One of the most common questions we are asked is "How much draft do I need to put the texture on that I want ?"
The general rule of thumb is that you require 1 ½ degrees of draft for each .001" of texture finish depth, however, there are other considerations that must be taken into account, due to the many new resins and polymers, molding improvements, and various other factors that come into play in modern plastic molding. Examples of situations that require additional draft are thin wall part design and high pressure molding.
Some important considerations to keep in mind are :
- Is the vertical wall in question an inside or outside wall ? If it is an inside wall, the part will shrink onto it during molding, so you will need more draft in order to apply a texture, or apply the texture at a lighter depth.
- Certain plastics have very little shrinkage and will therefore not shrink away from outside walls as easily as other plastics. Thermosets, Ryanite, Glass Filled Nylon, Glass Filled Polypropylene, ABS, Polycarbonate, etc. will usually require more draft in order to mold parts without scuff or drag marks.
- If the core is very simple, and there is nothing on the core to hold the part in place during ejection, the part will tend to hang onto the cavity, creating scuff marks. The part may require more draft, or perhaps texture could be applied to the core side. This helps hold the part onto the core during ejection. This method has been used very successfully to solve this sort of problem.
In cases where it may not be possible to build in the necessary draft for the texture required, we have developed the technology necessary to gradually change the depth of the texture from full depth on the horizontal areas to a very low percentage of depth on the vertical areas where the draft is severely reduced. This can usually be accomplished in such a subtle way that the change in depth is not obvious or even noticeable, yet it accomplishes better moldability. If you require this technology, planning is required, so please get us involved as early as possible in the project so that we can plan together to give you the very best combination of moldability, aesthetics and delivery on your textured parts.
During the texturing or Accugrave processes, metal is always removed from the tooling. In some cases, on a vertical wall in the texture zone, there is a seal off above or within the zone, or you require a band left untextured above the texture at the parting line. These situations create an "undercut" condition or a condition where negative draft has been created due to the metal removed during texturing. This condition can cause scuffing, dragging, sticking of the part, or in a worst case , die lock.
This condition can be difficult to address once the mold is ready to texture, so it is therefore best handled at the design stage if at all possible. One of the best methods would be to consider setting your vertical sealoff areas back away from the vertical wall being textured by a distance equivalent to the expected metal removal of the texture selected. This would alleviate undercut concerns in those areas, regardless of the amount of metal removal. Please call us as soon as possible to help you plan for the proper distance required to move the sealoffs back prior to texturing, as each texture has a different metal removal factor. Some are minor, but many of the newer Automotive textures require significant metal removal in order to satisfy the visual appearance requirements of the OEM¡¯s master for the texture. It is not uncommon to see a texture that finishes at .002" or .003" with .015" metal removal, so it can be a major issue.
Metal Removal versus Finished Depth
It is very important to note that the finish depth of any texture pattern does not necessarily indicate that only that much metal has been removed to arrive at that depth. This is especially true of Automotive textures, many of which have significantly different metal removal amounts as compared to their finish depths. Since a visual standard must be matched for Automotive approvals, many patterns require many stages of etching to make the molded part look like the master. Each stage removes metal, and even areas that are at a lower depth percentage on the part to accommodate draft considerations have significant metal removal. This factor needs to be addressed for potential undercut situations, for matches from a P20 cavity to an H13 slide or insert, and for potential post-texture hand work to hand blend areas that have been treated defensively during texturing to keep metal removal to a minimum. If you have a concern regarding this condition on your mold, please call us to discuss solutions.
Deep or Blind Ribs
Some parts have a strengthening rib, fin or flange that is formed by a slot or groove in the cavity. When these grooves or ribs are thin and deep (1/8" wide by 1/2" deep for example), it can be very difficult, if not impossible, to texture inside the rib or groove. If at all possible, identify this condition at design stage, and if the mold will be textured, design a slide or other component to move away from the zone, allowing access to the walls of the groove or rib for texturing.
We often encounter this condition, and will try our best if it is at all borderline, but cannot guarantee results. There is no way to inspect or predict the accuracy of our work inside such a rib, and the results of our efforts will only become apparent at the post-texture tryout of the mold. Even then, if the texture is not satisfactory in the rib, the options for repair are slim. Try to avoid this situation when designing the mold if it is going to be textured.