--nas01-usermydocuments-rivory-my_documents-my_pictures-rjg__valve_gate-terumo_part_in_mold-resized-600.jpgDesign standards for plastic injection molded components are important for maintaining manufacturability of the parts to be injection molded .   These standards make it easier moving forward creating the mold design and mold fabrication. 

Wall Thickness and Sink Marks

Whenever possible, plastic parts should be designed with a nominal or consistent wall thickness. When injection molded parts shrink with cooling, the differences in the thickness of the part will cause the part to shrink at different rates.  Thicker areas of the part will shrink more than thinner areas. This can sometimes result in deformation of the part. Another potential issue includes sink marks - which are areas of the part that are "dimpled" due to excessive shrinkage. The solution to these problems is to remove or "core out" thick areas of the part so as to avoid these conditions.

Draft Angles and Ribs

Because plastic molded parts are injected into mold cavities (voids) and over cores (standing steel), they require draft in order to release from the mold. Draft is the amount of taper on the vertical walls of the plastic part. Without draft, a part will either not eject from the mold - or sustain damage during ejection.  Typically, draft angles between 1° and 2° are required, but can vary depending on part restrictions and specifications.

A plastic injection molded part that has been designed with a nominal wall thickness may not be as strong as a thicker part, which means ribs are needed for strength. When using ribs on the part, thicknesses should be 70% of the relative part thickness so as to avoid sink marks. This vertical part surface should also have draft.   

Using Bosses and Threads for Plastic Injection Molding

Bosses are used in parts that will be part of an assembly (i.e. receptacle for a screw or threaded insert or as the locator for a mating pin on another port). Stand-alone bosses need designs using the same design guidelines for ribs. Bosses should be cored to the bottom of the boss to maintain uniform wall thickness. Gussets are then used to stabilize the boss.

Threads allow you to fasten your plastic part to another plastic part or to a non-plastic product by using common fasteners. When designing molded-in threads, avoid feathered edges and include radius roots.

Special Features for Plastic Injection Molded Parts

The two halves of an injection mold separate in opposite directions.  Plastic part features also need to be able to release in that same direction. Any part features like holes or undercuts or shoulders that can't release in the direction of mold separation will require either redesign or special attention.

The axis of a hole in the part must be parallel to the direction of mold separation. There are a few exceptions.  A hole whose axis is slightly angled from the direction of mold separation could be created with a "split pin" design, where half of the coring required to make the hole is on one side of the mold with the other half on the opposite side. 

Also, mold components called "side actions" can be used. These pull coring in a direction other than the direction of mold separation. This feature adds flexibility to the part design and increases the price of the mold.  

Special Design Considerations for Insert Molding:

  • Inserts should have no sharp corners  
  • They should be round and have rounded knurling  
  • An undercut should be provided for pullout strength
  • The insert should protrude at least 0.016 inches into the mold cavity 
  • The boss diameter should be 1.5 times the insert diameter - except for inserts with a diameter greater than .5 inches.  For these diameters - the boss wall should be created with the total part thickness and specific grade of material in mind 
  • Keep the metal insert small relative to the plastic surrounding it    
  • The thickness of the material beneath it should be equal to at least one-sixth of the diameter of the insert to minimize sink marks

The toughened grades of the various resins should be evaluated. These grades offer higher elongation than standard grades, with a greater resistance to cracking. Most engineering plastics can be used in insert molding.  Lower shrinkage rates for some materials, however, give them an advantage in the insert molding application. 

Because glass and mineral reinforced resins have the advantage of even lower mold shrinkages than their base resins, they too have been used successfully for insert molding applications.   

Over-Molding Design Considerations 

The wall thickness of the substrate and over-mold should be as uniform as possible to obtain the best molding cycle.   If the part requires the use of thick TPE sections, they need to be cored out to minimize shrinkage problems, reduce the part weight and lower cycle time.    The TPE thickness should be less than or equal to the thickness of the substrate to prevent warpage; this is especially critical for long, flat geometries.

Long draws should have a 3-5° draft per side to aid component ejection. 

Properly designed deep undercuts are possible with TPEs if the part does not have sharp corners.  Advanced coring is used when the mold opens and the elastomer is allowed to deflect as it is ejected. 

Every Part Design is Unique

Every part design is unique based on the requirements and function of the part.  Crescent Industries’engineers are ready to help with advice and suggestions to make your next injection molded product or device a success.

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DuPont Engineering Polymers General Design Principles

Polyone Overmold Design Guide

Topics: custom injection molding