We, as a manufacturer of components and parts for the bottom of shoes, hereby define our vision of the basic rules for the installation and fastening of plastic heels by Customers.
Our opinion regarding this part of shoe assembly operations is based on:
So, we give the main theses (preliminary molding operations and positioning and centering operations are not considered).
Heels should be tightly pressed to the heel of the sole and be sure to be glued to it. There should be no gaps between the sole and the heel.
Heels can be attached with screwed nails and screws, but it is more efficient to use a combined scheme (a central screw with nails along the contour of the heel). Nails are hammered in with an inclination into the inside of the heel so that their points do not come out on the side surface of the heel.
The distance from the edge of the insole to the point of installation of the nail is 7-10 mm. Minimum Thickness the lapis walls of our heels are 7 mm, but more often than 10 mm. That is, its (lapis wall) bearing capacity is sufficient to securely hold the screwed nails and / or the central screw. However, we strongly recommend using a metal heel plate (heel plate, eg PK-86, PK-87). Its use allows you to evenly distribute the destructive forces from fasteners over the entire area of the lapis.
Before screwing in the central screw, pre-drilling the body of the heel is mandatory. The diameter of the drill should be equal to the inner diameter of the screw thread (i.e. the outer diameter of the screw minus the height of two turns). Pre-drilling will ensure full screwing (without breaking the walls of the material) and maximum pull-out resistance. When pre-drilling and screwing the central screw, it is very important to maintain the coaxial orientation of the screw with respect to the axis of the heel!
When fastening the heel, it is not allowed to direct the axis of the central screw towards the outer wall of the heel. In this case, the internal stress created by the screw (especially without pre-drilling) is directed towards the adjacent surface of the heel and contributes to the cracking of the plastic. We recommend using (as central) screws for wood and plastic. Their design characteristics (height and angles of sharpening turns, ratio of pitch to inner diameter of the thread, etc.) guarantee the best performance of the subsequent operation of the entire assembly.
The minimum screw diameter recommended for installation is 4.5 mm. Length - depending on the design of the heel. The length of the central screw, in any case, should ensure that the body of the screw passes through the wall of the lapis with a margin! In the case of using electric screwdrivers, it is highly desirable to use devices with anti-rotation protection. Otherwise, you will have to rely on the skill of the shoemaker to avoid breaking the material around the walls of the screw. In the case of repeated turning of the screw in place, the whole logical meaning of its use is lost!
The installation point of the center screw should be positioned approximately in the center of the lapis. This position (in the case of using our insole) coincides with the end
U-shaped groove in the heel part of the arch support. In addition, the thickness of the lapis wall of our heels in this place is the maximum. If more than one screw is installed, be aware that the heel material around the screw body is subjected to deformation loads. Therefore, to maintain its load-bearing capacity, the distance between the centers of adjacent screws must exceed at least three screw diameters. In the case of using the machine method of fastening the heel, it is necessary to take a balanced approach to the choice of fastening accessories (nails, staples, etc.) and machine adjustments. Particular attention should be paid when working with transparent heels, given the high density and structural rigidity of polycarbonate, it is possible to split the product as a reaction to shock and expansion loads on the material. In this case, it is required either to change the driving conditions (nail diameter, heel temperature, machine parameters), or switch to a manual fastening method.
Among other things, it is worth considering the environmental conditions in the production room. Reducing the ambient temperature in the workshop below + 10*C significantly affects the reduction of the impact strength of all plastics.
