Plastic Part Tooling Strategies for Medical Device Development

Time and Cost Reduction for New Product Development

For consumer products with plastic parts, there are many choices for methods of fabrication, all with costs and benefits; for medical products with plastic parts, these choices carry added complexity due to the complex nature of medical devices and the materials and precision needed, along with the regulatory environment. This article presents a guide to stepping through the decisions needed to choose the optimal manufacturing technique for your medical device design’s plastic parts, considering business, financial, and technical constraints from early concept development to low- and high-volume manufacturing.

Part Volume for Plastic Molding

In today’s manufacturing environment there are a multitude of choices for plastic molding.

Very often the part volumes needed in the early stage of a product’s life do not warrant the high costs of production molding tooling. As such this paper presents options such that it is possible to chart a clear path through the tooling strategy to achieve the needed part performance in a time and cost sensitive manner. Such decisions are typically based not only on engineering factors such as part strength, cosmetics, cleanliness etc., but also organizational requirements, typically product volume, tooling budget constraints, approved tooling suppliers, and project schedule.


Evaluate several different approaches to procuring plastic parts in any development phase. There is more than one way to get it done.


Key information to nail down before talking to suppliers includes answers to the following questions:

  • What will be annual volume of the part in production?
  • What part properties, (Medical grade, high strength, low cost, tight tolerances etc.,) are needed?
  • Will domestic suppliers meet you needs, or will offshore supplier be considered?

Plastic Part Molding Technologies

NOTE: These general rules of the road vary by part complexity and size.

Urethane castings are a great way to generate a small volume of realistic looking parts quickly (10-20 days,) at a low total cost, (Including mold cost,) but not low part cost. Urethane parts are cast in a silicone mold. The mold is typically created from a 3D printed and sanded master part. Typically, 8-15 parts can be made from a silicone mold (before degrading/breaking,) depending on part size, complexity, and draft angles. The master 3D printed part can typically create 3-5 silicone molds. Urethane parts usually require painting in order to have good cosmetics. The urethane material is not as strong or tough as injection molded plastic parts. Dimensional accuracy and flatness/straightness are not as good as injection molded parts.

Reaction injection molding (RIM) is a low pressure (~100psi) molding process that combines two components, (Think epoxy,) to create the part. RIM is typically used for lower volume larger parts where injection molding tooling cost would be exorbitant. Finishing and painting is required as the molded part surfaces often have pits, and the natural color is a marbled gray. While the mold is typically opened/closed by a press, side cores and other features are usually hand placed. RIM is typically cost competitive up to about 800 parts/year. Above these quantities injection molding or structural foam becomes more economical. As such large parts that need good rigidity such at the fascia of ATM’s, large medical equipment covers, and medial mobile cart covers are typical for RIM. The highly manual molding process allows for complex features such as multiple side pulls and undercuts. In addition, the low-cost mold (Low pressure), and medium tool time creates a highly competitive option for large parts and high security/high strength parts. The downside to RIM is that the part price remains high even when volume increase due to the manual nature of the molding and part finishing.

Prototype plastic injection molding (Class 105) is the go-to solution for getting dozens of parts up to approximately 10,000 total from the mold, (After this the mold can start to wear.) The intended production plastic material can be used. With many organizations mold changes can be made quickly and relatively inexpensively. Cost for a mold is greater than what you would pay for urethan cast parts, but the upside is that one the tool is paid for, many parts can be made. Part cost is usually in the range of a few dollars per (small) part, which is much cheaper than urethane. Tool cost typically ranges from $3k to $20k depending on size and complexity. Prototype injection molding typically uses aluminum cavity blocks that either fit into a standard mold base or are the entire side of the mold. Aluminum can be rapidly milled, ram EDM’d, (Electric Discharge Machining,) or wire EDM’d, thus allowing for a relatively economical and rapid way to get to a medium to high precision and repeatable part, (Depending on supplier). Competitive tool turn around time is 5-15 working days. Part finish will typically not be as cosmetic as a hard tooled parts but is usually sufficient for prototype builds.

Structural foam uses plastic injection molding equipment and mold making techniques; the difference is that a foaming agent is added to the plastic such that larger parts, thick wall parts, or varying cross-section parts can be filled with a smaller injection molding machine; (Larger injection volume and lower clamp force/lower injection pressure.) This also reduces the plastic material weight and cost. Structural foam is applicable for volumes of dozens on up. Tooling time is in medium range typically: 3-8 weeks depending on the supplier and complexity of the part. Mold cost is significantly higher than RIM due to the higher injection pressures requiring a precision machined mold that is designed to be used in an injection molded machine. Structural foam parts are not cosmetic as molded, (Think plastic utility carts.) Painting is needed if there are cosmetic requirements.

Mid range plastic injection molding tooling (Class 102, 103, 104) represents the middle ground in production volumes. With tool lives ranging from 100k to 1MM maximum shots, these types of tools are commonly used when the volumes do not require more expensive tools. There are many steel options that can be used, different tool builders have varying favorites. Typically steels used due to their ease of machining are P20 or 17-4. The steel is used as machined in the mold, as opposed to hardening and coating before molding. There is a wide range of part and mold costs in these categories.

Gas assist injection molding uses plastic injection molding machinery with the addition of a gas assist unit. The gas injection into the mold after the plastic allows thick-walled parts to be molded with less plastic volume and no sink. High injection molding tooling costs typically relegate this technology to higher part volumes.

Hard tooled plastic injection molding (Class 101) is used when either very precise and accurate parts are needed, or there are higher annual volumes. For a very precise part, the lowest annual volume is typically 5000/yr. Hard tooling with multiple cavities is the high-volume leading technology. Through the prototype stage the mold dimensions are dialed in such that when the production tool is cut, there is very little guess work when it comes to meeting the dimensional requirements. For high cavitation, (8+ cavities,) tools, typically a four-cavity tool is built first to verify cavity fill and cooling using a hot runner system. “Hard” refers to the steel used in the cavity. It is typically hardened tool steel such as A2 or 420SS with an anti-wear coating such as titanium-nitride or Nickel-PTFE. Mold cost is high (starting at $50k, average is $120k,) and build time starts at 8 weeks, average is 12-14 weeks. Any mold changes needed will be expensive. With all that investment, comes significant rewards, high volumes of parts at a low cost with precision dimensions and finish. The high quality does not happen automatically, however. It is important to engage a supplier with a robust quality control system such that they can dial in the mold and molding parameters to produce the part in specification with out asking the customer for tolerance expansion etc.

Examples

Softball size medical part that will be injection molded in production, 15 prototype parts needed, annual volume anticipated to be 5000/year

  • Prototype tooling: With the low volume of prototypes needed, urethane casting is a good option since there is not a need for specific material properties.

  • Middle step: Aluminum proto tooling would be adequate to get the project through V&V testing.

  • Production tooling: Being a medical part, a supplier with ISO certifications and a clean room may be required. Aluminum proto tooling would be the most economical but may not provide the needed level of repeatability. Often medical parts need stainless steel tooling to meet cleanliness requirements. For these volumes a class 104 tool may be applicable.

Soccer ball size part, high strength material is needed, 150 prototype parts needed, annual volume 150,000/year (NOT medical)

  • Prototype tooling: With the strength requirement and higher volume of protos, urethanes will not work. As such going to proto injection molding is a good way to get started. This way you will have functional parts that can be dialed in as needed.

  • Production tooling: At these volumes a class 102 or 101 tool is applicable. Work with your molder on specific needs from the tool for the application.

Large part (20”x30”) with thick walls, 8 prototype parts needed, annual volume 500/year

  • Prototype tooling: Low volume and not high strength is a great fit for urethane cast parts.

  • Production tooling: RIM is a good fit here. With the low volumes and thick walls, RIM will be very cost competitive. While structural foam could be used to make these parts, the tooling cost would be much higher than RIM.

Summary

The information presented here provides general best practices based on the technologies most commonly used and so most cost competitive in industry. Suppliers will have varying levels of expertise and price competitiveness at different volumes (i.e. 5000/yr vs 150,000/yr.) The most important thing you can do to be successful in your plastic part tooling strategy is to understand your suppliers’ capabilities, strengths, and weaknesses, and work with suppliers you can trust.

Tensentric is a team of highly experienced engineers developing a wide range of medical devices and in vitro diagnostic systems. Tensentric has completed over 300 development projects for clients in the medical device and IVD space since the company’s inception in 2009 and is ISO 13485:2016 certified for design and manufacturing. With capabilities for BSL-2 lab use, manufacturing process development, rapid prototyping, human factors validation and consulting, and in-house design for injection molding expertise, Tensentric is uniquely suited to a wide variety of medical device design, development, and manufacturing application.

  |  boulder, co   |  www.tensentric.com