R.J. Crawford

Simulation of the stretch blow molding process of PET bottles

Simulations of various stretch blow and blow moldings of axisymmetric PET bottles has been carried out using ABAQUS. A creep constitutive model with material data developed for a thermoforming process was used in the finite element analysis. Simulations using shell and solid elements were compared with experimental moldings. The creep material model, when combined with solid elements and a very high coefficient of friction, provided the best predictions for bottle side wall thickness, strains, blowing pressure, and general material movement. It was found that, using ABAQUS, the predicted wall thickness distribution of the material in an injection blow molded bottle agreed well with the values obtained using commercial process conditions.

Rotational molding of liquid plastic systems: An assessment of material moldability

This article describes the outcome of an investigation into the rotational molding of liquid plastics. Three liquid plastic systems were assessed: (1) a nylon block copolymer, Nyrim; (2) two grades of polyvinyl chloride plastisol, Acrol DS409A and Hydro PRC/65/0307/9270; and (3) two grades of polyurethane, Hyperlast 7850506 and Hyperlast 7853184. Initially, resin flow behavior was assessed in the uniaxial rotation mode. Characteristic flow regimes were identified for each liquid. The onset and duration of a particular regime depends on the initial viscosity of the liquid, its repooling behavior, and its curing profile. For a particular material, the finished part appearance (hydrocysts, bubbles, fault-free, etc.) depends on the mold rotation speed and material shot size, as well as mold geometry. A fault-free operating criterion is presented for the uniaxial rotation of liquid plastics. Uniaxial rotation tests are useful for predicting general trends in biaxial rotation of liquid plastics. The most important factors in determining the state of a finished part made by biaxial rotation are: (i) resin initial viscosity and curing profile; (ii) mold rotation speed; (iii) shot size; and (iv) mold shape. An ideal viscosity profile for a rotational molding liquid resin, based on information gained from both uniaxial and biaxial rotation tests, is proposed. The rheology of each material was also examined and the profiles observed were related to the molding behavior of each liquid.

Rotational Moulding of Liquid Polymers

The vast majority of rotationally moulded articles are produced from powdered polymers. However, the moulding process developed originally from the use of liquid polymers and nowadays there is a renewed interest in such systems because of some unique advantages that they offer. This paper compares the behaviour of three different liquid polymer systems—nylon 6 by ring-opening caprolactam, polyvinyl chloride plastisol and polyurethane. The flow behaviour of each material is examined with particular reference to wall thickness distributions and bubble formation in the product. On the basis of this, criteria for the production of fault-free mouldings have been established. The interrelationships between mould shape and resin viscosity are also examined and an ideal viscosity—time—temperature profile is proposed for liquid polymer systems. Finally, a general comparison of the materials is made with regard to material handling, safety, cycle times, etc.

Development of reactive rotational moulding process

Abstract A recent revival of industrial interest in rotational moulding of reactive liquid polymers has led to renewed research efforts in this area of polymer processing. This paper describes the design and construction of a reactive liquid polymer feed system for use in reactive rotational moulding. This feed system allows multiple material shots to be added to the mould without the need to stop mould rotation. The paper also describes work to develop a control technique that explores the exploitation of changes in ultrasound signal properties to monitor viscosity during cure in reactive rotational moulding (RRM). Tests have been conducted on dicyclopentadiene (DCPD) in a static off-line rig, to simulate the conditions that are expected during rotational moulding. The signals propagated through the DCPD indicate a rise in ultrasonic velocity during cure, associated with mechanical property changes owing to increased crosslinking. This technique is non-intrusive, and measurements can be obtained real-time for the duration of cure.