Draw Ratio

Tooling Geometry

High values necessitate modifications in the plug assist or air pressure parameters to maintain wall thickness uniformity. Designers must account for the radius of the corners since tight angles increase local stress during the expansion phase. When a sheet enters a mould, the contact points solidify first while the remaining sections stretch to reach the final cavity walls.

Increased contact speed prevents premature cooling. Uneven cooling creates areas of weakness where the polymer density drops below the required threshold for structural integrity. Controlling the distribution of mass through temperature zoning allows for more predictable outcomes despite aggressive expansion requirements.

Resin Selection

Raw material suppliers provide datasheets containing the elongation at break, which serves as the upper boundary for what a specific polymer grade withstands under heat. Virgin resin batches typically offer more consistent rheological properties compared to regrind mixes, which suffer from shorter chain lengths after multiple thermal histories. Polymer chains that align too rapidly during the stretching phase lose the ability to redistribute stress, resulting in localized tears or rupture.

Moulders evaluate the melt flow index to predict how easily a material fills the tools while resisting the thinning associated with deep draws. High impact grades often contain elastomer modifiers that improve the stretching limit, though these additions raise the cost per pound of the final product.

Processing Limit

Excessive material stretching results in a part that fails to meet thickness specifications across its base and sidewalls, leading to high rejection rates and increased scrap costs. A moulder cannot force a material beyond its physical limits without compromising the safety and service life of the resulting container or housing. Monitoring the pressure profile allows operators to detect when the material reaches its limit, providing a chance to adjust heater banks or cooling times before the production run deviates from the intended target.

Consistent part weight remains the best evidence that the stretching operation stayed within the safe processing window for that specific geometry. Precise thermal management during the initial heating cycle keeps the polymer in a pliable state, which ensures the material fills deep recesses without excessive thinning. Proper optimization of these variables ensures the final component holds its shape under mechanical load.