why 20 important points on plastics design

As can be seen, there are innumerable rules of a definite nature which govern correct die design for plastic molding,and a thorough understanding of all these fundamentals is highly essential to the competent manager, sales engineer,draftsman, and estimator.

Every item is different and every new proposition presents a problem in itself which requires utilization of a general knowledge of die design. The sales engineer should know at a glance whether or not a proposition presented to him is practical from a molding stand-point and should also be prepared to suggest any changes in design which might facilitate the molding operation.

plastics design

The estimator should carefully check the prints for possible changes while he is preparing his quotations, and the tool room should also be on the alert for any errors which might have inadvertently passed the attention of the engineering department. With a thorough checking of the work it becomes quite improbable that anything but the best design will result, and therefore the most efficient mold will undoubtedly be constructed.


  1.  The design of plastic molded parts is important from what two standpoints?
  2. Why are interior undercuts in a molded part prohibitive?
  3. Is it possible to mold parts with exterior undercuts?
  4. Define a split-cavity mold and name an instance where such would have to be used.
  5. What is meant by parting line?
  6. On a molded part are inside edges and corners more practical when sharp or should they have a small radius? Why?
  7. Why is a slight vertical draft usually recommended for deep molded parts? Approximately what should this draft be?
  8. What can be done to the design of the mold to make the part adhere to either the top or bottom as desired?
  9. Why is the use of long narrow holes in a piece undesirable?
  10. What is the objection to having slant or side holes in a molded part?
  11. Give two reasons why a thick wall section in the design will increase the cost of producing the part.
  12. What is apt to occur to a piece, or to the mold, when it is designed with very thin sections on the vertical dimensions?
  13. What is a shrink block and when is it used?
  14. Discuss the advantages and disadvantages of threaded metal inserts as compared with having the threaded section an integral part of the mold itself.
  15. What factors in the design of a part determine whether or not inserts should be used?
  16. When a hob is used in making the die, are raised or depressed letters on the finished pieces less expensive? Is the same true when the cavities have been machined?
  17. Is there any objection to specifying thin edges or sharp points in the design of the mold cavities? Why?
  18. Why are closer tolerances obtainable on horizontal than on vertical dimensions?
  19. What precaution must be taken when inserts are placed in corners of a molded part?
  20. Why should parting lines on curved surfaces be avoided if possible?