Accurately Measure Plastic Molded Parts

With injection molding, heated and melted plastic is shaped by injecting it into a mold. As the plastic material is injected at high pressure, thin-walled products and products with complex shapes can be formed at high speed, making this method suitable for commercial production. It is mainly used for molding of thermoplastic resins, but infrequently it is also used for molding of thermosetting resins.

In the next step, the molded part is removed from the machine and passes through various processes and inspections to become the plastic product. Major products that are produced by injection molding include the following.

• Smartphone covers
• Electric appliance housings, plastic model kits
• Toilet seats
• Automobile bumpers and instrument panels

Injection molding can be described as the most common method of plastic molding because it is used in molding and commercial production of such a wide range of plastic products of all sizes. Injection molding is further classified into the following methods according to the product which is manufactured.

With blow molding, heated and melted plastic is extruded in a tube shape and placed into the mold, then air is blown into the inside to inflate and shape the plastic. This method is suitable for producing plastic molded products that have hollow interiors, and it is also referred to as “hollow molding” because of the production method. Major products produced by blow molding include the following.

• Plastic drink bottles
• Containers for products such as liquid cosmetics and liquid detergents
• Tubes
• Tube joints

When the product will be filled with liquid, it may be necessary to select the type of plastic according to the liquid characteristics. For example, when it is necessary to prevent oxidation caused by permeation of oxygen, a plastic which has a gas barrier property is used. When chemical resistance is required, multiple molding materials, including plastics that do not deteriorate when exposed to chemicals, are molded in layer form.
There are various blow molding methods used according to the product being produced. This section explains the most typical blow molding methods.

The basic structure and principle of an extrusion blow molding machine are shown below.

A : Parison (hot parison) B : Mold

• 1
  The molten plastic is extruded.
• 2
  A die is used to form a cylindrical shape called a parison (hot parison).
• 3
  The parison is placed inside the mold before it cools and solidifies.
• 4
  Air is blown into it to form the desired shape.

The basic structure and principle of an injection blow molding machine are shown below. One typical product is plastic drink bottles made from polyethylene terephthalate (PET).

A : Stretch rod B : Mold C : Preform (cold parison) D : Heater

• 1
  Injection molding of thermoplastic resin is used to create a preform (cold parison) in a test tube shape in advance.
• 2
  The preform is heated and is set into the mold using a stretch rod.
• 3
  High-pressure air is blown into the preform to create the desired shape.

In order to prevent oxidizing and deterioration of the contents and to improve the strength, the material used for the parison is produced by co-extruding ethylene-vinyl alcohol copolymer (EVOH) or other material that has a high gas barrier property. This molding method is therefore suited for molding products such as plastic containers for cooking oil or seasonings, or gasoline tanks.
The basic principle of multi-layer blow molding is shown below.

A : Polyethylene (PE) B : Bonding layer C : Ethylene vinyl alcohol (EVOH) copolymer

• 1
  Co-extrude two or more types of plastic material to form a multi-layer preform (cold parison).
• 2
  Heat the preform (cold parison).
• 3
  Air is blown into it to perform blow molding.

Warpage and waviness of plastic molded parts means that the workpiece is curved like a bow. These problems are also called “bending” or “twisting.”Warpage consists of forward warpage and reverse warpage, according to the direction of warping.
Waviness refers to surface irregularities produced by warpage in various directions. The primary causes of warpage and waviness in plastic molded parts are the following.

• Differences in plastic shrinking
• Uneven mold temperature
• Differences in the shrinkage rates of the molten plastics resulting from the direction of flow

Warpage occurs when differences in plastic shrinking occur due to variation of temperature or pressure inside the cavity. The amount of plastic shrinkage inside the cavity is proportional to temperature and inversely proportional to pressure. As a result, when a workpiece has both a thin part and a thick part, because the thin part has a lower temperature, the amount of shrinkage is smaller, and more shrinkage occurs at the thick part because it stores more heat. The difference in temperature results in a difference in plastic shrinking inside the cavity, causing the mode of deformation known as warpage.

Strain in a plastic molded product is a phenomenon in which the entire workpiece becomes twisted or warped. It is caused primarily by residual stress. Residual stress in plastic molding means internal strain which remains in a plastic molded product. It includes tensile residual stress in which there is a force which acts toward the outside, and compressive residual stress which acts toward the inside.
Tensile residual stress is stress which pulls the inside of the molded product toward the outside. It occurs when molten plastic that was injected into the mold at high pressure begins cooling and solidifying inside the cavity starting from the outside. Compressive residual stress is stress occurring when the pressure applied to mold in order to form the product acts to compress the plastic toward the inside.

A plastic product where there is residual stress may not only lose its dimensional accuracy due to shrinkage or expansion, but may also cause deformation or cracking when the residual stress of the plastic product reacts to factors such as cutting, grinding, welding or other heating, and treatment or finishing using a coating or solvent. In addition to during manufacturing and processing, special caution is required because the shape may change due to strain which is produced as the product ages.
For this reason, the process called annealing may be used to eliminate residual stress.

When measuring warpage and waviness using a coordinate measuring machine, it is ordinarily necessary for the end of the probe to contact four or more points at the corners of the target measurement surface.
In case of a plate for example, generally six to eight points are measured. When the measurement area is large, measurement accuracy can be improved by increasing the number of measured points to collect more measurement data.
However, measurement of warpage and waviness involves the following problems.

• Because it is necessary to measure by contacting individual points, it is difficult to identify the shape of the entire target.
• Measuring more points to acquire more measurement data requires a lot of time, and it is not possible to identify the detailed shape of the entire target.

Hand-held tools such as calipers are very convenient for measurement. However, there are multiple causes which result in measurement error or variation in the measurement data.
With calipers, measurement conditions such as the contact force (measuring force) when measuring each point by hand and selection of the measured points differ depending on the operator. This results in variation in the measurement values and makes it difficult to obtain quantitative measurements. For warpage and waviness over a wide area, many points must be measured and inspections require a long time.

When a plastic product with a complex shape is measured using a CMM or calipers, measurement requires much time because of the many points that must be measured. Because only individual points are measured, it is difficult to measure the 3D shape of the target.
With the VL Series, you only need to place the target on the stage and scan it. The 3D shape of the target can be captured by non-contact means and no positioning is required. As profile measurement is also possible at any part of the target, it is possible to visualize and identify the locations and precise numerical values of shape defects.
This allows easy checking of the mold and molding conditions for trial products, and allows the causes of assembly failures to be smoothly identified and corrected in commercial products. Because the shape data is measured quantitatively, it is possible to easily control warpage and waviness based on tolerances, and to use the measurements for trend analysis.

It is possible to compare a product’s 3D-CAD data with the acquired measurement data to visualize any differences between the actual product and the design. For objects that were difficult to measure by conventional means, comparing the measurement results with the 3D-CAD data allows previously undetected defects to be identified. This is effective for inspections and also for cause analysis of defective fitting parts.
Because a wide range of dimensional measurements are supported, precisely measuring the dimensions at locations of concern allows problems to be analyzed in detail.

Cross-sections that are difficult to measure by conventional means can be created by non-destructive means that allow detailed measurement and analysis. Reference planes can be set as needed in the 3D shape data and cross-section measurement can be performed from every direction.

Discover how the VL-700 3D scanner revolutionizes quality control across industries, ensuring unmatched efficiency and reliability in your measurement processes. Request a demo today!

In this way, the VL Series can deliver dramatic improvements in thickness measurement efficiency and defect analysis.