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Design Recommendations and general practices for metal casting

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General
Accuracy and Feature Sizes
Drafting, Blending and Fillets
Surface Finish
Weld Repair
Heat Treatment
Straightening Fixtures
Machining


GENERAL:
The following is your set of instructions and recommendations to streamline the use of Soligen as a "factory linked to your CAD system". These design recommendations should apply to cast parts that are made either by DSPC or conventionally. DSPC does not require draft or core supports, and therefore DSPC made parts can be identical to cast parts regardless of their casting method, however, in preparation for production it is always better to consider the final production geometry. Thus if the goal is to use sand casting, lost foam, permanent mold, investment casting or die casting, the net shape as other casting considerations should be taken into account when designing the part's geometry.

Once we obtain your approval of the first article parts (which were created directly from your CAD file in the patternless DSPC process), our design team develops production tooling (patterns or dies) from the same CAD file as your approved first article. This production tooling is then used at our foundries to fabricate production quantities of your parts utilizing traditional casting practices. Thus, Soligen is able to provide you with a smooth transition from design to production, since both the first article parts and the short production runs conform to your CAD file.

Soligen uses either gravity or low pressure casting processes to produce DSPC parts, therefore casting rules of thumb should be observed. The DSPC process is capable of producing a net shape casting. When you send a file, unless you otherwise instruct us in writing, we will produce a cast metal part that will match your STL file (except for un-castable features)*. If the part is to be machined by you, please send a cast part file, or a drawing with instructions that specify:

    • Machining stock.
    • Machining datum points and references
    • Holes to be plugged
    • Areas where you do not want us to put gating or risers (for part accuracy)

* If we detect non castable features, Soligen will contact you to receive your authorization prior to any modification of the CAD file.

Soligen can add machining stock, plug holes, etc., and will do it upon your request. The first two hours of such CAD work are included in your price. You should also be aware that sharp corners and non blended transitions from thin sections to thick sections are sources for casting defects (such as porosity, inclusions and shrinkage). Such areas are also sensitive to cracks and distortion in heat treatment. Avoid sharp corners, a minimum radius of 1 mm (0.040") is recommended. Radii are a function of the thickness of the sections that they connect. For wall thickness, walls thinner than about 2 mm (0.080) in your design can result in misruns or improper filling. Please consult with a casting design handbook for further information. Our sales representative or program engineer will be glad to recommend references.

CASTING ACCURACY AND FEATURE SIZES:
As a good practice, you should generally assume that the accuracy of the part will conform to standard sand casting in order to reduce problems later in your manufacturing cycle. For tolerances on non-heat treated cast parts, refer to the following table:

Dimension Tolerance (inches)
Equal or smaller than 1 inch +/- 0.021"
Between 1" and 12" 0.021 + 0.002" per inch
Greater than 12" 0.021 + 0.005" per inch
Angles +/- 30'

While the DSPC process tends to be more accurate than standard sand casting, please be specific in your cast parts accuracy requirements.

DRAFTING, BLENDING AND FILLETS:
DSPC castings do not require drafting and fillets, because there is no tooling per se which must draw. The DSPC part will therefore be identical to your STL file in this respect, i.e., if drafts and fillets will be in the part only if they are in the part file. Sand castings are often modified from the original geometry so that the tooling can be withdrawn from the sand without damaging the imprint. As a good practice you should plan to allow a draft of at least 1 degree on walls with low aspect ratio and up to 2 degrees when available.

Another important consideration in cast part design is to make an effort to keep the wall thickness of the part as uniform as practically possible. That is because a transition from thick to thin wall section is sensitive to shrinkage due to uneven solidification. Sharp corners and unblended transitions are the most sensitive. You should therefore attempt to blend and fillet intersecting sections. To increase the likelihood of getting a sound casting the design should call for smooth blending and as large a fillet diameter as possible. Finally, if you plan to order multiple pieces i.e., sand casting or low pressure permanent mold casting (short production runs) and are not aware of the constraints under which a pattern maker will operate, please make your requirements clear and indicate so at the time that the request for quotation (RFQ) is placed. Our technical team will be happy to assist you.

SURFACE FINISH: DSPC and production casting are finished to remove all gating. Certain cast parts require core support holes, or core removal "windows" that need to be welded into the part. In such cases Soligen will grind all external welding traces and attempt to make the surface as close to the original design as possible. Minor casting defects are welded and ground smooth prior to sand or shot blasting. Glass bead finishing, hand sanding and even polishing is available upon request. Please see DSPC capabilities for the surfaces of DSPC parts.

WELD REPAIR: DSPC is not different from any other cast metal part. You should also be aware that it is industry practice to make minor weld repair to short runs of castings in order to improve their appearance and to correct minor defects. Unless specifically instructed to avoid weld repair at the time of the request for quotation, Soligen will assume that industry practices apply. Therefore, if your part is structural and you want us to avoid weld repair or restrict it to areas or certain features, please indicate so with written weld relief comments. For core supports and core removal windows welding is performed prior to heat treatment. Small surface defects in non critical area may be welded after heat treatment.

HEAT TREATMENT: Soligen recommends that all aluminum 356 and 357 castings should be heat treated, in case of producing cylinder heads from non heat treated alloys such as 319 and 319.2 same heat treatment is recommended. Steel parts require normalizing. Soligen recommends that unless you are in an extreme hurry to receive your parts (heat treatment takes 2-4 business days depending on the alloy), or in case that the heat treatment is recommended during the machining process, you specify and allow us to heat treat your parts. All heat treatments are performed at certified vendors and certificates are available upon request. For materials with special requirements or exotic specifications, we prefer to be given a temperature profile rather than a spec.

STRAIGHTENING FIXTURES: Depending on the geometry some cast parts may deform during the heat treatment process. In such cases, it is customary in the foundry industry to perform straightening. Parts that are prone to deformation are parts with "open frame structure" and parts, which consist of geometrical features that can sag, or parts that contain geometry sensitive to internal stress concentration while being solidified. Long thin walls, unsupported cantilevers, as well as non-blended changes in wall thickness, are natural sources for distortions. In obvious cases Soligen's design team may incorporate "tie bars" that assist in converting open to close frame structures. These tie bars will be removed after heat treatment and in some cases after rough machining. Soligen's design team will be glad to advise you on this matter including technical discussions and prices concerning the required fixtures. Please be advised that failure to specify straightening and proper aging may result in warpage or distortion after machining, additional heat treatment cycles or aging of the finished parts as residual stresses release.

MACHINING: Soligen provides accurate machining of parts such as multi cylinder heads, manifolds, steering components, valves and pumps. For accurate machining quotes, Soligen requests that you provide machining drawings or specifications outlining the machining tolerances. Soligen welcomes as much information as you permit on the part including its use in the final product, mating parts and loading factors on the part. Part of our Parts Now program service is to work with our customers on manufacturability and value analysis. As a rule of thumb, machining surfaces should have a machining stock of 3 mm, and small features of less than 3 mm or holes with high aspect ratio should be plugged and later machined. STL and IGES files are notoriously weak in their usefulness for adding stock for machining or finishing, and it will almost always be more expensive for the customer to get Soligen to modify such files rather than doing it within your company. Please be advised that large surfaces should include more machining stock, to allow for parallelism and perpendicularity to the datum points. Holes which require accurate drilling should simply be filled. It is often a good idea to put an undersized dimple where the hole should be in order to make the machinist's job easier.