There are two things that you want to mitigate when designing a part for injection molding, tooling (mold) cost, and to avoid sink/warp of the finished part. We can build a mold just about any way you want, but that doesn’t always mean that it is cost effective or that the tool will yield the desired part.

The moment the liquid plastic enters the mold it’s going to start shrinking, and it is not going to do it uniformly unless you control it. This is how molded parts become warped, or end up with sink marks.

The good news is that Eck has you covered. Our expert sales engineers can guide you on how your part can be designed to reduce costs, create high quality uniform parts all while making sure it meets your functional needs! Below are general guidelines

Size

Every company has limitations on the size of the part that can be mold. Currently the maximum part that Eck Plastics can mold is roughly.

  • 50 sq. in. (127 sq. cm.)
  • Maximum part volume of approximately 20 cu. in.
Draft
To prevent parts from being damaged during ejection from the mold, a taper is applied to the faces of the part that parallel with direction of the mold opening. Recommended draft is as follows:


  • Draft all surfaces parallel to the direction of mold separation

  • Angle walls and other attributes that are formed in both mold halve to assist ejection and retain uniform wall thickness

  • As a general rule, use the standard one degree of draft plus one additional degree of draft for every 0.001 inch of texture depth

  • Use a draft angle of at least one-half degree for most materials. Design permitting, use one degree of draft for easy part ejection. SAN resins typically require one to two degrees of draft.


Coring and Wall Thickness
Parts that are too thick will result in warping and sink marks. By shelling out thick areas a more uniform part can be achieved while also reducing material costs. Rigidity can still be maintained in a shelled part by placing ribs inside the shelled out part. Wall thickness details are below:

Sink marks and warpage best avoided by designing your part with uniform wall thicknesses. Even further different polymers shrink at different rates. To yield the best results, Eck Plastics recommends using the wall thicknesses listed below:
Resin / MaterialInches
ABS0.045 - 0.140
Acetal0.030 - 0.120
Acrylic0.025 - 0.500
Liquid Crystal Polymer0.030 - 0.120
Long-fiber Reinforced Plastics0.075 - 0.150
Nylon0.030 - 0.115
PC (Polycarbonate)0.040 - 0.150
Polyester0.025 - 0.125
Polyethylene0.030 - 0.200
Polyphenylene Sulfide0.020 - 0.180
Polypropylene0.025 - 0.150
Polystyrene0.035 - 0.150
Polyurethane0.080 - 0.150
Tolerances

The entire industry accepts that tolerances specified on a print for injection molded parts should vary with respect with the polymer used, and the overall size of the part.  This is because industry wide tolerances greatly rely on the molder’s ability to predict how the polymer will shrink as it changes from a liquid to solid both inside the mold, and after the part has been ejected.  Below is a chart that specifies generally accepted tolerances for the injection molding industry.

Resin / MaterialTolerances 
FineCommercial
ABSPlus or minus .003 for the first 1 inch in length plus an additional .001 for every inch thereafter.Plus or minus .005 for the first inch in length plus .00125 for every inch thereafter
AcetalPlus or minus .005 for the first inch in length plus .00125 for every inch thereafter Plus or minus .005 for the first inch in length plus .00125 for every inch thereafter
AcrylicPlus or minus .0035 for the first inch in length plus .001 for every inch thereafter Plus or minus .005 for the first inch in length plus .00125 for every inch thereafter
Liquid Crystal PolymerPlus or minus .005 for the first inch in length plus .00125 for every inch thereafter Plus or minus .005 for the first inch in length plus .00125 for every inch thereafter
Long-fiber Reinforced PlasticsPlus or minus .005 for the first inch in length plus .00125 for every inch thereafter Plus or minus .005 for the first inch in length plus .00125 for every inch thereafter
NylonPlus or minus .0025 for the first inch in length plus .00125 for every inch thereafter Plus or minus .00475 for the first inch in length plus .00125 for every inch thereafter
PC (Polycarbonate)Plus or minus .0025 for the first inch in length plus .00075 for every inch thereafter Plus or minus .004 for the first inch in length plus .001 for every inch thereafter
PolyesterPlus or minus .003 for the first inch in length plus .0006 for every inch thereafter Plus or minus .004 for the first inch in length plus .00125 for every inch thereafter
PolyethylenePlus or minus .00425 for the first inch in length plus .0015 for every inch thereafter Plus or minus .007 for the first inch in length plus .002 for every inch thereafter
Polyphenylene SulfidePlus or minus .005 for the first inch in length plus .00125 for every inch thereafter Plus or minus .005 for the first inch in length plus .00125 for every inch thereafter
PolypropylenePlus or minus .00425 for the first inch in length plus .00125 for every inch thereafter Plus or minus .007 for the first inch in length plus .002 for every inch thereafter
PolystyrenePlus or minus .003 for the first inch in length plus .00075 for every inch thereafter Plus or minus .0045 for the first inch in length plus .00125 for every inch thereafter
PolyurethanePlus or minus .005 for the first inch in length plus .00125 for every inch thereafter Plus or minus .005 for the first inch in length plus .00125 for every inch thereafter

Corner Design
The general practice of designing the outside corner radius of a part to one wall thickness larger than the inside radius will maintain a constant thickness through corners.
Thickness Transitions
By rounding or tapering transitions in part thickness will minimize read-through and possible surface blemishes. Additionally, the practice of material blending may reduce molded-in stresses and stress concentration associated with abrupt changes in thickness.
Ribs


Providing ribs within a design reinforces strength and stiffness in molded parts without increasing overall wall thickness. Proper rib design involves five main criteria: thickness, height, location, quantity, and mold-ability.

Other uses for ribs are as follows:

  • They act as stops or guides for mechanisms

  • Ribs locate and captivate components within an assembly

  • They provide alignment in mating parts
  • Rib Thickness

    There are many factors that determine the appropriate rib thickness for a part. Often if ribs are too thick they can cause sink, a cosmetic problems on the opposite wall surface. The material, rib thickness, surface texture, color, proximity to a gate, and a variety of processing conditions determine the severity of the sink.

    Rib Location & Quantity:


    • Proper thought should be used when deciding on the location and quantity of ribs within a part. For example, ribs added to increase part strength and prevent breakage may reduce the ability of the part to absorb impacts without failure.

    • Furthermore, a grid of ribs added to ensure part flatness may lead to mold-cooling difficulties and warpage.

    • Typically much easier to add than remove, ribs should be applied sparingly in the original design and added as needed to fine tune performance.


    Bosses
    Bosses are projections on a part designed to add strength, to facilitate alignment during assembly or allow for fastening additional parts. The most common variety consists of cylindrical projections with holes designed to receive screws, threaded inserts, or other types of fastening hardware.
    General rule for design is the outside diameter of bosses should remain within 2.0 to 2.4 times the outside diameter of the screw or insert. It is suggested to avoid bosses that merge into sidewalls because they can form thick sections that cause lead to sink.
    Normally, the boss hole should extend to the base-wall level, even if the full depth is not needed for assembly. Shallower holes can leave thick sections, resulting in sink. Deeper holes reduce the base wall thickness, leading to filling problems, knit lines, or surface blemishes. Because of the required draft, tall bosses (those greater than five times their outside diameter) can create a filling problem at their top or a thick section at their base. Additionally, the cores in tall bosses can be difficult to cool and support. Think about coring a tall boss from two sides or extending tall gussets to the standoff height instead of than the whole boss.