You have a comprehensive 3D model, and the molder can see the shape of the part from it. The part CAD helps the molder assess geometry, undercuts, and potential tooling requirements, but it may not fully communicate how the part must fit, function, or be tested within the assembly. So the quote could change at a later stage. But all this information is covered in the assembly information. This guide covers what assembly information should be included in a molding RFQ.
| Question | Answer |
|---|---|
| What assembly information should be included in a molding RFQ? | Explain how the part fits into the bigger assembly: what it connects to, the joining method (weld, screw, snap-fit, etc.), and any sealing requirements that apply. |
Which Assembly Details Need to be Included in a Molding RFQ?
Without an understanding of the assembly process, suppliers are making different assumptions about fit, hardware, testing, and scope. The key is to include the following information to allow suppliers to quote the same requirements.
| Assembly information | What to provide | Why the supplier needs it |
|---|---|---|
| Mating parts | CAD, drawings, or samples | Shows actual interfaces |
| Assembly method | Snap-fit, screw, weld, adhesive, press-fit, etc. | Determines secondary process and design requirements |
| Critical interfaces | Holes, bosses, slots, sealing faces, datums | Identifies functional dimensions |
| Tolerances | Assembly-critical dimensions and stack-up | Helps assess molding capability |
| Hardware | Screws, nuts, inserts, clips, seals | Defines supplied components and operations |
| Assembly sequence | Order in which parts are joined | Identifies access and interference issues |
| Functional tests | Leak, pull, torque, fit, electrical or other tests | Defines acceptance requirements |
| Supplier scope | Molding only or molding + assembly | Prevents quotation scope differences |
| Assembly quantity | Expected assembled quantity and production volume | Supports labor and automation assumptions |
| Packaging | Component or finished-assembly packaging | Affects handling and final cost |
Dimensions and Tolerances of Critical Assemblies
Specify which features control fit, hole, boss, snap hook and catch, and sealing face dimensions. Now indicate these features on the drawing so they are not lost in the details.
Then specify the tolerance for each of the critical dimensions.
General molding tolerances may be sufficient for some interfaces. Identify assembly-critical dimensions and specify any additional tolerance requirements needed to achieve the intended fit and function. The required level of dimensional control depends on the design, material, mating components, and functional requirements. For example, a snap-fit or press-fit may require specific dimensional control when the fit, retention force, or assembly performance depends on it, while other designs may work within standard molding tolerances. If multiple parts are involved in an assembly, indicate the relevant tolerance stack-up so that the molder understands what fit they can expect. Consider the specified materials and operating conditions as well, since factors such as moisture and temperature can affect dimensional stability and fit. A molder cannot evaluate a suggested tolerance for a particular resin without knowing which features are critical.
Assembly Method and Secondary Operations
You need to provide enough details so that the molder is not left with any guesswork. Mention how the part will be joined to other components, such as with screws, snap fittings, press fits, or heat staking. Also specify if the assembly uses vibration or ultrasonic welding, adhesive bonding, insert molding, or overmolding. You can also add any post-molding tasks, such as drilling or pad printing.
Also provide the specified resin grade and the materials of the mating parts, along with relevant operating temperature and moisture conditions. If the assembly will be repeatedly assembled and disassembled, mention that requirement as well. Material compatibility can affect joining methods such as welding, while temperature and moisture can affect dimensional fit. For example, nylon can change dimensions as it absorbs moisture, which may affect how mating features fit.
The portion itself is altered by the joining technique. Such an ultrasonic weld needs a specific joint design, such as an energy director. Whereas a press fit may require appropriate dimensional control of the mating features. It depends on the intended fit and function. Similarly, insert molding requires the molder to load and position metal inserts during each cycle, which has an impact on cycle time and cost.
Here, it is necessary to mention that, in any case, if the method is fixed but the design is not yet finalized, mention that too. Define the functional requirement and let the molder suggest a solution. You can say, "the joint must hold 50 N of pull" instead of discussing the particular arrangement of the ribs. When the requirement is clear, the molder can suggest a more affordable or moldable option better than the one shown in your drawing.
Add the Mating Parts in addition to the Molded-Part CAD.
Sending the CAD of the molded part indicates to the supplier what needs to be molded. It only tells what the plastic part looks like, and does not explain how that part interacts with the other parts around it.
What should be provided?
Depending on the nature of the work, this may include any of the following:
- CAD files for the parts that connect to your molded part
- Assembly CAD showing how the parts fit together
- 2D drawings with key dimensions and tolerances
- A BOM listing the related parts and hardware
- Drawings for screws, clips, inserts, and other hardware
- Seal or gasket details, including size and material
- PCB, connector, sensor, or other electronic details if alignment is important
- Physical samples, if they can help explain the fit or function
It is not necessary to provide the entire product, however. If a component has no bearing on the molded part, leave it out. Only include those components that have an effect on the assembly or performance of the product.
What happens if the mating phase is still unlocked?
There is the possibility that the mating/assembly design is not finalized yet, but you still want to send the RFQ and get a quote. In such conditions, just tell the supplier which dimensions are fixed and which can change. Mark unfinished CAD or assembly drawings as preliminary and provide your current expected clearances and tolerances. Also tell the supplier about anything that could affect tooling or cost.
State Who Is Responsible for Assembly
The RFQ should clearly state who is responsible for molding, secondary operations, assembly, testing, and packaging. This prevents suppliers from making different assumptions about what they need to include in the quote.
Molding supplier responsibility
You can list every operation the supplier is expected to perform. This may include:
- Injection molding
- Insert loading or insert molding
- Trimming, drilling, or tapping
- Heat staking or ultrasonic welding
- Component assembly
- Functional testing
- Inspection
- Packaging
Buyer-Supplied Components and Responsibilities
Specify any components the buyer will supply or processes it will carry out, such as screws, seals, metal inserts, PCBs, connectors, or others. Note whether the supplier has to install the component or whether the assembly is the buyer’s responsibility. If a particular operation is outside the supplier’s scope, state that clearly in the RFQ.
Include Functional Testing and Assembly Acceptance Requirements
The injection molding RFQ requirements should specify any test that is to take place on completed assemblies. A supplier may test each molded part and still have to perform additional tests on the completed assemblies.
Part inspection is not the same as assembly testing
Dimensional inspection of molded parts ensures that the molded part is according to the drawing. This can be related to dimensional accuracy, hole size, wall thickness, appearance, or any other specification. However, passing these tests is not always an indicator of an assembled product’s final performance.
For example, a molded housing can pass all dimensional inspections and still fail a leak test after assembly. Similarly, a snap-fit may meet dimensional requirements but fail a retention test.
Only include tests that apply to the actual product and its intended use.
What to specify
The RFQ should provide the test technique and acceptability limit for each necessary test. It should also mention how many units need to be tested and how often testing is required, such as first-piece, sample-based, or production testing.
Additionally, the RFQ should include whether the provider is required to produce test reports and who supplies the test equipment. Add the requirement that assembled samples be authorized before production can proceed.
Don’t let the supplier guess what the acceptability limitations are. If a product requirement has already been established, give it.
Myth vs. Reality: Common Assembly Assumptions in an RFQ
One of the things that can be overlooked while creating an injection molding RFQ is the assembly information for RFQ. While it is assumed by buyers that the CAD file, drawing, or general tolerancing notes will carry all required information for the supplier to proceed. But, in practice, some assembly requirements for injection molding need to be stated separately.
| Myth | Reality |
|---|---|
| The supplier can understand the assembly from the plastic CAD. | Just by looking at the single part model, one does not get the hint how it connects to the rest of the product. For better understanding, information about mating parts, assembly drawing, or interface dimensions is needed. |
| The general tolerance on the drawing is enough. | Different dimensions require different levels of control. You need to pay extra attention to the ones that affect fit, alignment, sealing, or how the parts stay together. If you see any dimension is important, you can clearly mark it on the drawing to let the supplier know that it matters. |
| We can decide the assembly method after the mold is built. | How the parts combine has a great impact on the part and the mold design. For example, ultrasonic welding may need a specific area for the parts to be joined, whereas a press-fit depends on both parts to be of correct sizes so that they fit together. |
| The supplier will include assembly if we need it. | It’s not necessary. Just let the supplier know what you want the quote to cover, whether that is molding, extra operations, assembly, testing, packaging, or all of these. |
| If the molded part passes inspection, the assembly will work. | There is the possibility that a part can meet its drawing dimensions, but still have problems during assembly. It all depends on the product, the finished assembly may need to be checked for things like leaks, fit, holding strength, or torque. Let the supplier know which tests are needed and what the finished assembly needs to pass. |
How Missing Assembly Information Can Affect Quoting
Even if there is a small gap in the assembly information, it can leave room for different interpretations. One supplier may understand the requirement one way, while another may think it differently. Thus, there will be quotes that look different even though everyone is working from the same part design.
So, before comparing the numbers, make sure you understand what each supplier has assumed and what is there in the quoted scope.
| Missing Information | Possible quote impact |
|---|---|
| Assembly features | Mold and tooling complexity |
| Secondary operations | Labor and part cost |
| Testing requirements | Inspection and testing cost |
| Assembly scope | Labor, equipment, and setup |
| Fixtures or automation | Tooling and setup cost |
| Purchased components | Material and handling cost |
If one supplier assumes manual assembly and another adds price as per custom automation, then both the quotes will look extremely different. The scenario will remain the same if they are quoting the exact same plastic part.
Final Takeaway
The supplier doesn’t want information about how to design or manufacture everything. They just want to understand how the molded part will be used. So, provide the assembly details to make them understand the requirements and come up with the right and final comparable quotes. However, in the absence of the exact assembly method, you can even ask them to recommend a suitable method.
Frequently Asked Questions
Is it necessary to give a physical assembly sample with an RFQ?
It is better to provide a physical sample as it helps suppliers understand how the parts should fit and work together. Along with that, a physical sample can also show things like assembly force, alignment, or appearance. Understanding these just from CAD and drawings is relatively difficult or may not be possible.
Should you add an assembly sequence in an RFQ?
It is best to include it if you know how to put the parts together, i.e., you know the order. This will help the supplier understand whether the sequence could affect access, fit, tooling, fixtures, automation, or testing. But if you haven’t decided on the sequence yet, let the supplier know, and you can even ask them to recommend an approach.
When should you include assembly drawings in an RFQ?
When the molded part needs to fit or work with other components, in that scenario including an assembly drawing is important. It gives the supplier a clearer picture of how the parts come together and helps them understand the important fit and interface details.