How To Design TPE Parts For Injection Molding

• Material selection
• Driving environmental conditions identified
• Material performance extremes contrasted to requirements
• Material selection validated
• Part design process accounts for characteristics of selected TPE

Consult a good TPE information source to guide this part of the process.

Part Design Process

Designing a TPE part for injection molding will include the process steps common for developing a successful part design.The broad breakdown of the design process is into the following steps:

Kitchenware EPDM/PP TPV Industrial Design Example

The focus here will be on this last portion of the design process where the injection molding process design factors must be accounted for when designing details for the TPE parts.

Design For Injection Molding

The TPE part designed for injection molding needs to incorporate design features that work within the mold limitations, processing effects and economics associated with this process. Many issues required for rigid thermoplastics are roughly equivalent with TPEs. But since TPEs are softer and often quite flexible there are some factors that can be more forgiving and others that are less forgiving. These differences include:

Undercut Design Guidelines for TPEs

There are some processing options that may be limited to the specific TPE and grade chosen so the raw material supplier should be consulted.

The mold design process and process parameters are also important areas to developing a TPE part design. Mold filling analysis and mold design techniques are used to establish the best gate method and location. The runner system style is also a factor in the gating decisions. Molding cycle conditions, cooling and part warp can be evaluated prior to mold construction with computer mold filling analysis (MFA). Knit and weld line locations can be optimized for the gate location with MFA. Optimized tool vent locations can be established with MFA for TPE part molds. After these steps, typically, a prototype mold is prepared to validate the process and make parts to test.

TPE Wall Thickness Guidelines

There are a variety of other guidelines for TPE part design that are basically parallel to those used for rigid thermoplastics. Parts mold most consistently where there is a uniform wall thickness. Violating this guideline can result in over packed or under filled sections in the part and likely part warping. Design softer TPE grade parts with 0.25° draft angles and harder TPE grades with 0.5° draft angles to avoid ejection difficulties. Ribs thickness should be 50% of the wall thickness. Similarly, bosses should have a thickness of 50% of the wall. Undercuts of 0.7 mm for harder TPE grades to 2.0 mm for softer TPE grades can be ejected directly without using a special mold design articulation feature to relieve the interference. Typically part wall thicknesses should be limited to the range of 0.5 to 6 mm thick to avoid filling problems or extended cooling requirements. Part corners should have a radius, e.g. 0.5 mm, to avoid the air trap and subsequent burning that can occur.

Part Design Examples in Various Markets & Applications

TPE parts are used in an extremely wide variety of applications and markets. Example TPE parts that are injection molded include the following to cite a few.

Auto Gear Case Seal using EPDM/PP TPV

TPE injection molded parts are so ubiquitous that there may be very few industries where one cannot find them. The many markets could not all be listed here, but they include:

Conclusions

Those who are developing TPE injection molded parts will benefit from an understanding of TPE part design guidelines. There area a number of steps in the part development process that must be included in order to have a successful part. Designing to account for the injection molding process is critical to achieving good parts and to avoid long term processing and part quality issues. Well designed TPE parts will continue to be used more widely in commercial applications as they continue to prove their performance in the rapidly growing TPE market.

• Part Aesthetics and Differentiation
• Industrial design
• Overall design validated with feedback

• Criteria established for part function
• Design to meet functions
• Validate design functions
• Injection molding process design factors
• Mold design capabilities and limits accounted for
• Moldfilling conducted on part design
• Process economics and quality capabilities optimized
• Demolding, assembly and automation optimized

• Softer TPEs require large ejector pins
• TPEs have higher surface friction with more flexible material
• Deep textures are hard to eject and require larger draft angles, but may not be as difficult as for a rigid thermoplastic
• Short cooling times cause deformation, ejector pin marks since the softer TPE is easier to deform until fully cooled
• Ribs require a slightly higher draft angle
• Poor release draft angles can cause a vacuum and part collapse during ejection can occur easier in TPEs
• Runner systems are very flexible and can hang during ejection
• Flexible TPEs can often be readily ejected even with significant undercuts
• Avoids need for more elaborate mold splits or collapsible cores
• Allows for air ejection in some cases
• For overmolding use softer TPE over rigid plastic substrate to avoid deforming the softer TPE portion, and the TPE grade must be compatible to bond to the plastic
• Gussets to support bosses may be less problematic in the softer TPEs, where the high stress concentration points flex instead of cracking as in rigid thermoplastics

• Seals for plumbing
• Vacuum seals
• Oil seals
• O-rings
• Bumpers for appliances, automotive and furniture
• Power transmission / torque coupler
• Mounts
• Sport grips
• Overmolded soft touch surfaces
• Tool handles
• Flexible keypads
• Armor cladding for electronic devices
• Syringe plunger seal
• Connectors
• Electrical plugs
• Strain relief on small electrical appliances
• Tray liners
• Air bag door covers
• Shifter and hand brake grips
• Auto headlamp seals
• Kitchenware
• Eyedropper bulbs
• Drive axle seals
• Appliance motor mounts

• Appliances
• Architecture
• Automotive
• Medical
• Hardware
• Houseware
• Electronics
• Trucks
• Industrial equipment
• Hose
• Electrical
• Stationery
• Plumbing
• Sporting goods
• Power transmission
• Irrigation
• Cosmetics
• Electrical