he moulded plastic sample has a wide range of features that make it easy to understand the material properties.
Material stiffness:
The sample features a cantilever in the centre (marked with a blue dot in the illustration), which you can press to feel how easily it bends. You can also try bending and twisting the sample. It has a thickness of 2 mm (0.078 inch), which is typical for moulded plastic parts.
Density:
The volume of the sample mould cavity is 19.61 cm³. The moulded plastic samples are slightly smaller due to shrinkage during moulding. The amount of shrinkage is specific to each plastic material.
The density of the plastics we produce samples of varies between 0.9 and 1.5 g/cm³, so the weight of a sample ranges from 18 to 30 grams. You can feel the difference in weight when lifting the samples.
Live integral hinge:
The sample includes a live integral hinge in the corner.
Some plastic grades support the functionality of a live hinge. This feature, for example, allows a container and its lid to be molded as a single part.
For a live hinge to function properly, the plastic material must have:
- High elongation at break – to provide toughness to the hinge.
- Good flex fatigue resistance – to prevent the hinge from breaking after repeated flexing.
- Excellent flow characteristics during molding – to ensure the plastic can fill the thin hinge geometry in the mold.
Common materials suitable for live hinges include polypropylene (PP), polyethylene (PE), and various types of thermoplastic elastomers (TPEs).
For many of our materials, the hinge may break the first time you try to bend it because the material is not suitable for live hinges.
If the hinge is missing from the sample upon receipt, it indicates that the material's flow was insufficient during molding or that the hinge broke off during demolding. This is especially common with materials containing fillers such as glass fiber or wood fiber.
Surface quality
The sample has different surface qualities:
- Moderate spark-eroded surface: VDI 30
- Rather rough spark-eroded surface: VDI 36
- Well-polished surface: SPI-A1
- Sand-blasted surface: approx. VDI 21 or SPI-D2
- Non-finished machined surface
By comparing the samples, you can see how surface qualities appear in different materials
and the effect of fillers.
Plastic fluidity during moulding:
The viscosity or fluidity of plastic materials is indicated by the melt flow index (MFI). Materials with a higher MFI have lower viscosity when melted and flow more easily through the mould, allowing them to fill narrow gaps and small cavities. However, they are also more prone to causing flash by seeping between the mould’s parting lines.
The moulded sample features a window divided into nine sections, each with a different thickness ranging from 0.05 mm to 0.45 mm. The material’s fluidity can be assessed based on how many sections are filled: if only a few are filled, the melt flow index is low—and vice versa.
Plastic shrinkage:
Due to shrinkage during cooling, the actual size of the moulded plastic samples varies by material grade. The samples are typically 0.2% to 2.5% smaller than the mould cavity. In semi-crystalline and glass-reinforced plastics, shrinkage is slightly lower in the flow direction than perpendicular to it.
All samples were produced using the same mould, so any dimensional differences directly reflect material shrinkage. On the underside of each sample, two pairs of ribs are spaced exactly 70 mm apart in the mould. A measurable variation between these distances indicates directional shrinkage.
Sensitivity to sink marks:
Thicker wall sections cool and shrink more slowly than surrounding areas, often leading to visible sink marks on the product surface.
Sink marks commonly form at the bases of ribs and bosses. Their visibility can be reduced by increasing surface roughness, while their severity can be minimized by adjusting rib thickness. A typical guideline is to keep rib thickness at 60% of the adjoining wall thickness, though this can vary by material.
The sample includes three surface finishes, each with three ribs of varying thicknesses, allowing assessment of the material’s tendency to form sink marks.
Warpage:
Good plastic part design aims for even mould filling and uniform cooling. A common issue is inefficient heat dissipation at inner corners, which continue to shrink after surrounding areas have set. This leads to warping, commonly known as the corner effect.
Warping sensitivity varies by material. The sample includes two curved features (1, 2) for evaluating this: the rounded arc (2), representing better design, should closely match the mould geometry, while the sharper corner (1) is more prone to inward warpage.
Additionally, two ribs (3) at the rear edge help assess distortion and waviness here indicates higher warpage sensitivity. The overall straightness of the sample is also a good indicator of the material’s tendency to warp.
Effect of draft angles on surface finish:
Plastic parts should have draft angles to prevent surface scratches, reduce processing issues, and shorten cycle times. The optimal draft angle depends on the material and surface finish.
The samples left and right sides have different surface textures: VDI 30 (moderate spark erosion) and VDI 36 (rougher finish). Each side features areas with draft angles of 0.5°, 1°, and 2°. If scratches appear on certain surfaces, it indicates that the draft angle and surface roughness combination is unsuitable for the material used.
Technically, there is no disadvantage to using a larger draft angle. Draft angles should only be reduced when absolutely necessary and with clear justification.
Creep and stress corrosion:
Creep is the permanent deformation of a material under prolonged stress, where the part does not return to its original shape. It is influenced by time, temperature, stress, and material type. All plastics are prone to creep to some degree, though semi-crystalline materials show greater resistance. Creep must be considered in the design of plastic springs, snap-fits, and press-fits.
The sample includes a small creep indicator that can be deflected using, for example, a screw head or drill bit. After one week, the degree to which it returns to its original position indicates the material’s creep resistance.
If the indicator breaks or detaches, it may be due to stress corrosion, which is a failure mechanism where sustained stress and environmental factors (like solvents or fats) cause cracking. Amorphous materials are particularly vulnerable, and cracked bosses are a typical symptom. Stress corrosion should be considered in all plastic designs.
Friction:
Materials vary in their friction properties. Semi-crystalline plastics like POM and PA are known for their excellent performance in bearing applications. In other cases, a secure hand grip may be more important.
While surface roughness can influence grip, a rougher surface doesn’t always mean higher friction. Sometimes a polished surface may offer better grip.
The sample illustrates how different surface finishes affect the tactile feel of a material. You can also evaluate friction by rubbing the sample against metal or another plastic sample.
