attention to the water usage will help identify this. Moist heat setters should use significant amounts of water, so identifying a falloff in water usage should prompt the heat setting machine to be checked for blocked pipes. Good maintenance of heat setters is important, however to check the actual conditions to which the lasted upper are subjected, SATRA has developed a temperature and humidity detector suitable for this purpose.
The SATRA Sensor (STD 228) is a temperature and humidity sensor which, when mounted in a modified last (SATRA STD 228SL), can be inserted into a production line. While passing through a heat setter, it records the actual conditions that are experienced by the lasted upper during the heat setting process. The data collection is time-based, so the duration of time to which the upper is subjected to the specified temperature and humidity can also be checked. After using the STD 228 Sensor through a heat setter, the data can be downloaded to a PC via a mini USB socket. Software supplied with SATRA Sensor can be used for data processing and presentation of results.
The SATRA plastimeter
SATRA supplies another aid to assist in correct heat setting – the SATRA STD 449 plastimeter. Used in conjunction with SATRA test method TM18:1995 – ‘Shape retention – dome plastimeter method’, it provides a means to determine the shape retention of a material after it has been stretched into a dome shape and subjected to a setting treatment. A disc of material is bonded to a brass ring and then clamped into the device. The specimen is then deformed by forcing a non-metallic dome (the height of which can be accurately measured by a removable device that is included). The dome is held in this position while the test assembly is subjected to the chosen heat setting process, after which the forming dome is withdrawn carefully, leaving the specimen in a domed state.
The height of the specimen dome is re-measured, typically after one hour and after 24 hours following removal of the forming dome. The reduction in dome height is used to calculate the percentage set value for the specimen under the heat setting conditions to which it was subjected. The test can be repeated with different heat setting parameters until the optimum conditions are found for this material. This test can also be carried out with a combination of materials – for example, an upper together with a lining or backer – to check the overall set for the combination.
Heat setting is an important process to ensure the correct shape and fit of many types of footwear. However, ensuring that the correct parameters are set on a heat setter and then achieved in production are important considerations, for which the SATRA devices described in this article can provide valuable assistance.
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How can we help?
Please email firstname.lastname@example.org to receive a quotation for a SATRA Sensor (STD 228), a SATRA Sensor mounted in a last (STD 228SL) or a SATRA plastimeter (STD 449).
This article was originally published on page 12 of the May 2014 issue of SATRA Bulletin.