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Breaking the rules, whom to blame?
Definition: Simple rules 1) basic principle, 2) non complicated guideline
The basics of part design are fairly simple, in that the part has to be that of what the designer needs for the functionality / performance of the part. But within plastics there are a few basic rules which are always broken and cause many issues unto themselves. The breaking of these 4 simple rules by the designer causes all within the molding and plastic part production time and money. Though the blame game may proceed to the poor processor it is really the designer whom should take the blame, if in fact the rules are ignored. On a side note all should review and comment freely at a design review so that all is on the table so to speak before even steel is cut.
The four simple ones are (no particular order)
1- Uniform wall
2- Flow thick to thin
3- Radius all inside corners
4- Use draft
Uniform walls- It is a simple one; the wall thickness of the part should be uniform. It is probably the most broken of the all the rules due to what the designer needs or does. One major reason in the support for uniform walls is fairly simple in that the part can now cool at the same rate. With uniform cooling and wall thickness there shall be more of a uniform shrinkage within our part.
Uniform walls within the part design may require the use of cores, or changing of the design slightly to facilitate the uniformity of the wall thickness. The downside is tool construction cost which with the use of cores, and new details will drive the cost up. The plus side of this is that the part shall be produced with fewer issues and probably higher yields due to a well thought out solution to issues of non-uniform wall. Cycle time may also see the improvement.
Flow thick to thin- This allows for the packing out of the thick section and minimization of voids in the part. Too many times the flow is thin to thick and the issue is those small bubbles within the part, sinks and or voids. Sometimes even shorts can be attributable to the flow and wall thickness issue. If wall thickness must be varied than transition the change in wall thickness over a distance that is 3 times the wall thickness. What this means is if the wall was 0.100" we would transition over 0.300" of an inch. It must be noted that the above means that the gate location is in the thick section.
When flowing from thin the wall freezes prior to that in the thick, thus it cannot be packed out, and in addition equates to a greater pressure drop, packing ability within the part.
Radius- It should be that all inside corners be radiused, though on a part print this may only be called out in the notes section. In many cases it just says radius, break corners and is not called out in detail. A sharp inside corner within our part acts as a stress raiser. The best example to be given is that Nylon with no radius on the inside corner will break at 0.5 foot pounds force whereas 0.01" radius will break at 2 foot pounds of force ,0.020" radius breaks at 4 foot pounds force and at 0.040"the break is at 12 foot pounds force. (Conditioned to 50% R.H.)
Some rules of thumb are:
Radius = ½ of T (thickness of wall)
Radius = 0.25-0.75 of T (thickness of wall)
Understand that the radius should not be so big that it now affects the part itself; also it is important to know that sometimes thinking outside of the box as to where to put the radius is important. One of the issues is that when thinking outside the box the downside is tooling cost goes up as we may have a special core, or other design detail that is now time consuming to manufacture.
Draft- Draft is the angle of the side wall and or product that allows it to have taper. This is in the direction of ejection of the part from both the cavity and core. In some cases no draft is allowed on a part, this should be a red flag or caution to the molder / processor and also for the tool maker if they are in contact with the molder / processor.
Draft is usually called out by the material supplier on the design protocol for the material in question. The big rule of thumb is that a minimum draft is ½ degree per inch and a standard would be 1 degree per inch.
Should texture be on the part/mold for appearance sake than for every 0.001" of texture depth in the part we need 1 degree of draft per inch. For example if our depth of texture is 0.003" than 3 degrees plus our original ½ to 1 degree for a total of 3.5 to 4 degree of draft. Note that in some literature there is a disregarding for our original draft, using practical experience is best for making this decision.
IF IN DESIGNING THE TOOL /PART IT IS TO BE A POLISHED SURFACE BUT IN THE BACK OF YOUR MIND IT MAY NEED TO BE TEXTURED FOR A COSMETICAL REASON CONSIDER DRAFT NOW TO THE WORST CASE SENERIO!!!
The reason for the above statement is that in black parts especially a blush or scuff is present or fingerprints on the finished part, adding texture hides this, but if the tool has minimal to no draft to start with adding texture without the draft necessary for the said texture will cause many more issues in the part. The number one is how we get it out without scuffing! The number two is the cost of mold modifications and or cycle times when our wall thickness exceeds our nominal.
While the above give only 4 there are many basic rules to part design that need be followed which allow for a mold design and or production of the plastic part to be produced by method of choice. The Breaking of the rules is typical and common, but understanding this going into the project is the key, and all should sign off and then solve as a team.
SLSILVEY
12122013
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