Q1: What is the root cause of TPV shrinkage?

A: TPV shrinkage is fundamentally caused by the crystallization of its plastic phase (typically PP) during cooling. The rubber phase (e.g., EPDM) is already crosslinked and shrinks little; the PP phase rearranges its molecular chains and crystallizes upon cooling, reducing in volume significantly. This is the main source of TPV shrinkage. With this in mind, all shrinkage-reduction strategies revolve around either suppressing PP crystallization or compensating for its shrinkage.

Q2: What can be done at the material formulation level?

A: There are three main approaches:

Adding inorganic fillers: Rigid fillers such as calcium carbonate and talc occupy space and restrict PP molecular chain contraction, effectively reducing shrinkage. Smaller particle size and higher loading generally give better results. However, care must be taken as they may affect scratch resistance or mechanical properties, and coupling agents are usually needed.

Adjusting PP phase content and grade: Reducing the PP ratio or switching to a lower-crystallinity PP grade can lower overall shrinkage. Patents indicate that incorporating specific components into thermoplastic elastomers makes the molding shrinkage rate easier to reduce.

Selecting low-shrinkage grades directly: TPV shrinkage varies significantly between suppliers, typically ranging from 1.2% to 1.6% (grades with higher filler content tend to be at the lower end). Choosing an already-optimized low-shrinkage grade is the most convenient solution.

Q3: In injection molding, which parameter has the greatest impact on shrinkage?

A: Pack pressure (holding pressure) is the core factor. Pack pressure should be sufficient, typically set at 50%–70% of injection pressure, to continuously compensate for the volumetric shrinkage of the melt during cooling. Pack time should cover the gate freeze-off time — this can be judged by weighing the part; when weight no longer increases with extended pack time, the gate has frozen.

Q4: How does temperature affect TPV shrinkage?

A: The higher the melt temperature, the greater the shrinkage after cooling. While ensuring adequate flowability, appropriately lowering the injection temperature and mold temperature (e.g., by 5–20°C) helps reduce shrinkage. However, be careful: excessively low mold temperature may cause internal stress or incomplete filling.

Q5: What compensation is needed in mold design?

A: TPV shrinkage is anisotropic — shrinkage in the flow direction differs from that in the perpendicular direction (for example, one grade may show 3.4% parallel shrinkage versus only 0.8% perpendicular). Before cutting the mold, shrinkage allowances must be reserved based on measured data, rather than scaling by a single shrinkage rate.

Q6: Are there physical methods to directly "hold" the part and prevent it from shrinking?

A: Yes. For long extruded profiles such as sealing strips, embedding copper wire or rigid plastic strips inside can physically constrain the part and maintain dimensional stability. For injection-molded parts, adding a skeleton or insert can also reduce shrinkage.

Q7: What are the key considerations for gate and wall thickness design?

A: The gate should be located at the thickest wall section to ensure pack pressure is effectively transmitted to the far end. Thick walls cool slowly and shrink more; adding reinforcing ribs can reduce surface sink marks. Wall thickness transitions should be gradual to avoid asymmetric shrinkage caused by uneven cooling.

Q8: If all the above measures are taken and shrinkage still exceeds the limit, what else can be done?

A: Consider post-treatment. After demolding, place the part at a constant temperature (e.g., 80°C for several hours) to allow residual shrinkage to release before shipment, preventing further deformation after delivery. You can also work with your material supplier to develop a custom low-shrinkage grade tailored to your specific part's wall thickness and runner structure.