Finishes on Plastic Molded Parts

jeepdog

Starting to get Involved
#1
We are having a discussion at work to wit: We have drawing for a plastic molded part with a 125 finish requirement. The argument is whether that is a legitimate call out for plastic materials. I don't see why not (short of calling out the micron style finishes which basically amounts to the same thing after converting) we are old school.
 
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normzone

Trusted Information Resource
#2
Is that callout a default in the drawing template, or a specific flag on a surface, or a note in a table?

Drawing templates are a common source of abuse and inappropriate requirements. Many lazy engineering departments travel that road.

For that matter, don't even get me started on purchase order templates. Some of the big aerospace outfits are some of the worst offenders with uncontrolled versions and wildly varying default requirements, often wholly inappropriate.

Ever since copy and paste was invented, reviewing requirements became a different game.
 

jeepdog

Starting to get Involved
#3
Thanks for responding
yes its likely a default but I have no problem with the finish callout. We are arguing whether its proper to call out a finish on plastic parts. The argument one side makes is, the mold will produce the finish so we shouldn't call it out on the parts!!??
I contend that we will be getting the parts not the mold and the mold better be producing parts w/ a 125 finish as required.

So the question is, is it proper to callout a finish on plastic parts?
 

Golfman25

Trusted Information Resource
#4
Thanks for responding
yes its likely a default but I have no problem with the finish callout. We are arguing whether its proper to call out a finish on plastic parts. The argument one side makes is, the mold will produce the finish so we shouldn't call it out on the parts!!??
I contend that we will be getting the parts not the mold and the mold better be producing parts w/ a 125 finish as required.

So the question is, is it proper to callout a finish on plastic parts?
I have never seen it done. Finish will be a result of the material used and the polish on the mold. Not much you can do to change it. Where on metal parts, I can sand, grind, lap, polish to get different finishes.
 

jeepdog

Starting to get Involved
#6
OK, to be clear, we are ordering the parts and I would expect smooth parts. So to me, the onus for the polishing and the monitoring of the mfg. process is strictly on the manufacturer and my inspection should expect the parts to have a 125 finish. I wouldn't buy a chair with bumpy armrests (bad analogy but you get it)!

Anyway thank you all for responding. Also for the proud liberal, me too!
 

normzone

Trusted Information Resource
#7
I contend that we will be getting the parts not the mold and the mold better be producing parts w/ a 125 finish as required.

So the question is, is it proper to callout a finish on plastic parts?
Well, let's see .... did they create the mold to your specifications, or is this something they sell to multiple customers?

There is some process variation possible in molded parts, although it's not a popular topic for discussion. People want to believe that each part, each day, each shift, each year will be like the first. I've paid my dues in the molding mines, and that is not always the case.

Another way to answer your question is, has the mold and the first pieces out of it been validated to produce parts that meet the surface finish requirement?

Lastly, if that surface finish went to a four or a five hundred, would it affect your use or opinion of the part?
 

Ronen E

Problem Solver
Staff member
Moderator
#8
There is some process variation possible in molded parts, although it's not a popular topic for discussion. People want to believe that each part, each day, each shift, each year will be like the first. I've paid my dues in the molding mines, and that is not always the case.
:agree1: I agree 100%.

Process settings and process variation can get varied finishes out of the same tool, using the same raw material (not to mention varying raw material lots).

If you are ordering a part and you need a specific finish on it, it's legitimate to call it out regardless of type of material or manufacturing technology. If your supplier has an issue with that (eg you provided a tool incapable of producing the required part), let it be heard and discussed to the point. Surface finish is no different from any dimension etc., in that regard.
 

AgnieszkaSz

Involved In Discussions
#9
It happens often in injection moulding that the drawing shows details pertinent to mould finish, not part finish. I have seen several drawings with such confused information.
 

Eredhel

Quality Manager
#10
I obviously don't know these particular parts but we occasionally CNC plastic parts. Sometimes in the future life of a part you will get options to machine from raw material rather than being required to use the original cast or mold. In a case like that you'll need the callout for surface finish. I don't even know if these particular parts can be machined but there is at least one reason to have surface finish callouts in plastic.
 
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