So Cheap, Yet so Expensive
Modern day injection molding practices can get many (if not most) plastic parts made very cheap – under $1.00 – but there’s a catch: mass manufacture using injection molding comes with a hefty up front fee, often ranging from 10-100 thousand dollars. So is this worth it? And where is this money doing?
We will discuss 4 major factors in Mold Costs:
- Product Size
- Shape & Complexity
- Mold Material
- Surface Finish
Then, we’ll discuss a few additional items that play into the overall cost.
Injection molding is a common manufacturing process where a material is melted and injected into mold, or cavity, the liquid solidifies into the shape of the cavity, and is then extracted. Although this can be done with a variety of materials, we’ll talk more specifically about plastic injection molding today as this is one of the most common and most versatile materials in products today.
A Brief Look at a Common Practice
Plastic injection molding is the most common method of mass producing plastics. To discuss the costs associated with this manufacturing process, I’ll provide a brief summary, but Bill Hammack has a great video detailing this if you want to learn more. Very simply, a mold is made by taking a block of material (typically stainless steel or aluminum) and cutting out (machining) the desired product shape (cavity).
Once made, a mold can be used to make from tens of thousands of units to millions of units (shots) of product. A simple mold can cost as little as $1,000 but as we will discuss shortly, these prices can quickly escalate based on a series of factors. In addition, a finished product will typically be comprised of multiple plastic components, further adding to this up front cost.
Based on the considerable up front costs of injection molding, it is mainly feasible for large production runs. These costs may also be reimbursed by the factory (depending on the factory) at certain production quantities milestones (to be discussed in the negotiation stage, not the production stage).
Factors in Mold Costs
There are many factors that play into the cost of a mold. Here are the biggest ones to consider:
1. Size of product
What: Simply, the volume of material required to make the appropriate size of cavity will be larger
What can you do: This depends on the type of product – often times, there is nothing to be done. If you have come to the manufacture stage and have designed a large product, it is likely for a good reason and you may not be able to do anything. If this is a functional component, and aesthetics are not important, this is something you may be able to adjust. If you do, this is likely a design decision and the design team should be consulted on any changes.
2. Shape & Complexity
What: A complex shape is going to take longer to cut out, and require more oversight to manage machining, setup, and mold deformations. (Note, two-shot molding will not be considered in the scope of this discussion.)
What you can do: Again, this goes back to the design process as any change will be significant and noticeable. Any unnecessary complexities should already be dealt with or reviewed with the design team. What you can do, is address large sweeping curves and extremely sharp corners as these will increase cost due to tooling time and may compound with surface finish requirements – these should be standardized unless a key part of the design.
3. Production Quantities
What: Mold material will determine how long the mold lasts – using softer aluminum is easier on the machining tools, and cheaper, but will not last as long due to the wear on this softer material. Harder stainless steel is more expensive to obtain, and takes longer to machine but will last longer as well.
What you can do: Know your priorities. Are you looking for the best deal on a long term investment or do you need it done cheap now, and are ok with spending more later on?
4. Surface Finish
What: Machining marks left on the mold will be visible on the molded part. If a surface is visible, it should be finished to one degree or another. However, the finer the finish, the more it will cost. Large surface areas with a high polish can get very expensive. Similarly, textured surfaces are extremely expensive. These finishing processes are more precise, remove much less material than standard machining processes, and require more oversight and thus take more time/cost more money.
What you can do: You can remove any unnecessary polishes and textures (ones that are not critical to function, such as seals, or to aesthetics, such as surfaces not visible to the end user). In addition, know what you want – A matte finish will go a long way in decreasing cost, and may actually fit your product/brand better.
A few bonus items that affect the costs of a mold:
Tolerancing – Tolerancing is important if it is finer than typical machining tolerances. These will differ based on the common practice (to be considered if you are looking oversees), but +/- 0.005” is typical and is important for smaller, functional components.
Shrinkage – Shrinkage will also factor into tolerance and is dependent on part size (about 0.002in/in), and similar to tolerance is more important for functional components.
Multi-cavity Molds & Family Molds – based on the batch production size or cost reduction efforts, a single mold can make multiple parts. This can make sense if you are running large batch sizes or your part size is very small. Family molds have multiple cavities in a single mold to make different parts (rather than multiple of the exact same part). This must be done carefully as different part geometries often cool at different rates making for sub-optimal results.
Mold Base Compatibility – Your mold is your property and a considerable barrier to entry for competition. Practically speaking, that means you can take it to a different factory (and/or country) if you don’t like your current factory… right? Well not always, some factories will create molds that only fit their machines. Creating universal base and interface will cost more if this is not standard practice at the factory you are speaking to. This will not be a problem if you have found a good factory the first time, so take the time to make sure the factory you use is a good fit for you.
Design for Injection Molding – with your mold cost will come dedicated time to design the mold. This is not part of Design for Manufacture and Assembly (which is product focused), this involves the design of the overall mold – the cavity itself, the orientation and location of the split line, the placement of ejector pins/holes, placement of sprue and runners, cooling channels, and overall mold fit.
T0/T1/T2 Samples & Tooling Changes – Every injection mold is different. It’s new, and new things don’t work the first time. It is standard to go through 2 iterations of mold changes to deal with imperfections, these are referred to as T0, T1, and T2 samples and tooling modifications are made between each to address any necessary changes.
Setup Costs and Labor – simply, all the time associated with addressing each of the above items and everything in between.
Do what you can that is within your realm of expertise. However, do not be afraid to find a professional to help you with this. Factories are a great resource and have the expertise. Working with a designer that can address some of the earlier design decisions can make a significant impact.
About the Author
Ventrify is a product design and manufacture firm that helps entrepreneurs bring product ideas from concept to market. We take in fledgling ideas and bring them through our iterative design process to create products our clients can be proud of. Then, we work with manufacturing facilities across the world to bring our clients the highest quality products at competitive prices.